Found 25 papers in cond-mat
Date of feed: Mon, 22 Jan 2024 01:30:00 GMT

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Observing Topological Insulator Phases with a Programmable Quantum Simulator. (arXiv:2401.10362v1 [quant-ph])
Or Katz, Lei Feng, Diego Porras, Christopher Monroe

Topological insulators exhibit fascinating properties such as the appearance of edge states protected by symmetries. The Su-Schrieffer-Heeger (SSH) model is a canonical description of a one-dimensional quantum topological insulator. We experimentally implement a modified SSH model with long-range interacting spin systems in one-dimensional trapped ion crystals of up to $22$ spins. An array of tightly focused laser beams generates site-specific Floquet fields that control the bond dimerization of the spins, which when subject to reflection symmetry, exhibit signatures of topologically-protected edge states. We study the evolution of highly excited configurations with various ranges of the spin-spin interaction, revealing the nontrivial role of many-body fermionic-interaction terms on the resulting dynamics. These results allow direct quantum simulations of topological quantum degrees of freedom expected in exotic materials, but here with high control of individual spins and their interaction range.


Composition dependence of bulk properties in the Co-intercalated transition-metal dichalcogenide Co$_{1/3}$TaS$_2$. (arXiv:2401.10421v1 [cond-mat.str-el])
Pyeongjae Park, Woonghee Cho, Chaebin Kim, Yeochan An, Maxim Avdeev, Kazuki Iida, Ryoichi Kajimoto, Je-Geun Park

Spontaneous Hall conductivity has recently been reported in the triangular lattice antiferromagnet Co$_{1/3}$TaS$_2$ under a zero magnetic field. This phenomenon originates from the distinctive noncoplanar triple-Q magnetic ground state, possessing uniform real-space Berry curvature characterized by scalar spin chirality. We investigated the physical properties of Co$_{1/3}$TaS$_2$ by judiciously controlling the composition, revealing a drastic change in its bulk properties, even by slight variations in cobalt composition, despite the same crystal structure. For $0.299 < x < 0.325$, Co$_x$TaS$_2$ keeps all the characteristics of the ground state consistent with the previous studies -- two antiferromagnetic phase transitions at $T_{N1}$ and $T_{N2} (< T_{N1})$, a large spontaneous Hall conductivity (${\sigma}_{xy} (H=0)$), and a weak ferromagnetic moment along the c-axis. However, samples with $x > 0.330$ exhibit distinct bulk properties, including the absence of both ${\sigma}_{xy} (H=0)$ and the weak ferromagnetic moment. Our neutron diffraction data reveal that Co$_x$TaS$_2$ with $x > 0.330$ develops coplanar helical magnetic order with $q_{m1} = (1/3, 0, 0)$. This is entirely different from what has been seen in $x < 0.325$, explaining the observed composition dependence.


Nanoscale Conducting and Insulating Domains on YbB$_6$. (arXiv:2401.10424v1 [cond-mat.mes-hall])
Aaron Coe (1), Zhi-Huai Zhu (1), Yang He (1), Dae-Jeong Kim (2), Zachary Fisk (2), Jason Hoffman (1), Jennifer Hoffman (1 and 3)

YbB$_6$ is a predicted topological insulator, with experimental evidence for conducting surface states of yet-unproven origin. However, its lack of a natural cleavage plane, and resultant surface-dependent polarity, has obscured its study. We use scanning tunneling microscopy to image the cleaved surface of YbB$_6$, exhibiting several coexisting terminations with distinct atomic structures. Our spectroscopic measurements show band-bending between the terminations, resulting in both conducting and fully-gapped regions. In the conductive regions, we observe spectral peaks that are suggestive of van Hove singularities arising from Rashba spin-split quantum well states. The insulating regions rule out the possibility that YbB$_6$ is a strong topological insulator, while the spin-polarized conducting regions suggest possible utility for spintronic devices.


Treatment and Aging Studies of GaAs(111)B Substrates for van der Waals Chalcogenide Film Growth. (arXiv:2401.10425v1 [cond-mat.mtrl-sci])
Mingyu Yu, Jiayang Wang, Sahani A. Iddawela, Molly McDonough, Jessica L. Thompson, Susan B Sinnott, Danielle Reifsnyder Hickey, Stephanie Law

GaAs(111)B is a semiconductor substrate widely used in research and commercial fields due to its low cost, mature synthesis technology, and excellent properties for manufacturing electronic devices. It is not only used to grow three-dimensional (3D) strongly-bonded materials, but has also been used as a substrate for layered, van der Waals (vdW)-bonded chalcogenide film growth. However, GaAs(111)B wafers cannot be directly used for growing epitaxial vdW chalcogenide films for two reasons: (1) the GaAs surface has a substantial number of dangling bonds that need to be passivated for vdW layers growth; (2) the substrate surface is covered with a thin epi-ready oxide layer which must be removed before film growth. In this paper, we optimize the method for deoxidizing GaAs(111)B substrates under a Se overpressure and successfully create a smooth, deoxidized, and passivated substrate for subsequent growth of vdW chalcogenide materials. We demonstrate the benefits of this method for the growth of vdW chalcogenide thin films using GaSe as a representative of vdW chalcogenides. In addition, we find that severely aged substrates have difficulty maintaining a smooth surface during the deoxidation and passivation process and cause GaSe crystals to nucleate in random shapes and orientations. We describe a method using water droplet testing to determine the age of the substrate. Finally, X-ray photoelectron spectroscopy (XPS) characterization reveals that the natural aging of GaAs(111)B in the air results in an increase in surface oxides, Ga2O3 and As2O3, while exposure to ultraviolet (UV)-ozone not only enhances the contents of these two oxides but also generates a new oxide, As2O5. Our research contributes to expanding the compatibility of GaAs(111)B with diverse growth materials and the production of high-quality heterostructure devices.


Spontaneous localization at a potential saddle point from edge state reconstruction in a quantum Hall point contact. (arXiv:2401.10433v1 [cond-mat.mes-hall])
Liam A. Cohen, Noah L. Samuelson, Taige Wang, Kai Klocke, Cian C. Reeves, Takashi Taniguchi, Kenji Watanabe, Sagar Vijay, Michael P. Zaletel, Andrea F. Young

Quantum point contacts (QPCs) are an essential component in mesoscopic devices. Here, we study the transmission of quantum Hall edge modes through a gate-defined QPC in monolayer graphene. We observe resonant tunneling peaks and a nonlinear conductance pattern characteristic of Coulomb-blockaded localized states. The in-plane electric polarizability reveals the states are localized at a classically-unstable electrostatic saddle point. We explain this unexpected finding within a self-consistent Thomas-Fermi model, finding that localization of a zero-dimensional state at the saddle point is favored whenever the applied confinement potential is sufficiently soft compared to the Coulomb energy. Our results provide a direct demonstration of Coulomb-driven reconstruction at the boundary of a quantum Hall system.


Fractal Quantum Transport in MoS2 Superlattices. (arXiv:2401.10436v1 [cond-mat.mes-hall])
Aitor Garcia-Ruiz, Ming-Hao Liu

We study the band structure and quantum transport of MoS2 on a nanometer-scale periodic potential under magnetic fields. Using the continuum model, we compute the band structure of the system with and without magnetic fields. We found the formation of a self-similar fractal band gaps for values of the magnetic field of about 1 T. Additionally, we simulate the quantum transport along a realistic two-terminal device affected by the same potential and find a remarkably good consistency between these two approaches. Our results can be extended to provide a theoretical understanding on the transport phenomenon on transition metal-dichalcogenides.


Observation of tunable topological polaritons in a cavity waveguide. (arXiv:2401.10450v1 [physics.optics])
Dong Zhao, Ziyao Wang, Linyun Yang, Yuxin Zhong, Xiang Xi, Zhenxiao Zhu, Maohua Gong, Qingan Tu, Yan Meng, Bei Yan, Ce Shang, Zhen Gao

Topological polaritons characterized by light-matter interactions have become a pivotal platform in exploring new topological phases of matter. Recent theoretical advances unveiled a novel mechanism for tuning topological phases of polaritons by modifying the surrounding photonic environment (light-matter interactions) without altering the lattice structure. Here, by embedding a dimerized chain of microwave helical resonators (electric dipole emitters) in a metallic cavity waveguide, we report the pioneering observation of tunable topological phases of polaritons by varying the cavity width which governs the surrounding photonic environment and the strength of light-matter interactions. Moreover, we experimentally identified a new type of topological phase transition which includes three non-coincident critical points in the parameter space: the closure of the polaritonic bandgap, the transition of the Zak phase, and the hybridization of the topological edge states with the bulk states. These results reveal some remarkable and uncharted properties of topological matter when strongly coupled to light and provide an innovative design principle for tunable topological photonic devices.


Raman scattering signatures of spinons and triplons in frustrated antiferromagnets. (arXiv:2401.10452v1 [cond-mat.str-el])
O. R. Bellwood, H. L. Nourse, B. J. Powell

Magnetically frustrated spin systems compose a significant proportion of topological quantum spin liquid candidates. Evidence for spin liquids in these materials comes largely from the detection of fractionalised spin-1/2 quasiparticles, known as spinons. However, the one-dimensional Heisenberg chain, which is topologically trivial, also hosts spinons. Thus, observing spinons does not necessarily signify long-range entanglement. Here, we show that spinons arising from one-dimensional physics leave a clear fingerprint in magnetic Raman scattering. We achieve this by calculating the magnetic Raman intensity of coupled Heisenberg chains. Our findings are in excellent agreement with the magnetic Raman scattering measurements on the anisotropic triangular antiferromagnet Ca$_3$ReO$_5$Cl$_2$.


Two-dimensional quantum droplets in binary quadrupolar condensates. (arXiv:2401.10481v1 [cond-mat.quant-gas])
Aowei Yang, Jiahao Zhou, Xiaoqing Liang, Guilong Li, Bin Liu, Huan-Bo Luo, Boris A Malomed, Yongyao Li

We study the stability and characteristics of two-dimensional (2D) quasi-isotropic quantum droplets (QDs) of fundamental and vortex types, formed by binary Bose-Einstein condensate with magnetic quadrupole-quadrupole interactions (MQQIs). The magnetic quadrupoles are built as pairs of dipoles and antidipoles polarized along the x-axis. The MQQIs are induced by applying an external magnetic field that varies along the x-axis. The system is modeled by the Gross-Pitaevskii equations including the MQQIs and Lee-Huang-Yang correction to the mean-field approximation. Stable 2D fundamental QDs and quasi-isotropic vortex QDs with topological charges S<4 are produced by means of the imaginary-time-integration method for configurations with the quadrupoles polarized parallel to the systems two-dimensional plane. Effects of the norm and MQQI strength on the QDs are studied in detail. Some results, including an accurate prediction of the effective area, chemical potential, and peak density of QDs, are obtained in an analytical form by means of the Thomas-Fermi approximation. Collisions between moving QDs are studied by means of systematic simulations.


Photonic Supercoupling in Silicon Topological Waveguides. (arXiv:2401.10508v1 [physics.optics])
Ridong Jia, Yi Ji Tan, Nikhil Navaratna, Abhishek Kumar, Ranjan Singh

Electromagnetic wave coupling between photonic systems relies on the evanescent field typically confined within a single wavelength. Extending evanescent coupling distance requires low refractive index contrast and perfect momentum matching for achieving a large coupling ratio. Here, we report the discovery of photonic supercoupling in a topological valley Hall pair of waveguides, showing a substantial improvement in coupling efficiency across multiple wavelengths. Experimentally, we realize ultra-high coupling ratios between waveguides through valley-conserved vortex flow of electromagnetic energy, attaining 95% coupling efficiency for separations of up to three wavelengths. This demonstration of photonic supercoupling in topological systems significantly extends the coupling distance between on-chip waveguides and components, paving the path for the development of supercoupled photonic integrated devices, optical sensing, and telecommunications.


Comment on "Machine Learning the Operator Content of the Critical Self-Dual Ising-Higgs Gauge Model'', arXiv:2311.17994v1. (arXiv:2401.10563v1 [cond-mat.stat-mech])
Claudio Bonati, Andrea Pelissetto, Ettore Vicari

We critically discuss the results reported in arXiv:2311.17994v1 by L. Oppenheim, M. Koch-Janusz, S. Gazit, and Z. Ringel, on the multicritical behavior of the three-dimensional Ising-Gauge model at the multicritical point. We argue that their results do not contradict the theoretical scenario put forward in ``Multicritical point of the three-dimensional ${\mathbb Z}_2$ gauge Higgs model'', Phys. Rev. B 105, 165138 (2022), arXiv:2112.01824, that predicted a multicritical behavior controlled by the stable $XY$ fixed point of an effective three-dimensional ${\mathbb Z}_2\oplus {\mathbb Z}_2$ Landau-Ginzburg-Wilson $\Phi^4$ field theory. Actually, their results, as well as all numerical results reported so far in the literature, are consistent with a multicritical $XY$ scenario.


Multipole and fracton topological order via gauging foliated SPT phases. (arXiv:2401.10677v1 [cond-mat.str-el])
Hiromi Ebisu, Masazumi Honda, Taiichi Nakanishi

Spurred by recent development of fracton topological phases, unusual topological phases possessing fractionalized quasi-particles with mobility constraints, the concept of symmetries has been renewed. In particular, in accordance with the progress of multipole symmetries, associated with conservation of multipoles, such as dipole or quadruple moments as well as global charges, there have been proposed topological phases with such symmetries. These topological phases are unconventional as excitations are subject to mobility constraints corresponding to the multipole symmetries. We demonstrate a way to construct such phases by preparing layers of symmetry protected topological (SPT) phases and implementing gauging a global symmetry. After gauging, the statistics of a fractional excitation is altered when crossing the SPT phases, resulting in topological phases with the multipole symmetries. The way we construct the phases allows us to have a comprehensive understanding of field theories of topological phases with the multipole symmetries and other fracton models.


Resurgence of superconductivity and the role of $d_{xy}$ hole band in FeSe$_{1-x}$Te$_x$. (arXiv:2401.10769v1 [cond-mat.supr-con])
Archie B. Morfoot, Timur K. Kim, Matthew D. Watson, Amir A. Haghighirad, Shiv J. Singh, Nick Bultinck, Amalia I. Coldea

Iron-chalcogenide superconductors display rich phenomena caused by orbital-dependent band shifts and electronic correlations. Additionally, they are potential candidates for topological superconductivity due to the band inversion between the Fe $d$ bands and the chalcogen $p_z$ band. Here we present a detailed study of the electronic structure of the nematic superconductors FeSe$_{1-x}$Te$_x$ ($0<x<0.4$) using angle-resolved photoemission spectroscopy to understand the role of orbital-dependent band shifts, electronic correlations and the chalcogen band. We assess the changes in the effective masses using a three-band low energy model, and the band renormalization via comparison with DFT band structure calculations. The effective masses decrease for all three-hole bands inside the nematic phase followed by a strong increase for the band with $d_{xy}$ orbital character. Interestingly, this nearly-flat $d_{xy}$ band becomes more correlated as it shifts towards the Fermi level with increasing Te concentrations and as the second superconducting dome emerges. Our findings suggests that the $d_{xy}$ hole band, which is very sensitive to the chalcogen height, could be involved in promoting an additional pairing channel and increasing the density of states to stabilize the second superconducting dome in FeSe$_{1-x}$Te$_x$. This simultaneous shift of the $d_{xy}$ hole band and enhanced superconductivity is in contrast with FeSe$_{1-x}$S$_x$.


Inverse Primitive Path Analysis. (arXiv:2401.10813v1 [cond-mat.soft])
Carsten Svaneborg

The primitive-path analysis (PPA) {[}R. Everaers et al. Science 303, 823, (2004){]} is an algorithm that transforms a model polymer melt into its topologically equivalent mesh by removing excess contour length stored in thermal fluctuations. Here we present an inverse PPA algorithm that gradually reintroduces contour length in a PPA mesh to produce an topologically equilvalent polymer melt. This enables the generation of model polymer materials with well controlled topology. As an illustration, we generate knitted model polymer materials with a 2D cubic lattice of entanglement points using a synthetic PPA mesh as a starting point. We also show how to combine PPA and inverse PPA to accelerate stress relaxation approximately by an order of magnitude in simulation time. This reduces the computational cost of computational studies of structure-property relations for polymer materials.


Helical Luttinger liquid on a space-time lattice. (arXiv:2401.10828v1 [cond-mat.str-el])
V. A. Zakharov, J. Tworzydlo, C. W. J. Beenakker, M. J. Pacholski

The Luttinger model is a paradigm for the breakdown due to interactions of the Fermi liquid description of one-dimensional massless Dirac fermions. Attempts to discretize the model on a one-dimensional lattice have failed to reproduce the established bosonization results, because of the fermion-doubling obstruction: A local discretization of the Hamiltonian introduces a spurious second species of low-energy excitations, while a nonlocal discretization opens a single-particle gap at the Dirac point. Here we show how to work around this obstruction, by discretizing both space and time to obtain a local action for a helical Luttinger liquid with Hubbard interaction. The approach enables quantum Monte Carlo simulations of interacting relativistic fermions.


Spectral signatures of non-trivial topology in a superconducting circuit. (arXiv:2401.10876v1 [cond-mat.mes-hall])
L. Peyruchat (1 and 2), R. H. Rodriguez (1 and 2), J.-L. Smirr (2), R. Leone (3), Ç. Ö. Girit (1 and 2) ((1) Quantronics Group, Université Paris Saclay, CEA, CNRS, SPEC, (2) JEIP, USR 3573 CNRS, Collège de France, PSL University, (3) Laboratoire de Physique et Chimie Théoriques, Université de Lorraine, CNRS)

Topology, like symmetry, is a fundamental concept in understanding general properties of physical systems. In condensed matter systems, non-trivial topology may manifest itself as singular features in the energy spectrum or the quantization of observable quantities such as electrical conductance and magnetic flux. Using microwave spectroscopy, we show that a superconducting circuit with three Josephson tunnel junctions in parallel can possess energy degeneracies indicative of $\textrm{\emph{intrinsic}}$ non-trivial topology. We identify three topological invariants, one of which is related to a hidden quantum mechanical supersymmetry. Depending on fabrication parameters, devices are gapless or not, and fall on a simple phase diagram which is shown to be robust to perturbations including junction imperfections, asymmetry, and inductance. Josephson tunnel junction circuits, which are readily fabricated with conventional microlithography techniques, allow access to a wide range of topological systems which have no condensed matter analog. Notable spectral features of these circuits, such as degeneracies and flat bands, may be leveraged for quantum information applications, whereas quantized transport properties could be useful for metrology applications.


Non-Analytic Magnetic Response and Intrinsic Ferromagnetic Clusters in a Dirac Spin Liquid Candidate. (arXiv:2401.10888v1 [cond-mat.str-el])
B.S. Shivaram, J. Prestigiacomo, Aini Xu, Zhenyuan Zeng, Trevor D. Ford, Itamar Kimchi, Shiliang Li, Patrick A. Lee

Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the 2D Kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] (YCOB-Cl), where the vast mismatch in size of Y and Cu avoids subsitutional disorder, otherwise present in kagome materials, has emerged as a favorable candidate. In crystals of this system, with $x<$ 0.4 and no long range order, we report an unusual field dependent magnetization $M(B)$, where $M/B$ changes linearly with $|B|$, the absolute value of the field, in contrast to the expected quadratic behavior. Model calculations with a distribution of ferromagnetic (FM) clusters faithfully capture observed features suggesting such clusters to be intrinsic to real QSL materials. YCOB-Cl has a field enhanced $T^2$ heat capacity as expected for a Dirac QSL but lacks a linear $T$ behavior in the spin susceptibility. By demonstrating that FM clusters dominate the contribution to the susceptibility but not the heat capacity, our work paves the way towards reconciling the apparent inconsistency with a Dirac QSL.


Detecting charge transfer at defects in 2D materials with electron ptychography. (arXiv:2301.04469v4 [cond-mat.mtrl-sci] UPDATED)
Christoph Hofer, Jacob Madsen, Toma Susi, Timothy J. Pennycook

Charge transfer between atoms is fundamental to chemical bonding but has remained very challenging to detect directly in real space. Atomic-resolution imaging of charge density is not sufficient by itself, as the change in the density due to bonding is very subtle compared to the total local charge density. Sufficiently high sensitivity, precision and accuracy are required, which we demonstrate here for the detection of charge transfer at defects in two-dimensional WS\textsubscript{2} via high-speed electron ptychography and its ability to correct errors due to residual lens aberrations.


Topological Unwinding in an Exciton-Polariton Condensate Array. (arXiv:2307.06550v2 [cond-mat.quant-gas] UPDATED)
Guitao Lyu, Yuki Minami, Na Young Kim, Tim Byrnes, Gentaro Watanabe

The phase distribution in a Bose-Einstein condensate can realize various topological states which can be classified according to distinct winding numbers. While states with different winding numbers are topologically protected in the linear Schr\"odinger equation, when nonlinearities are introduced, violations of the topological protection can occur, leading to unwinding. Exciton-polariton condensates constitute a weakly nonlinear open-dissipative system that is well suited to studying such physics. Here we show that a one-dimensional array of exciton-polariton condensates displays a spontaneous phase unwinding from a $\pi$- to zero-state. We clarify that this collective mode transition is caused by the combined effect of nonlinearity and topological defects in the condensates. While the mode-switching phenomenon previously observed in our experiment [C.W. Lai $\it et \ al.$, Nature (London) $\bf 450$, 529 (2007)] was interpreted as the single-particle mode competition, we offer a new explanation in terms the collective phase unwinding of metastable states. Reanalyzing the experimental data, we find an evidence of the collective phase unwinding. Our results open a route towards active control of the mode switching of exciton-polariton condensates by manipulating the topological defects, which may be employed as one of the basic technologies of prospective quantum polaritonic devices.


Topological constraints on the dynamics of vortex formation in a two-dimensional quantum fluid. (arXiv:2308.02305v2 [physics.optics] UPDATED)
Thibault Congy, Pierre Azam, Robin Kaiser, Nicolas Pavloff

We present experimental and theoretical results on formation of quantum vortices in a laser beam propagating in a nonlinear medium. Topological constrains richer than the mere conservation of vorticity impose an elaborate dynamical behavior to the formation and annihilation of vortex-antivortex pairs. We identify two such mechanisms, both described by the same fold-Hopf bifurcation. One of them is particularly efficient although it is not observed in the context of liquid helium films or stationary systems because it relies on the compressible nature of the fluid of light we consider and on the non-stationarity of its flow.


Infrared resonance Raman of bilayer graphene: signatures of massive fermions and band structure on the 2D peak. (arXiv:2310.04071v2 [cond-mat.mes-hall] UPDATED)
Lorenzo Graziotto, Francesco Macheda, Tommaso Venanzi, Guglielmo Marchese, 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. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each sub-structure and, comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.


Foliated BF theories and Multipole symmetries. (arXiv:2310.06701v2 [cond-mat.str-el] UPDATED)
Hiromi Ebisu, Masazumi Honda, Taiichi Nakanishi

Due to the recent studies of the fracton topological phases, which host deconfined quasi-particle excitations with mobility restrictions, the concept of symmetries have been updated. Focusing on one of such new symmetries, multipole symmetries, including global, dipole, and quadruple symmetries, and gauge fields associated with them, we construct a new sets of $\mathbb{Z}_N$ $2+1d$ foliated BF theories, where BF theories of conventional topological phases are stacked in layers with couplings between them. By investigating gauge invariant non-local operators, we show that our foliated BF theories exhibit unusual ground state degeneracy depending on the system size; it depends on the greatest common divisor between $N$ and the system size. Our result provides a unified insight on UV lattice models of the fracton topological phases and other unconventional ones in view of foliated field theories.


Theory of infrared double-resonance Raman spectrum in graphene: the role of the zone-boundary electron-phonon enhancement. (arXiv:2310.09188v2 [cond-mat.mes-hall] UPDATED)
Lorenzo Graziotto, Francesco Macheda, Thibault Sohier, Matteo Calandra, Francesco Mauri

We theoretically investigate the double-resonance Raman spectrum of monolayer graphene down to infrared laser excitation energies. By using first-principles density functional theory calculations, we improve upon previous theoretical predictions based on conical models or tight-binding approximations, and rigorously justify the evaluation of the electron-phonon enhancement found in Ref. [Venanzi, T., Graziotto, L. et al., Phys. Rev. Lett. 130, 256901 (2023)]. We proceed to discuss the effects of such enhancement on the room temperature graphene resistivity, hinting towards a possible reconciliation of theoretical and experimental discrepancies.


Anomalous Hall effect with plateaus observed in a magnetic Weyl semimetal NdAlGe at low temperatures. (arXiv:2312.00222v2 [cond-mat.str-el] UPDATED)
Naoki Kikugawa, Shinya Uji, Taichi Terashima

In the $R$Al(Si,Ge) ($R$: lanthanides) family, both spatial inversion and time-reversal symmetries are broken. This may offer opportunities to study Weyl-fermion physics in nontrivial spin structures emerging from a noncentrosymmetric crystal structure. In this study, we investigated the anomalous Hall effect (AHE) in NdAlGe via magnetotransport, magnetization, and magnetic torque measurements down to 40 mK (0.4 K for magnetization). The single crystals grown by a laser-heated floating-zone method exhibit a single magnetic phase transition at $T_{\rm M}$ = 13.5 K, where the $T_{\rm M}$ is the transition temperature. With the magnetic field parallel to the easy $\lbrack$001$\rbrack$ axis, the AHE gradually evolves as the temperature decreases below $T_{\rm M}$. The anomalous Hall conductivity (AHC) reaches $\sim$320 $\Omega^{-1}$cm$^{-1}$ at 40 mK in the magnetically saturated state. Except in low-temperature low-field plateau phases, the AHC and magnetization are proportional, and their ratio agrees with the ratios for conventional ferromagnets, suggesting that the intrinsic AHE occurs by the Karplus-Luttinger mechanism. Below $\sim$0.6 K, the curves of Hall resistivity against the field exhibit plateaus at low fields below $\sim$0.5 T, correlating with the plateaus in the magnetization curve. For the first plateau, the magnetization is one order of magnitude smaller than the magnetically saturated state, whereas the AHE is more than half that in the saturated state. This finding under well below $T_{\rm M}$ suggests that the AHE at the first plateau is not governed by the magnetization and may be interpreted based on a multipole or spin chirality.


Symmetry-Driven Valleytronics in Single-Layer Tin Chalcogenides. (arXiv:2401.08339v2 [cond-mat.mtrl-sci] UPDATED)
Vo Khuong Dien, Pham Thi Bich Thao, Nguyen Thi Han, Nguyen Thanh Tien

The concept of valleytronics has recently gained considerable research attention due to its intriguing physical phenomena and practical applications in optoelectronics and quantum information. In this study, by employing GW-BSE calculations and symmetry analysis, we demonstrate that single-layer orthorhombic SnS and SnSe possess high carrier mobility and exceptional excitonic effects. Especially, these materials display spontaneous linearly polarized optical selectivity, a behavior that differs from the valley-selective circular dichroism observed in the hexagonal lattices. Specifically, when subjected to a zigzag polarization of light, only the A exciton (stemming from the X valley) becomes optically active, while the B exciton (arising from the Y valley) remains dark. The armchair-polarized light triggers the opposite behavior. This selective optical excitation arises from the symmetry of the bands under mirror symmetry. Additionally, the study reveals a strong coupling between valley physics and ferroelectricity in layered tin chalcogenides, enabling the manipulation of electronic transport and exciton polarization. Layered tin chalcogenides thus emerge as promising candidates for both valleytronic and ferroelectric materials.