Found 31 papers in cond-mat
Date of feed: Mon, 04 Dec 2023 01:30:00 GMT

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Mechanical control of quantum transport in graphene. (arXiv:2312.00177v1 [cond-mat.mes-hall])
A. C. McRae, G. Wei, L. Huang, S. Yigen, V. Tayari, A. R. Champagne

Two-dimensional materials (2DMs) are fundamentally electro-mechanical systems. Their environment unavoidably strains them and modifies their quantum transport properties. For instance, a simple uniaxial strain could completely turn off the conductivity of ballistic graphene or switch on/off the superconducting phase of magic-angle bilayer graphene. Here we report measurements of quantum transport in strained graphene which agree quantitatively with models based on mechanically-induced gauge potentials. We mechanically induce in-situ a scalar potential, which modifies graphene's work function by up to 25 meV, and vector potentials which suppress the ballistic conductivity of graphene by up to 30 % and control its quantum interferences. To do so, we developed an experimental platform able to precisely tune both the mechanics and electrostatics of suspended graphene transistors at low-temperature over a broad range of strain (up to 2.6 %). This work opens many opportunities to experimentally explore quantitative strain effects in 2DM quantum transport and technologies.


Proximity effect of s-wave superconductor on inversion broken Weyl Semi-Metal. (arXiv:2312.00187v1 [cond-mat.supr-con])
Robert Dawson, Vivek Aji

Inducing superconductivity in systems with unconventional band structures is a promising approach for realising unconventional superconductivity. Of particular interest are single interface or Josephson Junction architectures involving Weyl semimetals, which are predicted to host odd parity, potentially topological, superconducting states. These expectations rely crucially on the tunneling of electronic states at the interface between the two systems. In this study, we revisit the question of induced superconductivity in an inversion broken WSM via quantum tunneling, treating the interface as an effective potential barrier. We determine the conditions under which the gap function couples to the Weyl physics and its properties within the WSM. Our simulations show that the mismatch in the nature of the low energy electronic states leads to a rapid decay of the superconductivity within the semi-metal.


Anomalous Hall effect with plateaus observed in a magnetic Weyl semimetal NdAlGe at low temperatures. (arXiv:2312.00222v1 [cond-mat.str-el])
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.


Effects of domain walls and chiral supercurrent in quantum anomalous Hall Josephson junctions. (arXiv:2312.00331v1 [cond-mat.mes-hall])
Junjie Qi, Haiwen Liu, Jie Liu, Hua Jiang, Dong E. Liu, Chui-Zhen Chen, Ke He, X. C. Xie

The intriguing interplay between topology and superconductivity has attracted significant attention, given its potential for realizing topological superconductivity. In this study, we investigate the transport properties of the chiral Josephson effect in the quantum anomalous Hall insulators (QAHIs)-based junction. We reveal a systematic crossover from edge-state to bulk-state dominant supercurrents, with a notable $0-\pi$ transition observed under non-zero magnetic flux through chemical potential adjustments. This transition underscores the competition between bulk and chiral edge transport. Furthermore, we identify an evolution among three distinct quantum interference patterns: from a $2\Phi_0$-periodic oscillation pattern, to a $\Phi_0$-periodic oscillation pattern, and then to an asymmetric Fraunhofer pattern ($\Phi_0 = h/2e$ is the flux quantum, $h$ the Planck constant, and $e$ the electron charge). Subsequently, we examine the influence of domains on quantum interference patterns. Intriguingly, a distinctive Fraunhofer-like pattern emerges due to coexistence of chiral edge states and domain wall states, even when the chemical potential is within gap. These results not only advance the theoretical understanding but also pave the way for the experimental discovery of the chiral Josephson effect based on QAHI doped with magnetic impurities.


Hybrid Higher-Order Topological Skin Modes in the Two-Dimensional Su-Schrieffer-Heeger Model with Nonreciprocal Hoppings. (arXiv:2312.00371v1 [cond-mat.mes-hall])
Hiromasa Wakao

The coexistence of edge states and skin effects provides the topologically protected localized states at the corners of two-dimensional systems. In this paper, we realize such corner states in the two-dimensional Su-Schrieffer-Heeger model with the nonreciprocal hoppings. For the characterization of the real line gap topology, we introduce the $\mathbb{Z}_4$ Berry phase protected by generalized four-fold rotational symmetry. From the physical picture of the adiabatic connection, we find that the value of the $\mathbb{Z}_4$ Berry predicts the position of edge states. Additionally, by using the winding number, we characterize the point gap topology of the edge spectra. From the results of these characterizations by the first-order topological invariants, we find that the pair of values of the $\mathbb{Z}_4$ Berry phase and the winging number yields the position of the topologically protected localized states.


Magneto-transport in the monolayer MoS2 material system for high-performance field-effect transistor applications. (arXiv:2312.00378v1 [cond-mat.mes-hall])
Anup Kumar Mandia, Rohit Kumar, Seung-Cheol Lee, Satadeep Bhattacharjee, Bhaskaran Muralidharan

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities and magnetic fields.


Unravelling spontaneous Bloch-type skyrmion in centrosymmetric two-dimensional magnets. (arXiv:2312.00423v1 [cond-mat.mtrl-sci])
Jingman Pang, Xiaohang Niu, Hong Jian Zhao, Yun Zhang, Laurent Bellaiche

The realization of magnetic skyrmions in two-dimensional (2D) magnets holds great promise for both fundamental research and device applications. Despite recent progress, two-dimensional skyrmion hosts are still limited, due to the fact that most 2D magnets are centrosymmetric and thus lack Dzyaloshinskii-Moriya interaction (DMI). We show here, using a general analysis based on symmetry, that Bloch-type skyrmions can, in fact, be stabilized in 2D magnets, due to the interplay between in-plane component (dx) of second nearest-neighbor DMI and magnetic anisotropy. Its validity is demonstrated in the Cr2Ge2Te6 monolayer, which is also verified by recent experiments. Our work gives a clear direction for experimental studies of 2D magnetic materials to stabilize skyrmions and should greatly enrich the research on magnetic skyrmions in 2D lattices.


Comparative Analysis of Tight-Binding models for Transition Metal Dichalcogenides. (arXiv:2312.00498v1 [cond-mat.mtrl-sci])
Bert Jorissen, Lucian Covaci, Bart Partoens

We provide a comprehensive analysis of the prominent tight-binding (TB) models for transition metal dichalcogenides (TMDs) available in the literature. We inspect the construction of these TB models, discuss their parameterization used and conduct a thorough comparison of their effectiveness in capturing important electronic properties. Based on these insights, we propose a novel TB model for TMDs designed for enhanced computational efficiency. Utilizing $MoS_2$ as a representative case, we explain why specific models offer a more accurate description. Our primary aim is to assist researchers in choosing the most appropriate TB model for their calculations on TMDs.


Weak Electronic Correlations Observed in Magnetic Weyl Semimetal Mn$_3$Ge. (arXiv:2312.00511v1 [cond-mat.mtrl-sci])
Susmita Changdar, Susanta Ghosh, Anumita Bose, Indrani Kar, Achintya Low, Patrick Le Fevre, François Bertran, Awadhesh Narayan, Setti Thirupathaiah

Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we systematically studied the electronic band structure of Mn$_3$Ge in the vicinity of the Fermi level. We observe several bands crossing the Fermi level, confirming the metallic nature of the studied system. We further observe several flat bands along various high symmetry directions, consistent with the DFT calculations. The calculated partial density of states (PDOS) suggests a dominant Mn $3d$ orbital contribution to the total valence band DOS. With the help of orbital-resolved band structure calculations, we qualitatively identify the orbital information of the experimentally obtained band dispersions. Out-of-plane electronic band dispersions are explored by measuring the ARPES data at various photon energies. Importantly, our study suggests relatively weaker electronic correlations in Mn$_3$Ge compared to Mn$_3$Sn.


Longitudinal optical conductivity of graphene in van der Waals heterostructures composed of graphene and transition metal dichalcogenides. (arXiv:2312.00543v1 [cond-mat.mes-hall])
Ruoyang Cui, Yaojin Li

Placing and twisting graphene on transition metal dichalcogenides (TMDC) forms a van der Waals (vdW) heterostructure. The occurrence of Zeeman splitting and Rashba spin-orbit coupling (SOC) changes graphene's linear dispersion and conductivity. Hence, this paper studies the dependence of graphene's longitudinal optical conductivity on Rashba SOC, the twist-angle and temperature. At zero temperature, a main conductivity peak exists. When Rashba SOC increases, a second peak occurs, with both extremes presenting an enhanced height and width, and the frequencies where the two peaks arise will increase because the energy gap and the possibility of electron transition increase. Altering the twist-angle from 0 to 30$^{\circ}$, the conductivity is primarily affected by chalcogen atoms. Moreover, when temperature increases to room temperature, besides a Drude peak due to the thermal excitation, a new band arises in the conductivity owing to the joint effect of the thermal transition and the photon transition.


Phonon-Limited Transport in 2D Materials: A Unified Approach for ab initio Mobility and Current Calculations. (arXiv:2312.00577v1 [cond-mat.mes-hall])
Jonathan Backman, Youseung Lee, Mathieu Luisier

This paper presents an ab initio methodology to account for electron-phonon interactions in 2D materials, focusing on transition metal dichalcogenides (TMDCs). It combines density functional theory and maximally localized Wannier functions to acquire material data and relies on the linearized Boltzmann transport equation (LBTE) and the non-equilibrium Green's functions (NEGF) method to determine the transport properties of materials and devices, respectively. It is shown that for MoS$_2$, both LBTE and NEGF return very close mobility values, without the need to adjust any parameter. The excellent agreement between both approaches results from the inclusion of non-diagonal entries in the electron-phonon scattering self-energies. The NEGF solver is then used to shed light on the "current vs. voltage" characteristics of a monolayer MoS$_2$ transistor, highlighting how the interactions with phonons impact both the current magnitude and its distribution. The mobility of other TMDCs is considered as well, demonstrating the capabilities of the proposed technique to assess the potential of 2D channel materials in next-generation logic applications.


Edge modes, extended TQFT, and measurement based quantum computation. (arXiv:2312.00605v1 [hep-th])
Gabriel Wong

Quantum teleportation can be used to define a notion of parallel transport which characterizes the entanglement structure of a quantum state \cite{Czech:2018kvg}. This suggests one can formulate a gauge theory of entanglement. In \cite{Wong:2022mnv}, it was explained that measurement based quantum computation in one dimension can be understood in term of such a gauge theory (MBQC). In this work, we give an alternative formulation of this "entanglement gauge theory" as an extended topological field theory. This formulation gives a alternative perspective on the relation between the circuit model and MBQC. In addition, it provides an interpretation of MBQC in terms of the extended Hilbert space construction in gauge theories, in which the entanglement edge modes play the role of the logical qubit.


Hyperdeterminants and Composite fermion States in Fractional Chern Insulators. (arXiv:2312.00636v1 [cond-mat.str-el])
Xiaodong Hu, Di Xiao, Ying Ran

Fractional Chern insulators (FCI) were proposed theoretically about a decade ago. These exotic states of matter are fractional quantum Hall states realized when a nearly flat Chern band is partially filled, even in the absence of an external magnetic field. Recently, exciting experimental signatures of such states have been reported in twisted MoTe$_2$ bilayer systems. Motivated by these experimental and theoretical progresses, in this paper, we develop a projective construction for the composite fermion states (either the Jain's sequence or the composite Fermi liquid) in a partially filled Chern band with Chern number $C=\pm1$, which is capable of capturing the microscopics, e.g., symmetry fractionalization patterns and magnetoroton excitations. On the mean-field level, the ground states' and excitated states' composite fermion wavefunctions are found self-consistently in an enlarged Hilbert space. Beyond the mean-field, these wavefunctions can be projected back to the physical Hilbert space to construct the electronic wavefunctions, allowing direct comparison with FCI states from exact diagonalization on finite lattices. We find that the projected electronic wavefunction corresponds to the \emph{combinatorial hyperdeterminant} of a tensor. When applied to the traditional Galilean invariant Landau level context, the present construction exactly reproduces Jain's composite fermion wavefunctions. We apply this projective construction to the twisted bilayer MoTe$_2$ system. Experimentally relevant properties are computed, such as the magnetoroton band structures and quantum numbers.


Interplay between Haldane and modified Haldane models in $\alpha$-$T_{3}$ lattice: Band structures, phase diagrams and edge states. (arXiv:2312.00642v1 [cond-mat.str-el])
Kok Wai Lee, Pei-Hao Fu, Yee Sin Ang

We study the topological properties of the Haldane and modified Haldane models in $\alpha$-$T_{3}$ lattice. The band structures and phase diagrams of the system are investigated. Individually, each model undergoes a distinct phase transition: (i) The Haldane-only model experiences a topological phase transition from the Chern insulator ($\mathcal{C} = 1$) phase to the higher Chern insulator ($\mathcal{C} = 2$) phase; while (ii) the modified-Haldane-only model experiences a phase transition from the topological metal ($\mathcal{C} = 2$) phase to the higher Chern insulator ($\mathcal{C} = 2$) phase and we show that $\mathcal{C}$ is insufficient to characterize this system because $\mathcal{C}$ remains unchanged before and after the phase transition. By plotting the Chern number and $\mathcal{C}$ phase diagram, we show that in the presence of both Haldane and modified Haldane models in the $\alpha$-$T_{3}$ lattice, the interplay between the two models manifests three distinct topological phases, namely the $\mathcal{C} = 1$ Chern insulator (CI) phase, $\mathcal{C} = 2$ higher Chern insulator (HCI) phase and $\mathcal{C} = 2$ topological metal (TM) phase. These results are further supported by the $\alpha$-$T_{3}$ zigzag edge states calculations.


Molecular Dynamics Study of Electro-Osmotic Flow in a Nanochannel with Molybdenum Disulfide Walls. (arXiv:2312.00767v1 [cond-mat.soft])
S.M.Kazem Manzoorolajdad, Hossein Hamzehpour, Jalal Sarabadani

The electro-osmotic flow (EOF) in a neutral system consisting of an aqueous NaCl solution confined in a nanochannel with two parallel Molybdenum disulfide ($\textrm{MoS}_{\textrm{2}}$) walls and in the presence of an external electric field parallel to the channel walls, is investigated for the first time. The results indicate that the thickness of the Stern layer grows as the negative electric surface charge density on the nanochannel walls increases. The Stern layer becomes thinner as the salt concentration is increased. Moreover, the EOF occurs under the no-slip condition on the walls. In addition, by increasing the surface charge density the average of the flow velocity across the nanochannel initially grows (Debye--H$\ddot{\textrm{u}}$ckel regime) and reaches its maximum value. Then, by further increasing the surface charge density the water flow rate decreases (intermediate regime), and gets the zero value and becomes negative (reverse flow regime) at even larger values of the surface charge densities. Comparing the results of the previous work wherein the channels are composed of the black phosphorene walls with those of the present study for a channel composed of $\textrm{MoS}_{\textrm{2}}$ surfaces, show that for the latter case the reverse flow occurs at a lower surface charge density and with a greater value of the peak velocity with respect to the change in the surface charge density for the former case.


Interference of chiral Andreev edge states. (arXiv:1907.01722v3 [cond-mat.mes-hall] UPDATED)
Lingfei Zhao, Ethan G. Arnault, Alexey Bondarev, Andrew Seredinski, Trevyn Larson, Anne W. Draelos, Hengming Li, Kenji Watanabe, Takashi Taniguchi, François Amet, Harold U. Baranger, Gleb Finkelstein

The search for topological excitations such as Majorana fermions has spurred interest in the boundaries between distinct quantum states. Here, we explore an interface between two prototypical phases of electrons with conceptually different ground states: the integer quantum Hall insulator and the s-wave superconductor. We find clear signatures of hybridized electron and hole states similar to chiral Majorana fermions, to which we refer as chiral Andreev edge states (CAES). They propagate along the interface in the direction determined by magnetic field and their interference can turn an incoming electron into an outgoing electron or a hole, depending on the phase accumulated by the CAES along their path. Our results demonstrate that these excitations can propagate and interfere over a significant length, opening future possibilities for their coherent manipulation.


Birth Quota of Non-Generic Degeneracy Points. (arXiv:2202.05825v2 [cond-mat.mes-hall] UPDATED)
Gergő Pintér, György Frank, Dániel Varjas, András Pályi

Weyl points are generic and stable features in the energy spectrum of Hamiltonians that depend on a three-dimensional parameter space. Non-generic isolated two-fold degeneracy points, such as multi-Weyl points, split into Weyl points upon a generic perturbation that removes the fine-tuning or protecting symmetry. The number of the resulting Weyl points is at least $|Q|$, where $Q$ is the topological charge associated to the non-generic degeneracy point. Here, we show that such a non-generic degeneracy point also has a birth quota, i.e., a maximum number of Weyl points that can be born from it upon any perturbation. The birth quota is a local multiplicity associated to the non-generic degeneracy point, an invariant of map germs known from singularity theory. This holds not only for the case of a three-dimensional parameter space with a Hermitian Hamiltonian, but also for the case of a two-dimensional parameter space with a chiral-symmetric Hamiltonian. We illustrate the power of this result for electronic band structures of two- and three-dimensional crystals. Our work establishes a strong connection between singularity theory and topological band structures, and more broadly, parameter-dependent quantum systems.


Graphene-based quantum Hall interferometer with self-aligned side gates. (arXiv:2206.05623v2 [cond-mat.mes-hall] UPDATED)
Lingfei Zhao, Ethan G. Arnault, Trevyn F. Q. Larson, Zubair Iftikhar, Andrew Seredinski, Tate Fleming, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein

The vanishing band gap of graphene has long presented challenges for making high-quality quantum point contacts (QPCs) -- the partially transparent p-n interfaces introduced by conventional split-gates tend to short the QPC. This complication has hindered the fabrication of graphene quantum Hall Fabry-P\'erot interferometers, until recent advances have allowed split-gate QPCs to operate utilizing the highly resistive $\nu=0$ state. Here, we present a simple recipe to fabricate QPCs by etching a narrow trench in the graphene sheet to separate the conducting channel from self-aligned graphene side gates. We demonstrate operation of the individual QPCs in the quantum Hall regime, and further utilize these QPCs to create and study a quantum Hall interferometer.


Loss and decoherence at the quantum Hall - superconductor interface. (arXiv:2210.04842v2 [cond-mat.mes-hall] UPDATED)
Lingfei Zhao, Zubair Iftikhar, Trevyn F.Q. Larson, Ethan G. Arnault, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein

We perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the quantum Hall (QH) regime. We find that the probability of Andreev conversion from electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our study may pave the road to engineering future generation of hybrid devices for exploiting superconductivity proximity in chiral channels.


Ion-Implanted $^8$Li Nuclear Magnetic Resonance in Highly Oriented Pyrolytic Graphite. (arXiv:2301.07821v4 [cond-mat.mtrl-sci] UPDATED)
John O. Ticknor, Jonah R. Adelman, Aris Chatzichristos, Martin H. Dehn, Luca Egoriti, Derek Fujimoto, Victoria L. Karner, Robert F. Kiefl, C. D. Philip Levy, Ruohong Li, Ryan M. L. McFadden, Gerald D. Morris, Mohamed Oudah, Monika Stachura, Edward Thoeng, W. Andrew MacFarlane

We report $\beta$-detected nuclear magnetic resonance of ultra-dilute $^{8}$Li$^{+}$ implanted in highly oriented pyrolytic graphite (HOPG). The absence of motional narrowing and diffusional spin-lattice relaxation implies Li$^+$ is not appreciably mobile up to 400 K, in sharp contrast to the highly lithiated stage compounds. However, the relaxation is remarkably fast and persists down to cryogenic temperatures. Ruling out extrinsic paramagnetic impurities and intrinsic ferromagnetism, we conclude the relaxation is due to paramagnetic centers correlated with implantation. While the resulting effects are not consistent with a Kondo impurity, they also differ from free paramagnetic centers, and we suggest that a resonant scattering approach may account for much of the observed phenomenology.


$\alpha$-induction for bi-unitary connections. (arXiv:2302.05577v3 [math.QA] UPDATED)
Yasuyuki Kawahigashi

The tensor functor called $\alpha$-induction arises from a Frobenius algebra object, or a Q-system, in a braided unitary fusion category. In the operator algebraic language, it gives extensions of endomorphism of $N$ to $M$ arising from a subfactor $N\subset M$ of finite index and finite depth giving a braided fusion category of endomorpshisms of $N$. It is also understood in terms of Ocneanu's graphical calculus. We study this $\alpha$-induction for bi-unitary connections, which give a characterization of finite-dimensional nondegenerate commuting squares and gives certain 4-tensors appearing in recent studies of 2-dimensional topological order. We show that the resulting $\alpha$-induced bi-unitary connections are flat if we have a commutative Frobenius algebra, or a local Q-system. Examples related to chiral conformal field theory and the Dynkin diagrams are presented.


Quantum Velocity Limits for Multiple Observables: Conservation Laws, Correlations, and Macroscopic Systems. (arXiv:2305.03190v3 [cond-mat.stat-mech] UPDATED)
Ryusuke Hamazaki

How multiple observables mutually influence their dynamics has been a crucial issue in statistical mechanics. We introduce a new concept, "quantum velocity limits," to establish a quantitative and rigorous theory for non-equilibrium quantum dynamics for multiple observables. Quantum velocity limits are universal inequalities for a vector the describes velocities of multiple observables. They elucidate that the speed of an observable of our interest can be tighter bounded when we have knowledge of other observables, such as experimentally accessible ones or conserved quantities, compared with the conventional speed limits for a single observable. We first derive an information-theoretical velocity limit in terms of the generalized correlation matrix of the observables and the quantum Fisher information. The velocity limit has various novel consequences: (i) conservation law in the system, a fundamental ingredient of quantum dynamics, can improve the velocity limits through the correlation between the observables and conserved quantities; (ii) speed of an observable can be bounded by a nontrivial lower bound from the information on another observable; (iii) there exists a notable non-equilibrium tradeoff relation, stating that speeds of uncorrelated observables, e.g., anti-commuting observables, cannot be simultaneously large; (iv) velocity limits for any observables on a local subsystem in locally interacting many-body systems remain convergent even in the thermodynamic limit. Moreover, we discover another distinct velocity limit for multiple observables on the basis of the local conservation law of probability current, which becomes advantageous for macroscopic transitions of multiple quantities.


Raman Spectroscopy of Monolayer to Bulk PtSe2 Exfoliated Crystals. (arXiv:2307.15520v3 [cond-mat.mtrl-sci] UPDATED)
Marin Tharrault, Eva Desgué, Dominique Carisetti, Bernard Plaçais, Christophe Voisin, Pierre Legagneux, Emmanuel Baudin

Raman spectroscopy is widely used to assess the quality of 2D materials thin films. This report focuses on $\rm{PtSe_2}$, a noble transition metal dichalcogenide which has the remarkable property to transit from a semi-conductor to a semi-metal with increasing layer number. While polycrystalline $\rm{PtSe_2}$ can be grown with various crystalline qualities, getting insight into the monocrystalline intrinsic properties remains challenging. We report on the study of exfoliated 1 to 10 layers $\rm{PtSe_2}$ by Raman spectroscopy, featuring record linewidth. The clear Raman signatures allow layer-thickness identification and provides a reference metrics to assess crystal quality of grown films.


Consequences of the gauging of Weyl symmetry and the two-dimensional conformal anomaly. (arXiv:2309.09598v2 [hep-th] UPDATED)
Omar Zanusso

We discuss the generalization of the local renormalization group approach to theories in which Weyl symmetry is gauged. These theories naturally correspond to scale invariant - rather than conformal invariant - models in the flat space limit. We argue that this generalization can be of use when discussing the issue of scale vs conformal invariance in quantum and statistical field theories. The application of Wess-Zumino consistency conditions constrains the form of the Weyl anomaly and the beta functions in a nonperturbative way. In this work we concentrate on two dimensional models including also the contributions of the boundary. Our findings suggest that the renormalization group flow between scale invariant theories differs from the one between conformal theories because of the presence of a new charge that appears in the anomaly. It does not seem to be possible to find a general scheme for which the new charge is zero, unless the theory is conformal in flat space. Two illustrative examples involving flat space's conformal and scale invariant models that do not allow for a naive application of the standard local treatment are given.


Tight-binding theory of spin-spin interactions, Curie temperatures, and quantum Hall effects in topological (Hg,Cr)Te in comparison to non-topological (Zn,Cr)Te, and (Ga,Mn)N. (arXiv:2310.19856v2 [cond-mat.mtrl-sci] UPDATED)
Cezary Śliwa, Tomasz Dietl

Earlier theoretical results on $p$-$d$ and $d$-$d$ exchange interactions for zinc-blende semiconductors with Cr$^{2{+}}$ and Mn$^{3{+}}$ ions are revisited and extended by including contributions beyond the dominating ferromagnetic (FM) superexchange term [i.e., the interband Bloembergen-Rowland-Van Vleck contribution and antiferromagnetic (AFM) two-electron term], and applied to topological Cr-doped HgTe and non-topological (Zn,Cr)Te and (Ga,Mn)N in zinc-blende and wurtzite crystallographic structures. From the obtained values of the $d$-$d$ exchange integrals $J_{ij}$, and by combining the Monte-Carlo simulations with the percolation theory for randomly distributed magnetic ions, we determine magnitudes of Curie temperatures $T_{\text{C}}(x)$ for $\mathrm{Zn}_{1-x}\mathrm{Cr}_x\mathrm{Te}$ and $\mathrm{Ga}_{1-x}\mathrm{Mn}_x\mathrm{N}$ and compare to available experimental data. Furthermore, we find that competition between FM and AFM $d$-$d$ interactions can lead to a spin-glass phase in the case of $\mathrm{Hg}_{1-x}\mathrm{Cr}_x\mathrm{Te}$. This competition, along with a relatively large magnitude of the AF $p$-$d$ exchange energy $N_0\beta$ can stabilize the quantum spin Hall effect, but may require the application of a tilted magnetic field to observe the quantum anomalous Hall effect in HgTe quantum wells doped with Cr.


Self-duality under gauging a non-invertible symmetry. (arXiv:2310.19867v2 [hep-th] UPDATED)
Yichul Choi, Da-Chuan Lu, Zhengdi Sun

We discuss two-dimensional conformal field theories (CFTs) which are invariant under gauging a non-invertible global symmetry. At every point on the orbifold branch of $c=1$ CFTs, it is known that the theory is self-dual under gauging a $\mathbb{Z}_2\times \mathbb{Z}_2$ symmetry, and has $\mathsf{Rep}(H_8)$ and $\mathsf{Rep}(D_8)$ fusion category symmetries as a result. We find that gauging the entire $\mathsf{Rep}(H_8)$ fusion category symmetry maps the orbifold theory at radius $R$ to that at radius $2/R$. At $R=\sqrt{2}$, which corresponds to two decoupled Ising CFTs (Ising$^2$ in short), the theory is self-dual under gauging the $\mathsf{Rep}(H_8)$ symmetry. This implies the existence of a topological defect line in the Ising$^2$ CFT obtained from half-space gauging of the $\mathsf{Rep}(H_8)$ symmetry, which commutes with the $c=1$ Virasoro algebra but does not preserve the fully extended chiral algebra. We bootstrap its action on the $c=1$ Virasoro primary operators, and find that there are no relevant or marginal operators preserving it. Mathematically, the new topological line combines with the $\mathsf{Rep}(H_8)$ symmetry to form a bigger fusion category which is a $\mathbb{Z}_2$-extension of $\mathsf{Rep}(H_8)$. We solve the pentagon equations including the additional topological line and find 8 solutions, where two of them are realized in the Ising$^2$ CFT. Finally, we show that the torus partition functions of the Monster$^2$ CFT and Ising$\times$Monster CFT are also invariant under gauging the $\mathsf{Rep}(H_8)$ symmetry.


Finite Temperature Entanglement Negativity of Fermionic Symmetry Protected Topological Phases and Quantum Critical Points in One Dimension. (arXiv:2310.20566v2 [cond-mat.str-el] UPDATED)
Wonjune Choi, Michael Knap, Frank Pollmann

We study the logarithmic entanglement negativity of symmetry-protected topological (SPT) phases and quantum critical points (QCPs) of one-dimensional noninteracting fermions at finite temperatures. In particular, we consider a free fermion model that realizes not only quantum phase transitions between gapped phases but also an exotic topological phase transition between quantum critical states in the form of the fermionic Lifshitz transition. The bipartite entanglement negativity between adjacent fermion blocks reveals the crossover boundary of the quantum critical fan near the QCP between two gapped phases. Along the critical phase boundary between the gapped phases, the sudden decrease in the entanglement negativity signals the fermionic Lifshitz transition responsible for the change in the topological nature of the QCPs. In addition, the tripartite entanglement negativity between spatially separated fermion blocks counts the number of topologically protected boundary modes for both SPT phases and topologically nontrivial QCPs at zero temperature. However, the long-distance entanglement between the boundary modes vanishes at finite temperatures due to the instability of SPTs, the phases themselves.


Projective symmetries of three-dimensional TQFTs. (arXiv:2311.01637v2 [math.QA] UPDATED)
Jackson Van Dyke

Quantum field theory has various projective characteristics which are captured by what are called anomalies. This paper explores this idea in the context of fully-extended three-dimensional topological quantum field theories (TQFTs).

Given a three-dimensional TQFT (valued in the Morita 3-category of fusion categories), the anomaly identified herein is an obstruction to gauging a naturally occurring orthogonal group of symmetries, i.e. we study 't Hooft anomalies. In other words, the orthogonal group almost acts: There is a lack of coherence at the top level. This lack of coherence is captured by a "higher (central) extension" of the orthogonal group, obtained via a modification of the obstruction theory of Etingof-Nikshych-Ostrik-Meir [ENO10]. This extension tautologically acts on the given TQFT/fusion category, and this precisely classifies a projective (equivalently anomalous) TQFT. We explain the sense in which this is an analogue of the classical spin representation. This is an instance of a phenomenon emphasized by Freed [Fre23]: Quantum theory is projective.

In the appendices we establish a general relationship between the language of projectivity/anomalies and the language of topological symmetries. We also identify a universal anomaly associated with any theory which is appropriately "simple".


Nonequilibrium protection effect and spatial localization of noise-induced fluctuations under gas flow scattering on partially penetrable obstacle. (arXiv:2311.11658v2 [cond-mat.stat-mech] UPDATED)
S.P. Lukyanets, O.V. Kliushnichenko

We consider a nonequilibrium transition that leads to the formation of nonlinear steady-state structures due to the gas flow scattering on a partially penetrable obstacle. The resulting nonequilibrium steady state (NESS) corresponds to a two-domain gas structure attained at certain critical parameters. We use a simple mean-field model of the driven lattice gas with ring topology to demonstrate that this transition is accompanied by the emergence of local invariants related to a complex composed of the obstacle and its nearest gas surrounding, which we refer to as obstacle edges. These invariants are independent of the main system parameters and behave as local first integrals, at least qualitatively. As a result, the complex becomes insensitive to the noise of external driving field within the overcritical domain. The emerged invariants describe the conservation of the number of particles inside the obstacle and strong temporal synchronization or correlation of gas states at obstacle edges. Such synchronization guarantees the equality to zero of the total edge current at any time. The robustness against external drive fluctuations is shown to be accompanied by strong spatial localization of induced gas fluctuations near the domain wall separating the depleted and dense gas phases. Such a behavior can be associated with nonequilibrium protection effect and synchronization of edges. The transition rates between different NESSs are shown to be different. The relaxation rates from one NESS to another take complex and real values in the sub- and overcritical regimes, respectively. The mechanism of these transitions is governed by the generation of shock waves at the back side of the obstacle. In the subcritical regime, these solitary waves are generated sequentially many times, while only a single excitation is sufficient to rearrange the system state in the overcritical regime.


Weyl orbits as probe of chiral separation effect in magnetic Weyl semimetals. (arXiv:2311.12712v2 [cond-mat.mes-hall] UPDATED)
M.A.Zubkov

We consider magnetic Weyl semimetals. First of all we review relation of intrinsic anomalous Hall conductivity, band contribution to intrinsic magnetic moment, and the conductivity of chiral separation effect (CSE) to the topological invariants written in terms of the Wigner transformed Green functions (with effects of interaction and disorder taken into account). Next, we concentrate on the CSE. The corresponding bulk axial current would result in accumulation of particles and holes of opposite chiralities at the surface of the sample. However, this accumulation is compensated by the flow of the states in momentum space along the Fermi arcs. Together with the bulk CSE current this flow forms closed Weyl orbits. Their detection can be considered as experimental discovery of chiral separation effect. Previously it was proposed to detect Weyl orbits through the observation of quantum oscillations \cite{Potter_2014} . We propose the alternative way to detect existence of Weyl orbits through the observation of their contributions to Hall conductance.


Optically Induced Monopoles, Knots, and Skyrmions in Quantum Gases. (arXiv:2311.15972v2 [cond-mat.quant-gas] UPDATED)
Toni Annala, Tommi Mikkonen, Mikko Möttönen

We propose, and theoretically analyze, a practical protocol for the creation of topological monopole configurations, quantum knots, and skyrmions in Bose--Einstein condensates by employing fictitious magnetic fields induced by the interaction of the atomic cloud with coherent light fields. It is observed that a single coherent field is not enough for this purpose, but instead we find incoherent superpositions of several coherent fields that introduce topological point charges. We numerically estimate the experimentally achievable strengths and gradients of the induced fictitious magnetic fields and find them to be adjustable at will to several orders of magnitude greater than those of the physical magnetic fields employed in previous experimental studies. This property together with ultrafast control of the optical fields paves the way for advanced engineering of topological defects in quantum gases.