Found 39 papers in cond-mat
Date of feed: Mon, 09 Oct 2023 00:30:00 GMT

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Intervalley coherence and intrinsic spin-orbit coupling in rhombohedral trilayer graphene. (arXiv:2310.03781v1 [cond-mat.mes-hall])
Trevor Arp, Owen Sheekey, Haoxin Zhou, C.L. Tschirhart, Caitlin L. Patterson, H. M. Yoo, Ludwig Holleis, Evgeny Redekop, Grigory Babikyan, Tian Xie, Jiewen Xiao, Yaar Vituri, Tobias Holder, Takashi Taniguchi, Kenji Watanabe, Martin E. Huber, Erez Berg, Andrea F. Young

Rhombohedral graphene multilayers provide a clean and highly reproducible platform to explore the emergence of superconductivity and magnetism in a strongly interacting electron system. Here, we use electronic compressibility and local magnetometry to explore the phase diagram of this material class in unprecedented detail. We focus on rhombohedral trilayer in the quarter metal regime, where the electronic ground state is characterized by the occupation of a single spin and valley isospin flavor. Our measurements reveal a subtle competition between valley imbalanced (VI) orbital ferromagnets and intervalley coherent (IVC) states in which electron wave functions in the two momentum space valleys develop a macroscopically coherent relative phase. Contrasting the in-plane spin susceptibility of the IVC and VI phases reveals the influence of graphene's intrinsic spin-orbit coupling, which drives the emergence of a distinct correlated phase with hybrid VI and IVC character. Spin-orbit also suppresses the in-plane magnetic susceptibility of the VI phase, which allows us to extract the spin-orbit coupling strength of $\lambda \approx 50\mu$eV for our hexagonal boron nitride-encapsulated graphene system. We discuss the implications of finite spin-orbit coupling on the spin-triplet superconductors observed in both rhombohedral and twisted graphene multilayers.

Gapped Phases with Non-Invertible Symmetries: (1+1)d. (arXiv:2310.03784v1 [hep-th])
Lakshya Bhardwaj, Lea E. Bottini, Daniel Pajer, Sakura Schafer-Nameki

We propose a general framework to characterize gapped infra-red (IR) phases of theories with non-invertible (or categorical) symmetries. In this paper we focus on (1+1)d gapped phases with fusion category symmetries. The approach that we propose uses the Symmetry Topological Field Theory (SymTFT) as a key input: associated to a field theory in d spacetime dimensions, the SymTFT lives in one dimension higher and admits a gapped boundary, which realizes the categorical symmetries. It also admits a second, physical, boundary, which is generically not gapped. Upon interval compactification of the SymTFT by colliding the gapped and physical boundaries, we regain the original theory. In this paper, we realize gapped symmetric phases by choosing the physical boundary to be a gapped boundary condition as well. This set-up provides computational power to determine the number of vacua, the symmetry breaking pattern, and the action of the symmetry on the vacua. The SymTFT also manifestly encodes the order parameters for these gapped phases, thus providing a generalized, categorical Landau paradigm for (1+1)d gapped phases. We find that for non-invertible symmetries the order parameters involve multiplets containing both untwisted and twisted sector local operators, and hence can be interpreted as mixtures of conventional and string order parameters. We also observe that spontaneous breaking of non-invertible symmetries can lead to vacua that are physically distinguishable: unlike the standard symmetries described by groups, non-invertible symmetries can have different actions on different vacua of an irreducible gapped phase. This leads to the presence of relative Euler terms between physically distinct vacua. We also provide a mathematical description of symmetric gapped phases as 2-functors from delooping of fusion category characterizing the symmetry to Euler completion of 2-vector spaces.

Categorical Landau Paradigm for Gapped Phases. (arXiv:2310.03786v1 [cond-mat.str-el])
Lakshya Bhardwaj, Lea E. Bottini, Daniel Pajer, Sakura Schafer-Nameki

We propose a unified framework to classify gapped infra-red (IR) phases with categorical symmetries, leading to a generalized, categorical Landau paradigm. This is applicable in any dimension and gives a succinct, comprehensive, and computationally powerful approach to classifying gapped symmetric phases. The key tool is the symmetry topological field theory (SymTFT), which is a one dimension higher TFT with two boundaries, which we choose both to be topological. We illustrate the general idea for (1+1)d gapped phases with categorical symmetries and suggest higher-dimensional extensions.

Influence of disorder on antidot vortex Majorana states in 3D topological insulators. (arXiv:2310.03810v1 [cond-mat.mes-hall])
Rafał Rechciński, Aleksei Khindanov, Dmitry I. Pikulin, Jian Liao, Leonid P. Rokhinson, Yong P. Chen, Roman M. Lutchyn, Jukka I. Väyrynen

Topological insulator/superconductor two-dimensional heterostructures are promising candidates for realizing topological superconductivity and Majorana modes. In these systems, a vortex pinned by a pre-fabricated antidot in the superconductor can host Majorana zero-energy modes (MZMs), which are exotic quasiparticles that may enable quantum information processing. However, a major challenge is to design devices that can manipulate the information encoded in these MZMs. One of the key factors is to create small and clean antidots, so that the MZMs, localized in the vortex core, have a large gap to other excitations. If the antidot is too large or too disordered, the level spacing for the subgap vortex states may become smaller than temperature. In this paper, we numerically investigate the effects of disorder, chemical potential, and antidot size on the subgap vortex spectrum, using a two-dimensional effective model of the topological insulator surface. Our model allows us to simulate large system sizes with vortices up to 1.8 $\mu$m in diameter. We also compare our disorder model with the transport data from existing experiments. We find that the spectral gap can exhibit a non-monotonic behavior as a function of disorder strength, and that it can be tuned by applying a gate voltage.

Serrated plastic flow in slowly-deforming complex concentrated alloys: universal signatures of dislocation avalanches. (arXiv:2310.03828v1 [cond-mat.mtrl-sci])
Kamran Karimi, Amin Esfandiarpour, Stefanos Papanikolaou

Under plastic flow, multi-element high/medium-entropy alloys (HEAs/MEAs) commonly exhibit complex intermittent and collective dislocation dynamics owing to inherent lattice distortion and atomic-level chemical complexities. Using atomistic simulations, we report on an avalanche study of slowly-driven model face-centered cubic (fcc) NiCoCrFeMn and NiCoCr chemically complex alloys aiming for microstructural/topological characterization of associated dislocation avalanches. The results of our avalanche simulations reveal a close correspondence between the observed serration features in the stress response of the deforming HEA/MEA and the incurred slip patterns within the bulk crystal. We show that such correlations become quite pronounced within the rate-independent (quasi-static) regime exhibiting scale-free statistics and critical scaling features as universal signatures of dislocation avalanches.

Collective excitations and screening in two-dimensional tilted nodal-line semimetals. (arXiv:2310.03835v1 [cond-mat.mes-hall])
Hamid Rahimpoor, Saeed H. Abedinpour

Topological nodal-line semimetals are characterized by symmetry-protected one-dimensional band-touching lines or loops, which give rise to their peculiar Fermi surfaces at low energies. Furthermore, if time-reversal or inversion symmetry breaking tilts the bands, anisotropic Fermi surfaces hosting electron and hole carriers simultaneously can also appear. We analytically investigate the linear density-density response function of a two-dimensional tilted nodal-line semimetal in the intrinsic and doped regimes. Despite the anisotropic electronic bands, the polarizability remains isotropic in our model system. We find that the plasmon dispersion in the long wavelength limit exhibits a standard behavior that is proportional to the square root of the wave vector, characteristic of two-dimensional electron liquids. Tilting tends to enhance the plasmon frequency, and the Drude weight does not depend on the carrier density at low doping levels. In these regimes, unlike the intrinsic and highly-doped ones, the static polarizability has two distinct singularities at finite wave vectors. This results in beat patterns in the Friedel oscillations.

Persistent polarization oscillations in ring-shape polariton condensates. (arXiv:2310.03836v1 [physics.optics])
A. V. Yulin, E. S. Sedov, A. V. Kavokin, I. A. Shelykh

We predict the limit cycle solution for a ring-shape bosonic condensate of exciton-polaritons confined in an optically induced rotating trap. The limit cycle manifests itself with polarization oscillations on a characteristic timescale of tens of picoseconds. The effect arises due to the interplay between orbital motion and the polarization degree of freedom. It is specific to spinor bosonic condensates and would be absent in a scalar case, where a bi-stability of stationary solutions would be observed instead. This work offers a tool of initialisation and control of qubits based on superpositions of polariton condensates characterised by different topologic charges.

Dispersive Drumhead States in Nodal-Line Semimetal Junctions. (arXiv:2310.03896v1 [cond-mat.mes-hall])
Francesco Buccheri, Reinhold Egger, Alessandro De Martino

We consider a smooth interface between a topological nodal-line semimetal and a topologically trivial insulator (e.g., the vacuum) or another semimetal with a nodal ring of different radius. Using a low-energy effective Hamiltonian including only the two crossing bands, we show that these junctions accommodate a two-dimensional zero-energy level and a set of two-dimensional dispersive bands, corresponding to states localized at the interface. We characterize the spectrum, identifying the parameter ranges in which these states are present, and highlight the role of the nodal radius and the smoothness of the interface. We also suggest material-independent ways to detect and identify these states, using optical conductivity and infrared absorption spectroscopy in magnetic field.

Anisotropy of thermal conductivity oscillations in relation to the Kitaev spin liquid phase. (arXiv:2310.03917v1 [cond-mat.str-el])
Heda Zhang, Hu Miao, Thomas Z Ward, David G Mandrus, Stephen E Nagler, Michael A McGuire, Jiaqiang Yan

In the presence of external magnetic field, the Kitaev model could either hosts gapped topological anyon or gapless Majorana fermions. In $\alpha$-RuCl$_3$, the gapped and gapless cases are only separated by a thirty-degree rotation of the in-plane magnetic field vector. The presence/absence of the spectral gap is key for understanding the thermal transport behavior in $\alpha$-RuCl$_3$. Here, we study the anisotropy of the oscillatory features of thermal conductivity in $\alpha$-RuCl$_3$. We examine the oscillatory features of thermal conductivities (k//a, k//b) with fixed external fields and found distinct behavior for the gapped (B//a) and gapless (B//b) scenarios. Furthermore, we track the evolution of thermal resistivity ($\lambda_{a}$) and its oscillatory features with the rotation of in-plane magnetic fields from B//b to B//a. The thermal resistivity $\lambda (B,\theta)$ display distinct rotational symmetries before and after the emergence of the field induced Kitaev spin liquid phase. These experiment data suggest close correlations between the oscillatory features of thermal conductivity, the underlying Kitaev spin liquid phase and the fermionic excitation it holds.

Hyperbolic phonon-polariton electroluminescence in graphene-hBN van der Waals heterostructures. (arXiv:2310.03926v1 [cond-mat.mes-hall])
Qiushi Guo, Iliya Esin, Cheng Li, Chen Chen, Song Liu, James H. Edgar, Selina Zhou, Eugene Demler, Gil Refael, Fengnian Xia

Phonon-polaritons are electromagnetic waves resulting from the coherent coupling of photons with optical phonons in polar dielectrics. Due to their exceptional ability to confine electric fields to deep subwavelength scales with low loss, they are uniquely poised to enable a suite of applications beyond the reach of conventional photonics, such as sub-diffraction imaging and near-field energy transfer. The conventional approach to exciting phonon-polaritons through optical methods, however, necessitates costly mid-infrared and terahertz coherent light sources along with near-field scanning probes, and generally leads to low excitation efficiency due to the substantial momentum mismatch between phonon-polaritons and free-space photons. Here, we demonstrate that under proper conditions, phonon-polaritons can be excited all-electrically by flowing charge carriers. Specifically, in hexagonal boron nitride (hBN)/graphene heterostructures, by electrically driving charge carriers in ultra-high-mobility graphene out of equilibrium, we observe bright electroluminescence of hBN's hyperbolic phonon-polaritons (HPhPs) at mid-IR frequencies. The HPhP electroluminescence shows a temperature and carrier density dependence distinct from black-body or super-Planckian thermal emission. Moreover, the carrier density dependence of HPhP electroluminescence spectra reveals that HPhP electroluminescence can arise from both inter-band transition and intra-band Cherenkov radiation of charge carriers in graphene. The HPhP electroluminescence offers fundamentally new avenues for realizing electrically-pumped, tunable mid-IR and THz phonon-polariton lasers, and efficient cooling of electronic devices.

Turning non-magnetic two-dimensional molybdenum disulfide into room temperature magnets by the synergistic effect of strain engineering and charge injection. (arXiv:2310.03995v1 [cond-mat.mtrl-sci])
Jing Wu, Ruyi Guo, Daoxiong Wu, Xiuling Li, Xiaojun Wu

The development of two-dimensional (2D) room temperature magnets is of great significance to the practical application of spintronic devices. However, the number of synthesized intrinsic 2D magnets is limited and the performances of them are not satisfactory, e.g. typically with low Curie temperature and poor environmental stability. Magnetic modulation based on developed 2D materials, especially non-magnetic 2D materials, can bring us new breakthroughs. Herein, we report room temperature ferromagnetism in halogenated MoS2 monolayer under the synergistic effect of strain engineering and charge injection, and the combined implementation of these two processes is based on the halogenation of MoS2. The adsorbed halogen atoms X (X = F, Cl, and Br) on the surface leads to lattice superstretching and hole injection, resulting in MoS2 monolayer exhibiting half-metallic properties, with one spin channel being gapless in the band structure. The Curie temperature of halogenated MoS2 monolayer is 513~615 K, which is much higher than the room temperature. In addition, large magnetic anisotropy energy and good environmental stability make halogenated MoS2 display great advantages in practical spintronic nanodevices.

Evanescently coupled topological ring-waveguide systems for chip-scale ultrahigh frequency phononic circuits. (arXiv:2310.04008v1 [cond-mat.mes-hall])
Daiki Hatanaka, Hiroaki Takeshita, Motoki Kataoka, Hajime Okamoto, Kenji Tsuruta, Hiroshi Yamaguchi

Topological phononics enabling backscattering-immune transport is expected to improve the performance of electromechanical systems for classical and quantum information technologies. Nonetheless, most of the previous demonstrations utilized macroscale and low-frequency structures and thus offered little experimental insight into ultrahigh frequency phonon transport, especially in chip-scale circuits. Here, we report microwave phonon transmissions in a microscopic topological ring-waveguide coupled system, which is an important building block for wave-based signal processing. The elastic waves in the topological waveguide evanescently couple to the ring resonator, while maintaining the valley pseudospin polarization. The resultant waves are robust to backscattering even in the tiny hexagonal ring, generating a resonant phonon circulation. Furthermore, the evanescently coupled structure allows for a critical coupling, where valley-dependent ring-waveguide interference enables blocking of the topological edge transmission. Our demonstrations reveal the capability of using topological phenomena to manipulate ultrahigh frequency elastic waves in intricate phononic circuits for classical and quantum signal-processing applications.

Superconductivity with Tc 116 K discovered in antimony polyhydrides. (arXiv:2310.04033v1 [cond-mat.supr-con])
K. Lu, X. He, C.L. Zhang, Z.W. Li, S.J. Zhang, B. S. Min, J. Zhang, J.F. Zhao, L.C. Shi, Y. Peng, S.M. Feng, Q.Q. Liu, J. Song, R.C. Yu, X.C. Wang, Y. Wang, M. Bykov, C. Q. Jin

Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature $T_c = 116^\circ$K at 184 GPa. The investigation of SC at magnetic field revealed that the superconducting coherent length ~40 angstroms based on Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest Tc in addition to sulfur hydride among the polyhydrides of elements from main group IIIA to VIIA in periodic table.

Theory for Planar Hall Effect in Organic Dirac Fermion System. (arXiv:2310.04066v1 [cond-mat.str-el])
Yuki Nakamura, Takao Morinari

In a recent experiment on the interlayer magnetoresistance in the quasi-two-dimensional organic salt, $\alpha$-(BEDT-TTF)$_2$I$_3$, it has been observed that at low temperatures, interlayer tunneling attains phase coherence, leading to the emergence of a three-dimensional electronic structure. Theoretically and experimentally it has been suggested that the system exhibits characteristics of a three-dimensional Dirac semimetal as a consequence of broken time-reversal symmetry and inversion symmetry. Here, we perform a theoretical calculation of the magnetoconductivity under an in-plane magnetic field and demonstrate that the system displays a planar Hall effect. Our calculations are based on a realistic model for $\alpha$-(BEDT-TTF)$_2$I$_3$ incorporating interlayer tunneling and the tilt of the Dirac cone. Given that the planar Hall effect is anticipated as a consequence of chiral anomaly, our findings provide support for the classification of $\alpha$-(BEDT-TTF)$_2$I$_3$ as a three-dimensional Dirac semimetal.

Twisted Coupled Wire Model for moir\'e Sliding Luttinger Liquid. (arXiv:2310.04070v1 [cond-mat.str-el])
Yichen Hu, Yuanfeng Xu, Biao Lian

Recent experiments in twisted bilayer WTe$_2$ revealed the existence of anisotropic Luttinger liquid behavior. To generically characterize such anisotropic twisted bilayer systems, we study a model of twisted bilayer of 2D arrays of coupled wires, which effectively form an array of coupled moir\'e wires. We solve the model by transfer matrix method, and identify quasi-1D electron bands in the system at small twist angles. With electron interactions added, we show that the moir\'e wires have an effective Luttinger parameter $g_\text{eff}$ much lower than that of the microscopic wires. This leads to a sliding Luttinger liquid (SLL) temperature regime, in which power-law current voltage relations arise. For parameters partly estimated from WTe$_2$, a microscopic interaction $U\sim0.7$eV yields a temperature regime of SLL similar to that in the WTe$_2$ experiments.

Enhanced coupling between massive fermions and zone-boundary phonons probed by infrared resonance Raman in bilayer graphene. (arXiv:2310.04071v1 [cond-mat.mes-hall])
Lorenzo Graziotto, Francesco Macheda, Tommaso Venanzi, Simone Sotgiu, Taoufiq Ouaj, Elena Stellino, Claudia Fasolato, Paolo Postorino, Marvin Metzelaars, Paul Kögerler, Bernd Beschoten, Matteo Calandra, Michele Ortolani, Christoph Stampfer, Francesco Mauri, Leonetta Baldassarre

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. By lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the low-energy split bands close to the K point. The low excitation energy suppresses some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonant 2D peak. Studying the different intensities of the sub-structures and comparing experimental measurements with fully ab initio theoretical calculations, in the case of bilayer graphene we unveil an enhanced coupling between the massive fermions and the lattice vibrations at the K point, in analogy to what found for the massless fermions of monolayer graphene, and also suggesting that what governs the enhancement is the vicinity of the electron-hole pair momentum to K rather than how small the electron-hole pair energy is.

ZrOsSi: A $Z_2$ topological metal with a superconducting ground state. (arXiv:2310.04105v1 [cond-mat.supr-con])
S. K. Ghosh, B. Li, C. Xu, A. D. Hillier, P. K. Biswas, X. Xu, T. Shiroka

The silicide superconductors (Ta, Nb, Zr)OsSi are among the best candidate materials for investigating the interplay of topological order and superconductivity. Here, we investigate in detail the normal-state topological properties of (Ta, Nb, Zr)OsSi, focusing on ZrOsSi, by employing a combination of $^{29}$Si nuclear magnetic resonance (NMR) measurements and first-principles band-structure calculations. We show that, while (Ta, Nb)OsSi behave as almost ideal metals, characterized by weak electronic correlations and a relatively low density of states, the replacement of Ta (or Nb) with Zr expands the crystal lattice and shifts ZrOsSi towards an insulator. Our ab initio calculations indicate that ZrOsSi is a $Z_2$ topological metal with clear surface Dirac cones and properties similar to a doped strong topological insulator.

Electronic structure of MoS$_2$ revisited: a comprehensive assessment of $G_0W_0$ calculations. (arXiv:2310.04198v1 [cond-mat.mtrl-sci])
Ronaldo Rodrigues Pela, Cecilia Vona, Sven Lubeck, Ben Alex, Ignacio Gonzalez Oliva, Claudia Draxl

Two-dimensional MoS$_2$ combines many interesting properties that make the material a top candidate for a variety of applications. It exhibits a high electron mobility comparable to graphene, a direct fundamental band gap, relatively strongly bound excitons, and moderate spin-orbit coupling. For a thorough understanding of all these properties, an accurate description of the electronic structure is mandatory. Surprisingly, published band gaps of MoS$_2$ obtained with $GW$, the state-of-the-art in electronic-structure calculations, are quite scattered, ranging from 2.31 to 2.97 eV. The details of $G_0W_0$ calculations, such as the underlying geometry, the starting point, the inclusion of spin-orbit coupling, and the treatment of the Coulomb potential can critically determine how accurate the results are. In this manuscript, we employ the linearized augmented planewave + local orbital method to systematically investigate how all these aspects affect the quality of $G_0W_0$ calculations, and also provide a summary of literature data. We conclude that the best overall agreement with experiments and coupled-cluster calculations is found for $G_0W_0$ results with HSE06 as a starting point including spin-orbit coupling, a truncated Coulomb potential, and an analytical treatment of the singularity at $q=0$.

High-yield atmospheric water capture via bioinspired material segregation. (arXiv:2310.04254v1 [physics.flu-dyn])
Yiwei Gao, Santiago Ricoy, Addison Cobb, Ryan Phung, Areianna Lewis, Aaron Sahm, Nathan Ortiz, Sameer Rao, H. Jeremy Cho

Atmospheric water harvesting is urgently needed given increasing global water scarcity. Current sorbent-based devices that cycle between water capture and release have low harvesting rates. We envision a radically different multi-material architecture with segregated and simultaneous capture and release. This way, proven fast-release mechanisms that approach theoretical limits can be incorporated; however, no capture mechanism exists to supply liquid adequately for release. Inspired by tree frogs and airplants, our capture approach transports water through a hydrogel membrane ``skin'' into a liquid desiccant. We report an extraordinarily high capture rate of 5.50 $\text{kg}\,\text{m}^{-2}\,\text{d}^{-1}$ at a low humidity of 35%, limited by the convection of air to the device. At higher humidities, we demonstrate up to 16.9 $\text{kg}\,\text{m}^{-2}\,\text{d}^{-1}$, exceeding theoretical limits for release. Simulated performance of a hypothetical one-square-meter device shows that water could be supplied to two to three people in dry environments. This work is a significant step toward providing new resources to water-scarce regions.

Constrain relations for superfluid weight and pairings in a chiral flat band superconductor. (arXiv:2310.04325v1 [cond-mat.supr-con])
M. Thumin, G. Bouzerar

Within ten years, flat band (FB) superconductivity has gained a huge interest for its remarkable features and connection to quantum geometry. We investigate the superconductivity in a FB system whose orbitals are inequivalent and in which the gap and the quantum metric are tunable. The key feature of the present theoretical study is to show a unique and simple constrain relation that pairings obey. Furthermore, pairings and superfluid weight in partially filled FB are shown to be controlled by those of the half-filled lattice. We argue that the geometry of the lattice or the complexity of the hopping terms have no impact on the features revealed in this work as far as the system is bipartite.

Kagome KMn$_3$Sb$_5$ metal: Magnetism, lattice dynamics, and anomalous Hall conductivity. (arXiv:2310.04339v1 [cond-mat.str-el])
Sobhit Singh, A.C. Garcia-Castro

Kagome metals are reported to exhibit remarkable properties, including superconductivity, charge density wave order, and a large anomalous Hall conductivity, which facilitate the implementation of spintronic devices. In this work, we study a novel kagome metal based on Mn magnetic sites in a KMn$_3$Sb$_5$ stoichiometry. By means of first-principles density functional theory calculations, we demonstrate that the studied compound is dynamically stable, locking the ferromagnetic order as the ground state configuration, thus preventing the charge-density-wave state as reported in its vanadium-based counterpart KV$_3$Sb$_5$. Our calculations predict that KMn$_3$Sb$_5$ exhibits an out-of-plane (001) ferromagnetic response as the ground state, allowing for the emergence of topologically protected Weyl nodes near the Fermi level and nonzero anomalous Hall conductivity ($\sigma_{ij}$) in this centrosymmetric system. We obtain a tangible $\sigma_{xy} = 314$ S$\cdot$cm$^{-1}$ component, which is comparable to that of other kagome metals. Finally, we explore the effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic properties and find that, although the lattice parameters and $\sigma_{xy}$ moderately vary with increasing $+U$, KMn$_3$Sb$_5$ stands as an ideal stable ferromagnetic kagome metal with a large anomalous Hall conductivity response.

The long-range interacting Fermi polaron. (arXiv:2310.04351v1 [cond-mat.quant-gas])
Krzysztof Myśliwy, Krzysztof Jachymski

We construct the simplest density functional for the problem of a single impurity interacting with a Fermi gas via a long--ranged potential using the Thomas--Fermi approach. We find that the Fermi polaron is fully bosonized in two dimensions, as the model results in a suitable Landau--Pekar functional known from the Bose polaron problem which describes a self--interacting impurity. In other dimensions, the impurity self--interacts with an infinite number of its own images, and no bosonization occurs. We discuss applications of our theory for the $2d$ exciton--polaron and the ionic polaron problem and compute the effective mass for these cases, finding a self--trapping transition with order depending on the dimensionality.

Tuning topological superconductivity within the $t$-$J$-$U$ model of twisted bilayer cuprates. (arXiv:2310.04379v1 [cond-mat.supr-con])
Maciej Fidrysiak, Bartłomiej Rzeszotarski, Józef Spałek

We carry out a theoretical study of unconventional superconductivity in twisted bilayer cuprates as a function of electron density and layer twist angle. The bilayer $t$-$J$-$U$ model is employed and analyzed within the framework of a generalized variational wave function approach in the statistically-consistent Gutzwiller formulation. The constructed phase diagram encompasses both gapless $d$-wave state (reflecting the pairing symmetry of untwisted copper-oxides) and gapped $d+\mathrm{e}^{i\varphi}d$ phase that breaks spontaneously time-reversal-symmetry (TRS) and is characterized by nontrivial Chern number. We find that $d+\mathrm{e}^{i\varphi}d$ state occupies a non-convex butterfly-shaped region in the doping vs. twist-angle plane, and demonstrate the presence of previously unreported reentrant TRS-breaking phase on the underdoped side of the phase diagram. This circumstance supports the emergence of topological superconductivity for fine-tuned twist angles away from $45^\circ$. Our analysis of the microscopically derived Landau free energy functional points toward sensitivity of the superconducting order parameter to small perturbations close to the topological state boundary.

Matter-wave collimation to picokelvin energies with scattering length and potential shape control. (arXiv:2310.04383v1 [physics.atom-ph])
Alexander Herbst, Timothé Estrampes, Henning Albers, Robin Corgier, Knut Stolzenberg, Sebastian Bode, Eric Charron, Ernst M. Rasel, Naceur Gaaloul, Dennis Schlippert

We study the impact of atomic interactions on an in-situ collimation method for matter-waves. Building upon an earlier study with $^{87}$Rb, we apply a lensing protocol to $^{39}$K where the atomic scattering length can be tailored by means of magnetic Feshbach resonances. Minimizing interactions, we show an enhancement of the collimation compared to the strong interaction regime, realizing ballistic 2D expansion energies of 438(77) pK in our experiment. Our results are supported by an accurate simulation, describing the ensemble dynamics, which we further use to study the behavior of various trap configurations for different interaction strengths. Based on our findings we propose an advanced scenario which allows for 3D expansion energies below 16 pK by implementing an additional pulsed delta-kick collimation directly after release from the trapping potential. Our results pave the way to achieve state-of-the-art quantum state in typical dipole trap setups required to perform ultra-precise measurements without the need of complex micro-gravity or long baselines environments.

Direct Photon Scattering by Plasmons in BiTeI. (arXiv:2310.04394v1 [cond-mat.str-el])
A. C. Lee, S. Sarkar, K. Du, H.-H. Kung, C. J. Won, K. Wang, S.-W. Cheong, S. Maiti, G. Blumberg

We use polarization resolved Raman spectroscopy to show that for 3D giant Rashba system the bulk plasmon collective mode directly couples to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit although the standard theory predicts that the plasmon spectral weight should scale as the square of its quasi-momentum and hence be negligibly weak in the Raman spectra. Such plasmon coupling to the Raman response at $\mathbf q \rightarrow 0$ arises for a polar system with spin-orbit coupling when the incoming photon excitation is turned to a resonance with Rashba-split intermediates states involved in the resonant Raman process. As an example, we identify special features of BiTeI's chiral band structure that enable the appearance of plasmon mode in the Raman spectrum.

Possibility of a Topological Phase Transition in Two-dimensional $RP^3$ Model. (arXiv:2112.15053v2 [cond-mat.stat-mech] UPDATED)
Tsuyoshi Okubo, Naoki Kawashima

We study by large-scale Monte Carlo simulation the $RP^3$ model, which can be regarded as an effective low-energy model of a triangular lattice Heisenberg antiferromagnet. $Z_2$ vortices appear as elementary excitations in the triangular lattice Heisenberg antiferromagnet. Such $Z_2$ vortices are ubiquitous in other frustrated Heisenberg spin systems that have noncollinear long-range orders. In this study, we investigate a possible topological phase transition driven by the binding--unbinding of $Z_2$ vortices. By extracting important degrees of freedom, we map a frustrated spin system to an effective $RP^3$ model. From large-scale Monte Carlo simulation, we obtain an order parameter and a correlation length of up to $L=16384$. Concerning the existence of a $Z_2$-vortex transition, by extrapolating the order parameter to the thermodynamics limit assuming the $Z_2$-vortex transition, we obtain a finite transition temperature as $T_v/\tilde{J} \simeq 0.25$. Our estimate of the correlation length at $T_v$ is much larger than $L=16384$, which is beyond the previous estimate obtained with the triangular lattice Heisenberg model.

Spontaneous Formation of Exceptional Points at the Onset of Magnetism. (arXiv:2207.05097v2 [cond-mat.str-el] UPDATED)
Lorenzo Crippa, Giorgio Sangiovanni, Jan Carl Budich

We reveal how symmetry protected nodal points in topological semimetals may be promoted to pairs of generically stable exceptional points (EPs) by symmetry-breaking fluctuations at the onset of long-range order. This novel route to non-Hermitian (NH) topology is exemplified by a magnetic NH Weyl phase spontaneously emerging at the surface of a strongly correlated three-dimensional topological insulator when entering the ferromagnetic regime from a high temperature paramagnetic phase. Here, electronic excitations with opposite spin acquire significantly different life-times, thus giving rise to an anti-Hermitian structure in spin that is incompatible with the chiral spin texture of the nodal surface states, and hence facilitates the spontaneous formation of EPs. We present numerical evidence of this phenomenon by solving a microscopic multi-band Hubbard model non-perturbatively in the framework of dynamical mean-field theory.

Anti-site defect-induced disorder in compensated topological magnet MnBi$_{2-x}$Sb$_x$Te$_4$. (arXiv:2208.13374v3 [cond-mat.mes-hall] UPDATED)
Felix Lüpke, Marek Kolmer, Jiaqiang Yan, Hao Chang, Paolo Vilmercati, Hanno H. Weitering, Wonhee Ko, An-Ping Li

The gapped Dirac-like surface states of compensated magnetic topological insulator MnBi$_{2-x}$Sb$_x$Te$_4$ (MBST) are a promising host for exotic quantum phenomena such as the quantum anomalous Hall effect and axion insulating states. However, it has become clear that atomic defects undermine the stabilization of such quantum phases as they lead to spatial variations in the surface state gap and doping levels. The large number of possible defect configurations in MBST make studying the influence of individual defects virtually impossible. Here, we present a statistical analysis of the nanoscale effect of defects in MBST with $x=0.64$, by scanning tunneling microscopy/spectroscopy (STM/S). We identify (Bi,Sb)$_{\rm Mn}$ anti-site defects to be the main source of the observed doping fluctuations, leading towards the formation of nanoscale charge puddles and effectively closing the transport gap. Our findings will guide further optimization of this material system via defect engineering, to enable exploitation of its promising properties.

Hermitian Topologies originating from non-Hermitian braidings. (arXiv:2212.13736v2 [cond-mat.mes-hall] UPDATED)
W. B. Rui, Y. X. Zhao, Z. D. Wang

The complex energy bands of non-Hermitian systems braid in momentum space even in one dimension. Here, we reveal that the non-Hermitian braiding underlies the Hermitian topological physics with chiral symmetry under a general framework that unifies Hermitian and non-Hermitian systems. Particularly, we derive an elegant identity that equates the linking number between the knots of braiding non-Hermitian bands and the zero-energy loop to the topological invariant of chiral-symmetric topological phases in one dimension. Moreover, we find an exotic class of phase transitions arising from the critical point transforming different knot structures of the non-Hermitian braiding, which are not included in the conventional Hermitian topological phase transition theory. Nevertheless, we show the bulk-boundary correspondence between the bulk non-Hermitian braiding and boundary zero-modes of the Hermitian topological insulators. Finally, we construct typical topological phases with non-Hermitian braidings, which can be readily realized by artificial crystals.

Nearly flat Chern band in periodically strained monolayer and bilayer graphene. (arXiv:2302.07199v2 [cond-mat.mes-hall] UPDATED)
Xiaohan Wan, Siddhartha Sarkar, Kai Sun, Shi-Zeng Lin

The flat band is a key ingredient for the realization of interesting quantum states for novel functionalities. In this work, we investigate the conditions for the flat band in both monolayer and bilayer graphene under periodic strain. We find topological nearly flat bands with homogeneous distribution of Berry curvature in both systems. The quantum metric of the nearly flat band closely resembles that for Landau levels. For monolayer graphene, the strain field can be regarded as an effective gauge field, while for Bernal-stacked (AB-stacked) bilayer graphene, its role is beyond the description of gauge field. We also provide an understanding of the origin of the nearly flat band in monolayer graphene in terms of the Jackiw-Rebbi model for Dirac fermions with sign-changing mass. Our work suggests strained graphene as a promising platform for strongly correlated quantum states.

The Ginzburg-Landau theory of flat band superconductors with quantum metric. (arXiv:2303.15504v4 [cond-mat.supr-con] UPDATED)
Shuai A. Chen, K. T. Law

Recent experimental study unveiled highly unconventional phenomena in the superconducting twisted bilayer graphene (TBG) with ultra flat bands, which cannot be described by the conventional BCS theory. For example, given the small Fermi velocity of the flat bands, the superconducting coherence length predicted by BCS theory is more than 20 times shorter than the measured values. A new theory is needed to understand many of the unconventional properties of flat band superconductors. In this work, we establish a Ginzburg-Landau (GL) theory from a microscopic flat band Hamiltonian. The GL theory shows how the properties of the physical quantities such as the critical temperature, the superconducting coherence length, the upper critical field and the superfluid density are governed by the quantum metric of the Bloch states. One key conclusion is that the superconducting coherence length is not determined by the Fermi velocity but by the size of the optimally localized Wannier functions which is limited by quantum metric. Applying the theory to TBG, we calculated the superconducting coherence length and the upper critical fields. The results match the experimental ones well without fine tuning of parameters. The established GL theory provides a new and general theoretical framework for understanding flat band superconductors with quantum metric.

Charge-resolved entanglement in the presence of topological defects. (arXiv:2306.15532v2 [quant-ph] UPDATED)
David X. Horvath, Shachar Fraenkel, Stefano Scopa, Colin Rylands

Topological excitations or defects such as solitons are ubiquitous throughout physics, supporting numerous interesting phenomena like zero energy modes with exotic statistics and fractionalized charges. In this paper, we study such objects through the lens of symmetry-resolved entanglement entropy. Specifically, we compute the charge-resolved entanglement entropy for a single interval in the low-lying states of the Su-Schrieffer-Heeger model in the presence of topological defects. Using a combination of exact and asymptotic analytical techniques, backed up by numerical analysis, we find that, compared to the unresolved counterpart and to the pure system, a richer structure of entanglement emerges. This includes a redistribution between its configurational and fluctuational parts due to the presence of the defect and an interesting interplay with entanglement equipartition. In particular, in a subsystem that excludes the defect, equipartition is restricted to charge sectors of the same parity, while full equipartition is restored if the subsystem includes the defect, as long as the associated zero mode remains unoccupied. Additionally, by exciting zero modes in the presence of multiple defects, we observe a significant enhancement of entanglement in certain charge sectors, due to charge splitting on the defects. The two different scenarios featuring the breakdown of entanglement equipartition are underlied by a joint mechanism, which we unveil by relating them to degeneracies in the spectrum of the entanglement Hamiltonian. In addition, equipartition is shown to stem from an equidistant entanglement spectrum.

Topological transition from nodal to nodeless Zeeman splitting in altermagnets. (arXiv:2307.12380v3 [cond-mat.mes-hall] UPDATED)
Rafael M. Fernandes, Vanuildo S. de Carvalho, Turan Birol, Rodrigo G. Pereira

In an altermagnet, the symmetry that relates configurations with flipped magnetic moments is a rotation. This makes it qualitatively different from a ferromagnet, where no such symmetry exists, or a collinear antiferromagnet, where this symmetry is a lattice translation. In this paper, we investigate the impact of the crystalline environment on the magnetic and electronic properties of an altermagnet. We find that, because each component of the magnetization acquires its own angular dependence, the Zeeman splitting of the bands has symmetry-protected nodal lines residing on mirror planes of the crystal. Upon crossing the Fermi surface, these nodal lines give rise to pinch points that behave as single or double type-II Weyl nodes. We show that an external magnetic field perpendicular to these mirror planes can only move the nodal lines, such that a critical field value is necessary to collapse the nodes and make the Weyl pinch points annihilate. This unveils the topological nature of the transition from a nodal to a nodeless Zeeman splitting of the bands. We also classify the altermagnetic states of common crystallographic point groups in the presence of spin-orbit coupling, revealing that a broad family of magnetic orthorhombic perovskites can realize altermagnetism.

Long-range Ising spins models emerging from frustrated Josephson junctions arrays with topological constraints. (arXiv:2308.07143v2 [quant-ph] UPDATED)
Oliver Neyenhuys, Mikhail V. Fistul, Ilya M. Eremin

Geometrical frustration in correlated systems can give rise to a plethora of novel ordered states and intriguing phases. Here, we analyze theoretically vertex-sharing frustrated Kagome lattice of Josephson junctions and identify various classical and quantum phases. The frustration is provided by periodically arranged $0$- and $\pi$- Josephson junctions. In the frustrated regime the macroscopic phases are composed of different patterns of vortex/antivortex penetrating each basic element of the Kagome lattice, i.e., a superconducting triangle interrupted by three Josephson junctions. We obtain that numerous topological constraints, related to the flux quantization in any hexagon loop, lead to highly anisotropic and long-range interaction between well separated vortices (antivortices). Taking into account this interaction and a possibility of macroscopic "tunneling" between vortex and antivortex in single superconducting triangles we derive an effective Ising-type spin Hamiltonian with strongly anisotropic long-range interaction. In the classically frustrated regime we calculate numerically the temperature-dependent spatially averaged spins polarization, $\overline{m}(T)$, characterizing the crossover between the ordered and disordered vortex/antivortex states. In the coherent quantum regime we analyze the lifting of the degeneracy of the ground state and the appearance of the highly entangled states.

Generating Nanoporous Graphene from Point and Stone-Wales Defects: A Study with Dimensionally Restricted Molecular Dynamics (DR-MD). (arXiv:2308.13810v2 [cond-mat.mtrl-sci] UPDATED)
Ji Wei Yoon

Defects in graphene are both a boon and a bane for applications - they can induce uncontrollable effects but can also provide novel ways to manipulate the properties of pristine graphene. Nanoporous Graphene, which contains nanoscopic holes, has found impactful applications in sustainability domains, e.g. gas separation, water filtration membranes and battery technologies. For this report, we investigate pore formation in graphene with no defect, one and two mono-vacancies, and two di-vacancies using bespoke Dimensionally Restricted Molecular Dynamics (DR-MD) designed for the purpose. We show DR-MD to be superior to free-standing or substrate suspended configurations for simulating stable defected structures. Applying DR-MD, stable pore configurations are identified, and their formation mechanisms elucidated. We also investigated formation mechanisms due to two Stone-Wales 55-77 defects, and the formation energies of their linearly extended structures, along the zigzag and armchair directions, and when they are placed in different relative orientations. This study offers a way to identify stable porous defect structures in graphene and insights into atomistic pore formation mechanisms for an environmentally important material.

Time-Reversal Invariant Topological Moir\'e Flatband: A Platform for the Fractional Quantum Spin Hall Effect. (arXiv:2309.07222v2 [cond-mat.mes-hall] UPDATED)
Yi-Ming Wu, Daniel Shaffer, Zhengzhi Wu, Luiz H. Santos

Motivated by recent observation of the quantum spin Hall effect in monolayer germanene and twisted bilayer transition-metal-dichalcogenides (TMDs), we study the topological phases of moir\'e twisted bilayers with time-reversal symmetry and spin $s_z$ conservation. By using a continuum model description which can be applied to both germanene and TMD bilayers, we show that at small twist angles, the emergent moir\'e flatbands can be topologically nontrivial due to inversion symmetry breaking. Each of these flatbands for each spin projection admits a lowest-Landau-level description in the chiral limit and at magic twist angle. This allows for the construction of a many-body Laughlin state with time-reversal symmetry which can be stabilized by a short-range pseudopotential, and therefore serves as an ideal platform for realizing the so-far elusive fractional quantum spin Hall effect with emergent spin-1/2 U(1) symmetry.

Predicting the mechanical properties of spring networks. (arXiv:2309.07844v2 [cond-mat.soft] UPDATED)
Doron Grossman, Arezki Boudaoud

The elastic response of mechanical, chemical, and biological systems is often modeled using a discrete arrangement of Hookean springs, either modeling finite material elements or even the molecular bonds of a system. However, to date, there is no direct derivation of the relation between discrete spring network, and a general elastic continuum. Furthermore, understanding the networks' mechanical response requires simulations that may be expensive computationally. Here we report a method to derive the exact elastic continuum model of any discrete network of springs, requiring network geometry and topology only. We identify and calculate the so-called "non-affine" displacements. Explicit comparison of our calculations to simulations of different crystalline and disordered configurations, shows we successfully capture the mechanics even of auxetic materials. Our method is valid for residually stressed systems with non-trivial geometries, is easily generalizable to other discrete models, and opens the possibility of a rational design of elastic systems.

Electronic Phase Transformations and Energy Gap Variations in Uniaxial and Biaxial Strained Monolayer VS$_2$ TMDs: A Comprehensive DFT and Beyond-DFT Study. (arXiv:2309.08393v3 [cond-mat.mtrl-sci] UPDATED)
Oguzhan Orhan, Şener Özönder, Soner Ozgen

In the field of 2D materials, transition metal dichalcogenides (TMDs) are gaining attention for electronic applications. Our study delves into the H-phase monolayer VS$_2$ of the TMD family, analyzing its electronic structure and how strain affects its band structure using Density Functional Theory (DFT). Using a variety of computational methods, we provide an in-depth view of the electronic band structure. We find that strains between -5\% and +5\% significantly affect the energy gap, with uniaxial strains having a stronger effect than biaxial strains. Remarkably, compressive strains induce a phase shift from semiconducting to metallic, associated with symmetry breaking and changes in bond length. These findings not only deepen our understanding of the electronic nuances of monolayer VS$_2$ under varying strains but also suggest potential avenues for creating new electronic devices through strain engineering.

Generalized Ginsparg-Wilson relations. (arXiv:2309.08542v2 [hep-lat] UPDATED)
Michael Clancy, David B. Kaplan, Hersh Singh

We give a general derivation of Ginsparg-Wilson relations for both Dirac and Majorana fermions in any dimension. These relations encode continuous and discrete chiral, parity and time reversal anomalies and will apply to the various classes of free fermion topological insulators and superconductors (in the framework of a relativistic quantum field theory in Euclidean spacetime). We show how to formulate the exact symmetries of the lattice action and the relevant index theorems for the anomalies.

Found 1 papers in sci-rep

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Synthesis and optical properties of WS2 nanotubes with relatively small diameters
Kazuhiro Yanagi

Scientific Reports, Published online: 08 October 2023; doi:10.1038/s41598-023-44072-z

Synthesis and optical properties of WS2 nanotubes with relatively small diameters