Found 30 papers in cond-mat
Date of feed: Mon, 07 Aug 2023 00:30:00 GMT

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Beyond Kitaev physics in strong spin-orbit coupled magnets. (arXiv:2308.01943v1 [cond-mat.str-el])
Ioannis Rousochatzakis, Natalia B. Perkins, Qiang Luo, Hae-Young Kee

We review the recent advances and current challenges in the field of strong spin-orbit coupled Kitaev materials, with a particular emphasis on the physics beyond the exactly-solvable Kitaev spin liquid point. To that end, we give a comprehensive overview of the most relevant exchange interactions in $d^5$ and $d^7$ iridates and similar compounds, an exposition of their microscopic origin, and a systematic attempt to map out the most interesting correlated regimes of the multi-dimensional parameter space, guided by powerful symmetry and duality transformations as well as by insights from wide-ranging analytical and numerical studies. We also survey recent exciting results on quasi-1D models and discuss their relevance to higher-dimensional models. Finally, we highlight some of the key questions in the field as well as future directions.


Experimental observation of exceptional bound states in a classical circuit network. (arXiv:2308.01970v1 [quant-ph])
Deyuan Zou, Tian Chen, Haiyu Meng, Yee Sin Ang, Xiangdong Zhang, Ching Hua Lee

Exceptional bound (EB) states represent an unique new class of robust bound states protected by the defectiveness of non-Hermitian exceptional points. Conceptually distinct from the more well-known topological states and non-Hermitian skin states, they were recently discovered as a novel source of negative entanglement entropy in the quantum entanglement context. Yet, EB states have been physically elusive, being originally interpreted as negative probability eigenstates of the propagator of non-Hermitian Fermi gases. In this work, we show that EB states are in fact far more ubiquitous, also arising robustly in broad classes of systems whether classical or quantum. This hinges crucially on a newly-discovered spectral flow that rigorously justifies the EB nature of small candidate lattice systems. As a highlight, we present their first experimental realization through an electrical circuit, where they manifest as prominent stable resonant voltage profiles. Our work brings a hitherto elusive but fundamentally distinctive quantum phenomenon into the realm of classical metamaterials, and provides a novel pathway for the engineering of robust modes in otherwise sensitive systems.


Can Majorana zero modes in quantum Hall edges survive edge reconstruction?. (arXiv:2308.01980v1 [cond-mat.mes-hall])
Kishore Iyer, Amulya Ratnakar, Sumathi Rao, Sourin Das

Parafermion zero modes can be trapped in the domain walls of quantum Hall edges proximitized by superconductors and ferromagnets. The $\nu = 1/3$ fractional quantum Hall side strip arising due to edge reconstruction of a $\nu = 1$ edge doubles the number of topological sectors such that each of them is $Z_{2} \times Z_{2}$ degenerate. The many-body spectrum displays a $4\pi$ Josephson periodicity, with the states in each $Z_{2}$ being energetically decoupled. Signatures of the new states appear in the fractional Josephson current when the edge velocities are taken to be different.


Unconventional Metallic Magnetism: Non-analyticity and Sign-changing Behavior of Orbital Magnetization in ABC Trilayer Graphene. (arXiv:2308.01996v1 [cond-mat.mes-hall])
Mainak Das, Chunli Huang

We study an unique form of metallic ferromagnetism in which orbital moments surpasses the role of spin moments in shaping the overall magnetization. This system emerges naturally upon doping a topologically non-trivial Chern band in the recently identified quarter metal phase of rhombohedral trilayer graphene. Our comprehensive scan of the density-interlayer potential parameter space reveals an unexpected landscape of orbital magnetization marked by two sign changes and a line of singularities. The sign change originates from an intense Berry curvature concentrated close to the band-edge, and the singularity arises from a topological Lifshitz transition that transform a simply connected Fermi sea into an annular Fermi sea. Importantly, these variations occur while the groundstate order-parameter (i.e.~valley and spin polarization) remains unchanged. This unconventional relationship between the order parameter and magnetization markedly contrasts traditional spin ferromagnets, where spin magnetization is simply proportional to the groundstate spin polarization via the gyromagnetic ratio. We compute energy and magnetization curves as functions of collective valley rotation to shed light on magnetization dynamics and to expand the Stoner-Wohlfarth magnetization reversal model. We provide predictions on the magnetic coercive field that can be readily tested in experiments. Our results challenge established perceptions of magnetism, emphasising the important role of orbital moments in two-dimensional materials such as graphene and transition metal dichalcogenides, and in turn, expand our understanding and potential manipulation of magnetic behaviors in these systems.


The correlated insulators of magic angle twisted bilayer graphene at zero and one quantum of magnetic flux: a tight-binding study. (arXiv:2308.01997v1 [cond-mat.mes-hall])
Miguel Sánchez Sánchez, Tobias Stauber

Magic angle twisted bilayer graphene (MATBG) has become one of the prominent topics in Condensed Matter during the last few years, however, fully atomistic studies of the interacting physics are missing. In this work, we study the correlated insulator states of MATBG in the setting of a tight-binding model, under a perpendicular magnetic field of $0$ and $26.5$ T, corresponding to zero and one quantum of magnetic flux per unit cell. At zero field and for dopings of two holes ($\nu=-2$) or two electrons ($\nu=+2$) per unit cell, the Kramers intervalley coherent (KIVC) order is the ground state at the Hartree-Fock level, although it is stabilized by a different mechanism to that in continuum model. At charge neutrality, the spin polarized state is competitive with the KIVC due to the on-site Hubbard energy. We obtain a strongly electron-hole asymmetric phase diagram with robust insulators for electron filling and metals for negative filling. In the presence of magnetic flux, we predict an insulator with Chern number $-2$ for $\nu=-2$, a spin polarized state at charge neutrality and competing insulators with Chern numbers $+2$ and $0$ at $\nu=+2$. The stability of the $\nu=+2$ insulators is determined by the screening environment, allowing for the possibility of observing a topological phase transition.


Thermal pure matrix product state in two dimensions: tracking thermal equilibrium from paramagnet down to the Kitaev honeycomb spin liquid state. (arXiv:2308.02015v1 [cond-mat.str-el])
Matthias Gohlke, Atsushi Iwaki, Chisa Hotta

We show that the matrix product state (MPS) provides a thermal quantum pure state (TPQ) representation in equilibrium in two spatial dimensions over the whole temperature range. We use the Kitaev honeycomb model as a prominent example hosting a quantum spin liquid (QSL) ground state to target the two specific-heat peaks previously solved nearly exactly using the free Majorana fermionic description. Starting from the high-temperature random state, our TPQ-MPS wrapping the cylinder precisely reproduces these peaks, showing that the quantum many-body description based on spins can still capture the emergent itinerant Majorana fermions in a ${\mathbb Z}_2$ gauge field. The truncation process efficiently discards the high-energy states, eventually reaching the long-range entangled topological state.


Bond dissociation dynamics of single molecules on Ag(111). (arXiv:2308.02091v1 [physics.chem-ph])
Donato Civita, Jutta Schwarz, Stefan Hecht, Leonhard Grill

The breaking of a chemical bond is fundamental in most chemical reactions. To understand chemical processes in heterogeneous catalysis or on-surface polymerization the study of bond dissociation in molecules adsorbed on crystalline surfaces is advantageous. Single molecule studies of bond breaking can give details of the dissociation dynamics, which are challenging to obtain in mole-scale ensemble experiments. Bond breaking in single adsorbed molecules can be triggered using the energy of the tunnelling electrons in a scanning tunnelling microscope (STM) at selected positions to investigate the dissociation dynamics. Single bond dissociation dynamics has been deeply investigated only in small molecules, but not in larger molecules that exhibit distinct rotational degrees of freedom. Here, we use low temperature (7 K) STM to dissociate a single bromine atom from an elongated molecule (dibromo-terfluorene) adsorbed on a Ag(111) surface. This rod-like molecule allows to clearly identify not only displacement of the reaction fragments, but also their rotation. The results show that the molecular fragment binds to the nearest silver atom and only further rotation is allowed. Moreover, the excitation responsible for the bond breaking can propagate through the molecular backbone to dissociate a bromine atom that is not located at the pulse position. These results show the important role of the metal substrate in conditioning the bond dissociation dynamics. Our results might allow to improve the control of the synthesis of 2D materials and targeted engineering of molecular architectures.


Spin-valley locking in Kekul\'e-distorted graphene with Dirac-Rashba interactions. (arXiv:2308.02130v1 [cond-mat.mes-hall])
David A. Ruiz-Tijerina, Jesús Arturo Sánchez-Sánchez, Ramon Carrillo-Bastos, Santiago Galván y García, Francisco Mireles

The joint effects of Kekul\'e lattice distortions and Rashba-type spin-orbit coupling on the electronic properties of graphene are explored. We modeled the position dependence of the Rashba energy term in a manner that allows its seamless integration into the scheme introduced by Gamayun et al.[New J. Phys. 20, 023016 (2018)] to describe graphene with Kekul\'e lattice distortion. Particularly for the Kekul\'e-Y texture, the effective low energy Dirac Hamiltonian contains a new spin-valley locking term, in addition to the well-known Rashba-induced momentum-pseudospin and spin-pseudospin couplings, and the Kekul\'e-induced momentum-valley coupling term. We report on the low-energy band structure and Landau level spectra of Rashba-spin-orbit-coupled Kek-Y graphene, and propose an experimental scheme to discern between the presence of Rashba spin-orbit coupling, Kek-Y lattice distortion, and both, based on doping-dependent magnetotransport measurements.


First-principle study of spin transport property in $L1_0$-FePd(001)/graphene heterojunction. (arXiv:2308.02171v1 [cond-mat.mtrl-sci])
Hayato Adachi, Ryuusuke Endo, Hikari Shinya, Hiroshi Naganuma, Mitsuharu Uemoto

In our previous work, we synthesized a metal/2D material heterointerface consisting of $L1_0$-ordered iron-palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/$m$-Gr/FePd(001), where $m$ represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150-200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in $m$ not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite $\pi$-band-like behavior. Furthermore, we examine the impact of lateral displacements (sliding) at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.


Transport evidence of the three-dimensional Dirac semimetal phase in doped $\alpha$-Sn grown by molecular beam epitaxy. (arXiv:2308.02192v1 [cond-mat.mtrl-sci])
Yuanfeng Ding, Bingxin Li, Chen Li, Yan-Bin Chen, Hong Lu, Yan-Feng Chen

We report the quantum transport properties of the $\alpha$-Sn films grown on CdTe (001) substrates by molecular beam epitaxy. The $\alpha$-Sn films are doped with phosphorus to tune the Fermi level and access the bulk state. Clear Shubnikov-de Haas oscillations can be observed below 30 K and a nontrivial Berry phase has been confirmed. A nearly spherical Fermi surface has been demonstrated by angle-dependent oscillation frequencies. In addition, the sign of negative magnetoresistance which is attributed to the chiral anomaly has also been observed. These results provide strong evidence of the three-dimensional Dirac semimetal phase in $\alpha$-Sn.


Charge State-Dependent Symmetry Breaking of Atomic Defects in Transition Metal Dichalcogenides. (arXiv:2308.02201v1 [cond-mat.mtrl-sci])
Feifei Xiang, Lysander Huberich, Preston A. Vargas, Riccardo Torsi, Jonas Allerbeck, Anne Marie Z. Tan, Chengye Dong, Pascal Ruffieux, Roman Fasel, Oliver Gröning, Yu-Chuan Lin, Richard G. Hennig, Joshua A. Robinson, Bruno Schuler

The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS$_2$ by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). By substrate chemical gating different charge states of sulfur vacancies (Vac$_\text{S}$) and substitutional rhenium dopants (Re$_\text{Mo}$) can be stabilized. Vac$_\text{S}^{-1}$ as well as Re$_\text{Mo}^{0}$ and Re$_\text{Mo}^{-1}$ exhibit local lattice distortions and symmetry-broken defect orbitals attributed to a Jahn-Teller effect (JTE) and pseudo-JTE, respectively. By mapping the electronic and geometric structure of single point defects, we disentangle the effects of spatial averaging, charge multistability, configurational dynamics, and external perturbations that often mask the presence of local symmetry breaking.


Ultrafast nonadiabatic phonon renormalization in photoexcited single-layer MoS$_2$. (arXiv:2308.02207v1 [cond-mat.mtrl-sci])
Nina Girotto, Fabio Caruso, Dino Novko

Comprehending nonequilibrium electron-phonon dynamics at the microscopic level and at the short time scales is one of the main goals in condensed matter physics. Effective temperature models and time-dependent Boltzmann equations are standard techniques for exploring and understanding nonequilibrium state and the corresponding scattering channels. However, these methods consider only the time evolution of carrier occupation function, while the self-consistent phonon dressing in each time instant coming from the nonequilibrium population is ignored, which makes them less suitable for studying ultrafast phenomena where softening of the phonon modes plays an active role. Here, we combine ab-initio time-dependent Boltzmann equations and many-body phonon self-energy calculations to investigate the full momentum- and mode-resolved nonadiabatic phonon renormalization picture in the MoS$_2$ monolayer under nonequilibrium conditions. Our results show that the nonequilibrium state of photoexcited MoS$_2$ is governed by multi-valley topology of valence and conduction bands that brings about characteristic anisotropic electron-phonon thermalization paths and the corresponding phonon renormalization of strongly-coupled modes around high-symmetry points of the Brillouin zone. As the carrier population is thermalized towards its equilibrium state, we track in time the evolution of the remarkable phonon anomalies induced by nonequilibrium and the overall enhancement of the phonon relaxation rates. This work shows potential guidelines to tailor the electron-phonon relaxation channels and control the phonon dynamics under extreme photoexcited conditions.


Thermodynamics of interacting systems: the role of the topology and collective effects. (arXiv:2308.02255v1 [cond-mat.stat-mech])
Iago N. Mamede, Karel Proesmans, Carlos E. Fiore

We will study a class of system composed of interacting quantum dots (QDs) placed in contact with a hot and cold thermal baths subjected to a non-conservative driving worksource. Despite their simplicity, these models showcase an intricate array of phenomena, including pump and heat engine regimes as well as a discontinuous phase transition. We will look at three distinctive topologies: a minimal and beyond minimal (homogeneous and heterogeneous interaction structures). The former case is represented by stark different networks ("all-to-all" interactions and only a central interacting to its neighbors) and present exact solutions, whereas homogeneous and heterogeneous structures have been analyzed by numerical simulations. We find that the topology plays a major role on the thermodynamic performance if the individual energies of the quantum dots are small, in part due to the presence of first-order phase-transitions. If the individual energies are large, the topology is not important and results are well-described by a system with all-to-all interactions.


Disorder-Induced Phase Transitions in Three-Dimensional Chiral Second-Order Topological Insulator. (arXiv:2308.02256v1 [cond-mat.mes-hall])
Yedi Shen, Zeyu Li, Qian Niu, Zhenhua Qiao

Topological insulators have been extended to higher-order versions that possess topological hinge or corner states in lower dimensions. However, their robustness against disorder is still unclear. Here, we theoretically investigate the phase transitions of three-dimensional (3D) chiral second-order topological insulator (SOTI) in the presence of disorders. Our results show that, by increasing disorder strength, the nonzero densities of states of side surface and bulk emerge at critical disorder strengths of $W_{S}$ and $W_{B}$, respectively. The spectral function indicates that the bulk gap is only closed at one of the $R_{4z}\mathcal{T}$-invariant points, i.e., $\Gamma_{3}$. The closing of side surface gap or bulk gap is ascribed to the significant decrease of the elastic mean free time of quasi-particles. Because of the localization of side surface states, we find that the 3D chiral SOTI is robust at an averaged quantized conductance of $2e^{2}/h$ with disorder strength up to $W_{B}$. When the disorder strength is beyond $W_{B}$, the 3D chiral SOTI is then successively driven into two phases, i.e., diffusive metallic phase and Anderson insulating phase. Furthermore, an averaged conductance plateau of $e^{2}/h$ emerges in the diffusive metallic phase.


Topological Mott insulator in odd-integer filled Anderson lattice model with Hatsugai-Kohmoto interactions. (arXiv:2308.02292v1 [cond-mat.str-el])
Krystian Jabłonowski, Jan Skolimowski, Krzysztof Byczuk, Marcin M. Wysokiński

Recently, quantum anomalous Hall state at odd integer filling in moir\'e stacked MoTe$_2$/WSe$_2$ has been convincingly interpreted as a topological Mott insulator state appearing due to strong interactions in {\it band} basis [arXiv:2210.11486]. In this work, we aim to analyze the formation of a topological Mott insulator due to interactions in {\it orbital} basis instead, being more natural for systems where interactions originate from the character of $f$ or $d$ orbitals rather than band flatness. For that reason, we study an odd-integer filled Anderson lattice model incorporating odd-parity hybridization between orbitals with different degrees of correlations introduced in the Hatsugai-Kohmoto spirit. We demonstrate that a topological Mott insulating state can be realized in a considered model only when weak intra- and inter-orbital correlations involving dispersive states are taken into account. Interestingly, we find that all topological transitions between trivial and $\mathbb{Z}_2$ topological Mott insulating phases are not accompanied by a spectral gap closing, consistently with a phenomenon called {\it first-order topological transition}. Instead, they are signaled by a kink developed in spectral function at one of the time reversal invariant momenta. We believe that our approach can provide insightful phenomenology of topological Mott insulators in spin-orbit coupled $f$ or $d$ electron systems.


Quantum transport regimes in quartic dispersion materials with Anderson disorder. (arXiv:2308.02300v1 [cond-mat.dis-nn])
Mustafa Polat, Hazan Özkan, Hâldun Sevinçli

Mexican-hat-shaped quartic dispersion manifests itself in certain families of single-layer twodimensional hexagonal crystals such as compounds of groups III-VI and groups IV-V as well as elemental crystals of group V. Quartic band forms the valence band edge in various of these structures, and some of the experimentally confirmed structures are GaS, GaSe, InSe, SnSb and blue phosphorene. Here, we numerically investigate strictly-one-dimensional (1D) and quasi-one dimensional (Q1D) nanoribbons with quartic dispersion and systematically study the effects of Anderson disorder on their transport properties with the help of a minimal tight-binding model and Landauer formalism. We compare the analytical expression for the scaling function with simulation data to deduce about the domains of diffusion and localization regimes. In 1D, it is shown that conductance drops dramatically at the quartic band edge compared to a quadratic band. As for the Q1D nanoribbons, a set of singularities emerge close to the band edge, which suppress conductance and lead to short mean-free-paths and localization lengths. Interestingly, wider nanoribbons can have shorter mean-free-paths because of denser singularities. However, the localization lengths do not necessarily follow the same trend. The results display the peculiar effects of quartic dispersion on transport in disordered systems.


Topological constraints on the dynamics of vortex formation in a bi-dimensional quantum fluid. (arXiv:2308.02305v1 [physics.optics])
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 that the mere conservation of vorticity impose an elaborate dynamical behavior to the formation and annihilation of vortex/anti-vortex 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 linear systems because it relies on the finite compressibility and on the non-stationnarity of the fluid of light we consider.


Anisotropy of the spin Hall effect in a Dirac ferromagnet. (arXiv:2308.02336v1 [cond-mat.mes-hall])
Guanxiong Qu, Masamitsu Hayashi, Masao Ogata, Junji Fujimoto

We study the intrinsic spin Hall effect of a Dirac Hamiltonian system with ferromagnetic exchange coupling, a minimal model combining relativistic spin-orbit interaction and ferromagnetism. The energy bands of the Dirac Hamiltonian are split after introducing a Stoner-type ferromagnetic ordering which breaks the spherical symmetry of pristine Dirac model. The totally antisymmetric spin Hall conductivity (SHC) tensor becomes axially anisotropic along the direction of external electric field. Interestingly, the anisotropy does not vanish in the asymptotic limit of zero magnetization. We show that the ferromagnetic ordering breaks the spin degeneracy of the eigenfunctions and modifies the selection rules of the interband transitions for the intrinsic spin Hall effect. The difference in the selection rule between the pristine and the ferromagnetic Dirac phases causes the anisotropy of the SHC, resulting in a discontinuity of the SHC as the magnetization, directed orthogonal to the electric field, is reduced to zero in the ferromagnetic Dirac phase and enters the pristine Dirac phase.


Strongly Anisotropic Spin and Orbital Rashba Effect at a Tellurium - Noble Metal Interface. (arXiv:2308.02372v1 [cond-mat.mtrl-sci])
B. Geldiyev, M. Ünzelmann, P. Eck, T. Kißlinger, J. Schusser, T. Figgemeier, P. Kagerer, N. Tezak, M. Krivenkov, A. Varykhalov, A. Fedorov, L. Nicolaï, J. Minár, K. Miyamoto, T. Okuda, K. Shimada, D. Di Sante, G. Sangiovanni, L. Hammer, M. A. Schneider, H. Bentmann, F. Reinert

We study the interplay of lattice, spin and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy (STM) and quantitative low-energy electron diffraction (LEED-IV). Using spin- and angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT), we observe a Te-Au interface state with highly anisotropic Rashba-type spin-orbit splitting at the X point of the Brillouin zone. Based on a profound symmetry and tight-binding analysis, we show how in-plane square lattice symmetry and broken inversion symmetry at the Te-Au interface together enforce a remarkably anisotropic orbital Rashba effect which strongly modulates the spin splitting.


Isolated Majorana mode in a quantum computer from a duality twist. (arXiv:2308.02387v1 [quant-ph])
Sutapa Samanta, Derek S. Wang, Armin Rahmani, Aditi Mitra

Experimental investigation of the interplay of dualities, generalized symmetries, and topological defects is an important challenge in condensed matter physics and quantum materials. A simple model exhibiting this physics is the transverse-field Ising model, which can host a noninvertible topological defect that performs the Kramers-Wannier duality transformation. When acting on one point in space, this duality defect imposes the duality twisted boundary condition and binds a single Majorana zero mode. This Majorana zero mode is unusual as it lacks localized partners and has an infinite lifetime, even in finite systems. Using Floquet driving of a closed Ising chain with a duality defect, we generate this Majorana zero mode in a digital quantum computer. We detect the mode by measuring its associated persistent autocorrelation function using an efficient sampling protocol and a compound strategy for error mitigation. We also show that the Majorana zero mode resides at the domain wall between two regions related by a Kramers-Wannier duality. Finally, we highlight the robustness of the isolated Majorana zero mode to integrability and symmetry-breaking perturbations. Our findings offer an experimental approach to investigating exotic topological defects in Floquet systems.


Effect of thermal fuctuations on the nontrivial topology of the d+id superconducting phase. (arXiv:2308.02402v1 [cond-mat.supr-con])
A.G. Groshev, A.K. Arzhnikov

The behavior of the topological index, characterizing the properties of superconducting phases of quasi-two-dimensional systems with nontrivial topology, is investigated depending on the temperature and parameters of the effective non-Hermitian Hamiltonian. For this purpose, a method of calculating the topological index, based on a self-consistent functional-integral theory, is proposed. The method makes it possible to take into account thermal fluctuations and study the behavior of the topological index as a function of temperature and Hamiltonian parameters. The chiral d+id superconducting phase of a quasi-two-dimensional model with effective attraction between the electrons located at the nearest sites of a triangular lattice is considered. It is shown that the characteristic features in the energy dependence of the self-energy part, which arise when thermal fluctuations are taken into account, have a structure that does not lead to a change in the topological properties of the system. It is found that thermal fluctuations, as well as an increase in effective attraction in this system, contribute to the expansion of the temperature region, in which the value of the topological index is close to the integer C1=-2.


Cooperative quantum phenomena in light-matter platforms. (arXiv:2107.02674v4 [quant-ph] UPDATED)
Michael Reitz, Christian Sommer, Claudiu Genes

Quantum cooperativity is evident in light-matter platforms where quantum emitter ensembles are interfaced with confined optical modes and are coupled via the ubiquitous electromagnetic quantum vacuum. Cooperative effects can find applications, among other areas, in topological quantum optics, in quantum metrology or in quantum information. This tutorial provides a set of theoretical tools to tackle the behavior responsible for the onset of cooperativity by extending open quantum system dynamics methods, such as the master equation and quantum Langevin equations, to electron-photon interactions in strongly coupled and correlated quantum emitter ensembles. The methods are illustrated on a wide range of current research topics such as the design of nanoscale coherent light sources, highly-reflective quantum metasurfaces or low intracavity power superradiant lasers. The analytical approaches are developed for ensembles of identical two-level quantum emitters and then extended to more complex systems where frequency disorder or vibronic couplings are taken into account. The relevance of the approach ranges from atoms in optical lattices to quantum dots or molecular systems in solid-state environments.


Synthetic complex Weyl superconductors, chiral Josephson effect and synthetic half-vortices. (arXiv:2108.08182v2 [cond-mat.supr-con] UPDATED)
Zahra Faraei, S. A. Jafari

We show that the most generic form of spin-singlet superconducting order parameter for chiral fermions is of the $\Delta_s+i\gamma^5\Delta_5$ where $\Delta_s$ is the usual order parameter and $\Delta_5$ is the pseudo-scalar order parameter. After factoring out the $U(1)$ phase $e^{i\phi}$, this form of superconductivity admits yet additional complex structure in the plane of $(\Delta_s,\Delta_5)$. The polar angle $\chi$ in this plane dubbed chiral angle will be locked to the $U(1)$ phase $\phi$. We propose a synthetic setup based on stacking of topological insulators (TIs) and superconductors (SCs). Alternatively flux biasing the superconductors with a fluxes $\pm\Phi$ leads to $\Delta_5=\Delta_0 \sin(\chi)$, where $\Delta_0$ is the superconducting order parameter of the SC layers, and the chiral angle $\chi=\Phi/\Phi_0$ is directly given by the flux $\Phi$ in units of the flux quantum $\Phi_0=h/(2e)$. This can be used as a building block to construct a two-dimensional Josephson array. In this setup $\chi$ will be a background field defining a pseudoscalar $\Delta_5$ that can be tuned to desired configuration. While in a uniform background field $\Delta_5$ the dynamics of $\phi$ is given by standard XY model and its associated vortices, a {\em staggered} background $\pm\Delta_5$ (or equivalently $\chi$ and $\chi+\pi$ in alternating lattice sites) creates a new set of minima for the $\phi$ field that will support half-vortex excitations. An isolated single synthetic "half-vortex" in the $\chi$ field in an otherwise uniform background will bind a $\phi$-half-vortex. This is similar to the way a p-wave superconducting vortex core binds a Majorana fermion.


Bloch Oscillations Along a Synthetic Dimension of Atomic Trap States. (arXiv:2112.10648v2 [cond-mat.quant-gas] UPDATED)
Christopher Oliver, Aaron Smith, Thomas Easton, Grazia Salerno, Vera Guarrera, Nathan Goldman, Giovanni Barontini, Hannah M. Price

Synthetic dimensions provide a powerful approach for simulating condensed matter physics in cold atoms and photonics, whereby a set of discrete degrees of freedom are coupled together and re-interpreted as lattice sites along an artificial spatial dimension. However, atomic experimental realisations have been limited so far by the number of artificial lattice sites that can be feasibly coupled along the synthetic dimension. Here, we experimentally realise for the first time a very long and controllable synthetic dimension of atomic harmonic trap states. To create this, we couple trap states by dynamically modulating the trapping potential of the atomic cloud with patterned light. By controlling the detuning between the frequency of the driving potential and the trapping frequency, we implement a controllable force in the synthetic dimension. This induces Bloch oscillations in which atoms move periodically up and down tens of atomic trap states. We experimentally observe the key characteristics of this behaviour in the real space dynamics of the cloud, and verify our observations with numerical simulations and semiclassical theory. This experiment provides an intuitive approach for the manipulation and control of highly-excited trap states, and sets the stage for the future exploration of topological physics in higher dimensions.


Grain Boundary Effects in Dealloying Metals: A Multi-Phase Field Study. (arXiv:2206.01234v2 [cond-mat.mtrl-sci] UPDATED)
Nathan Bieberdorf, Mark D. Asta, Laurent Capolungo

A multi-phase field model is employed to study the microstructural evolution of an alloy undergoing liquid dealloying. The model proposed extends upon the original approach of Geslin et al. to consider dealloying in the presence of grain boundaries. The model is implemented using a semi-implicit time stepping algorithm using spectral methods, which enables simulating large 2D and 3D domains over long time-scales while still maintaining a realistic interfacial thickness. The model is exercised to demonstrate a mechanism of coupled grain-boundary migration to maintain equilibrium contact angles with this topologically-complex solid-liquid interface during dealloying. This mechanism locally accelerates dealloying by dissolving the less noble alloy metal from (and rejecting the more noble metal into) the migrating grain boundary, thereby enhancing the diffusion-coupled-growth of the liquid channel into the precursor. The deeper corrosion channel at the migrating grain boundary asymmetrically disrupts the ligament connectivity of the final dealloyed structure, in qualitative agreement with published experimental observations. It is shown that these grain boundary migration-assisted corrosion channels form even for precursors with small amounts of the dissolving alloy species, below the so-called \textit{parting limit}


Synergistic correlated states and nontrivial topology in coupled graphene-insulator heterostructures. (arXiv:2206.05659v4 [cond-mat.mes-hall] UPDATED)
Xin Lu, Shihao Zhang, Yaning Wang, Xiang Gao, Kaining Yang, Zhongqing Guo, Yuchen Gao, Yu Ye, Zheng Vitto Han, Jianpeng Liu

In this work, we study the synergistic correlated states in two distinct types of interacting electronic systems coupled by interlayer Coulomb interactions. We propose that this scenario can be realized in a type of Coulomb-coupled graphene-insulator heterostructures with gate tunable band alignment. We find that, by virtue of the interlayer Coulomb coupling between the interacting electrons in the two layers, electronic states that cannot be revealed in either individual layer would emerge in a cooperative and synergistic manner. Specifically, as a result of the band alignment, charge carriers can be transferred between graphene and the substrate under the control of gate voltages, which can yield a long-wavelength electronic crystal at the surface of the substrate. This electronic crystal exerts a superlattice Coulomb potential on the Dirac electrons in graphene, which generates subbands with reduced non-interacting Fermi velocity. As a result, $e$-$e$ Coulomb interactions within graphene would play a more important role, giving rise to a gapped Dirac state at the charge neutrality point, accompanied by interaction-enhanced Fermi velocity. Moreover, the superlattice potential can give rise to topologically nontrivial subband structures which are tunable by superlattice's constant and anisotropy. Reciprocally, the electronic crystal formed in the substrate can be substantially stabilized in such coupled bilayer heterostructure by virtue of the cooperative interlayer Coulomb coupling. We further perform high-throughput first principles calculations to identify a number of promising insulating materials as candidate substrates for graphene to demonstrate these effects.


Prediction of a novel type-I antiferromagnetic Weyl semimetal. (arXiv:2208.11412v2 [cond-mat.mtrl-sci] UPDATED)
Davide Grassano, Luca Binci, Nicola Marzari

Topological materials have been a main focus of studies in the past decade due to their protected properties that can be exploited for the fabrication of new devices. Among them, Weyl semimetals are a class of topological semimetals with non-trivial linear band crossing close to the Fermi level. The existence of such crossings requires the breaking of either time-reversal or inversion symmetry and is responsible for the exotic physical properties.

In this work we identify the full-Heusler compound InMnTi$_2$, as a promising, easy to synthesize, $T$- and $I$-breaking Weyl semimetal. This material exhibits several features that are comparatively more intriguing with respect to other known Weyl semimetals: the distance between two neighboring nodes is large enough to observe a wide range of linear dispersions in the bands, and only one kind of such node's pairs is present in the Brillouin zone. We also show the presence of Fermi arcs stable across a wide range of chemical potentials. Finally, the lack of contributions from trivial points to the low-energy properties makes the materials a promising candidate for practical devices.


Mixed singlet-triplet superconducting state within the moir\'e $t$-$J$-$U$ model as applied to the description of twisted WSe$_2$ bilayer. (arXiv:2302.13841v3 [cond-mat.str-el] UPDATED)
M. Zegrodnik, A. Biborski

We analyze an analog of the $t$-$J$-$U$ model as applied to the description of a single moir\'e flat band of twisted WSe$_2$ bilayer. To take into account the correlation effects induced by a significant strength of the Coulomb repulsion, we use the Gutzwiller approach and compare it with the results obtained by the Hartree-Fock method. We discuss in detail the graduate appearance of a two dome structure of the superconducting state in the phase diagram by systematically increasing the Coulomb repulsion integral, $U$. The two superconducting domes residing on both sides of a Mott insulating state can be reproduced for a realistic parameter range in agreement with the available experimental data. According to our analysis the paired state has a highly unconventional character with a mixed $d+id$ (singlet) and $p-ip$ (triplet) symmetry. Both components of the mixed paired state are of comparable amplitudes. However, as shown here, a transition between pure singlet and pure triplet pairing should be possible in the considered system by tuning the gate voltage, which controls the magnitude of the valley-dependent spin-splitting in the system.


First order phase transitions within Weyl type of materials at low temperatures. (arXiv:2305.09007v2 [cond-mat.str-el] UPDATED)
Y. M. P. Gomes, Everlyn Martins, Marcus Benghi Pinto, Rudnei O. Ramos

We analyze the possible dynamical chiral symmetry breaking patterns taking place within Weyl type of materials. Here, these systems are modeled by the (2+1)-dimensional Gross-Neveu model with a tilt in the Dirac cone. The optimized perturbation theory (OPT) is employed in order to evaluate the effective potential at finite temperatures and chemical potentials beyond the traditional large-$N$ limit. The nonperturbative finite-$N$ corrections generated by the OPT method and its associated variational procedure show that a first-order phase transition boundary, missed at large $N$, exists in the regime of low temperatures and large chemical potentials. This result, which represents our main finding, implies that one should hit a region of mixed phases when exploring the low-temperature range. The associated first order transition line, which starts at $T=0$, terminates at a tricritical point such that the transitions taking place at high $T$ are of the second kind. In particular, we discuss how the tilt in the Dirac cone affects the position of the tricritical point as well as the values of critical temperature and coexistence chemical potential among other quantities. Some experimental implications and predictions are also briefly discussed.


Spin Space Groups: Full Classification and Applications. (arXiv:2307.10364v2 [cond-mat.mes-hall] UPDATED)
Zhenyu Xiao, Jianzhou Zhao, Yanqi Li, Ryuichi Shindou, Zhi-Da Song

In this work, we exhaust all the spin-space symmetries, which fully characterize collinear, non-collinear, commensurate, and incommensurate spiral magnetism, and investigate enriched features of electronic bands that respect these symmetries. We achieve this by systematically classifying the so-called spin space groups (SSGs) - joint symmetry groups of spatial and spin operations that leave the magnetic structure unchanged. Generally speaking, they are accurate (approximate) symmetries in systems where spin-orbit coupling (SOC) is negligible (finite but weaker than the interested energy scale); but we also show that specific SSGs could remain valid even in the presence of a strong SOC. By representing the SSGs as O($N$) representations, we - for the first time - obtain the complete classifications of 1421, 9542, and 56512 distinct SSGs for collinear ($N=1$), coplanar ($N=2$), and non-coplanar ($N=3$) magnetism, respectively. SSG not only fully characterizes the symmetry of spin d.o.f., but also gives rise to exotic electronic states, which, in general, form projective representations of magnetic space groups (MSGs). Surprisingly, electronic bands in SSGs exhibit features never seen in MSGs, such as nonsymmorphic SSG Brillouin zone (BZ), where SSG operations behave as glide or screw when act on momentum and unconventional spin-momentum locking, which is completely determined by SSG, independent of Hamiltonian details. To apply our theory, we identify the SSG for each of the 1604 published magnetic structures in the MAGNDATA database on the Bilbao Crystallographic Server. Material examples exhibiting aforementioned novel features are discussed with emphasis. We also investigate new types of SSG-protected topological electronic states that are unprecedented in MSGs.