Found 47 papers in cond-mat
Date of feed: Thu, 20 Jul 2023 00:30:00 GMT

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Quantifying Alignment and Quality of Graphene Nanoribbons: A Polarized Raman Spectroscopy Approach. (arXiv:2307.09490v1 [cond-mat.mtrl-sci])
Rimah Darawish, Jan Overbeck, Klaus Müllen, Michel Calame, Pascal Ruffieux, Roman Fasel, Gabriela Borin Barin

Graphene nanoribbons (GNRs) are atomically precise stripes of graphene with tunable electronic properties, making them promising for room-temperature switching applications like field-effect transistors (FETs). However, challenges persist in GNR processing and characterization, particularly regarding GNR alignment during device integration. In this study, we quantitatively assess the alignment and quality of 9-atom-wide armchair graphene nanoribbons (9-AGNRs) on different substrates using polarized Raman spectroscopy. Our approach incorporates an extended model that describes GNR alignment through a Gaussian distribution of angles. We not only extract the angular distribution of GNRs but also analyze polarization-independent intensity contributions to the Raman signal, providing insights into surface disorder on the growth substrate and after substrate transfer. Our findings reveal that low-coverage samples grown on Au(788) exhibit superior uniaxial alignment compared to high-coverage samples, attributed to preferential growth along step edges, as confirmed by scanning tunneling microscopy (STM). Upon substrate transfer, the alignment of low-coverage samples deteriorates, accompanied by increased surface disorder. On the other hand, high-coverage samples maintain alignment and exhibit reduced disorder on the target substrate. Our extended model enables a quantitative description of GNR alignment and quality, facilitating the development of GNR-based nanoelectronic devices.

Flat-band spin density wave in twisted bilayer materials. (arXiv:2307.09506v1 [cond-mat.str-el])
Zhigang Song, Jingshan Qi, Olivia Liebman, Prineha Narang

Twisting is a novel technique for creating strongly correlated effects in two-dimensional bilayered materials, and can tunably generate nontrivial topological properties, magnetism, and superconductivity. Magnetism is particularly significant as it can both compete with superconductivity and lead to the emergence of nontrivial topological states. However, the origin of magnetism in twisted structures remains a subject of controversy. Using self-developed large-scale electronic structure calculations, we propose the magnetism in these twisted bilayer systems originates from spin splitting induced by the enhanced ratio of the exchange interaction to band dispersion.

Dissipative phase transitions and passive error correction. (arXiv:2307.09512v1 [quant-ph])
Yu-Jie Liu, Simon Lieu

We classify different ways to passively protect classical and quantum information, i.e. we do not allow for syndrome measurements, in the context of local Lindblad models for spin systems. Within this family of models, we suggest that passive error correction is associated with nontrivial phases of matter and propose a definition for dissipative phases based on robust steady state degeneracy of a Lindbladian in the thermodynamic limit. We study three thermalizing models in this context: the 2D Ising model, the 2D toric code, and the 4D toric code. In the low-temperature phase, the 2D Ising model hosts a robust classical steady state degeneracy while the 4D toric code hosts a robust quantum steady state degeneracy. We perturb the models with terms that violate detailed balance and observe that qualitative features remain unchanged, suggesting that $\mathbb{Z}_2$ symmetry breaking in a Lindbladian is useful to protect a classical bit while intrinsic topological order protects a qubit.

Impurity screening by defects in (1+1)$d$ quantum critical systems. (arXiv:2307.09519v1 [cond-mat.str-el])
Ying-Hai Wu, Hong-Hao Tu, Meng Cheng

We propose a novel mechanism of impurity screening in (1+1)$d$ quantum critical states described by conformal field theories (CFTs). An impurity can be screened if it has the same quantum numbers as some gapless degrees of freedom of the CFT. The common source of these degrees of freedom is the chiral primary fields of the CFT, but we uncover that topological defect lines of the CFT may also take this role. Theoretical analysis relies on the insight that the impurities can be interpreted as edge modes of certain symmetry-protected topological (SPT) states. By stacking a SPT state with a CFT, one or two interfaces on which the SPT edge modes reside are created. If screening occurs due to topological defect lines, a symmetry-enriched CFT with exotic boundary states are obtained. The boundary conditions that appear in these cases are difficult to achieve using previously known methods. Numerical simulations are performed in two quantum spin chains whose bulks are described by the SU(3)$_{1}$ and Spin(5)$_{1}$ CFTs and edges are coupled to spin-1/2 impurities. In both cases, low-energy eigenvalues are consistent with analytical predictions.

Controllable Creation of Skyrmion Bags in a Ferromagnetic Nanodisk. (arXiv:2307.09528v1 [cond-mat.mes-hall])
Lan Bo, Rongzhi Zhao, Chenglong Hu, Xichao Zhang, Xuefeng Zhang, Masahito Mochizuki

Skyrmion bags are composed of an outer skyrmion and arbitrary inner skyrmions, which have recently been observed in bulk chiral magnets, but still remain elusive in magnetic films. Here, we propose a method of creating skyrmion bags in a thin-film nanodisk, which includes three steps. Firstly, the size of outer skyrmion is enlarged by a vertical magnetic field, then inner skyrmions are nucleated at an off-center area by local current injection, and the system is finally reconstructed due to multiple inter-skyrmion potentials. Thus, skyrmion bags with topological charge up to forty can be created. Simulated Lorentz transmission electron microscopy images are given to facilitate the experimental demonstration. Our proposal is expected to inspire relevant experiments in magnetic films, and pave the way for potential spintronic applications based on skyrmion bags.

Yu-Shiba-Rusinov tips: imaging spins at the atomic scale with full magnetic sensitivity. (arXiv:2307.09534v1 [cond-mat.supr-con])
Felix Küster, Souvik Das, Stuart S. P. Parkin, Paolo Sessi

Measurements of magnetic properties at the atomic scale require probes capable of combining high spatial resolution with spin sensitivity. Spin-polarized scanning tunneling microscopy (SP-STM) fulfills these conditions by using atomically sharp magnetic tips. The imaging of spin structures results from the tunneling magneto-conductance that depends on the imbalance in the local density of spin-up and spin-down electrons. Spin-sensitive tips are generally formed from bulk materials or by coating non-magnetic tips with a thin magnetic layer. However, ferromagnetic materials generate stray magnetic fields which can influence the magnetic structure of the probed system, while the magnetization of antiferromagnetic materials is difficult to set tip by externally applied magnetic fields. Here, we use functionalized Yu-Shiba-Rusinov (YSR) tips prepared by attaching magnetic adatoms at the apex of a superconducting cluster to image magnetic interactions at the atomic scale. We demonstrate that YSR tips are capable of sensing different magnetization directions, conferring them full magnetic sensitivity. We additionally show that the finite size of the tip superconducting cluster makes it robust against relatively strong magnetic fields, making YSR tips capable of visualizing magnetic field driven transitions of the spin texture.

On the behaviour of a periodically forced and thermostatted harmonic chain. (arXiv:2307.09535v1 [math-ph])
Pedro Garrido, Tomasz Komorowski, Joel L. Lebowitz, Stefano Olla

We consider a chain consisting of $n+1$ pinned harmonic oscillators subjected on the right to a time dependent periodic force $\cF(t)$ while Langevin thermostats are attached at both endpoints of the chain. We show that for long times the system is described by a Gaussian measure whose covariance function is independent of the force, while the means are periodic. We compute explicitly the work and energy due to the periodic force for all $n$ including $n\to\infty$.

Large and tunable spin-orbit effect of 6p orbitals through structural cavities in crystals. (arXiv:2307.09545v1 [cond-mat.mtrl-sci])
Mauro Fava, William Lafargue-Dit-Hauret, Aldo H. Romero, Eric Bousquet

We explore from first-principles calculations the ferroelectric material Pb5Ge3O11 as a model for controlling the spin-orbit interaction (SOC) in crystalline solids. The SOC has a surprisingly strong effect on the structural energy landscape by deepening the ferroelectric double well. We observe that this effect comes from a specific Pb Wyckoff site that lies on the verge of a natural cavity channel of the crystal. We also find that a unique cavity state is formed by the empty 6p states of another Pb site at the edge of the cavity channel. This cavity state exhibits a sizeable spin splitting with a mixed Rashba-Weyl character and a topologically protected crossing of the related bands. We also show that the ferroelectric properties and the significant SOC effects are exceptionally robust against n-doping up to several electrons per unit cell. We trace the provenance of these original effects to the unique combination of the structural cavity channel and the chemistry of the Pb atoms with 6p orbitals localizing inside the channel.

Moir{\'e} pattern assisted geometric resonant tunneling in disordered twisted bilayer graphene. (arXiv:2307.09587v1 [cond-mat.mes-hall])
Zhe Hou, Ya-Yun Hu, Guang-Wen Yang

We investigate the mesoscopic transport through a twisted bilayer graphene (TBG) consisting of a clean graphene nanoribbon on the bottom and a disordered graphene disc on the top. We show that, with strong top-layer disorder the transmission through such a device shows a sequence of resonant peaks with respect to the rotation angle $\theta$, where at the resonance angles $\theta_c$ the disc region contains one giant hexagonal moir{\'e} supercell. A further investigation shows that the value of $\theta_c$ shows negligible dependence on the disorder strength, the Fermi energy, and the shape distortion, indicating the resonance is a robust geometric feature of the moir{\'e} supercell. We explain this geometric resonance based on the bound states formed inside the moir{\'e} supercell, with their averaged local density of states dominating at the AA stacking region while minimizing at the AB stacking region. By increasing the interlayer distance, the peak becomes less pronounced which further confirms the role of interlayer coupling. The results presented here suggest a new mechanism to tune the quantum transport signal through the twist angle in disordered moir{\'e} systems.

Theory of anomalous Hall effect in transition-metal pentatelluride $\mathrm{ZrTe}_{5}$ and $\mathrm{HfTe}_{5}$. (arXiv:2307.09708v1 [cond-mat.mes-hall])
Huan-Wen Wang, Bo Fu, Shun-Qing Shen

The anomalous Hall effect has considerable impact on the progress of condensed matter physics and occurs in systems with time-reversal symmetry breaking. Here we theoretically investigate the anomalous Hall effect in nonmagnetic transition-metal pentatelluride $\mathrm{ZrTe_{5}}$ and $\mathrm{HfTe}_{5}$. In the presence of Zeeman splitting and Dirac mass, there is an intrinsic anomalous Hall conductivity induced by the Berry curvature in the semiclassical treatment. In a finite magnetic field, the anomalous Hall conductivity rapidly decays to zero for constant spin-splitting and vanishes for the magnetic-field-dependent Zeeman energy. A semiclassical formula is derived to depict the magnetic field dependence of the Hall conductivity, which is beneficial for experimental data analysis. Lastly, when the chemical potential is fixed in the magnetic field, a Hall conductivity plateau arises, which may account for the observed anomalous Hall effect in experiments.

Emergence of two-fold spectral topology through non-Abelian gauge engineering. (arXiv:2307.09757v1 [cond-mat.mes-hall])
Ronika Sarkar, Ayan Banerjee, Awadhesh Narayan

Non-Abelian phenomena and non-Hermitian systems have both been widely explored in recent years. As a bridge between the two, we introduce and develop non-Abelian gauge engineering for realizing multi-fold spectral topology. As an example of our proposal, we engineer non-Hermiticity in the paradigmatic Su-Schrieffer-Heeger (SSH) model by introducing a generalized non-Abelian gauge, leading to an emergent two-fold spectral topology that governs the decoupled behaviour of the corresponding non-Hermitian skin effect. As a consequence of the non-Abelian gauge choice, our model exhibits a rich phase diagram consisting of distinct topological phases, which we characterize by introducing the notion of paired winding numbers, which, in turn, predict the direction of skin localization under open boundaries. We demonstrate that the choice of gauge parameters enables control over the directionality of the skin effect, allowing for it to be unilateral or bilateral. Furthermore, we discover non-dispersive flat bands emerging within the inherent SSH model framework, arising from the non-Abelian gauge. We also introduce a simplified toy model to capture the underlying physics, thereby giving direct physical insights. Our findings pave way for the exploration of unconventional spectral topology through non-Abelian gauges.

Coupling of the triple-Q state to the atomic lattice by anisotropic symmetric exchange. (arXiv:2307.09764v1 [cond-mat.mtrl-sci])
Felix Nickel, André Kubetzka, Soumyajyoti Haldar, Roland Wiesendanger, Stefan Heinze, Kirsten von Bergmann

We identify the triple-Q (3Q) state as magnetic ground state in Pd/Mn and Rh/Mn bilayers on Re(0001) using spin-polarized scanning tunneling microscopy and density functional theory. An atomistic model reveals that in general the 3Q state with tetrahedral magnetic order and zero net spin moment is coupled to a hexagonal atomic lattice in a highly symmetric orientation via the anisotropic symmetric exchange interaction, whereas other spin-orbit coupling terms cancel due to symmetry. Our experiments are in agreement with the predicted orientation of the 3Q state. A distortion from the ideal tetrahedral angles leads to other orientations of the 3Q state which, however, results in a reduced topological orbital magnetization compared to the ideal 3Q state.

Magneto-transport and electronic structures in MoSi2 bulks and thin films with different orientations. (arXiv:2307.09802v1 [cond-mat.mtrl-sci])
W. Afzal, F. Yun, Z. Li, Z. Yue, W. Zhao, L. Sang, G. Yang, Y. He, G. Peleckis, M. Fuhrer, X. Wang

We report a comprehensive study of magneto-transport properties in MoSi2 bulk and thin films. Textured MoSi2 thin films of around 70 nm were deposited on silicon substrates with different orientations. Giant magnetoresistance of 1000% was observed in sintered bulk samples while MoSi2 single crystals exhibit a magnetoresistance (MR) value of 800% at low temperatures. At the low temperatures, the MR of the textured thin films show weak anti-localization behaviour owing to the spin orbit coupling effects. Our first principle calculation show the presence of surface states in this material. The resistivity of all the MoSi2 thin films is significantly low and nearly independent of the temperature, which is important for electronic devices.

Spin-valley dependent double Andreev reflections in the proximitized graphene/superconductor junction. (arXiv:2307.09833v1 [cond-mat.mes-hall])
Lu Gao, Qiang Cheng, Qing-Feng Sun

We study the Andreev reflections and the quantum transport in the proximitized graphene/superconductor junction. The proximitized graphene possesses the pseudospin staggered potential and the intrinsic spin-orbit coupling induced by substrate, which are responsible for the spin-valley dependent double Andreev reflections and the anomalous transport properties in the junction. The pure specular Andreev reflection can happen in the superconducting gap for the $K\uparrow$ and $K'\downarrow$ electrons while the pure retro-Andreev reflection happens for the $K\downarrow$ and $K'\uparrow$ electrons. The coexisting two types of Andreev reflections related to the fixed spin-valley indices strongly depend on the chemical potential of the proximitized graphene. The condition of the emergence of the specific type of Andreev reflection for the electrons with the fixed spin-valley index is clarified. The spin-valley dependent Andreev reflections bring about the peculiar conductance spectra of the junction, which can help determine the values of the pseudospin staggered potential and the intrinsic spin-orbit coupling induced in graphene. Hence, our research results not only provide an experimental method to detect the induced potential and coupling in graphene but also establish the foundation of the superconductor electronics based on the spin-valley indices.

Lieb-Schultz-Mattis theorem for 1d quantum magnets with antiunitary translation and inversion symmetries. (arXiv:2307.09843v1 [cond-mat.str-el])
Yuan Yao, Linhao Li, Masaki Oshikawa, Chang-Tse Hsieh

We study quantum many-body systems in the presence of an exotic antiunitary translation or inversion symmetry involving time reversal. Based on a symmetry-twisting method and spectrum robustness,we propose that a half-integer spin chain which respects any of these two antiunitary crystalline symmetries in addition to the discrete $\mathbb{Z}_2\times\mathbb{Z}_2$ global spin-rotation symmetry must either be gapless or possess degenerate ground states. This explains the gaplessness of a class of chiral spin models not indicated by the Lieb-Schultz-Mattis (LSM) theorem and its known extensions. Moreover, we present symmetry classes with minimal sets of generators that give nontrivial LSM-type constraints, argued by the bulk-boundary correspondence in 2d symmetry-protected topological phases as well as lattice homotopy. Our results for detecting the ingappability of 1d quantum magnets from the interplay between spin-rotation symmetries and magnetic space groups are applicable to systems with a broader class of spin interactions, including the Dzyaloshinskii-Moriya and triple-product interactions.

Hyperbolic non-Abelian semimetal. (arXiv:2307.09876v1 [cond-mat.mes-hall])
Tarun Tummuru, Anffany Chen, Patrick M. Lenggenhager, Titus Neupert, Joseph Maciejko, Tomáš Bzdušek

We extend the notion of topologically protected semi-metallic band crossings to hyperbolic lattices in negatively curved space. Due to their distinct translation group structure, such lattices support non-Abelian Bloch states which, unlike conventional Bloch states, acquire a matrix-valued Bloch factor under lattice translations. Combining diverse numerical and analytical approaches, we uncover a quartic scaling in the density of states at low energies, and illuminate a nodal manifold of codimension five in the reciprocal space. The nodal manifold is topologically protected by a non-zero second Chern number, reminiscent of the characterization of Weyl nodes by the first Chern number.

Electrons in helical magnetic field: a new class of topological metals. (arXiv:2307.09884v1 [cond-mat.mtrl-sci])
Yu. B. Kudasov

Two theorems on electron states in helimagnets are proved. They reveal a Kramers-like degeneracy in helical magnetic field. Since a commensurate helical magnetic system is transitionally invariant with two multiple periods (ordinary translations and generalized ones with rotations), the band structure turns out to be topologically nontrivial. Together with the degeneracy, this gives an unusual spin structure of electron bands. A 2D model of nearly free electrons is proposed to describe conductive hexagonal palladium layers under an effective field of magnetically ordered CrO$_2$ spacers in PdCrO$_2$. The spin texture of the Fermi surface leads to abnormal conductivity.

Ultra-Fast All-Electrical Universal Nano-Qubits. (arXiv:2307.09890v1 [cond-mat.mes-hall])
David T. S. Perkins, Aires Ferreira

We propose how to create, control, and read-out real-space localized spin qubits in proximitized finite graphene nanoribbon (GNR) systems using purely electrical methods. Our proposed nano-qubits are formed of in-gap singlet-triplet states that emerge through the interplay of Coulomb and relativistic spin-dependent interactions in GNRs placed on a magnetic substrate. Application of an electric field perpendicular to the GNR heterostructure leads to a sudden change in the proximity couplings, i.e. a quantum quench, which enables us to deterministically rotate the nano-qubit to any arbitrary point on the Bloch sphere. We predict these spin qubits to undergo Rabi oscillations with optimal visibility and frequencies in excess of 10 GHz. Our findings open up a new avenue for the realization of graphene-based quantum computing with ultra-fast all-electrical methods.

Exchange interactions and intermolecular hybridization in a spin-1/2 nanographene dimer. (arXiv:2307.09930v1 [cond-mat.mes-hall])
N. Krane, E. Turco, A. Bernhardt, D. Jacob, G. Gandus, D. Passerone, M. Luisier, M. Juríček, R. Fasel, J. Fernández-Rossier, P. Ruffieux

Phenalenyl is a radical nanographene with triangular shape that hosts an unpaired electron with spin S = 1/2. The open-shell nature of phenalenyl is expected to be retained in covalently bonded networks. Here, we study a first step in that direction and report the synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization both on Au(111) and on a monolayer of NaCl on top of Au(111) by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that, together with a thorough theoretical analysis, are identified as the singlet-triplet excitation arising from interphenalenyl exchange. Two prominent features of our data permit to shed light on the nature of spin interactions in this system. First, the excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating a significant renormalization of the spin excitation energies due to exchange with the Au(111) electrons. Second, a position-dependent bias-asymmetry of the height of the inelastic steps is accounted for by an interphenalenyl hybridization of the singly occupied phenalenyl orbitals that is only possible via third neighbor hopping. This hybridization is also essential to activate kinetic interphenalenyl exchange. Our results set the stage for future work on the bottom-up synthesis of spin S = 1/2 spin lattices with large exchange interaction.

Dynamical Onset of Light-Induced Unconventional Superconductivity -- a Yukawa-Sachdev-Ye-Kitaev study. (arXiv:2307.09935v1 [cond-mat.supr-con])
Lukas Grunwald, Giacomo Passetti, Dante M. Kennes

We investigate the dynamical onset of superconductivity in the exactly solvable Yukawa-Sachdev-Ye-Kitaev model. It hosts an unconventional superconducting phase that emerges out of a non-Fermi liquid normal state, providing a toy model for superconductivity in a strongly correlated system. Analyzing dynamical protocols motivated by theoretical mechanisms proposed for light-induced superconductivity, that is light-induced cooling and the dressing of Hamiltonian parameters, we investigate the exact relaxation resulting out of undercooling and interaction quenches. While, in contrast to BCS theory, it is not possible for superconductivity to emerge following interaction quenches across the superconducting phase transition, we find that the dynamical relaxation of undercooled states universally leads to superconductivity. Despite the strong correlations, the emerging order parameter dynamics are well captured by a coarse grained Ginzburg-Landau theory for which we determine all parameters from microscopics.

Nonequilibrium phase transitions in Brownian $p$-state clock model. (arXiv:2307.09945v1 [cond-mat.stat-mech])
Chul-Ung Woo, Jae Dong Noh

We introduce a Brownian $p$-state clock model in two dimensions and investigate the nature of phase transitions numerically. As a nonequilibrium extension of the equilibrium lattice model, the Brownian $p$-state clock model allows spins to diffuse randomly in the two-dimensional space of area $L^2$ under periodic boundary conditions. We find three distinct phases for $p>4$: a disordered paramagnetic phase, a quasi-long-range-ordered critical phase, and an ordered ferromagnetic phase. In the intermediate critical phase, the magnetization order parameter follows a power law scaling $m \sim L^{-\tilde{\beta}}$, where the finite-size scaling exponent $\tilde{\beta}$ varies continuously. These critical behaviors are reminiscent of the double Berezinskii-Kosterlitz-Thouless~(BKT) transition picture of the equilibrium system. At the transition to the disordered phase, the exponent takes the universal value $\tilde\beta = 1/8$ which coincides with that of the equilibrium system. This result indicates that the BKT transition driven by the unbinding of topological excitations is robust against the particle diffusion. On the contrary, the exponent at the symmetry-breaking transition to the ordered phase deviates from the universal value $\tilde{\beta} = 2/p^2$ of the equilibrium system. The deviation is attributed to a nonequilibrium effect from the particle diffusion.

Magic-angle twisted bilayer graphene under orthogonal and in-plane magnetic fields. (arXiv:2307.09960v1 [cond-mat.mes-hall])
Gaëlle Bigeard, Alessandro Cresti

We investigate the effect of a magnetic field on the band structure of a bilayer graphene with a magic twist angle of 1.08{\deg}. The coupling of tight-binding model and Peierls phase allows the calculation of the energy bands of periodic two-dimensional systems. For an orthogonal magnetic field, the Landau levels turn out to be dispersive, especially for magnetic lengths comparable or larger than the twisted bilayer cell size. A high in-plane magnetic field modifies the low-energy bands and gap, which we demonstrate to be a direct consequence of the minimal coupling.

Detecting the interplay between self-statistics and braiding statistics in 3D topologically ordered phases through topological quantum field theory. (arXiv:2307.09983v1 [cond-mat.str-el])
Zhi-Feng Zhang, Qing-Rui Wang, Peng Ye

Low-energy effective theory of topological orders is topological quantum field theory (TQFT). While previous field-theoretical studies in $3$D (real space dimension) topological orders focus on either self-statistics, braiding statistics, shrinking rules, or fusion rules, it is yet to systematically put all topological data together and study their internal consistency. Here, we construct the topological $BF$ field theory with twisted terms (e.g., $AAdA$ and $AAB$) as well as a $K$-matrix $BB$ term, in order to simultaneously explore all such topological data and reach anomaly-free topological orders. We present general formulas and show how the $K$-matrix $BB$ term confines topological excitations, and how self-statistics of particles is transmuted. In order to reach anomaly-free topological orders, we explore how the principle of gauge invariance fundamentally influences the compatibility between braiding statistics and emergent fermions. For example, suppose the flux (loop excitation) and the charge (particle excitation) of gauge subgroup $\mathbb{Z}_{N_{i}}$ are denoted as $\phi_{i}$ and $q_{i}$ respectively in a $3$D bosonic topological order. The field-theoretical analysis simply tells us, within the present TQFTs, if $\phi_{i}$ nontrivially participates a multi-loop braiding or a Borromean-rings braiding, then fermionic $q_{i}$ is prohibited for ensuring the gauge invariance. Therefore, the possible ways of self-statistics assignment on particles are highly restricted once other topological data are given. Our analysis provides a field-theoretical approach to constructing anomaly-free topological orders in $3$D. Together with the previous efforts, our work paves the way toward a more complete field-theoretical analysis of $3$D topological orders, in analogy to the $K$-matrix Chern-Simons theory of $2$D topological orders.

Activating the fluorescence of a Ni(II) complex by energy transfer. (arXiv:2307.09984v1 [cond-mat.mes-hall])
Tzu-Chao Hung, Yokari Godinez-Loyola, Manuel Steinbrecher, Brian Kiraly, Alexander A. Khajetoorians, Nikos L. Doltsinis, Cristian A. Strassert, Daniel Wegner

Luminescence of open-shell 3d metal complexes is often quenched due to ultrafast intersystem crossing (ISC) and cooling into a dark metal-centered excited state. We demonstrate successful activation of fluorescence from individual nickel phthalocyanine (NiPc) molecules in the junction of a scanning tunneling microscope (STM) by resonant energy transfer from other metal phthalocyanines at low temperature. By combining STM, scanning tunneling spectroscopy, STM- induced luminescence, and photoluminescence experiments as well as time-dependent density functional theory, we provide evidence that there is an activation barrier for the ISC, which in most experimental conditions is overcome. We show that this is also the case in an electroluminescent tunnel junction where individual NiPc molecules adsorbed on an ultrathin NaCl decoupling film on a Ag(111) substrate are probed. However, when placing an MPc (M = Zn, Pd, Pt) molecule close to NiPc by means of STM atomic manipulation, resonant energy transfer can excite NiPc without overcoming the ISC activation barrier, leading to Q-band fluorescence. This work demonstrates that the thermally activated population of dark metal-centered states can be avoided by a designed local environment at low temperatures paired with a directed molecular excitation into vibrationally cold electronic states. Thus, we can envisage the use of luminophores based on more abundant transition metal complexes that do not rely on Pt or Ir.

Yu-Shiba-Rusinov bands in a self-assembled kagome lattice of magnetic molecules. (arXiv:2307.09993v1 [cond-mat.mes-hall])
Laetitia Farinacci, Gael Reecht, Felix von Oppen, Katharina J. Franke

Kagome lattices constitute versatile platforms for studying paradigmatic correlated phases. While molecular self-assembly of kagome structures on metallic substrates is promising, it is challenging to realize pristine kagome properties because of hybridization with the bulk degrees of freedom and modified electron-electron interactions. We suggest that a superconducting substrate offers an ideal support for a magnetic kagome lattice. Exchange coupling induces kagome-derived bands at the interface, which are protected from the bulk by the superconducting energy gap. We realize a magnetic kagome lattice on a superconductor by depositing Fe-porphin-chloride molecules on Pb(111) and using temperature-activated de-chlorination and self-assembly. This allows us to control the formation of smaller kagome precursors and long-range ordered kagome islands. Using scanning tunneling microscopy and spectroscopy at 1.6 K, we identify Yu-Shiba-Rusinov states inside the superconducting energy gap and track their hybridization from the precursors to larger islands, where the kagome lattice induces extended YSR bands. These YSR-derived kagome bands are protected inside the superconducting energy gap, motivating further studies to resolve possible spin-liquid or Kondo-lattice-type behavior.

Observation of Kekul\'e vortices induced in graphene by hydrogen adatoms. (arXiv:2307.10024v1 [cond-mat.mes-hall])
Y. Guan, C. Dutreix, H. Gonzales-Herrero, M. M. Ugeda, I. Brihuega, M. I. Katsnelson, O. V. Yazyev, V. T. Renard

Fractional charges are one of the wonders of the fractional quantum Hall effect, a liquid of strongly correlated electrons in a large magnetic field. Fractional excitations are also anticipated in two-dimensional crystals of non-interacting electrons under time-reversal symmetry, as bound states of a rotating bond order known as Kekul\'e vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realisations. Here, we report the observation of Kekul\'e vortices in the local density of states of graphene under time-reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms and have a purely electronic origin. Their 2{\pi} winding is reminiscent of the Berry phase {\pi} of the massless Dirac electrons. Remarkably, we observe that point scatterers with different symmetries such as divacancies can also induce a Kekul\'e bond order without vortex. Therefore, our local-probe study further confirms point defects as versatile building blocks for the control of graphene's electronic structure by kekul\'e order.

Emergence of Spinon Fermi Arcs in the Weyl-Mott Metal-Insulator Transition. (arXiv:2307.10102v1 [cond-mat.str-el])
Manuel Fernández López, Iñaki García-Elcano, Jorge Bravo-Abad, Jaime Merino

The Weyl-Mott insulator (WMI) has been postulated as a novel type of correlated insulator with non-trivial topological properties. We introduce a minimal microscopic model that captures generic features of the WMI transition in Weyl semimetals. The model hosts a bulk Mott insulator with spinon Fermi arcs on its surfaces which we identify as a WMI. At finite temperatures, we find an intermediate Weyl semimetallic phase with no quasiparticles which is consistent with the bad semimetallic behavior observed in pyrochlore iridates, A2Ir2O7, close to the Mott transition. Spinon Fermi arcs lead to a suppression of the bulk Mott gap at the surface of the WMI, in contrast to the gap enhancement found in conventional Mott insulators, which can be detected through angular resolved photoemission spectroscopy (ARPES).

An efficient displacement-based isogeometric formulation for geometrically exact viscoelastic beams. (arXiv:2307.10106v1 [math.NA])
Giulio Ferri, Diego Ignesti, Enzo Marino

We propose a novel approach to the linear viscoelastic problem of shear-deformable geometrically exact beams. The generalized Maxwell model for one-dimensional solids is here efficiently extended to the case of arbitrarily curved beams undergoing finite displacement and rotations. High efficiency is achieved by combining a series of distinguishing features, that are i) the formulation is displacement-based, therefore no additional unknowns, other than incremental displacements and rotations, are needed for the internal variables associated with the rate-dependent material; ii) the governing equations are discretized in space using the isogeometric collocation method, meaning that elements integration is totally bypassed; iii) finite rotations are updated using the incremental rotation vector, leading to two main benefits: minimum number of rotation unknowns (the three components of the incremental rotation vector) and no singularity problems; iv) the same $\rm SO(3)$-consistent linearization of the governing equations and update procedures as for non-rate-dependent linear elastic material can be used; v) a standard second-order accurate time integration scheme is made consistent with the underlying geometric structure of the kinematic problem. Moreover, taking full advantage of the isogeometric analysis features, the formulation permits accurately representing beams and beam structures with highly complex initial shape and topology, paving the way for a large number of potential applications in the field of architectured materials, meta-materials, morphing/programmable objects, topological optimizations, etc. Numerical applications are finally presented in order to demonstrate attributes and potentialities of the proposed formulation.

Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics. (arXiv:2307.10121v1 [cond-mat.soft])
Sung-Jo Kim, Žiga Kos, Eujin Um, Joonwoo Jeong

Swimming in low-Reynolds-number fluids requires the breaking of time-reversal symmetry and centrosymmetry. Microswimmers, often with asymmetric shapes, exhibit nonreciprocal motions or exploit nonequilibrium processes to propel. The role of surrounding fluids has also attracted attention because viscoelastic, non-Newtonian, and anisotropic properties of fluids matter in propulsion efficiency and navigation. Here we experimentally demonstrate that anisotropic fluids, nematic liquid crystals (NLC), can make a pulsating spherical bubble swim despite its centrosymmetric shape and time-symmetric motion. The NLC breaks the centrosymmetry by a deformed nematic director field with a topological defect accompanying the bubble. The nematodynamics renders the nonreciprocity in the pulsation-induced fluid flow. We also report the speed enhancement by confinement and the propulsion of another symmetry-broken bubble dressed by a bent disclination. Our experiments and theory elucidate another possible mechanism of moving bodies in complex fluids by spatiotemporal symmetry breaking.

Altermagnetic surface states: towards the observation and utilization of altermagnetism in thin films, interfaces and topological materials. (arXiv:2307.10146v1 [cond-mat.mtrl-sci])
Raghottam M Sattigeri, Giuseppe Cuono, Carmine Autieri

The altermagnetism influences the electronic states allowing the presence of non-relativistic spinsplittings. Since altermagnetic spin-splitting is present along specific k-paths of the 3D Brillouin zone, we expect that the altermagnetic surface states will be present on specific surface orientations. We unveil the properties of the altermagnetic surface states considering three representative space groups: tetragonal, orthorhombic and hexagonal. We calculate the 2D projected Brillouin zone from the 3D Brillouin zone. We study the surfaces with their respective 2D Brillouin zones establishing where the spin-splittings with opposite sign merge annihilating the altermagnetic properties and on which surfaces the altermagnetism is preserved. Looking at the three principal surface orientations, we find that for several cases two surfaces are blind to the altermagnetism, while the altermagnetism survives for one surface orientation. Which surface preserves the altermagnetism depends also on the magnetic order. We show that an electric field orthogonal to the blind surface can activate the altermagnetism. Our results predict which surfaces to cleave in order to preserve altermagnetism in surfaces or interfaces and this paves the way to observe non-relativistic altermagnetic spin-splitting in thin films via spin-resolved ARPES and to interface the altermagnetism with other collective modes. We open future perspectives for the study of altermagnetic effects on the trivial and topological surface states.

Direct observation of chiral edge current at zero magnetic field in odd-layer MnBi$_2$Te$_4$. (arXiv:2307.10150v1 [cond-mat.mes-hall])
Jinjiang Zhu, Yang Feng, Xiaodong Zhou, Yongchao Wang, Zichen Lian, Weiyan Lin, Qiushi He, Yishi Lin, Youfang Wang, Hongxu Yao, Hao Li, Yang Wu, Jing Wang, Jian Shen, Jinsong Zhang, Yayu Wang, Yihua Wang

The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. Its direct observation is assumed to require well-quantized Hall conductance, and is so far lacking. The recently discovered van der Waals antiferromagnet MnBi$_2$Te$_4$ is theorized as a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layer. The chirality of such edge current reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge-bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.

Fluctuation-driven, topology-stabilized order in a correlated nodal semimetal. (arXiv:2103.08489v2 [cond-mat.str-el] UPDATED)
Nathan C. Drucker, Thanh Nguyen, Fei Han, Xi Luo, Nina Andrejevic, Ziming Zhu, Grigory Bednik, Quynh T. Nguyen, Zhantao Chen, Linh K. Nguyen, Travis J. Williams, Matthew B. Stone, Alexander I. Kolesnikov, Songxue Chi, Jaime Fernandez-Baca, Tom Hogan, Ahmet Alatas, Alexander A. Puretzky, David B. Geohegan, Shengxi Huang, Yue Yu, Mingda Li

The interplay between strong electron correlation and band topology is at the forefront of condensed matter research. As a direct consequence of correlation, magnetism enriches topological phases and also has promising functional applications. However, the influence of topology on magnetism remains unclear, and the main research effort has been limited to ground state magnetic orders. Here we report a novel order above the magnetic transition temperature in magnetic Weyl semimetal (WSM) CeAlGe. Such order shows a number of anomalies in electrical and thermal transport, and neutron scattering measurements. We attribute this order to the coupling of Weyl fermions and magnetic fluctuations originating from a three-dimensional Seiberg-Witten monopole, which qualitatively agrees well with the observations. Our work reveals a prominent role topology may play in tailoring electron correlation beyond ground state ordering, and offers a new avenue to investigate emergent electronic properties in magnetic topological materials.

Intraspecific predator interference promotes biodiversity in ecosystems. (arXiv:2112.05098v3 [q-bio.PE] UPDATED)
Ju Kang, Shijie Zhang, Xin Wang

Explaining biodiversity is a fundamental issue in ecology. A long-standing puzzle lies in the paradox of the plankton: many species of plankton feeding on a limited type of resources coexist, apparently flouting the competitive exclusion principle (CEP), which holds that the number of predator (consumer) species cannot exceed that of the resources at steady state. Here, we present a mechanistic model and show that the intraspecific interference among the consumers enables a plethora of consumer species to coexist at constant population densities with only one or a handful of resource species. The facilitated biodiversity is resistant to stochasticity, either with the stochastic simulation algorithm or individual-based modeling. Our model naturally explains the classical experiments that invalidate CEP, quantitatively illustrates the universal S-shaped pattern of the rank-abundance curves across a wide range of ecological communities, and can be broadly used to resolve the mystery of biodiversity in many natural ecosystems.

How viscous bubbles collapse: topological and symmetry-breaking instabilities in curvature-driven hydrodynamics. (arXiv:2202.11125v3 [cond-mat.soft] UPDATED)
Benny Davidovitch, Avraham Klein

The duality between deformations of elastic bodies and non-inertial flows in viscous liquids has been a guiding principle in decades of research. However, this duality is broken when a spheroidal or other doubly-curved liquid film is suddenly forced out of mechanical equilibrium, as occurs e.g. when the pressure inside a liquid bubble drops rapidly due to rupture or controlled evacuation. In such cases the film may evolve through a non-inertial yet geometrically-nonlinear surface dynamics, which has remained largely unexplored. We reveal the driver of such dynamics as temporal variations in the curvature of the evolving surface. Focusing on the prototypical example of a floating bubble that undergoes rapid depressurization, we show that the bubble surface evolves via a topological instability and a subsequent front propagation, whereby a small planar zone nucleates and expands in the spherically-shaped film, bringing about hoop compression and triggering another, symmetry-breaking instability and radial wrinkles that grow in amplitude and invade the flattening film. Our analysis reveals the dynamics as a non-equilibrium branch of "Jellium" physics, whereby a rate-of-change of surface curvature in a viscous film is akin to charge in an electrostatic medium that comprises polarizable and conducting domains. We explain key features underlying recent experiments and highlight a qualitative inconsistency between the prediction of linear stability analysis and the observed "wavelength" of surface wrinkles. Our analysis points to the existence of a nonlinear curvature-driven mechanism for pattern selection in viscous flows.

Discovering dynamic laws from observations: the case of self-propelled, interacting colloids. (arXiv:2203.14846v4 [cond-mat.soft] UPDATED)
Miguel Ruiz-Garcia, C. Miguel Barriuso Gutierrez, Lachlan C. Alexander, Dirk G. A. L. Aarts, Luca Ghiringhelli, Chantal Valeriani

Active matter spans a wide range of time and length scales, from groups of cells and synthetic self-propelled particles to schools of fish, flocks of birds, or even human crowds. The theoretical framework describing these systems has shown tremendous success at finding universal phenomenology. However, further progress is often burdened by the difficulty of determining the forces that control the dynamics of the individual elements within each system. Accessing this local information is key to understanding the physics dominating the system and to create the models that can explain the observed collective phenomena. In this work, we present a machine-learning model, a graph neural network, that uses the collective movement of the system to learn the active and two-body forces controlling the individual dynamics of the particles. We verify our approach using numerical simulations of active brownian particles, considering different interaction potentials and levels of activity. Finally, we apply our model to experiments of electrophoretic Janus particles, extracting the active and two-body forces that control the dynamics of the colloids. Due to this, we can uncover the physics dominating the behavior of the system. We extract an active force that depends on the electric field and also area fraction. We also discover a dependence of the two-body interaction with the electric field that leads us to propose that the dominant force between these colloids is a screened electrostatic interaction with a constant length scale. We expect that this methodology can open a new avenue for the study and modeling of experimental systems of active particles.

Winding theta and destructive interference of instantons. (arXiv:2207.03008v2 [hep-th] UPDATED)
Mendel Nguyen, Yuya Tanizaki, Mithat Ünsal

While the $\theta$ dependence of field theories is $2\pi$ periodic, the ground-state wavefunctions at $\theta$ and $\theta+2\pi$ often belong to different classes of symmetry-protected topological states. When this is the case, a continuous change of the $\theta$ parameter can introduce an interface that supports a nontrivial field theory localized on the wall. We consider the $2$d $\mathbb{C}P^{N-1}$ sigma model as an example and construct a weak-coupling setup of this interface theory by considering the small $S^1$ compactification with nonzero winding $\theta$ parameter and a suitable symmetry-twisted boundary condition. This system has $N$ classical vacua connected by fractional instantons, but the anomaly constraint tells us that the fractional-instanton amplitudes should vanish completely to have $N$-fold degeneracy at the quantum level. We show how this happens in this purely bosonic system, uncovering that the integration over the zero modes annihilates the fractional instanton amplitudes, which is sharp contrast to what happens when the $\theta$ angle is constant. Moreover, we provide another explanation of this selection rule by showing that the $N$ perturbative vacua acquire different charges under the global symmetry with the activation of the winding $\theta$ angle. We also demonstrate a similar destructive interference between instanton effects in the $\mathbb{C}P^{N-1}$ quantum mechanics with the Berry phase.

Bipolar single-molecule electroluminescence and electrofluorochromism. (arXiv:2210.11118v2 [cond-mat.mes-hall] UPDATED)
Tzu-Chao Hung, Roberto Robles, Brian Kiraly, Julian H. Strik, Bram A. Rutten, Alexander A. Khajetoorians, Nicolas Lorente, Daniel Wegner

Understanding the fundamental mechanisms of optoelectronic excitation and relaxation pathways on the single-molecule level has only recently been started by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced luminescence (STML). In this paper, we investigate cationic and anionic fluorescence of individual zinc phthalocyanine (ZnPc) molecules adsorbed on ultrathin NaCl films on Ag(111) by using STML. They depend on the tip-sample bias polarity and appear at threshold voltages that are correlated with the onset energies of particular molecular orbitals, as identified by STS. We also find that the fluorescence is caused by a single electron tunneling process. Comparing with results from density functional theory calculations, we propose an alternative many-body picture to describe the charging and electroluminescence mechanism. Our study provides aspects toward well-defined voltage selectivity of bipolar electrofluorochromism, as well as fundamental insights regarding the role of transiently charged states of emitter molecules within OLED devices.

Skyrmion Jellyfish in Driven Chiral Magnets. (arXiv:2211.01714v5 [cond-mat.mes-hall] UPDATED)
Nina del Ser, Vivek Lohani

Chiral magnets can host topological particles known as skyrmions, which carry an exactly quantised topological charge $Q=-1$. In the presence of an oscillating magnetic field ${\bf B}_1(t)$, a single skyrmion embedded in a ferromagnetic background will start to move with constant velocity ${\bf v}_{\text{trans}}$. The mechanism behind this motion is similar to the one used by a jellyfish when it swims through water. We show that the skyrmion's motion is a universal phenomenon, arising in any magnetic system with translational modes. By projecting the equation of motion onto the skyrmion's translational modes and going to quadratic order in ${\bf B}_1(t)$, we obtain an analytical expression for ${\bf v}_{\text{trans}}$ as a function of the system's linear response. The linear response and consequently ${\bf v}_{\text{trans}}$ are influenced by the skyrmion's internal modes and scattering states, as well as by the ferromagnetic background's Kittel mode. The direction and speed of ${\bf v}_{\text{trans}}$ can be controlled by changing the polarisation, frequency and phase of the driving field ${\bf B}_1(t)$. For systems with small Gilbert damping parameter $\alpha$, we identify two distinct physical mechanisms used by the skyrmion to move. At low driving frequencies, the skyrmion's motion is driven by friction, and $v_{\text{trans}}\sim\alpha$, whereas at higher frequencies above the ferromagnetic gap, the skyrmion moves by magnon emission, and $v_{\text{trans}}$ becomes independent of $\alpha$.

Spin and electronic excitations in $4f$ atomic chains on Au(111) substrates. (arXiv:2212.08772v2 [cond-mat.mes-hall] UPDATED)
David W. Facemyer, Naveen K. Dandu, Alex Taekyung Lee, Vijay R. Singh, Anh T. Ngo, Sergio E. Ulloa

High spin systems, like those that incorporate rare-earth $4f$ elements (REEs), are increasingly relevant in many fields. Although research in such systems is sparse, the large Hilbert spaces they occupy are promising for many applications. In this work, we examine a one-dimensional linear array of europium (Eu) atoms on a Au(111) surface and study their electronic and magnetic excitations. Ab initio calculations using VASP with PBE+U are employed to study the structure. We find Eu atoms to have a net charge when on gold, consistent with a net magnetic momemt of $\simeq 3.5 \mu_B$. Examining various spin-projection configurations, we can evaluate first and second neighbor exchange energies in an isotropic Heisenberg model between spin-$\frac{7}{2}$ moments to obtain $J_1 \approx -1.2 \, \mathrm{K}$ and $J_2 \approx 0.2 \, \mathrm{K}$ for the relaxed-chain atomic separation of $a \approx 5$ $\mathrm{\dot{A}}$. These parameters are used to obtain the full spin excitation spectrum of a physically realizable four-atom chain. The large $|J_1|/J_2$ ratio results in a highly degenerate ferromagnetic ground state that is split by a significant easy plane single ion anisotropy of $0.6$ K. Spin-flip excitations are calculated to extract differential conductance profiles as those obtained by scanning tunneling microscopy techniques. We uncover interesting behavior of local spin excitations, especially as we track their dispersion with applied magnetic fields.

Cell augmentation framework for topological lattices. (arXiv:2301.10376v3 [cond-mat.mtrl-sci] UPDATED)
Mohammad Charara, Stefano Gonella

Maxwell lattices are characterized by an equal number of degrees of freedom and constraints. A subset of them, dubbed topological lattices, are capable of localizing stress and deformation on opposing edges, displaying a polarized mechanical response protected by the reciprocal-space topology of their band structure. In two dimensions, the opportunities for topological polarization have been largely restricted to the kagome and square lattice benchmark configurations, due to the non-triviality of generating arbitrary geometries that abide by Maxwell conditions. In this work, we introduce a generalized family of augmented topological lattices that display full in-plane topological polarization. We explore the robustness of such polarization upon selection of different augmentation criteria, with special emphasis on augmented configurations that display dichotomous behavior with respect to their primitive counterparts. We corroborate our results via intuitive table-top experiments conducted on a lattice prototype assembled from 3D-printed mechanical links.

General scatterings and electronic states in the quantum-wire network of moir\'e systems. (arXiv:2303.00759v3 [cond-mat.mes-hall] UPDATED)
Chen-Hsuan Hsu, Daniel Loss, Jelena Klinovaja

We investigate electronic states in a two-dimensional network consisting of interacting quantum wires, a model adopted for twisted bilayer systems. We construct general operators which describe various scattering processes in the system. In a twisted bilayer structure, the moir\'e periodicity allows for generalized umklapp scatterings, leading to a class of correlated states at certain fractional fillings. We identify scattering processes which can lead to an insulating gapped bulk with gapless chiral edge modes at fractional fillings, resembling the quantum anomalous Hall effect recently observed in twisted bilayer graphene. Finally, we demonstrate that the description can be useful in predicting spectroscopic and transport features to detect and characterize the chiral edge modes in the moir\'e-induced correlated states.

Magnetic-field-induced corner states in quantum spin Hall insulators. (arXiv:2303.09260v2 [cond-mat.mes-hall] UPDATED)
Sergey S. Krishtopenko, Frédéric Teppe

We address the general problem of magnetic-field-induced corner states in quantum spin Hall insulators (QSHIs). Our analytical findings reveal that when applied to the QSHIs in zinc-blende semiconductor quantum wells (QWs), the presence of corner states extends beyond the anticipated range of meeting edges, surpassing the limitations imposed by crystal symmetry. We clearly demonstrate that, in the most general scenario, magnetic field-induced corner states in QSHIs are not topological. However, we find that the presence of crystal symmetry can stabilize these states only under specific orientations of the in-plane magnetic field and meeting edges. Therefore, contrary to previous assumptions, our research unveils that QSHIs in the presence of a magnetic field cannot be accurately considered as higher-order topological insulators. Furthermore, the lack of an inversion center in zinc-blende semiconductor QWs enables the emergence of corner states through the influence of a perpendicular magnetic field.

Classifying topology in photonic heterostructures with gapless environments. (arXiv:2303.17135v2 [physics.optics] UPDATED)
Kahlil Y. Dixon, Terry A. Loring, Alexander Cerjan

Photonic topological insulators exhibit bulk-boundary correspondence, which requires that boundary-localized states appear at the interface formed between topologically distinct insulating materials. However, many topological photonic devices share a boundary with free space, which raises a subtle but critical problem as free space is gapless for photons above the light-line. Here, we use a local theory of topological materials to resolve bulk-boundary correspondence in heterostructures containing gapless materials and in radiative environments. In particular, we construct the heterostructure's spectral localizer, a composite operator based on the system's real-space description that provides a local marker for the system's topology and a corresponding local measure of its topological protection; both quantities are independent of the material's bulk band gap (or lack thereof). Moreover, we show that approximating radiative outcoupling as material absorption overestimates a heterostructure's topological protection. As the spectral localizer is applicable to systems in any physical dimension and in any discrete symmetry class, our results show how to calculate topological invariants, quantify topological protection, and locate topological boundary-localized resonances in topological materials that interface with gapless media in general.

Magnetic States of Graphene Proximitized Kitaev Materials. (arXiv:2305.12116v2 [cond-mat.str-el] UPDATED)
Jingtian Shi, A.H. MacDonald

Single layer $\alpha$-ruthenium trichloride ($\rm\alpha-RuCl_3$) has been proposed as a potential quantum spin liquid. Graphene/$\rm RuCl_3$ heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of $\rm RuCl_3$ layer and graphene electronic properties. We perform self-consistent Hartree-Fock calculations on a Hubbard-Kanamori model of the $4d^5$ $t_{2g}$ electrons of $\rm\alpha-RuCl_3$ and confirm that out-of-plane ferromagnetic and zigzag antiferromagnetic states are energetically competitive. We show that the influence of hybridization between graphene and $\rm\alpha-RuCl_3$ bands is strongly sensitive to the magnetic configuration of $\rm RuCl_3$ and the relative orientations of the two layers. We argue that strong hybridization leads to graphene magneto-resistance and that it may tilt the balance between closely competing magnetic states. Our analysis can be applied to any van der Waals heterobilayer system with weak interlayer hybridization and allows for arbitrary lattice constant mismatch and relative orientation.

Quantum geometry and bounds on dissipation in slowly driven quantum systems. (arXiv:2306.17220v2 [quant-ph] UPDATED)
Iliya Esin, Étienne Lantagne-Hurtubise, Frederik Nathan, Gil Refael

We show that heat production in slowly driven quantum systems is linked to the topological structure of the driving protocol through the Fubini-Study tensor. Analyzing a minimal model of a spin weakly coupled to a heat bath, we find that dissipation is controlled by the quantum metric and a "quality factor" characterizing the spin's precession. Utilizing these findings, we establish lower bounds on the heating rate in two-tone protocols, such as those employed in topological frequency converters. Notably, these bounds are determined by the topology of the protocol, independent of its microscopic details. Our results bridge topological phenomena and energy dissipation in slowly driven quantum systems, providing a design principle for optimal driving protocols.

Unconventional quantum oscillations and evidence of non-trivial electronic states in quasi-two-dimensional electron system at complex oxide interfaces. (arXiv:2307.04854v2 [cond-mat.mtrl-sci] UPDATED)
Km Rubi, Manish Duman, Shengwei Zeng, Andrew Ammerlaan, Femke Bangma, Mun K. Chan, Michel Goiran, Ariando Ariando, Suvankar Chakraverty, Walter Escoffier, Uli Zeitler, Neil Harrison

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present novel insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov-de Haas oscillations in two different systems: EuO/KTaO$_3$ (EuO/KTO) and LaAlO$_3$/SrTiO$_3$ (LAO/STO). To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at both interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these intriguing findings by considering the existence of non-trivial electronic bands, for which the $E-k$ dispersion incorporates both linear and parabolic dispersion relations. In addition to providing experimental evidence for topological-like electronic states in KTO-2DES and STO-2DES, the unconventional oscillations presented in this study establish a new paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillations frequency exhibits quadratic dependence on the magnetic field.

Topological interface states -- a possible path towards a Landau-level laser in the THz regime. (arXiv:2307.05116v2 [cond-mat.mes-hall] UPDATED)
Mark O. Goerbig

Volkov-Pankratov surface bands arise in smooth topological interfaces, i.e. interfaces between a topological and a trivial insulator, in addition to the chiral surface state imposed by the bulk-surface correspondence of topological materials. These two-dimensional bands become Landau-quantized if a magnetic field is applied perpendicular to the interface. I show that the energy scales, which are typically in the 10-100 meV range, can be controlled both by the perpendicular magnetic field and the interface width. The latter can still be varied with the help of a magnetic-field component in the interface. The Landau levels of the different Volkov-Pankratov bands are optically coupled, and their arrangement may allow one to obtain population inversion by resonant optical pumping. This could serve as the elementary brick of a multi-level laser based on Landau levels. Moreover, the photons are absorbed and emitted either parallel or perpendicular to the magnetic field, respectively in the Voigt and Faraday geometry, depending on the Volkov-Pankratov bands and Landau levels involved in the optical transitions.

Found 6 papers in prb
Date of feed: Thu, 20 Jul 2023 03:17:03 GMT

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

Variable range hopping in a nonequilibrium steady state
Preeti Bhandari, Vikas Malik, and Moshe Schechter
Author(s): Preeti Bhandari, Vikas Malik, and Moshe Schechter

We propose a Monte Carlo simulation to understand electron transport in a nonequilibrium steady state (NESS) for the lattice Coulomb Glass model, created by continuous excitation of single electrons to high energies followed by relaxation of the system. Around the Fermi level, the NESS state approxi…

[Phys. Rev. B 108, 024203] Published Wed Jul 19, 2023

Quasilinear magnetoresistance and de Haas–van Alphen quantum oscillations in a ${\mathrm{LuPb}}_{2}$ single crystal
Feng Yang, Shilong Li, Shengwei Chi, Xiaoxu Wang, Shan Jiang, Huakun Zuo, Lingxiao Zhao, Gang Xu, and Zengwei Zhu
Author(s): Feng Yang, Shilong Li, Shengwei Chi, Xiaoxu Wang, Shan Jiang, Huakun Zuo, Lingxiao Zhao, Gang Xu, and Zengwei Zhu

Materials with square lattices composed of group IV or V elements provide a promising platform for topological phases to emerge. We present the study on single crystals of ${\mathrm{LuPb}}_{2}$, which is a compound based on the Pb square net. The de Haas–van Alphen effect measurements reveal clear q…

[Phys. Rev. B 108, 035137] Published Wed Jul 19, 2023

Interplay between magnetism and band topology in the kagome magnets $R{\mathrm{Mn}}_{6}{\mathrm{Sn}}_{6}$
Y. Lee, R. Skomski, X. Wang, P. P. Orth, Y. Ren, Byungkyun Kang, A. K. Pathak, A. Kutepov, B. N. Harmon, R. J. McQueeney, I. I. Mazin, and Liqin Ke
Author(s): Y. Lee, R. Skomski, X. Wang, P. P. Orth, Y. Ren, Byungkyun Kang, A. K. Pathak, A. Kutepov, B. N. Harmon, R. J. McQueeney, I. I. Mazin, and Liqin Ke

The kagome magnets RMn6Sn6 have recently emerged as a new topological materials platform. By elucidating the topological nature of the band structure, the authors conclude that the observed anomalous Hall conductivity is unrelated to the previously speculated quasi-two-dimensional Dirac points. The microscopic origin of magnetocrystalline anisotropy is explored at various levels: phenomenological, analytical, and ab initio. The authors discovered how the special Mn coordination of the rare-earth atoms results in significant high-order anisotropy.

[Phys. Rev. B 108, 045132] Published Wed Jul 19, 2023

Classification and construction of interacting fractonic higher-order topological phases
Jian-Hao Zhang, Meng Cheng, and Zhen Bi
Author(s): Jian-Hao Zhang, Meng Cheng, and Zhen Bi

The notion of higher-order topological phases can have interesting generalizations to systems with subsystem symmetries that exhibit fractonic dynamics for charged excitations. In this work, we systematically study the higher-order topological phases protected by a combination of subsystem symmetrie…

[Phys. Rev. B 108, 045133] Published Wed Jul 19, 2023

Anomalous one-dimensional quantum confinement effect in graphene nanowrinkle
Jong-Guk Ahn, Jee Hyeon Kim, Minhui Lee, Yousoo Kim, Jaehoon Jung, and Hyunseob Lim
Author(s): Jong-Guk Ahn, Jee Hyeon Kim, Minhui Lee, Yousoo Kim, Jaehoon Jung, and Hyunseob Lim

A theoretical principle for explaining the peculiarity in “edge-free” wrinkled graphene has not been firmly established. Herein, we perform DFT calculations to verify the graphene nanowrinkle (GNW) feature on metal as a model system based on experimental observation. We unveil that the interfacial i…

[Phys. Rev. B 108, 045412] Published Wed Jul 19, 2023

Eight-dimensional topological systems simulated using time-space crystalline structures
Yakov Braver, Egidijus Anisimovas, and Krzysztof Sacha
Author(s): Yakov Braver, Egidijus Anisimovas, and Krzysztof Sacha

We demonstrate the possibility of using time-space crystalline structures to simulate eight-dimensional systems based on only two physical dimensions. A suitable choice of system parameters allows us to obtain a gapped energy spectrum, making topological effects become relevant. The nontrivial topol…

[Phys. Rev. B 108, L020303] Published Wed Jul 19, 2023

Found 1 papers in prl
Date of feed: Thu, 20 Jul 2023 03:17:02 GMT

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

Extended Spatial Coherence of Interlayer Excitons in ${\mathrm{MoSe}}_{2}/{\mathrm{WSe}}_{2}$ Heterobilayers
Mirco Troue, Johannes Figueiredo, Lukas Sigl, Christos Paspalides, Manuel Katzer, Takashi Taniguchi, Kenji Watanabe, Malte Selig, Andreas Knorr, Ursula Wurstbauer, and Alexander W. Holleitner
Author(s): Mirco Troue, Johannes Figueiredo, Lukas Sigl, Christos Paspalides, Manuel Katzer, Takashi Taniguchi, Kenji Watanabe, Malte Selig, Andreas Knorr, Ursula Wurstbauer, and Alexander W. Holleitner

Evidence of coherent light emission from excitons in a 2D-material structure could inspire new quantum-technology applications.

[Phys. Rev. Lett. 131, 036902] Published Wed Jul 19, 2023

Found 2 papers in nano-lett
Date of feed: Wed, 19 Jul 2023 13:04:48 GMT

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

[ASAP] Chemical Potential Characterization of Symmetry-Breaking Phases in a Rhombohedral Trilayer Graphene
Xiangyan Han, Qianling Liu, Yijie Wang, Ruirui Niu, Zhuangzhuang Qu, Zhiyu Wang, Zhuoxian Li, Chunrui Han, Kenji Watanabe, Takashi Taniguchi, Zhida Song, Jinhai Mao, Zheng Vitto Han, Zizhao Gan, and Jianming Lu

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Nano Letters
DOI: 10.1021/acs.nanolett.3c01262

[ASAP] Twist-Induced Modification in the Electronic Structure of Bilayer WSe2
Ding Pei, Zishu Zhou, Zhihai He, Liheng An, Han Gao, Hanbo Xiao, Cheng Chen, Shanmei He, Alexei Barinov, Jianpeng Liu, Hongming Weng, Ning Wang, Zhongkai Liu, and Yulin Chen

TOC Graphic

Nano Letters
DOI: 10.1021/acs.nanolett.3c01672

Found 2 papers in science-adv
Date of feed: Wed, 19 Jul 2023 19:00:10 GMT

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

Coupled topological flat and wide bands: Quasiparticle formation and destruction
Haoyu Hu and Qimiao Si
Science Advances, Volume 9, Issue 29, July 2023.

Ultrafast van der Waals diode using graphene quantum capacitance and Fermi-level depinning
Sungjae Hong, Chang-Ui Hong, Sol Lee, Myeongjin Jang, Chorom Jang, Yangjin Lee, Livia Janice Widiapradja, Sam Park, Kwanpyo Kim, Young-Woo Son, Jong-Gwan Yook, Seongil Im
Science Advances, Volume 9, Issue 29, July 2023.

Found 1 papers in nat-comm

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

Water nanolayer facilitated solitary-wave-like blisters in MoS2 thin films
< author missing >

Found 1 papers in comm-phys

Search terms: (topolog[a-z]+)|(graphit[a-z]+)|(rhombohedr[a-z]+)|(graphe[a-z]+)|(chalcog[a-z]+)|(landau)|(weyl)|(dirac)|(STM)|(scan[a-z]+ tunne[a-z]+ micr[a-z]+)|(scan[a-z]+ tunne[a-z]+ spectr[a-z]+)|(scan[a-z]+ prob[a-z]+ micr[a-z]+)|(MoS.+\d+|MoS\d+)|(MoSe.+\d+|MoSe\d+)|(MoTe.+\d+|MoTe\d+)|(WS.+\d+|WS\d+)|(WSe.+\d+|WSe\d+)|(WTe.+\d+|WTe\d+)|(Bi\d+Rh\d+I\d+|Bi.+\d+.+Rh.+\d+.+I.+\d+.+)|(BiTeI)|(BiTeBr)|(BiTeCl)|(ZrTe5|ZrTe.+5)|(Pt2HgSe3|Pt.+2HgSe.+3)|(jacuting[a-z]+)

Unusual crossover from Bardeen-Cooper-Schrieffer to Bose-Einstein-condensate superconductivity in iron chalcogenides
Takasada Shibauchi

Communications Physics, Published online: 19 July 2023; doi:10.1038/s42005-023-01289-8

The BCS-BEC crossover is typically observed using ultracold atomic systems but recent experiments suggest investigations may also be possible using strongly correlated systems. Here, the authors use FeSe1−xSx to investigate the evolution of the superconducting state in the BCS-BEC crossover regime observing multiband nature of the BCS-BEC crossover with a suppression of the nematic order upon S-substitution.