${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is a promising representative of intrinsic antiferromagnetic topological insulators, which could enable rare quantum mechanical effects like the quantum anomalous Hall effect. Especially at low temperatures, numerous studies have been reported, demonstrating the great potential of this compound in the magnetically ordered state below $T_{\mathrm{N}}$. Among recent findings, the alloy compound $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ has been suggested to be an interesting candidate for the realization of an ideal Weyl semimetal state. By exchanging Bi by Sb, the electronic structure is influenced in terms of a shift of the Fermi energy, and a decrease in the energy gap has been predicted. In this work, we investigate and compare the optical conductivity of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ single crystals with various Sb doping levels $x$ by infrared reflectivity measurements. We observe a big impact of the Sb content on the low-energy excitations characterizing the metallic state of our samples. Different $x=0.26$ crystals show significant differences in their optical response and also a strong position dependence. The findings are compared to the recently reported optical conductivity spectrum of the pure compound ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.

The second-order topological insulator, which has $(d-2)$-dimensional topological hinge or corner states, has been observed in three-dimensional materials, but has yet not been observed in a two-dimensional system. In this work we theoretically propose the realization of a second-order topological insulator in the van der Waals heterostructure of ${\mathrm{CoBr}}_2/{\mathrm{Pt}}_2{\mathrm{HgSe}}_3/{\mathrm{CoBr}}_2$. ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ is a large gap ${\mathbb{Z}}_2$ topological insulator. With an in-plane exchange field from neighboring ${\mathrm{CoBr}}_2$, a large global band gap of above 70 meV opens up at the edge. The corner states, which are robust against edge disorders and irregular shapes, are confirmed in the nanoflake. We further show that the second-order topological states can also be realized in the heterostructure of the jacut- ingaite family ${\mathbb{Z}}_2$ topological insulators. We believe that our work will be beneficial for the experimental realization of second-order topological insulators in van der Waals layered materials.

We have calculated the optical conductivity of monolayer ${\beta }_{12}$-borophene in the presence of an inversion symmetry breaking (ISB) field. We theoretically formulate such a field to effectively be induced by a substrate through an orbital hybridization effect in the experimental growth process. In the electronic band structure, the effect of the ISB field on the anisotropic dispersion appears as a perturbed manipulation of the energy scale. Different orientations of dispersion of triplet and Dirac fermions in ${\beta }_{12}$-borophene cause anisotropic optical interband transitions. Simulated ISB-induced optical conductivity using the Kubo formula produces various optical redshift and blueshift spectra. The longitudinal (Hall) component of optical conductivity presents a pure redshift (blueshift) spectrum with the ISB field, whereas the coexistence of both redshift and blueshift spectra emerge for the transverse component. This paper is a handful for practical applications in two-dimensional-based optoelectronic devices.

Exceptional points (EPs) of both eigenvalue and eigenvector degeneracy offer remarkable properties of the non-Hermitian systems based on the Jordanian form of Hamiltonians at EPs. Here we propose the perturbation theory able to underpin the physics in the vicinity of the higher-order EPs. The perturbation theory unveils lifting of degeneracy and origin of the different phases merging at the EP. It allows us to analyze the photonic local density of states and resonance energy transfer, determining their spectral behaviors in a general form. Resonant energy transfer is investigated in analytical and numerical examples. We analytically find the resonance energy transfer rate near the third-order EP occurring in the system of three coupled cavities and reveal singularities caused by the interplay of the perturbation and frequency detuning from degenerate eigenfrequency. Numerical simulation of the coupled-resonator system reveals the vital role of a mirror for switching to the EP of the doubled order and corresponding enhancement of the resonance energy transfer rate. Our investigation sheds light on the behavior of nanophotonic systems in non-Hermitian environments.

We consider a laterally confined two-dimensional electron gas (2DEG), placed inside a gyrotropic cavity. Splitting of the circularly polarized electromagnetic modes leads to the emergence of the ground-state spontaneous magnetization, anomalous Hall effect and chiral edge currents in 2DEG. We examine the dependence of the magnetization and edge current density on the system size for two particular cases of the confining potential: infinite wall and parabolic potentials. We show that paramagnetic and diamagnetic contributions to the edge currents have qualitatively different dependences on the system size. For sufficiently large systems the emergent magnetic moment scales with the number of particles and takes a universal value independent of the system shape, the form of confining potential, and the presence of interparticle interactions. These findings pave the route to the design quantum electrodynamic engineering of the material properties of the mesoscopic electron systems.

A candidate material for strongly correlated topological materials, ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ has attracted much attention, but its ground state remains controversial. Compared with the typical Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Pt}}_3{\mathrm{Bi}}_4$, two possibilities of ground states are proposed: ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ is either a spin–orbit-driven topological semimetal or a Kondo insulator with less Kondo coupling strength than platinum. Here, we performed density functional theory $\left(\mathrm{DFT}\right)+\mathrm{dynamical}$ mean field theory (DMFT) calculations on ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ under different pressures to clarify its ground state, as pressure can tune the strength of Kondo coupling without affecting the strength of spin-orbit coupling. We found ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ has a metallic ground state and becomes insulating with increasing pressure at a low temperature. And as the pressure increased to 2 GPa, a hybridization energy gap can be observed at 10 K. As the pressure increased to 5 GPa, the electronic structure of ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ is even similar to that of the Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Pt}}_3{\mathrm{Bi}}_4$ under ambient pressure, and a clear hybridization energy gap $(\sim 3\mathrm{meV})$ appears at 20 K. Our results not only demonstrate that the key factor controlling the different ground states between the two compounds is Kondo physics, rather than spin-orbit coupling, but also confirm that ${\mathrm{Ce}}_3{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ is an ideal material to tuning the ground states by changing the strength of hybridization by pressure.

We study the Coulomb drag between two strange-metal layers using the Einstein-Maxwell-Dilaton model from holography. We show that the low-temperature dependence of the drag resistivity is ${\rho }_D\propto T^4$, which strongly deviates from the quadratic dependence of Fermi liquids. We also present numerical results at room temperature, using typical parameters of the cuprates, to provide an estimate of the magnitude of this effect for future experiments. We find that the drag resistivity is enhanced by the plasmons characteristic of the two-layer system.

The antiferromagnetic ordering in Mott insulators upon lowering the temperature is accompanied by a transfer of the single-particle spectral weight to lower energies and a shift of the Mott gap to higher energies (magnetic blueshift, MBS). The MBS is governed by the double exchange and the exchange mechanisms. Both mechanisms enhance the MBS upon increasing the number of orbitals. By performing a polynomial fit to numerical dynamical mean-field theory data we provide an expansion for the MBS in terms of hopping and exchange coupling of a prototype Hubbard-Kondo-Heisenberg model and discuss how the results can be generalized for application to realistic Mott or charge-transfer insulator materials. This allows the estimation of the MBS of the charge gap in real materials in an extremely simple way that avoids extensive theoretical calculations. The approach is exemplarily applied to $\alpha$-MnTe, NiO, and ${\mathrm{BiFeO}}_3$ and a MBS of about 130, 360, and 157 meV is found, respectively. The values are compared with the previous theoretical calculations and the available experimental data. Our ready-to-use formula for the MBS will simplify future studies searching for materials with a strong coupling between the antiferromagnetic ordering and the charge excitations, which is paramount to realizing a coupled spin-charge coherent dynamics on a femtosecond timescale.

The response to compression of the clathrate type-II structure Ge(cF136) is investigated by means of ab initio small-cell metadynamics at different temperatures and pressures. Amorphous intermediates obtained from the collapse of ${\mathrm{Ge}}_{136}$ rapidly crystallize into different end products, depending on the choice of thermodynamic parameters. At lower pressure $p=2.5$ GPa the metastable metallic bct-5 phase competes against $\beta$-Sn [Ge(tI4)], which forms at higher pressures. Upon lowering the temperature, amorphous intermediates are instrumental to the formation of denser structural motifs, from which metallic bct-5 can form. Therein, anisotropic box fluctuations promote phase formation. The metadynamics runs are analyzed in depth using a set of topological descriptors, including coordination sequence and ring statistics. Differences in the structural landscape history and amorphous intermediates are critical for the selective formation of particular metastable polymorphs. Their understanding opens possibilities towards turning crystal structure predictions into actual materials, via identifying viable kinetic routes.

We study the effects of electron-electron interactions on the charge excitation spectrum of the spinful Su-Schrieffer-Heeger (SSH) model, a prototype of a one-dimensional bulk obstructed topological insulator. In view of recent progress in the fabrication of dopant-based quantum simulators we focus on experimentally detectable signatures of interacting topology in finite lattices. To this end we use Lanczos-based exact diagonalization to calculate the single-particle spectral function in real space which generalizes the local density of states to interacting systems. Its spatial and spectral resolution allows for the direct investigation and identification of edge states. By studying the noninteracting limit, we demonstrate that the topological in-gap states on the boundary are robust against both finite-size effects as well as random bond and onsite disorder which suggests the feasibility of simulating the SSH model in engineered dopant arrays in silicon. While edge excitations become zero-energy spinlike for any finite interaction strength, our analysis of the spectral function shows that the single-particle charge excitations are gapped out on the boundary. Despite the loss of topological protection, we find that these edge excitations are quasiparticlelike as long as they remain within the bulk gap. Above a critical interaction strength of $U_c\approx 5t$ these quasiparticles on the boundary lose their coherence which is explained by the merging of edge and bulk states. This is in contrast to the many-body edge excitations which survive the limit of strong coupling, as established in the literature. Our findings show that for moderate repulsive interactions the nontrivial phase of the interacting SSH model can be detected through remnant signatures of topological single-particle states using single-particle local measurement techniques such as scanning tunneling spectroscopy.

Monte Carlo simulations of hybrid quantum-classical models such as the double exchange Hamiltonian require calculating the density of states of the quantum degrees of freedom at every step. Unfortunately, the computational complexity of exact diagonalization grows as a function of the system's size $N$, making it prohibitively expensive for any realistic system. We consider leveraging data-driven methods, namely, neural networks, to replace the exact diagonalization step in order to speed up sample generation. We explore a model that learns the free energy for each spin configuration and a second one that learns the Hamiltonian's eigenvalues. We implement data augmentation by taking advantage of the Hamiltonian's symmetries to artificially enlarge our training set and benchmark the different models by evaluating several thermodynamic quantities. While all models considered here perform exceedingly well in the one-dimensional case, only the neural network that outputs the eigenvalues is able to capture the right behavior in two dimensions. The simplicity of the architecture we use in conjunction with the model agnostic form of the neural networks can enable fast sample generation without the need of a researcher's intervention.

Simulating models for quantum correlated matter unveils the inherent limitations of deterministic classical computations. In particular, in the case of quantum Monte Carlo methods, this is manifested by the emergence of negative weight configurations in the sampling, that is, the sign problem (SP). There have been several recent calculations which exploit the SP to locate underlying critical behavior. Here, utilizing a metric that quantifies phase-space ergodicity in such sampling, the Hamming distance, we suggest a significant advance on these ideas to extract the location of quantum critical points in various fermionic models, in spite of the presence of a severe SP. Combined with other methods, exact diagonalization in our case, it elucidates both the nature of the different phases as well as their location, as we demonstrate explicitly for the honeycomb and triangular Hubbard models, in both their U(1) and SU(2) forms. Our approach exemplifies a possible path allowing the exploration of the phase diagram of a variety of fermionic quantum models hitherto considered to be impractical via quantum Monte Carlo simulations.

The structure and chemical state of a boron chain, isoelectronic to polycumulene, were obtained with a combination of positron diffraction, photoelectron spectroscopy, and first-principles calculations. In the present work, an array of boron chains was found in a copper boride layer on a metal crystal substrate. The band structure of this material was determined to be electronically doped and hybridized by neighboring copper atoms. These boron atom chains exhibit a unique electrophilic character that is specific to the element. The properties of boron chains in the copper boride layer and the substrate effect are systematically investigated together.

Within the framework of the time-dependent Ginzburg-Landau theory we show that circularly polarized terahertz or far-infrared radiation induces a dc supercurrent that influences the dynamics of vortex-antivortex pair formation in a mesoscopic superconductor undergoing rapid thermal quenching. The Lorentz force arising from the supercurrents promotes vortex-antivortex separation and allows survival of the vortex polarity defined by the helicity of light. Based on this idea, we propose a two-stage irradiation scheme that provides a powerful method for controlled all-optical generation of Abrikosov vortices.

Different topological phases of quantum systems have become areas of increased focus in recent decades. In particular, the question of how to realize and manipulate systems with a nontrivial first Chern number is pursued both experimentally and theoretically. Here we go beyond typical spin-1/2 systems and consider both single and coupled spin-1 systems as a means of realizing higher first Chern numbers and studying the emergence of different topological phases. We show that rich topological phase diagrams can be realized by coupling two spin-1 systems using both numerical and analytical methods. Furthermore, we consider a concrete realization of spin-1 systems using a superconducting circuit. This realization includes nonstandard spin-spin interaction terms that may endanger the topological properties. We argue, however, that the realistic circuit Hamiltonian including all terms should be expected to show the rich phase structure as well. This puts our theoretical predictions within reach of state-of-the-art experimental setups.

We study the efficiencies of dampinglike and fieldlike torques in Nb/Ni and Ta/Ni bilayers and compare them with $\mathrm{Nb}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$(Py) and Ta/Py as a function of the thickness of a ferromagnet. In both these systems, we observe that the sign of the dampinglike torque is negative for Py whereas it is positive for nickel. The negative sign of the dampinglike torque in Nb/Py and Ta/Py is consistent with the negative sign of the spin Hall effect in niobium and tantalum. The sign reversal of the dampinglike torque in Nb/Ni and Ta/Ni suggests a strong generation of the orbital current from niobium and tantalum, which is converted into the spin current inside nickel and produces the orbital torque. We also find that both the dampinglike and fieldlike efficiencies change signs when nickel is replaced by Py.

The Dzyaloshinskii-Moriya interaction (DMI) is a key factor in chiral spin textures and has great potential in spintronics-based applications. The asymmetric bubble domain expansion under an in-plane magnetic field is a common method for DMI evaluation. However, abnormal bubble expansions occur occasionally in the creep regime, which may lead to deviations in the DMI evaluation, implying various physical phenomena in domain wall (DW) kinetics. In this study, DMI evaluation of ferrimagnetic Pt/Co/Tb trilayers was conducted via DW motion in the creep and flow regimes, where the in-plane fields of minimum DW velocities are significantly different for 1.5 nm Co. The accuracy of the extracted DMI effective field in the flow regime was confirmed by Brillouin light scattering measurements, where the DMI coefficient was approximately $1\mathrm{mJ}/{\mathrm{m}}^2$. In the creep regime, a typical flattened bubble shape was observed; accordingly, the dominant direction of the DW motion velocity changes with increasing the in-plane magnetic fields. Based on the bubble morphology and DW anisotropy energy analyses, we attributed the bubble expansion phenomena in the creep regime to the vertical Bloch lines. Our study provides alternatives for a better understanding of DW kinetics in DMI evaluations, which is essential for spin-texture applications.

Antiferromagnetic manganese-based nitride antiperovskites, such as ${\mathrm{Mn}}_3\mathrm{NiN}$, hold a triangular frustrated magnetic ordering, thanks to their kagome lattice formed by the Mn atoms along the (111) plane. As such, the magnetic frustration imposes a nontrivial interplay between the symmetric and asymmetric magnetic interactions, which can only reach equilibrium in a noncollinear magnetic configuration. Consequently, the associated electronic interactions and their possible tuning by external constraints, such as applied epitaxial strain, play a crucial role in defining the microscopic and macroscopic properties of such topological condensed matter systems. In this paper, we explored and explained the effect of the epitaxial strain imposed within the (111) plane, in which the magnetic and crystallographic symmetry operations are kept fixed, and only the magnitude of the ionic and electronic interactions are tuned. We found a tangible enhancement in the anomalous Hall conductivity along the (111) plane (${\sigma }_{111}^{\mathrm{AHE}}$) for compression values, whereas, for tension, the AHC is dramatically reduced. As such, the ${\sigma }_{111}^{\mathrm{AHE}}$ component fetches a maximum increase of 26%, with respect to the unstrained structure, for a compression value close to $-1.5\%$. Our findings indicate a distinct correlation between the anomalous Hall conductivity and the Berry curvature along the (111) plane as a function of the strain. Here, the nondivergent Berry curvature acts as the source and the strain as the control mechanism of this anomalous transport phenomenon.

Weyl semimetals are well known for their anomalous transport effects caused by a large fictitious magnetic field generated by the nonzero Berry curvature. We performed the analysis of the electrical transport measurements of the magnetic Weyl semimetal ${\mathrm{Mn}}_3\mathrm{Ge}$ in the a-b and a-c planes. We have observed negative longitudinal magnetoresistance (LMR) at a low magnetic field ($B<1.5$ T) along all the axes. The high-field LMR shows different behavior along the $x$ and $z$ axes. A similar trend has been observed in the case of planar Hall effect (PHE) measurements as well. The nature of high-field LMR along the $x$ axis changes near 200 K. The dominant carrier concentration type and metallic to semimetallic transition also occur near 200 K. These observations suggest two main conclusions: (i) the high-field LMR in ${\mathrm{Mn}}_3\mathrm{Ge}$ is driven by the metallic-semimetallic nature of the compound and (ii) ${\mathrm{Mn}}_3\mathrm{Ge}$ compound goes through an electronic band topological transition near 200 K. Single-crystal neutron diffraction does not show any change in the magnetic structure below 300 K. However, the in-plane lattice parameter ($a$) shows a maximum near 230 K. Therefore it is possible that the change in electronic band structure near 200 K is driven by the $a$ lattice parameter of the compound.

Weyl semimetals and nodal line semimetals are characterized by linear electronic bands touching at zero-dimensional points and one-dimensional lines, respectively. Recently, it has been predicted that nodal line semimetals can be driven into tunable Floquet-Weyl semimetals by circularly polarized light. Here we study the occurrence of interface states between two regions of a nodal line semimetal illuminated by two beams of light with opposite circular polarizations. Within a minimal model we find remarkable modifications of the energy structure by tuning the polarized light, such as the possible generation of Van Hove singularities. Moreover, by adding a $\delta$ doping of magnetic impurities along the interfacial plane, we show the occurrence of a switchable and topologically nontrivial, vortexlike pseudospin pattern of the interface states.