By means of group theory, topological quantum chemistry, first-principles, and Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb lattice phonon spectra. Taking the pure crystal structure as an input, we show that eleven distinct phases are possible, five of which necessarily have nontrivial topology according to topological quantum chemistry. Another four of them are also identified as topological using Wilson loops in an analytical model that includes all the symmetry allowed force constants up to third-nearest neighbors, making a total of nine topological phases. We then compute the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As, and Sb in this structure and construct its phase diagram. Despite the large proportion of topological phases found in the analytical model, all of the crystals lie in a trivial phase. By analyzing the force constants space using Monte Carlo calculations, we elucidate why topological phonon phases are physically difficult to realize in real materials with this crystal structure.

The exploring of ultrafast demagnetization is a significant subject for understanding the versatility of spin dynamics. However, the laser-induced ultrafast demagnetization process in chiral magnet FeGe has remained elusive. Here, we report the ultrafast demagnetization processes of FeGe film with various magnetic structures using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Our work reveals a transition from one-step ultrafast demagnetization (type I) to two-step demagnetization (type II) process by changing ambient temperature, laser fluence, and magnetic field. We confirm that the transition from one-step to two-step ultrafast demagnetization mainly results from the weak electron-phonon (e-p) coupling. In the skyrmion phase region, TRMOKE curves demonstrate a two-step ultrafast demagnetization process. The two-step ultrafast demagnetization will further enhance the magnetization loss, and consequently facilitate the magnetization switching and pave a pathway to engineer chiral magnetic devices on picosecond timescales.

Vortices are topological defects associated with superfluids and superconductors, which, when mobile, dissipate energy destroying the dissipationless nature of the superfluid. The nature of this “quantum dissipation” is rooted in the quantum physical nature of the problem, which has been the subject of an extensive literature. However, this has mostly been focused on the measures applicable in weakly interacting systems wherein they are tractable via conventional methods. Recently, it became possible to address such dynamical quantum thermalization problems in very strongly interacting systems using the holographic duality discovered in string theory, mapping the quantum problem on a gravitational problem in one higher dimension, having as benefit offering a more general view on how dissipation emerges from such intricate quantum physical circumstances. We study here the elementary problem of a single vortex in two space dimensions, set in motion by a sudden quench in the background superflow formed in a finite-density Reissner-Nordstrom holographic superfluid. This reveals a number of surprising outcomes addressing questions of principle. By fitting the trajectories unambiguously to the Hall-Vinen-Iordanskii phenomenological equation of motion we find that these are characterized by a large inertial mass at low temperature that, however, diminishes upon raising temperature. For a weak drive the drag is found to increase when temperature is lowered, which reveals a simple shear drag associated with the viscous metallic vortex cores, supplemented by a conventional normal fluid component at higher temperatures. For a strong drive we discover a unique dynamical phenomenon: the core of the vortex deforms accompanied by a large increase of the drag force.

Topology in condensed matter systems, inherited from the crystalline symmetry, has aroused great research interest in recent years, yet most studies are focused on the theoretical conceptual perspective and experimental verification. Herein, we present a practical application direction of the topological orders, as the intrinsic properties of the crystalline symmetry, for precise structural differentiation. Based on first-principles calculations, we studied the electronic bands and topological properties of $\alpha$-nitrogen under the long contradictory structures $P{2}_{1}3$ and $Pa\overline{3}$. Their comparable band structures come from their great crystal structure similarity, together leading to the difficulty of precise structural determination from conventional techniques. However, when we look at the intrinsic topology from these two structures, a strong different topological feature is observed between them. A large chiral Fermi arc state is present in the $P{2}_{1}3$ structure, connecting the triple point at $\mathrm{\Gamma }$ and the Dirac point at $R$, whereas there is no such arc state in the $Pa\overline{3}$ structure. Indeed, this Fermi arc state in the $P{2}_{1}3$ structure corresponds to one of two arc states from the $\pm 2$ topological charge points at the $\mathrm{\Gamma }$ and $R$ points, where the triple point and Dirac point do not carry any charge in $Pa\overline{3}$ because of its centrosymmetric symmetry. This distinct topological behavior between these two structures can provide a definitive means for its final structure determination.

Using angle-resolved photoelectron spectroscopy (ARPES), we study the band structures of two-dimensional electron gases (2DEGs) formed at Te-doped surfaces of InAs(110), as a result of band bending. The selected surface system is very clean and stable; therefore, ARPES spectra may be registered in short times, with minimal adsorption of residual gases and no photon beam-induced damage. We record a set of data that allows for a detailed analysis of the 2DEG band shapes, their thermal dependencies, and for an assessment of the many-body interactions. We find the electron-phonon interaction extremally weak and the electron-electron interactions hidden in dominant electron-donor interactions. While 2DEG band shapes are observed to be variable in correlation with the 2DEG density, this is tracked down to the variability of the band-bending potential and nonparabolicity of the system, and not to the many-body interactions. Thus, many-body renormalization of the 2DEG bands is not required, and the bands are described well by using the one-electron theoretical frame.

Recent experimental results suggest that a particular hydrodynamic theory describes charge fluctuations at long wavelengths in the square-lattice Hubbard model. Due to the continuity equation, the correlation functions for the charge and the current are directly connected: the parameters of the effective hydrodynamic model thus determine the optical conductivity. Here we investigate the validity of the proposed hydrodynamic theory in the full range of parameters of the Hubbard model. In the noninteracting case, there is no effective hydrodynamics, and the charge fluctuations present a rich variety of nonuniversal behaviors. At weak coupling, the optical conductivity is consistent with the hydrodynamic theory: at low frequency one observes a Lorentzian-shaped Drude peak, but the high-frequency asymptotics is necessarily different; the high-temperature limit for the product of the two hydrodynamic model parameters is also in agreement with numerical data. At strong coupling, we find that a generalization of the proposed hydrodynamic law is consistent with our quantum Monte Carlo, as well as the finite-temperature Lanczos results from literature. Most importantly, the temperature dependence of the hydrodynamic parameters as well as the dc resistivity are found to be very similar in the weak- and the strong-coupling regimes.

Real-time nonequilibrium Green functions (NEGFs) have been very successfully used to simulate the dynamics of correlated many-particle systems far from equilibrium. However, NEGF simulations are computationally expensive since the effort scales cubically with the simulation duration. Recently, we introduced the G1–G2 scheme that allows for a dramatic reduction to time-linear scaling [N. Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020); J.-P. Joost et al., Phys. Rev. B 101, 245101 (2020)]. Here we tackle another problem: the rapid growth of the computational effort with the system size. In many situations where the system of interest is coupled to a bath, to electric contacts, or to similar macroscopic systems for which a microscopic resolution of the electronic properties is not necessary, efficient simplifications are possible. This is achieved by the introduction of an embedding self-energy—a concept that has been successful in standard NEGF simulations. Here, we demonstrate how the embedding concept can be introduced into the G1–G2 scheme, allowing us to drastically accelerate NEGF embedding simulations. The approach is compatible with all advanced self-energies that can be represented by the G1–G2 scheme [as described in J.-P. Joost et al., Phys. Rev. B 105, 165155 (2022)] and retains the memoryless structure of the equations and their time-linear scaling. As a numerical illustration, we investigate the charge transfer between a Hubbard nanocluster and an additional site which is of relevance for the neutralization of ions in matter.

In comparison to transport of spin polarization in ferromagnets, transport of electric polarization in ferroelectrics remains less explored. Taking an excitonic insulator model of electronic ferroelectricity as a prototypical example, we theoretically investigate the low-energy dynamics and transport of electric polarization by microscopically constructing the Ginzburg-Landau action. We show that, because of the scalar nature of the excitonic order parameter, only the longitudinal fluctuations are relevant to the transport of electric polarization. We also formulate the electric-polarization diffusion equation, in which the electric-polarization current is defined purely electronically without recourse to the lattice degrees of freedom.

Designing novel crystalline materials composed of light and nontoxic elements, but with ultralow thermal conductivity insensitive to temperature, has been a long-standing challenge. One effective strategy is to utilize superatoms as building blocks to introduce hierarchical bonding and vibration for suppressing phonon velocity and enhancing high-order scattering. However, far fewer theoretical efforts have been made to understand the lattice dynamics and thermal transport in superatom-based materials. Herein, different from most of the existing works reported hitherto on atom-superatom hybrid systems, we carry out a comprehensive study on the phonon interaction and thermal transport in superatomic crystal $\mathrm{P}{\mathrm{H}}_4\mathrm{Al}{\mathrm{Br}}_4$ solely consisting of superatoms (superalkali and superhalogen), by employing homogeneous nonequilibrium molecular dynamics with active-learning potential, combined with density functional theory and unified theory of phonon thermal transport. We find that the supersalt $\mathrm{P}{\mathrm{H}}_4\mathrm{Al}{\mathrm{Br}}_4$ crystal exhibits amorphouslike ultralow lattice thermal conductivity of $0.329-0.286\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ from 200 to 600 K. In particular, due to the strong quartic anharmonicity, the contribution of four-phonon scattering reaches 38% of total scattering rates at 300 K, while the phonon coherence's contribution is comparable with that of population, resulting from significant phonon localization. These theoretical findings demonstrate that superatom-assembled materials exhibit features distinguished from atom-based crystals and provide a unique platform for exploring ultralow thermal conductivity with enhanced two-channel thermal transport.

In this paper, to better understand the physical origin of spontaneous out-of-plane (OOP) polarization in the wildly investigated two-dimensional (2D) material monolayer (ML) ${\mathrm{In}}_2{\mathrm{Se}}_3$, we studied its lattice dynamics behaviors via performing a series of first-principles calculations. Cooperating with a fitted phenomenological model, it is clearly shown that, although the depolarization field is nonzero, spontaneous OOP polarization will survive through the anharmonic coupling with other lattices modes. Additionally, our results indicated that the conventional proper ferroelectric (FE) transition of in-plane (IP) polarization plays a role in the primary driving force for the transition in lattice. Tuning the stability of the IP polar modes can be considered an effective pathway of controlling the FE behavior in ML ${\mathrm{In}}_2{\mathrm{Se}}_3$ and related III-VI group 2D materials.

Flat band electronic modes in twisted graphene bilayers are responsible for superconducting and other highly correlated electron-electron phases. Although some hints were known of a possible connection between the quantum Hall effect and zero flat band modes, it was not clear how such connection appears. Here the electronic behavior in twisted bilayer graphene is studied using the chiral model Hamiltonian. As a result, it is proved that for high-order magic angles, the zero flat band modes converge into coherent Landau states with a dispersion ${\sigma }^2=1/3\alpha$, where $\alpha$ is a coupling parameter that incorporates the twist angle and energetic scales. Then it is proved that the square of the Hamiltonian, which is a $2\times 2$ matrix operator, turns out to be equivalent in a first approximation to a two-dimensional quantum harmonic oscillator. The interlayer currents between graphene's bipartite lattices are identified with the angular momentum term while the confinement potential is an effective quadratic potential. By considering the zero-mode equation, the boundary conditions, and a scaling argument, a limiting quantization rule for high-order magic angles is obtained, i.e., ${\alpha }_{m+1}-{\alpha }_m=3/2$, where $m$ is the order of the angle. From there, an equipartition and quantization of the kinetic, confinement, and angular momentum contributions is found. All these results are in very good agreement with numerical calculations.

The honeycomb-lattice cobaltate ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ has recently been proposed to be a proximate Kitaev quantum spin liquid (QSL) candidate. However, non-Kitaev terms in the Hamiltonian lead to a zigzag-type antiferromagnetic (AFM) order at low temperatures. Here, we partially substitute magnetic ${\mathrm{Co}}^{2+}$ with nonmagnetic ${\mathrm{Zn}}^{2+}$ and investigate the chemical doping effect in tuning the magnetic ground states of ${\mathrm{Na}}_3{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{SbO}}_6$. X-ray diffraction characterizations reveal no structural transition but quite tiny changes on the lattice parameters over our substitution range $0\le x\le 0.4$. Magnetic susceptibility and specific-heat results both show that AFM transition temperature is continuously suppressed with increasing Zn content $x$ and neither long-range magnetic order nor spin freezing is observed when $x\ge 0.2$. More importantly, a linear term of the specific heat representing fermionic excitations is captured below 5 K in the magnetically disordered regime as opposed to the $C_{\mathrm{m}}\propto T^3$ behavior expected for bosonic excitations in the AFM state. Based on the data above, we establish a magnetic phase diagram of ${\mathrm{Na}}_3{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{SbO}}_6$. Our results indicate the presence of gapless fractional excitations in the samples with no magnetic order, evidencing a potential QSL state induced by doping in a Kitaev system.

We numerically study the one-dimensional long-range transverse field Ising model (TFIM) in the antiferromagnetic (AFM) regime at zero temperature using generalized Hartree-Fock (GHF) theory. The spin-spin interaction extends to all spins in the lattice and decays as $1/r^{\alpha }$, where $r$ denotes the distance between two spins and $\alpha$ is a tunable exponent. We map the spin operators to Majorana operators and approximate the ground state of the Hamiltonian with a fermionic Gaussian state (FGS). Using this approximation, we calculate the ground-state energy and the entanglement entropy, which allows us to map the phase diagram for different values of $\alpha$. In addition, we compute the scaling behavior of the entanglement entropy with the system size to determine the central charge at criticality for the case of $\alpha >1$. For $\alpha <1$ we find a logarithmic divergence of the entanglement entropy even far away from the critical point, a feature of systems with long-range interactions. We provide a detailed comparison of our results to outcomes of density matrix renormalization group (DMRG) and the linked cluster expansion (LCE) methods. In particular, we find excellent agreement of GHF with DMRG and LCE in the weak long-range regime $\alpha \ge 1$, and qualitative agreement with DMRG in the strong-long range regime $\alpha \le 1$. Our results highlight the power of the computationally efficient GHF method in simulating interacting quantum systems.

Topological matters with lattice disclinations have been widely investigated in Euclidean space. In recent years, exotic topological phases in hyperbolic lattices, which are regular tessellations in the curved space with a constant negative curvature, have been theoretically proposed and experimentally observed, while the investigation of topologically defective hyperbolic lattices is still lacking. Here, we study topological states in the hyperbolic lattice with polygonal defects. It is shown that the topologically one-way boundary state can still exist in the defective hyperbolic Haldane model. Interestingly, we find that only a single bulk site can induce the formation of boundary-dominated one-way propagations in defective hyperbolic lattices. In experiments, we fabricate the defective hyperbolic circuit with a single bulk site to detect the boundary-dominated topological state. Frequency-dependent impedance responses clearly illustrate the existence of nontrivial band gaps and midgap topological boundary states. Furthermore, the backscattering-immune propagation protected by a single bulk site is observed by measuring the dynamics of voltage packet. Our work suggests a useful platform to study topological phases in defective hyperbolic lattices, and may have potential applications in designing high-efficient topological devices with an extremely small bulk region.

Recently, abundant transport phenomena characterizing the surface states of Weyl semimetals (WSMs) have been reported. To generate these phenomena, electrons have to complete a closed intersurface orbit. Due to the unavoidable impurities in real materials, this orbit could be destroyed by the impurity scattering, which limits the detection of the surface states in WSMs. Here, we investigate the RKKY interaction between magnetic impurities, solely mediated by a single surface band, in semi-infinite WSMs. It is found that peculiar oscillations and slowly decaying laws of the RKKY interaction can act as the signals to capture the dispersive nature of the surface states of WSMs. The underlying physics is attributed to two effects: the band-edge effect and the bending effect of the surface band, which can control the RKKY interaction individually or compete with each other to produce more complex magnetic behaviors. In addition, the band-edge effect together with the finite Fermi energy would result in another interesting oscillation with battering pattern. All the results are significantly different from that in previous literatures where surface states have to couple with bulk states (or other surface states of different spins) to generate nonzero magnetic interaction. Compared to the previous models of surface states, the model here is more practical and is helpful for the deeper understanding of the surface magnetic properties in WSMs.

We theoretically study the influence of the $k$-cubic Rashba spin-orbit coupling (SOC) on the correlation effects of magnetic impurities by combining the variational method and the Hirsch-Fye quantum Monte Carlo (HFQMC) simulations. Markedly different from the normal $k$-linear Rashba SOC, even a small cubic Rashba term can greatly alter the band structure and induce a Van Hove singularity in a wide range of energy; thus, the single impurity local moment becomes largely tunable. The cubic Rashba SOC adopted in this paper breaks the rotational symmetry, but the host material is still invariant under the operations ${\mathcal{R}}^z\left(\pi \right), {\mathcal{IR}}^z(\pi /2), {\mathcal{M}}_{xz}, {\mathcal{M}}_{yz}$, where ${\mathcal{R}}^z\left(\theta \right)$ is the rotation of angle $\theta$ about the $z$ axis, $\mathcal{I}$ is the inversion operator, and ${\mathcal{M}}_{xz}$ (${\mathcal{M}}_{yz}$) is the mirror reflection about the $x\text{−}z$ ($y\text{−}z$) principal plane. Saliently, various components of spin-spin correlation between the single magnetic impurity and the conduction electrons show three- or sixfold rotational symmetry. This unique feature is due to the triple winding of the spins with a $2\pi$ rotation of $\mathbf{k}$, which is a hallmark of the cubic Rashba effect, and can possibly be an identifier to distinguish the cubic Rashba SOC from the normal $k$-linear Rashba term in experiments. Although the cubic Rashba term drastically alters the electronic properties of the host, we find that the spatial decay rate of the spin-spin correlation function remains essentially unchanged. Moreover, the carrier-mediated Ruderman-Kittel-Kasuya-Yosida interactions between two magnetic impurities show twisted features, the ferromagnetic diagonal terms dominate when two magnetic impurities are very close, but the off-diagonal terms become important at long distances.

Coherent engineering of landscape potential in crystalline materials is a rapidly evolving research field. Ultrafast optical pulses can manipulate low-frequency shear phonons in van der Waals layered materials through the dynamical dressing of electronic structure and photoexcited carrier density. In this work, we provide a diagrammatic formalism for nonlinear Raman force and implement it to shear phonon dynamics in bilayer graphene. We predict a controllable splitting of double degenerate shear phonon modes due to light-induced phonon mixing and renormalization according to a coherent nonlinear Raman force mechanism. Intriguingly, we obtain a light-induced shear phonon softening that facilitates structural instability at a critical field amplitude for which the shear phonon frequency vanishes. The phonon splitting and instability strongly depend on the laser intensity, frequency, chemical potential, and temperature of photoexcited electrons. This study motivates future experimental investigation of the optical fine tuning and regulation of shear phonons and layer stacking order in layered van der Waals materials.

Spin current generation and manipulation remain the key challenge of spintronics, in which relativistic spin-orbit coupling (SOC) plays a ubiquitous role. In this paper, we demonstrate that hidden Rashba spins in the nonmagnetic, centrosymmetric lattice of multilayer ${\mathrm{SnSe}}_2$ can be efficiently activated by spin-orbit scattering introduced by Se vacancies. Via vacancy scattering, conduction electrons with hidden spin-momentum locked polarizations acquire out-of-plane magnetization components, which effectively break the chiral symmetry between the two Se sublattices of an ${\mathrm{SnSe}}_2$ monolayer when electron spins start precession in the strong built-in Rashba SOC field. The resulting spin separations are manifested in quantum transport as vacancy concentration- and temperature-dependent crossovers from weak antilocalization to weak localization, with the distinctive spin relaxation mechanism of the D'yakonov-Perel' type. In a nonlocal geometry, the generated spins exhibit a long diffusion length exceeding 5 $\mu \mathrm{m}$, when fast momentum scattering protects the effective spin polarizations by driving the random-walk evolution of spin precessions subject to rapidly changing Rashba SOC field. Our study shows the great potential of two-dimensional systems with hidden-spin textures for spintronics.

We investigate the energy spectral phase transitions arising in one-dimensional superlattices under an imaginary gauge field and possessing $M$ sites in each unit cell in the large $M$ limit. It is shown that in models displaying nearly flat bands, a smooth phase transition, from quasi-entirely-real to complex energies, can be observed as the imaginary gauge field is increased, and that the phase transition becomes sharper and sharper (exact) as $M$ is increased. In this limiting case, for superlattices with random or incommensurate disorder, the spectral phase transition corresponds to a localization-delocalization transition of the eigenfunctions within each unit cell, dubbed non-Hermitian delocalization transition and originally predicted by Hatano and Nelson [N. Hatano and D. R. Nelson, Phys. Rev. Lett. 77, 570 (1996)]. However, it is shown here that in superlattices without disorder, a spectral phase transition can be observed as well, which does not correspond to a non-Hermitian delocalization phase transition. The predicted phenomena could be observed in non-Hermitian photonic quantum walks, where synthetic superlattices with controllable $M$ and imaginary gauge fields can be realized with existing experimental apparatus.