Found 44 papers in cond-mat
Date of feed: Wed, 23 Aug 2023 00:30:00 GMT

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Superconductivity in a Topological Lattice Model with Strong Repulsion. (arXiv:2308.10935v1 [cond-mat.str-el])
Rahul Sahay, Stefan Divic, Daniel E. Parker, Tomohiro Soejima, Sajant Anand, Johannes Hauschild, Monika Aidelsburger, Ashvin Vishwanath, Shubhayu Chatterjee, Norman Y. Yao, Michael P. Zaletel

The highly tunable nature of synthetic quantum materials -- both in the solid-state and cold atom contexts -- invites examining which microscopic ingredients aid in the realization of correlated phases of matter such as superconductors. Recent experimental advances in moir\'e materials suggest that unifying the features of the Fermi-Hubbard model and quantum Hall systems creates a fertile ground for the emergence of such phases. Here, we introduce a minimal 2D lattice model that incorporates exactly these features: time-reversal symmetry, band topology, and strong repulsive interactions. By using infinite cylinder density matrix renormalization group methods (cylinder iDMRG), we investigate the ground state phase diagram of this model. We find that it hosts an interaction-induced quantum spin Hall (QSH) insulator and demonstrate that weakly hole-doping this state gives rise to a superconductor at a finite circumference, with indications that this behavior persists on larger cylinders. At the aforementioned circumference, the superconducting phase is surprisingly robust to perturbations including additional repulsive interactions in the pairing channel. By developing a technique to probe the superconducting gap function in iDMRG, we phenomenologically characterize the superconductor. Namely, we demonstrate that it is formed from the weak pairing of holes atop the QSH insulator. Furthermore, we determine the pairing symmetry of the superconductor, finding it to be $p$-wave -- reminiscent of the unconventional superconductivity reported in experiments on twisted bilayer graphene (TBG). Motivated by this, we elucidate structural similarities and differences between our model and those of TBG in its chiral limit. Finally, to provide a more direct experimental realization, we detail an implementation of our Hamiltonian in a system of cold fermionic alkaline-earth atoms in an optical lattice.

Coulomb-driven band unflattening suppresses $K$-phonon pairing in moir\'e graphene. (arXiv:2308.10938v1 [cond-mat.supr-con])
Glenn Wagner, Yves H. Kwan, Nick Bultinck, Steven H. Simon, S.A. Parameswaran

It is a matter of current debate whether the gate-tunable superconductivity in twisted bilayer graphene is phonon-mediated or arises from electron-electron interactions. The recent observation of the strong coupling of electrons to so-called $K$-phonon modes in angle-resolved photoemission spectroscopy experiments has resuscitated early proposals that $K$-phonons drive superconductivity. We show that the bandwidth-enhancing effect of interactions drastically weakens both the intrinsic susceptibility towards pairing as well as the screening of Coulomb repulsion that is essential for the phonon attraction to dominate at low temperature. This rules out purely $K$-phonon-mediated superconductivity with the observed transition temperature of $\sim 1$ K. We conclude that the unflattening of bands by Coulomb interactions challenges any purely phonon-driven pairing mechanism, and must be addressed by a successful theory of superconductivity in moir\'e graphene.

Experimental demonstration of an integrated on-chip p-bit core utilizing stochastic Magnetic Tunnel Junctions and 2D-MoS$_{2}$ FETs. (arXiv:2308.10989v1 [cond-mat.mes-hall])
John Daniel, Zheng Sun, Xuejian Zhang, Yuanqiu Tan, Neil Dilley, Zhihong Chen, Joerg Appenzeller

Probabilistic computing is a novel computing scheme that offers a more efficient approach than conventional CMOS-based logic in a variety of applications ranging from optimization to Bayesian inference, and invertible Boolean logic. The probabilistic-bit (or p-bit, the base unit of probabilistic computing) is a naturally fluctuating entity that requires tunable stochasticity; by coupling low-barrier stochastic Magnetic Tunnel Junctions (MTJs) with a transistor circuit, a compact implementation is achieved. In this work, through integrating stochastic MTJs with 2D-MoS$_{2}$ FETs, the first on-chip realization of a key p-bit building block displaying voltage-controllable stochasticity is demonstrated. In addition, supported by circuit simulations, this work provides a careful analysis of the three transistor-one magnetic tunnel junction (3T-1MTJ) p-bit design, evaluating how the characteristics of each component influence the overall p-bit output. This understanding of the interplay between the characteristics of the transistors and the MTJ is vital for the construction of a fully functioning p-bit, making the design rules presented in this article key for future experimental implementations of scaled on-chip p-bit networks.

Stability of a quantum skyrmion: projective measurements and the quantum Zeno effect. (arXiv:2308.11014v1 [quant-ph])
Fabio Salvati, Mikhail I. Katsnelson, Andrey A. Bagrov, Tom Westerhout

Magnetic skyrmions are vortex-like quasiparticles characterized by long lifetime and remarkable topological properties. That makes them a promising candidate for the role of information carriers in magnetic information storage and processing devices. Although considerable progress has been made in studying skyrmions in classical systems, little is known about the quantum case: quantum skyrmions cannot be directly observed by probing the local magnetization of the system, and the notion of topological protection is elusive in the quantum realm. Here, we explore the potential robustness of quantum skyrmions in comparison to their classical counterparts. We theoretically analyze the dynamics of a quantum skyrmion subject to local projective measurements and demonstrate that the properties of the skyrmionic quantum state change very little upon external perturbations. We further show that by performing repetitive measurements on a quantum skyrmion, it can be completely stabilized through an analog of the quantum Zeno effect.

Strain-Induced Polarization Enhancement in BaTiO$_3$ Core-Shell Nanoparticles. (arXiv:2308.11044v1 [cond-mat.mtrl-sci])
Eugene A. Eliseev, Anna N. Morozovska, Sergei V. Kalinin, Dean R. Evans

Despite fascinating experimental results, the influence of defects and elastic strains on the physical state of nanosized ferroelectrics is still poorly explored theoretically. One of unresolved theoretical problems is the analytical description of the strongly enhanced spontaneous polarization, piezoelectric response, and dielectric properties of ferroelectric oxide thin films and core-shell nanoparticles induced by elastic strains and stresses. In particular, the 10-nm quasi-spherical BaTiO$_3$ core-shell nanoparticles reveal a giant spontaneous polarization up to 130 mu_C/cm2, where the physical origin is a large Ti off-centering. The available theoretical description cannot explain the giant spontaneous polarization observed in these spherical nanoparticles. This work analyzes polar properties of BaTiO$_3$ core-shell spherical nanoparticles using the Landau-Ginzburg-Devonshire approach, which considers the nonlinear electrostriction coupling and large Vegard strains in the shell. We reveal that a spontaneous polarization greater than 50 mu_C/cm2 can be stable in a (10-100) nm BaTiO$_3$ core at room temperature, where a 5 nm paraelectric shell is stretched by (3-6)% due to Vegard strains, which contribute to the elastic mismatch at the core-shell interface. The large polarization corresponds to very high tetragonality ratios (1.02-1.04), which is further increased by higher Vegard strains and/or intrinsic surface stresses leading to unphysically high tetragonality ratios. The nonlinear electrostriction coupling and the elastic mismatch at the core-shell interface are key physical factors of the spontaneous polarization enhancement in the core. Doping with the highly-polarized core-shell nanoparticles can be useful in optoelectronics and nonlinear optics to increase beam coupling efficiency, electric field enhancement, reduced switching voltages, ionic contamination elimination, and catalysis.

Exciton Superposition across Moir\'e States in a Semiconducting Moir\'e Superlattice. (arXiv:2308.11054v1 [cond-mat.mes-hall])
Zhen Lian, Dongxue Chen, Yuze Meng, Xiaotong Chen, Ying Su, Rounak Banerjee, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Chuanwei Zhang, Yong-Tao Cui, Su-Fei Shi

Moir\'e superlattices of semiconducting transition metal dichalcogenides (TMDCs) enable unprecedented spatial control of electron wavefunctions in an artificial lattice with periodicities more than ten times larger than that of atomic crystals, leading to emerging quantum states with fascinating electronic and optical properties. The breaking of translational symmetry further introduces a new degree of freedom inside each moir\'e unit cell: high symmetry points of energy minima called moir\'e sites, behaving as spatially separated quantum dots. The superposition of a quasiparticle wavefunction between different moir\'e sites will enable a new platform for quantum information processing but is hindered by the suppressed electron tunneling between moir\'e sites. Here we demonstrate the superposition between two moir\'e sites by constructing an angle-aligned trilayer WSe2/monolayer WS2 moir\'e heterojunction. The two moir\'e sites with energy minimum allow the formation of two different interlayer excitons, with the hole residing in either moir\'e site of the first WSe2 layer interfacing the WS2 layer and the electron in the third WSe2 layer. An external electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe2 layer, realizing the continuous tuning of interlayer exciton hopping between two moir\'e sites. Therefore, a superposition of the two interlayer excitons localized at different moir\'e sites can be realized, which can be resolved in the electric-field-dependent optical reflectance spectra, distinctly different from that of the natural trilayer WSe2 in which the moir\'e modulation is absent. Our study illustrates a strategy of harnessing the new moir\'e site degree of freedom for quantum information science, a new direction of twistronics.

Nonanalytic Corrections to the Landau Diamagnetic Susceptibility. (arXiv:2308.11057v1 [cond-mat.str-el])
R. David Mayrhofer, Andrey V. Chubukov

We analyze potential non-analytic terms in the Landau diamagnetic susceptibility, $\chi_{dia}$, at a finite temperature $T$ and/or finite magnetic field $H$. To do this, we express the diamagnetic susceptibility as $\chi_{dia} = (e/c)^2 \lim_{Q\rightarrow0} \Pi^{JJ}_\perp (Q)/Q^2$, where $\Pi^{JJ}_\perp$ is the transverse component of the static current-current correlator, and evaluate $\Pi^{JJ}_\perp (Q)$ for a system of fermions with Hubbard interaction to second order in Hubbard $U$ by combining self energy, Maki-Thompson, and Aslamazov-Larkin diagrams. We find that at $T=H=0$, the expansion of $\Pi^{JJ}_\perp (Q)/Q^2$ in $U$ is regular, but at a finite $T$ and/or $H$, it contains $U^2 T$ and/or $U^2 |H|$ terms. Similar terms have been previously found for the paramagnetic Pauli susceptibility. We obtain the full expression for the non-analytic $\delta \chi_{dia} (H,T)$ when both $T$ and $H$ are finite, and show that the $H/T$ dependence is similar to that for the Pauli susceptibility.

Photoinduced topological phase transitions in Kitaev-Heisenberg honeycomb ferromagnets with the Dzyaloshinskii-Moriya interaction. (arXiv:2308.11077v1 [cond-mat.str-el])
Zhengguo Tang, Heng Zhu, Hongchao Shi, Bing Tang

We theoretically study topological properties of Floquet magnon in a laser-irradiated Kitaev-Heisenberg honeycomb ferromagnet with the Dzyaloshinskii-Moriya interaction by means of the Floquet-Bloch theory. It is found that the Kitaev-Heisenberg ferromagnet can reveal two topological phases with different Chern numbers when it is irradiated by a circular-polarized light laser. Our results show that the topological phase of the system can be switched from one topological phase to another one via varying the light intensity. The intrinsic DMI plays a crucial role in the occurrence of photoinduced topological phase transition. It is shown that the sign reversal of the thermal hall conductivity is an important indicator on photoinduced topological phase transitions in the Kitaev-Heisenberg honeycomb ferromagnet.

Theory of a topological analogue of the magnetic bit. (arXiv:2308.11099v1 [cond-mat.mtrl-sci])
Xinyuan Xu, David Sénéchal, Ion Garate

In magnetic memories, the state of a ferromagnet is encoded in the orientation of its magnetization. The energy of the system is minimized when the magnetization is parallel or antiparallel to a preferred (easy) axis. These two stable directions define the logical bit. Under an external perturbation, the direction of magnetization can be controllably reversed and thus the bit flipped. Here, we theoretically design a topological analogue of the magnetic bit in the Su-Schrieffer-Heeger (SSH)-Holstein model, where we show that a transient external perturbation can lead to a permanent change in the electronic band topology.

Discovery of smectic charge and pair-density-wave orders in topological monolayer 1T$^\prime$-MoTe$_2$. (arXiv:2308.11101v1 [cond-mat.supr-con])
Li-Xuan Wei, Peng-Cheng Xiao, Fangsen Li, Li Wang, Bo-Yuan Deng, Fang-Jun Cheng, Fa-Wei Zheng, Ning Hao, Ping Zhang, Xu-Cun Ma, Qi-Kun Xue, Can-Li Song

Electronic liquid-crystal phases are observed in numerous strongly-correlated systems including high-temperature superconductors. However, identifying these exotic phases and understanding their interplay with superconductivity in topological materials remain challenging. Here we employ a cryogenic scanning tunneling microscopy to discover a smectic (stripe) charge order (CO) and a primary pair-density-wave (PDW) in topological monolayer 1T$^\prime$-MoTe$_2$. The two orders are spatially modulated unidirectionally at the same wavevector, but have a marked spatial phase difference of about 2$\pi$/5. Importantly, the primary PDW state features a two-gap superconductivity below the transition temperature of 6.0 K and induces another unique particle-hole-symmetric CO at twice the PDW wavevector. Combining these results and our density functional calculations, we reveal that the two smectic orders are primarily driven by nesting behaviors between electron and hole pockets. Our findings establish monolayer 1T$^\prime$-MoTe$_2$ as a topological paradigm for exploring electronic smecticity, which intertwines with multiple preexisting symmetry-breaking states.

Ultrastrong Light-Matter Coupling in 2D Metal-Chalcogenates. (arXiv:2308.11108v1 [physics.optics])
Surendra B. Anantharaman, Jason Lynch, Mariya Aleksich, Christopher E. Stevens, Christopher Munley, Bongjun Choi, Sridhar Shenoy, Thomas Darlington, Arka Majumdar, P. James Shuck, Joshua Hendrickson, J. Nathan Hohman, Deep Jariwala

Hybridization of excitons with photons to form hybrid quasiparticles, exciton-polaritons (EPs), has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at room temperatures which recently were observed in layered two-dimensional (2D) excitonic materials. Here, we report an extreme version of this phenomenon, an ultrastrong EP coupling, in a nascent, 2D excitonic system, the metal organic chalcogenate (MOCHA) compound named mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi Splitting in the ultrastrong coupling range (> 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show bright EP emission at room temperature as well as EP dispersions at low-temperatures. Importantly, we find lower EP emission linewidth narrowing to ~1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our results suggest that MOCHA materials are ideal for polaritonics in the deep green-blue part of the spectrum where strong excitonic materials with large optical constants are notably scarce.

Exploring Parity Magnetic Effects through Experimental Simulation with Superconducting Qubits. (arXiv:2308.11115v1 [quant-ph])
Yu Zhang, Yan-Qing Zhu, Jianwen Xu, Wen Zheng, Dong Lan, Giandomenico Palumbo, Nathan Goldman, Shi-Liang Zhu, Xinsheng Tan, Z.D.Wang, Yang Yu

We present the successful realization of four-dimensional (4D) semimetal bands featuring tensor monopoles, achieved using superconducting quantum circuits. Our experiment involves the creation of a highly tunable diamond energy diagram with four coupled transmons, and the parametric modulation of their tunable couplers, effectively mapping momentum space to parameter space. This approach enables us to establish a 4D Dirac-like Hamiltonian with fourfold degenerate points. Moreover, we manipulate the energy of tensor monopoles by introducing an additional pump microwave field, generating effective magnetic and pseudo-electric fields and simulating topological parity magnetic effects emerging from the parity anomaly. Utilizing non-adiabatic response methods, we measure the fractional second Chern number for a Dirac valley with a varying mass term, signifying a nontrivial topological phase transition connected to a 5D Yang monopole. Our work lays the foundation for further investigations into higher-dimensional topological states of matter and enriches our comprehension of topological phenomena.

Structural, morphological, and magnetic characterizations of (FexMn1-x)2O3 nanocrystals: A comprehensive stoichiometric determination. (arXiv:2308.11128v1 [cond-mat.mtrl-sci])
John C. Mantilla, Luiz C. C. M. Nagamine, Daniel Cornejo, Renato Cohen, Wesley de Oliveira, Paulo Souza, Sebastião W. da Silva, Fermin F.H. Aragón, Pedro L. Gastelois, Paulo C. Morais, José A.H. Coaquira

Iron manganese trioxide (FexMn1-x)2O3 nanocrystals were synthesized by the sol-gel method. The 80 K Mossbauer spectrum was well-fitted using two doublets representing the 8b and 24d crystallographic sites of the (FexMn1-x)2O3 phase and two weak extra sextets which were attributed to crystalline and amorphous hematite. Our findings showed formation of a bixbyite primary phase. The Raman spectrum exhibits six Raman active modes, typical of (Fe,Mn)2O3, and two extra Raman modes associated with the secondary hematite phase. X-ray photoelectron spectroscopy analysis confirmed the presence of oxygen vacancy onto the (FexMn1-x)2O3 particle surface, with varying oxidation states. X-band magnetic resonance data revealed a single broad resonance line in the whole temperature range (3.8 K - 300 K). The temperature dependence of both resonance field and resonance linewidth shows a remarkable change in the range of 40 - 50 K, herein credited to surface spin glass behavior. The model picture used assumes (FexMn1-x)2O3 nanoparticles with a core-shell structure. Results indicate that below about 50 K the spin system of shell reveals a paramagnetic to spin glass-like transition upon cooling, with a critical temperature estimated at 43 K. In the higher temperature range, the superparamagnetic hematite (secondary) phase contributes remarkably to the temperature dependence of the resonance linewidth. Zero-field-cooled (ZFC) and fieldcooled (FC) data show strong irreversibility and a peak in the ZFC curve at 33 K, attributed to a paramagnetic-ferrimagnetic transition of the main phase. Hysteresis curve at 5 K shows a low coercive field of 4 kOe, with the magnetization not reaching saturation at 70 kOe, suggesting the occurrence of a ferrimagnetic core with a magnetic disorder at surface, characteristic of core-shell spin-glass-like behavior.

Time-reversal symmetry-breaking flux state in an organic Dirac fermion system. (arXiv:2308.11141v1 [cond-mat.str-el])
Takao Morinari

We investigate symmetry breaking in the Dirac fermion phase of the organic compound $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressure, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene. The exchange interaction resulting from inter-molecule Coulomb repulsion leads to broken time-reversal symmetry and particle-hole symmetry while preserving translational symmetry. The system breaks time-reversal symmetry by creating fluxes in the unit cell. This symmetry-broken state exhibits a large Nernst signal as well as thermopower. We compute the Nernst signal and thermopower, demonstrating their consistency with experimental results.

Tuning of Electrical, Magnetic, and Topological Properties of Magnetic Weyl Semimetal Mn$_{3+x}$Ge by Fe doping. (arXiv:2308.11183v1 [cond-mat.mtrl-sci])
Susanta Ghosh, Achintya Low, Soumya Ghorai, Kalyan Mandal, Setti Thirupathaiah

We report on the tuning of electrical, magnetic, and topological properties of the magnetic Weyl semimetal (Mn$_{3+x}$Ge) by Fe doping at the Mn site, Mn$_{(3+x)-\delta}$Fe$_{\delta}$Ge ($\delta$=0, 0.30, and 0.62). Fe doping significantly changes the electrical and magnetic properties of Mn$_{3+x}$Ge. The resistivity of the parent compound displays metallic behavior, the system with $\delta$=0.30 of Fe doping exhibits semiconducting or bad-metallic behavior, and the system with $\delta$=0.62 of Fe doping demonstrates a metal-insulator transition at around 100 K. Further, we observe that the Fe doping increases in-plane ferromagnetism, magnetocrystalline anisotropy, and induces a spin-glass state at low temperatures. Surprisingly, topological Hall state has been noticed at a Fe doping of $\delta$=0.30 that is not found in the parent compound or with $\delta$=0.62 of Fe doping. In addition, spontaneous anomalous Hall effect observed in the parent system is significantly reduced with increasing Fe doping concentration.

Branched flows in active random walks and the formation of ant trail patterns. (arXiv:2308.11232v1 [])
King Hang Mok, Ragnar Fleischmann

Branched flow governs the transition from ballistic to diffusive motion of waves and conservative particle flows in spatially correlated random or complex environments. It occurs in many physical systems from micrometer to interstellar scales. In living matter systems, however, this transport regime is usually suppressed by dissipation and noise. In this article we demonstrate that, nonetheless, noisy active random walks, characterizing many living systems like foraging animals, and chemotactic bacteria, can show a regime of branched flow. To this aim we model the dynamics of trail forming ants and use it to derive a scaling theory of branched flows in active random walks in random bias fields in the presence of noise. We also show how trail patterns, formed by the interaction of ants by depositing pheromones along their trajectories, can be understood as a consequence of branched flow.

Collective Flows Drive Cavitation in Spinner Monolayers. (arXiv:2308.11280v1 [physics.flu-dyn])
Zaiyi Shen, Juho S. Lintuvuori

Hydrodynamic interactions can give rise to a collective motion of rotating particles. This, in turn, can lead to coherent fluid flows. Using large scale hydrodynamic simulations, we study the coupling between these two in spinner monolayers at weakly inertial regime. We observe an instability, where the initially uniform particle layer separates into particle void and particle rich areas. The particle void region corresponds to a fluid vortex, and it is driven by a surrounding spinner edge current. We show that the instability originates from a hydrodynamic lift force between the particle and fluid flows. The cavitation can be tuned by the strength of the collective flows. It is suppressed when the spinners are confined by a no-slip surface, and multiple cavity and oscillating cavity states are observed when the particle concentration is reduced.

Nonequilibrium Casimir-Polder Interaction Between Nanoparticles and Substrates Coated with Gapped Graphene. (arXiv:2308.11306v1 [cond-mat.mes-hall])
Galina L. Klimchitskaya, Constantine C. Korikov, Vladimir M. Mostepanenko, Oleg Yu. Tsybin

The out-of-thermal-equilibrium Casimir-Polder force between nanoparticles and dielectric substrates coated with gapped graphene is considered in the framework of the Dirac model using the formalism of the polarization tensor. This is an example of physical phenomena violating the time-reversal symmetry. After presenting the main points of the used formalism, we calculate two contributions to the Casimir-Polder force acting on a nanoparticle on the source side of a fused silica glass substrate coated with gapped graphene, which is either cooler or hotter than the environment. The total nonequilibrium force magnitudes are computed as a function of separation for different values of the energy gap and compared with those from an uncoated plate and with the equilibrium force in the presence of graphene coating. According to our results, the presence of a substrate increases the magnitude of the nonequlibrium force. The force magnitude becomes larger with higher and smaller with lower temperature of the graphene-coated substrate as compared to the equilibrium force at the environmental temperature. It is shown that with increasing energy gap the magnitude of the nonequilibrium force becomes smaller, and the graphene coating makes a lesser impact on the force acting on a nanoparticle from the uncoated substrate. Possible applications of the obtained results are discussed.

How to identify and characterize strongly correlated topological semimetals. (arXiv:2308.11318v1 [cond-mat.str-el])
Diana M. Kirschbaum, Monika Lužnik, Gwenvredig Le Roy, Silke Paschen

How strong correlations and topology interplay is a topic of great current interest. In this perspective paper, we focus on correlation-driven gapless phases. We take the time-reversal symmetric Weyl semimetal as an example because it is expected to have clear (albeit nonquantized) topological signatures in the Hall response and because the first strongly correlated representative, the noncentrosymmetric Weyl-Kondo semimetal Ce$_3$Bi$_4$Pd$_3$, has recently been discovered. We summarize its key characteristics and use them to construct a prototype Weyl-Kondo semimetal temperature-magnetic field phase diagram. This allows for a substantiated assessment of other Weyl-Kondo semimetal candidate materials. We also put forward scaling plots of the intrinsic Berry-curvature-induced Hall response vs the inverse Weyl velocity -- a measure of correlation strength, and vs the inverse charge carrier concentration -- a measure of the proximity of Weyl nodes to the Fermi level. They suggest that the topological Hall response is maximized by strong correlations and small carrier concentrations. We hope that our work will guide the search for new Weyl-Kondo semimetals and correlated topological semimetals in general, and also trigger new theoretical work.

A Multi-Technique Study of C2H4 Adsorption on Fe3O4(001). (arXiv:2308.11344v1 [cond-mat.mtrl-sci])
Lena Puntscher, Panukorn Sombut, Chunlei Wang, Manuel Ulreich, Jiri Pavelec, Ali Rafsanjani-Abbasi, Matthias Meier, Adam Lagin, Martin Setvin, Ulrike Diebold, Cesare Franchini, Michael Schmid, Gareth S. Parkinson

The adsorption/desorption of ethene (C2H4), also commonly known as ethylene, on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature programmed desorption (TPD), scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory (DFT) based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C2H4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C2H4 binds atop surface Fe sites up to a coverage of 2 molecules per (rt2xrt2)R45{\deg} unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C2H4 adsorption is found to be close to 4 molecules per (rt2xrt2)R45{\deg} unit cell.

Skyrmion motion in magnetic anisotropy gradients: Acceleration caused by deformation. (arXiv:2308.11361v1 [cond-mat.mes-hall])
Ismael Ribeiro de Assis, Ingrid Mertig, Börge Göbel

Magnetic skyrmions are nano-sized topologically non-trivial spin textures that can be moved by external stimuli such as spin currents and internal stimuli such as spatial gradients of a material parameter. Since the total energy of a skyrmion depends linearly on most of these parameters, like the perpendicular magnetic anisotropy, the exchange constant, or the Dzyaloshinskii-Moriya interaction strength, a skyrmion will move uniformly in a weak parameter gradient. In this paper, we show that the linear behavior changes once the gradients are strong enough so that the magnetic profile of a skyrmion is significantly altered throughout the propagation. In that case, the skyrmion experiences acceleration and moves along a curved trajectory. Furthermore, we show that when spin-orbit torques and material parameter gradients trigger a skyrmion motion, it can move on a straight path along the current or gradient direction. We discuss the significance of suppressing the skyrmion Hall effect for spintronic and neuromorphic applications of skyrmions. Lastly, we extend our discussion and compare it to a gradient generated by the Dzyaloshinskii-Moriya interaction.

Non-Hermitian topological ohmmeter. (arXiv:2308.11367v1 [cond-mat.mes-hall])
Viktor Könye, Kyrylo Ochkan, Anastasiia Chyzhykova, Jan Carl Budich, Jeroen van den Brink, Ion Cosma Fulga, Joseph Dufouleur

Measuring large electrical resistances forms an essential part of common applications such as insulation testing, but suffers from a fundamental problem: the larger the resistance, the less sensitive a canonical ohmmeter is. Here we develop a conceptually different electronic sensor by exploiting the topological properties of non-Hermitian matrices, whose eigenvalues can show an exponential sensitivity to perturbations. The ohmmeter is realized in an multi-terminal, linear electric circuit with a non-Hermitian conductance matrix, where the target resistance plays the role of the perturbation. We inject multiple currents and measure a single voltage in order to directly obtain the value of the resistance. The relative accuracy of the device increases exponentially with the number of terminals, and for large resistances outperforms a standard measurement by over an order of magnitude. Our work paves the way towards leveraging non-Hermitian conductance matrices in high-precision sensing.

Hamiltonian learning with real-space impurity tomography in topological moire superconductors. (arXiv:2308.11400v1 [cond-mat.mes-hall])
Maryam Khosravian, Rouven Koch, Jose L. Lado

Extracting Hamiltonian parameters from available experimental data is a challenge in quantum materials. In particular, real-space spectroscopy methods such as scanning tunneling spectroscopy allow probing electronic states with atomic resolution, yet even in those instances extracting effective Hamiltonian is an open challenge. Here we show that impurity states in modulated systems provide a promising approach to extracting non-trivial Hamiltonian parameters of a quantum material. We show that by combining the real-space spectroscopy of different impurity locations in a moire topological superconductor, modulations of exchange and superconducting parameters can be inferred via machine learning. We demonstrate our strategy with a physically-inspired harmonic expansion combined with a fully-connected neural network that we benchmark against a conventional convolutional architecture. We show that while both approaches allow extracting exchange modulations, only the former approach allows inferring the features of the superconducting order. Our results demonstrate the potential of machine learning methods to extract Hamiltonian parameters by real-space impurity spectroscopy as local probes of a topological state.

Water Structures Reveal Local Hydrophobicity on the In2O3(111) Surface. (arXiv:2308.11404v1 [cond-mat.mtrl-sci])
Hao Chen, Matthias A. Blatnik, Christian L. Ritterhoff, Igor Sokolović, Francesca Mirabella, Giada Franceschi, Michele Riva, Michael Schmid, Jan Čechal, Bernd Meyer, Ulrike Diebold, Margareta Wagner

Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis-acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In2O3(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, ncAFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets.

Adsorption configurations of Co-phthalocyanine on In2O3(111). (arXiv:2308.11423v1 [cond-mat.mtrl-sci])
Margareta Wagner, Fabio Calcinelli, Andreas Jeindl, Michael Schmid, Oliver T. Hofmann, Ulrike Diebold

Indium oxide offers optical transparency paired with electric conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of similar size as the unit cell. Non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), and density functional theory (DFT) were used to study the adsorption of Co-phthalocyanine (CoPc) on In2O3(111). Isolated CoPc molecules adsorb at two adsorption sites in a 7:3 ratio. The Co atom sits either on top of a surface oxygen ('F configuration') or indium atom ('S configuration'). This subtle change in adsorption site induces bending of the molecules, which is reflected in their electronic structure. According to DFT the lowest unoccupied molecular orbital of the undistorted gas-phase CoPc remains mostly unaffected in the F configuration but is filled by one electron in S configuration. At coverages up to one CoPc molecule per substrate unit cell, a mixture of domains with molecules in F and S configuration are found. Molecules at F sites first condense into a F-(2x2) structure and finally rearrange into a F-(1x1) symmetry with partially overlapping molecules, while S-sited molecules only assume a S-(1x1) superstructure.

Altermagnetic Orbital Chern Insulator in Twisted MoTe$_{2}$. (arXiv:2308.11454v1 [cond-mat.str-el])
Feng-Ren Fan, Cong Xiao, Wang Yao

In twisted MoTe$_{2}$, latest transport measurement has reported observation of quantum anomalous Hall effect at hole filling $\nu=-1$, which undergoes a topological phase transition to a trivial ferromagnet as layer hybridization gets suppressed by interlayer bias $D$. Here we show that this underlies the existence of an orbital Chern insulating state with gate ($D$) switchable sign in an altermagnetic spin background at hole filling $\nu=-2$. From momentum-space Hartree Fock calculations, we find this state has a topological phase diagram complementary to that of the $\nu=-1$ one: by sweeping $D$ from positive to negative, the Chern number of this $\nu=-2$ state can be switched between $+1$, $0$, and $-1$, accompanied by a sign change of a sizable orbital magnetization. In range of $D$ where this altermagnet is the ground state, the orbital magnetization allows magnetic field initialization of the spin altermagnetic order and the Chern number.

On Polymer Statistical Mechanics: From Gaussian Distribution to Maxwell-Boltzmann Distribution to Fermi-Dirac Distribution. (arXiv:2308.11482v1 [cond-mat.stat-mech])
Lixiang Yang

Macroscopic mechanical properties of polymers are determined by their microscopic molecular chain distribution. Due to randomness of these molecular chains, probability theory has been used to find their micro-states and energy distribution. In this paper, aided by central limit theorem and mixed Bayes rule, we showed that entropy elasticity based on Gaussian distribution is questionable. By releasing freely jointed chain assumption, we found that there is energy redistribution when each bond of a molecular chain changes its length. Therefore, we have to change Gaussian distribution used in polymer elasticity to Maxwell-Boltzmann distribution. Since Maxwell-Boltzmann distribution is only a good energy description for gas molecules, we found a mathematical path to change Maxwell-Boltzmann distribution to Fermi-Dirac distribution based on molecular chain structures. Because a molecular chain can be viewed as many monomers glued by covalent electrons, Fermi-Dirac distribution describes the probability of covalent electron occupancy in micro-states for solids such as polymers. Mathematical form of Fermi-Dirac distribution is logistic function. Mathematical simplicity and beauty of Fermi-Dirac distribution make many hard mechanics problems easy to understand. Generalized logistic function or Fermi-Dirac distribution function was able to understand many polymer mechanics problems such as viscoelasticity [1], viscoplasticity [2], shear band and necking [3], and ultrasonic bonding [4].

Combining ultrahigh index with exceptional nonlinearity in resonant transition metal dichalcogenide nanodisks. (arXiv:2308.11504v1 [physics.optics])
George Zograf, Alexander Yu. Polyakov, Maria Bancerek, Tomasz Antosiewicz, Betul Kucukoz, Timur Shegai

Second-order nonlinearity in solids gives rise to a plethora of unique physical phenomena ranging from piezoelectricity and optical rectification to optical parametric amplification, spontaneous parametric down-conversion, and the generation of entangled photon pairs. Monolayer transition metal dichalcogenides (TMDs), such as MoS$_2$, exhibit one of the highest known second-order nonlinear coefficients. However, the monolayer nature of these materials prevents the fabrication of resonant objects exclusively from the material itself, necessitating the use of external structures to achieve optical enhancement of nonlinear processes. Here, we exploit the 3R phase of a molybdenum disulfide multilayer for resonant nonlinear nanophotonics. The lack of inversion symmetry, even in the bulk of the material, provides a combination of a massive second-order susceptibility, an extremely high and anisotropic refractive index in the near-infrared region ($n>$~4.5), and low absorption losses, making 3R-MoS$_2$ highly attractive for nonlinear nanophotonics. We demonstrate this by fabricating 3R-MoS$_2$ nanodisks of various radii, which support resonant anapole states, and observing substantial ($>$ 100-fold) enhancement of second-harmonic generation in a single resonant nanodisk compared to an unpatterned flake of the same thickness. The enhancement is maximized at the spectral overlap between the anapole state of the disk and the material resonance of the second-order susceptibility. Our approach unveils a powerful tool for enhancing the entire spectrum of optical second-order nonlinear processes in nanostructured van der Waals materials, thereby paving the way for nonlinear and quantum high-index TMD-nanophotonics.

Salt-assisted vapor-liquid-solid growth of one-dimensional van der Waals materials. (arXiv:2308.11545v1 [cond-mat.mtrl-sci])
Thang Pham, Kate Reidy, Joachim D. Thomsen, Baoming Wang, Nishant Deshmukh, Michael A. Filler, Frances M. Ross

We have combined the benefits of two catalytic growth phenomena to form nanostructures of transition metal trichalcogenides (TMTs), materials that are challenging to grow in a nanostructured form by conventional techniques, as required to exploit their exotic physics. Our growth strategy combines the benefits of vapor-liquid-solid (VLS) growth in controlling dimension and growth location, and salt-assisted growth for fast growth at moderate temperatures. This salt-assisted VLS growth is enabled through use of a catalyst that includes Au and an alkali metal halide. We demonstrate high yields of NbS3 1D nanostructures with sub-ten nanometer diameter, tens of micrometers length, and distinct 1D morphologies consisting of nanowires and nanoribbons with [010] and [100] growth orientations, respectively. We present strategies to control the growth location, size, and morphology. We extend the growth method to synthesize other TMTs, NbSe3 and TiS3, as nanowires. Finally, we discuss the growth mechanism based on the relationships we measure between the materials characteristics (growth orientation, morphology and dimensions) and the growth conditions (catalyst volume and growth time). Our study introduces opportunities to expand the library of emerging 1D vdW materials and their heterostructures with controllable nanoscale dimensions.

Quantifying efficiency of remote excitation for surface enhanced Raman spectroscopy in molecular junctions. (arXiv:2308.11547v1 [cond-mat.mes-hall])
Shusen Liao, Yunxuan Zhu, Qian Ye, Stephen Sanders, Jiawei Yang, Alessandro Alabastri, Douglas Natelson

Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare SERS intensity and OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when photon count rates are comparable. By statistical analysis of 33 devices, coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow.

The prototypical organic-oxide interface: intra-molecular resolution of sexiphenyl on In$_2$O$_3$(111). (arXiv:2308.11550v1 [cond-mat.mtrl-sci])
Margareta Wagner, Jakob Hofinger, Martin Setvín, Lynn A. Boatner, Michael Schmid, Ulrike Diebold

The performance of an organic-semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. While an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material is essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In$_2$O$_3$(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide (ITO) is based on. The onset of nucleation and formation of the first monolayer are followed with atomically-resolved scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Annealing to 200$^\circ$C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests a twisted adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with poor long-range order. Eventually the molecules re-orient and form an ordered monolayer. This first monolayer has a densely packed, well-ordered (2$\times$1) structure with one 6P per In$_2$O$_3$(111) substrate unit cell, i.e., a molecular density of 5.64$\times$10$^{13}$ cm$^{-2}$.

From the chiral model of TBG to the Bistritzer--MacDonald model. (arXiv:2308.11555v1 [math-ph])
Simon Becker, Maciej Zworski

We analyse the splitting of exact flat bands in the chiral model of the twisted bilayer graphene (TBG) when the $AA'/BB'$ coupling of the full Bistritzer--MacDonald model is taken into account. The first-order perturbation caused by the $AA'/BB'$ potential the same for both bands and satisfies interesting symmetries, in particular it vanishes on the line defined by the $K$ points. The splitting of the flat bands is governed by the quadratic term which vanishes at the $K$ points.

Anomalous minimum and scaling behavior of localization length near an isolated flat band. (arXiv:1509.00881v3 [physics.optics] UPDATED)
Li Ge

Using one-dimensional tight-binding lattices and an analytical expression based on the Green's matrix, we show that anomalous minimum of the localization length near an isolated flat band, previously found for evanescent waves in a defect-free photonic crystal waveguide, is a generic feature and exists in the Anderson regime as well, i.e., in the presence of disorder. Our finding reveals a scaling behavior of the localization length in terms of the disorder strength, as well as a summation rule of the inverse localization length in terms of the density of states in different bands. Most interesting, the latter indicates the possibility of having two localization minima inside a band gap, if this band gap is formed by two flat bands such as in a double-sided Lieb lattice.

Evidence of nodal superconductivity in monolayer 1H-TaS$_2$ with hidden order fluctuations. (arXiv:2112.07316v2 [cond-mat.supr-con] UPDATED)
Viliam Vaňo, Somesh Chandra Ganguli, Mohammad Amini, Linghao Yan, Maryam Khosravian, Guangze Chen, Shawulienu Kezilebieke, Jose L. Lado, Peter Liljeroth

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, we show that pristine monolayer 1H-TaS$_2$ realizes a nodal superconducting state. By including non-magnetic disorder, we drive the nodal superconducting state to a conventional gapped s-wave state. Furthermore, we observe the emergence of many-body excitations close to the gap edge, signalling a potential unconventional pairing mechanism. Our results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconducting states.

Control of molecular orbital ordering using a van der Waals monolayer ferroelectric. (arXiv:2207.04245v2 [cond-mat.mtrl-sci] UPDATED)
Mohammad Amini, Orlando J. Silveira, Viliam Vaňo, Jose L. Lado, Adam S. Foster, Peter Liljeroth, Shawulienu Kezilebieke

Two-dimensional (2D) ferroelectric materials provide a promising platform for the electrical control of quantum states. In particular, due to their 2D nature, they are suitable for influencing the quantum states of deposited molecules via the proximity effect. Here, we report electrically controllable molecular states in phthalocyanine molecules adsorbed on monolayer ferroelectric material SnTe. In particular, we demonstrate that the strain and ferroelectric order in SnTe creates a transition between two distinct orbital orders in the adsorbed phthalocyanine molecules. By controlling the polarization of the ferroelectric domain using scanning tunneling microscopy (STM), we have successfully demonstrated that orbital order can be manipulated electrically. Our results show how ferroelastic coupling in 2D systems allows control of molecular states, providing a starting point for ferroelectrically switchable molecular orbital ordering and ultimately, electrical control of molecular magnetism.

Van Roosbroeck's equations with topological terms: the case of Weyl semimetals. (arXiv:2208.03379v2 [cond-mat.mes-hall] UPDATED)
Pierre-Antoine Graham, Simon Bertrand, Michaël Bédard, Robin Durand, Ion Garate

Van Roosbroeck's equations constitute a versatile tool to determine the dynamics of electrons under time- and space-dependent perturbations. Extensively utilized in ordinary semiconductors, their potential to model devices made from topological materials remains untapped. Here, we adapt van Roosbroeck's equations to theoretically study the bulk response of a Weyl semimetal to an ultrafast and spatially localized light pulse in the presence of a quantizing magnetic field. We predict a transient oscillatory photovoltage that originates from the chiral anomaly. The oscillations take place at the plasma frequency (THz range) and are damped by intervalley scattering and dielectric relaxation. Our results illustrate the ability of van Roosbroeck's equations to unveil the interplay between electronic band topology and fast carrier dynamics in microelectronic devices.

Hamiltonian inference from dynamical excitations in confined quantum magnets. (arXiv:2212.07893v3 [cond-mat.mes-hall] UPDATED)
Netta Karjalainen, Zina Lippo, Guangze Chen, Rouven Koch, Adolfo O. Fumega, Jose L. Lado

Quantum-disordered models provide a versatile platform to explore the emergence of quantum excitations in many-body systems. The engineering of spin models at the atomic scale with scanning tunneling microscopy and the local imaging of excitations with electrically driven spin resonance has risen as a powerful strategy to image spin excitations in finite quantum spin systems. Here, focusing on $S=1/2$ lattices as realized by Ti in MgO, we show that dynamical spin excitations provide a robust strategy to infer the nature of the underlying Hamiltonian. We show that finite-size interference of the dynamical many-body spin excitations of a generalized long-range Heisenberg model allows the underlying spin couplings to be inferred. We show that the spatial distribution of local spin excitations in Ti islands and ladders directly correlates with the underlying ground state in the thermodynamic limit. Using a supervised learning algorithm, we demonstrate that the different parameters of the Hamiltonian can be extracted by providing the spatially and frequency-dependent local excitations that can be directly measured by electrically driven spin resonance with scanning tunneling microscopy. Our results put forward local dynamical excitations in confined quantum spin models as versatile witnesses of the underlying ground state, providing an experimentally robust strategy for Hamiltonian inference in complex real spin models.

Topological inverse band theory in waveguide quantum electrodynamics. (arXiv:2301.05481v2 [physics.optics] UPDATED)
Yongguan Ke, Jiaxuan Huang, Wenjie Liu, Yuri Kivshar, Chaohong Lee

Topological phases play a crucial role in the fundamental physics of light-matter interaction and emerging applications of quantum technologies. However, the topological band theory of waveguide QED systems is known to break down, because the energy bands become disconnected. Here, we introduce a concept of the inverse energy band and explore analytically topological scattering in a waveguide with an array of quantum emitters. We uncover a rich structure of topological phase transitions, symmetric scale-free localization, completely flat bands, and the corresponding dark Wannier states. Although bulk-edge correspondence is partially broken because of radiative decay, we prove analytically that the scale-free localized states are distributed in a single inverse energy band in the topological phase and in two inverse bands in the trivial phase. Surprisingly, the winding number of the scattering textures depends on both the topological phase of inverse subradiant band and the odevity of the cell number. Our work uncovers the field of the topological inverse bands, and it brings a novel vision to topological phases in light-matter interactions.

Estimating Gibbs free energies via isobaric-isothermal flows. (arXiv:2305.13233v2 [physics.comp-ph] UPDATED)
Peter Wirnsberger, Borja Ibarz, George Papamakarios

We present a machine-learning model based on normalizing flows that is trained to sample from the isobaric-isothermal ensemble. In our approach, we approximate the joint distribution of a fully-flexible triclinic simulation box and particle coordinates to achieve a desired internal pressure. This novel extension of flow-based sampling to the isobaric-isothermal ensemble yields direct estimates of Gibbs free energies. We test our NPT-flow on monatomic water in the cubic and hexagonal ice phases and find excellent agreement of Gibbs free energies and other observables compared with established baselines.

Noninvertible anomalies in $SU(N)\times U(1)$ gauge theories. (arXiv:2305.14425v2 [hep-th] UPDATED)
Mohamed M. Anber, Erich Poppitz

We study $4$-dimensional $SU(N)\times U(1)$ gauge theories with a single massless Dirac fermion in the $2$-index symmetric/antisymmetric representations and show that they are endowed with a noninvertible $0$-form $\widetilde {\mathbb Z}_{2(N\pm 2)}^{\chi}$ chiral symmetry along with a $1$-form $\mathbb Z_N^{(1)}$ center symmetry. By using the Hamiltonian formalism and putting the theory on a spatial three-torus $\mathbb T^3$, we construct the non-unitary gauge invariant operator corresponding to $\widetilde {\mathbb Z}_{2(N\pm 2)}^{\chi}$ and find that it acts nontrivially in sectors of the Hilbert space characterized by selected magnetic fluxes. When we subject $\mathbb T^3$ to $\mathbb Z_N^{(1)}$ twists, for $N$ even, in selected magnetic flux sectors, the algebra of $\widetilde {\mathbb Z}_{2(N\pm 2)}^{\chi}$ and $\mathbb Z_N^{(1)}$ fails to commute by a $\mathbb Z_2$ phase. We interpret this noncommutativity as a mixed anomaly between the noninvertible and the $1$-form symmetries. The anomaly implies that all states in the torus Hilbert space with the selected magnetic fluxes exhibit a two-fold degeneracy for arbitrary $\mathbb T^3$ size. The degenerate states are labeled by discrete electric fluxes and are characterized by nonzero expectation values of condensates. In an Appendix, we also discuss how to construct the corresponding noninvertible defect via the ``half-space gauging'' of a discrete one-form magnetic symmetry.

Emergent Trion-Phonon Coupling in Atomically-Reconstructed MoSe$_2$-WSe$_2$ Heterobilayers. (arXiv:2306.01483v2 [cond-mat.mes-hall] UPDATED)
Sebastian Meier, Yaroslav Zhumagulov, Matthias Dietl, Philipp Parzefall, Michael Kempf, Johannes Holler, Philipp Nagler, Paulo E. Faria Junior, Jaroslav Fabian, Tobias Korn, Christian Schüller

In low-temperature resonant Raman experiments on MoSe$_2$-WSe$_2$ heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an energy, close to the interlayer shear mode (SM) of the heterobilayers, but with a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with the intralayer hybrid trions (HX$^-$) of the MoSe$_2$ and WSe$_2$ layers, only. No resonance with the neutral intralayer excitons is found. First-principles calculations reveal a strong coupling of Q-valley states, which are delocalized over both layers and participate in the HX$^-$, with the SM. This emerging trion-phonon coupling may be relevant for experiments on gate-controlled heterobilayers.

Degenerate flat bands in twisted bilayer graphene. (arXiv:2306.02909v2 [math-ph] UPDATED)
Simon Becker, Tristan Humbert, Maciej Zworski

We prove that in the chiral limit of the Bistritzer-MacDonald Hamiltonian, there exist magic angles at which the Hamiltonian exhibits flat bands of multiplicity four instead of two. We analyze the structure of the Bloch functions associated with the four bands, the corresponding Chern number, and show that there exist infinitely many degenerate magic angles for a generic choice of tunnelling potentials.

Imaginary phonon modes and phonon-mediated superconductivity in Y2C3. (arXiv:2308.00201v2 [cond-mat.supr-con] UPDATED)
Niraj K. Nepal, Paul C. Canfield, Lin-Lin Wang

For Y$_2$C$_3$ with a superconducting critical temperature (T$_c$) $\sim$18 K, zone-center imaginary optical phonon modes have been found for the high-symmetry $I$-$43d$ structure due to C dimer wobbling motion and electronic instability from a flat band near Fermi energy. After lattice distortion to the more stable lowest symmetry $P1$ structure, these stabilized low-energy phonon modes with mixed C and Y characters carry a strong electron-phonon coupling to give arise to the observed sizable T$_c$. Our work shows that compounds with the calculated dynamical instability should not be simply excluded in high-throughput search for new phonon-mediated superconductors.

Minimal model for double Weyl points, multiband quantum geometry, and singular flat band inspired by LK-99. (arXiv:2308.03751v2 [cond-mat.mes-hall] UPDATED)
Moritz M. Hirschmann, Johannes Mitscherling

Two common difficulties in the design of topological quantum materials are that the desired features lie too far from the Fermi level and are spread over a too large energy range. Doping-induced states at the Fermi level provide a solution, where non-trivial topological properties are enforced by the doping-reduced symmetry. To show this, we consider a regular placement of dopants in a lattice of space group (SG) 176 (P6$\text{}_3$/m), which reduces the symmetry to SG 143 (P3). Our two- and four-band models feature symmetry-enforced double Weyl points at $\Gamma$ and A with Chern bands for $k_z\neq 0,\pi$, Van Hove singularities, nontrivial multiband quantum geometry due to mixed orbital character, and a singular flat band. The excellent agreement with density-functional theory (DFT) calculations on copper-doped lead apatite ('LK-99') provides evidence that minimal topological bands at the Fermi level can be realized in doped materials.

Found 13 papers in prb
Date of feed: Wed, 23 Aug 2023 03:16:59 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]+)|(flatband)|(flat.{1}band)|(LK.{1}99)

Phase transition of a non-Abelian quasiperiodic mosaic lattice model with $p$-wave superfluidity
Jincui Zhao, Yujia Zhao, Ji-Guo Wang, Yueqing Li, and Xiao-Dong Bai
Author(s): Jincui Zhao, Yujia Zhao, Ji-Guo Wang, Yueqing Li, and Xiao-Dong Bai

It is now widely believed that $p$-wave superfluidity is the key to generate a novel critical phase in the non-Abelian Aubry-André-Harper model. However, we here establish that this belief is incorrect. In this work, we systemically investigate the phase transition of a non-Abelian quasiperiodic mos…

[Phys. Rev. B 108, 054204] Published Tue Aug 22, 2023

Higher-order topological and nodal superconducting transition-metal sulfides $M\mathrm{S} (M=\mathrm{Nb} \text{and} \mathrm{Ta})$
Yipeng An, Juncai Chen, Yong Yan, Jinfeng Wang, Yinong Zhou, Zhengxuan Wang, Chunlan Ma, Tianxing Wang, Ruqian Wu, and Wuming Liu
Author(s): Yipeng An, Juncai Chen, Yong Yan, Jinfeng Wang, Yinong Zhou, Zhengxuan Wang, Chunlan Ma, Tianxing Wang, Ruqian Wu, and Wuming Liu

Intrinsic topological superconducting materials are exotic and vital to develop the next-generation topological superconducting devices, topological quantum calculations, and quantum information technologies. Here, we predict the topological and nodal superconductivity of NiAs-type $M\mathrm{S}$ $(M…

[Phys. Rev. B 108, 054519] Published Tue Aug 22, 2023

Disorder effects on the quasiparticle and transport properties of two-dimensional Dirac fermionic systems
Bo Fu, Yanru Chen, Weiwei Chen, Wei Zhu, Ping Cui, Qunxiang Li, Zhenyu Zhang, and Qinwei Shi
Author(s): Bo Fu, Yanru Chen, Weiwei Chen, Wei Zhu, Ping Cui, Qunxiang Li, Zhenyu Zhang, and Qinwei Shi

Despite extensive existing studies, a complete understanding of the role of disorder in affecting the physical properties of two-dimensional Dirac fermionic systems remains a standing challenge, largely due to obstacles encountered in treating multiple scattering events for such inherently strong sc…

[Phys. Rev. B 108, 064207] Published Tue Aug 22, 2023

Quasiparticle and transport properties of disordered bilayer graphene
Yanru Chen, Bo Fu, Jinrong Xu, Qinwei Shi, Ping Cui, and Zhenyu Zhang
Author(s): Yanru Chen, Bo Fu, Jinrong Xu, Qinwei Shi, Ping Cui, and Zhenyu Zhang

In recent experimental and theoretical studies of graphene, disorder scattering processes have been suggested to play an important role in its electronic and transport properties. In a preceding paper, it has been shown that the nonperturbative momentum-space Lanczos method is able to accurately des…

[Phys. Rev. B 108, 064208] Published Tue Aug 22, 2023

Ab initio overestimation of the topological region in Eu-based compounds
Giuseppe Cuono, Raghottam M. Sattigeri, Carmine Autieri, and Tomasz Dietl
Author(s): Giuseppe Cuono, Raghottam M. Sattigeri, Carmine Autieri, and Tomasz Dietl

An underestimation of the fundamental band gap values by the density functional theory within the local density approximation and associated approaches is a well-known challenge of ab initio electronic structure computations. Motivated by recent optical experiments [D. Santos-Cottin et al., arXiv:2…

[Phys. Rev. B 108, 075150] Published Tue Aug 22, 2023

Investigating topological order using recurrent neural networks
Mohamed Hibat-Allah, Roger G. Melko, and Juan Carrasquilla
Author(s): Mohamed Hibat-Allah, Roger G. Melko, and Juan Carrasquilla

Recurrent neural networks (RNNs), originally developed for natural language processing, hold great promise for accurately describing strongly correlated quantum many-body systems. Here, we employ two-dimensional RNNs to investigate two prototypical quantum many-body Hamiltonians exhibiting topologic…

[Phys. Rev. B 108, 075152] Published Tue Aug 22, 2023

Experimental demonstration of the band compression effect in engineered kagome-honeycomb lattices
R. G. Yan, T. Z. Ji, W. L. Fan, Z. X. Zhang, H. T. Li, L. Sun, B. F. Miao, G. Chen, and H. F. Ding
Author(s): R. G. Yan, T. Z. Ji, W. L. Fan, Z. X. Zhang, H. T. Li, L. Sun, B. F. Miao, G. Chen, and H. F. Ding

Utilizing a low-temperature scanning-tunneling microscope, we construct Fe kagome-honeycomb lattices on Ag(111) and investigate their lattice parameter dependent electronic properties. The probed spectra exhibit the characteristic lattice peaks, which gradually merge and form a flat-band peak with d…

[Phys. Rev. B 108, 075153] Published Tue Aug 22, 2023

Tunable helical crystals
R. A. Niyazov, D. N. Aristov, and V. Yu. Kachorovskii
Author(s): R. A. Niyazov, D. N. Aristov, and V. Yu. Kachorovskii

We consider a superlattice formed by tunnel-connected identical holes, periodically placed in a two-dimensional topological insulator. We study tunneling transport through helical edges of these holes and demonstrate that the band structure of such helical crystal can be controlled by both gate elec…

[Phys. Rev. B 108, 075424] Published Tue Aug 22, 2023

Origin of strain tunability in flat valence band and ultrahigh shear piezoelectricity in superflexible non–van der Waals graphitic $\mathrm{Sc}X$ monolayers ($X=\mathrm{P}$, As, Sb)
Harshita Seksaria, Arneet Kaur, and Abir De Sarkar
Author(s): Harshita Seksaria, Arneet Kaur, and Abir De Sarkar

Utilizing the exceptional characteristics of two-dimensional (2D) materials for solid-state electronic devices presents an appealing strategy that could potentially address the need to prolong Moore's law. Evidently, the prevailing fraction of technically viable materials, which have already been su…

[Phys. Rev. B 108, 075426] Published Tue Aug 22, 2023

Theory of exciton-polariton condensation in gap-confined eigenmodes
Davide Nigro and Dario Gerace
Author(s): Davide Nigro and Dario Gerace

Exciton-polaritons are bosoniclike elementary excitations in semiconductors, which have been recently shown to display large occupancy of topologically protected polariton bound states in the continuum in suitably engineered photonic lattices [V. Ardizzone et al., Nature (London) 605, 447 (2022)], …

[Phys. Rev. B 108, 085305] Published Tue Aug 22, 2023

Stacking-layer-tuned topological phases in ${M}_{2}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{5} (M=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb})$ films
Yue Li, Yujin Jia, Bao Zhao, Hairui Bao, Hao Huan, Hongming Weng, and Zhongqin Yang
Author(s): Yue Li, Yujin Jia, Bao Zhao, Hairui Bao, Hao Huan, Hongming Weng, and Zhongqin Yang

With first-principles calculations and theoretical models, we reveal the connection between stacking order, film thickness, and topological behaviors in layered ${M}_{2}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{5} (M=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb})$ films. We find that single-layer ${M}_{2}{\mathrm{Bi}}_…

[Phys. Rev. B 108, 085428] Published Tue Aug 22, 2023

Topological superconductivity enhanced by exceptional points
R. Arouca, Jorge Cayao, and Annica M. Black-Schaffer
Author(s): R. Arouca, Jorge Cayao, and Annica M. Black-Schaffer

Majorana zero modes (MZMs) emerge as edge states in topological superconductors and are promising for topological quantum computation, but their detection has so far been elusive. Here we show that non-Hermiticity can be used to obtain dramatically more robust MZMs. The enhanced properties appear as…

[Phys. Rev. B 108, L060506] Published Tue Aug 22, 2023

Correlated Zak insulator in organic antiferromagnets
Takahiro Misawa and Makoto Naka
Author(s): Takahiro Misawa and Makoto Naka

Searching for topological insulators in solids is one of the main issues of modern condensed-matter physics since robust gapless edge or surface states of the topological insulators can be used as building blocks of next-generation devices. Enhancing spin-orbit couplings is a promising way to realiz…

[Phys. Rev. B 108, L081120] Published Tue Aug 22, 2023

Found 1 papers in prl
Date of feed: Wed, 23 Aug 2023 03:17:00 GMT

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Landau-Forbidden Quantum Criticality in Rydberg Quantum Simulators
Jong Yeon Lee, Joshua Ramette, Max A. Metlitski, Vladan Vuletić, Wen Wei Ho, and Soonwon Choi
Author(s): Jong Yeon Lee, Joshua Ramette, Max A. Metlitski, Vladan Vuletić, Wen Wei Ho, and Soonwon Choi

The Landau-Ginzburg-Wilson theory of phase transitions precludes a continuous transition between two phases that spontaneously break distinct symmetries. However, quantum mechanical effects can intertwine the symmetries, giving rise to an exotic phenomenon called deconfined quantum criticality (DQC)…

[Phys. Rev. Lett. 131, 083601] Published Tue Aug 22, 2023

Found 1 papers in sci-rep

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Author Correction: Demonstration of cross reaction in hybrid graphene oxide/tantalum dioxide guided mode resonance sensor for selective volatile organic compound
Sakoolkan Boonruang

Scientific Reports, Published online: 22 August 2023; doi:10.1038/s41598-023-40864-5

Author Correction: Demonstration of cross reaction in hybrid graphene oxide/tantalum dioxide guided mode resonance sensor for selective volatile organic compound

Found 1 papers in nat-comm

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Topological soliton molecule in quasi 1D charge density wave
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