The vibrational properties of ferrimagnetic ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ single crystals are investigated using Raman spectroscopy and density functional theory calculations. Eighteen Raman-active modes are identified, 14 of which are assigned according to the trigonal symmetry. Four additional peaks, obeying the $A_{1g}$ selection rules, are attributed to the overtones. The unconventional temperature evolution of the $A_{1g}^5$ mode self-energy suggests a competition between different short-range magnetic correlations that significantly impact the spin-phonon interaction in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. The research provides comprehensive insight into the lattice properties, studies their temperature dependence, and shows arguments for the existence of competing short-range magnetic phases in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$.

We report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Due to the ${180}^{\circ }$ Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the $d$-level inversion observed in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is absent in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ are substantially different from those of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

We develop a method of variational optimization of the infinite projected entangled pair states on the honeycomb lattice. The method is based on the automatic differentiation of the honeycomb-lattice corner transfer matrix renormalization group. We apply the approach to the antiferromagnetic Heisenberg spin-1/2 and ferromagnetic Kitaev models on the honeycomb lattice. The developed formalism gives quantitatively accurate results for the main physical observables and has a necessary potential for further extensions.

We report neutron-diffraction results on single-crystal ${\mathrm{CaMn}}_2{\mathrm{P}}_2$ containing corrugated Mn honeycomb layers, and we determine its ground-state magnetic structure. The diffraction patterns consist of prominent (1/6,1/6,$L$) reciprocal-lattice unit (r.l.u.; $L$ = integer) magnetic Bragg reflections, whose temperature-dependent intensities are consistent with a first-order antiferromagnetic phase transition at the Néel temperature $T_{\mathrm{N}}=70\left(1\right)$ K. Our analysis of the diffraction patterns reveals an in-plane $6\times 6$ magnetic unit cell with ordered spins that in the principal-axis directions rotate by ${60}^{\circ }$ steps between nearest neighbors on each sublattice that forms the honeycomb structure, consistent with the $P_{A}c$ magnetic space group. We find that a few other magnetic subgroup symmetries ($P_{A}2/c$, $P_{C}2/m$, $P_S\overline{1},P_{C}2,P_{C}m,P_{S}1$) of the paramagnetic $P\overline{3}m{11}^{\prime }$ crystal symmetry are consistent with the observed diffraction pattern. We relate our findings to frustrated $J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg honeycomb antiferromagnets with single-ion anisotropy and the emergence of Potts nematicity.

We study the temperature dependence of spin-Hall magnetoresistance (SMR) in antiferromagnetic-insulator/metal bilayer systems. We calculate the amplitude of the SMR signal by using a quantum Monte Carlo simulation and examine how the SMR depends on the amplitude of the spin, thickness of the antiferromagnetic-insulator layer, and randomness of the exchange interactions. Our results for simple quantum spin models provide a useful starting point for understanding SMR measurements on atomic layers of magnetic compounds.

A prototype of the monoaxial chiral magnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ exhibited two magnetostriction (MS) effects: (1) spontaneous MS due to the exchange interaction and spin-orbit coupling (SOC) at zero dc magnetic field ($H$) [Phys. Rev. B 102, 014446 (2020)] and (2) paramagnetic MS due to SOC and Zeeman energy at room temperature [Phys. Rev. B 105, 104412 (2022)]. ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ has various magnetic structures, such as helimagnetic, first chiral soliton lattice (CSL-1), second CSL (CSL-2), CSL-2 with irreversibility, and forced ferromagnetic phases, resulting from the Dzyaloshinskii–Moriya interaction as a function of $H$ below the magnetic ordering temperature ($T_{\mathrm{c}}$). In this study, we conducted powder x-ray diffraction analyses to investigate the effects of $H$ and temperature ($T$) on the MS at the atomic level. The $T$ dependence of the lattice constants reveals that below $T_{\mathrm{c}}$, the MS depends on the magnetic structure. The MS below $T_{\mathrm{c}}$ is discussed in this study in terms of both the hybridization between the $z^2$ orbital of $\mathrm{Nb}\left(4f\right)$ and delocalized $a_1$ orbital of Cr and the structural symmetry of the ${\mathrm{CrS}}_6$ octahedron.

We present a theoretical study of all-optical magnetization switching in rare-earth/transition-metal bilayer, $\mathrm{R}{\mathrm{E}}_n/\mathrm{T}{\mathrm{M}}_m$, consisting of $n$ RE monolayers and $m$ TM monolayers. Using the Landau-Lifshitz-Bloch equation, we numerically calculate the spin dynamics, including spin transport in the bilayer, upon incidence of a single laser pulse. It is shown that the spin transfer between the RE and TM layers with $n>1$ or $m>1$ is necessary for the switching. The calculation shows that the spin transfer makes conditions for the switching similar to those for the ferrimagnetic alloys. When the spin transfer between the monolayers is absent the intensity of the exchange scattering decreases and the switching is possible only in the $\mathrm{R}{\mathrm{E}}_1/\mathrm{T}{\mathrm{M}}_1$ bilayer. A relationship between the spin relaxation and temperature dependence of the switching is discussed.

We report a study on the specific heat and heat transport of ${\mathrm{Tb}}_2{\mathrm{Ti}}_{2-x}{\mathrm{Zr}}_x{\mathrm{O}}_7$ ($x=0$, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of ${\mathrm{Tb}}^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals have an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For the magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample.

The layered material ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, with alternating stacking honeycomb and triangular layers, is attracting considerable attention due to its rich physical properties. Here, using density functional theory and classical Monte Carlo (MC) methods, we systematically study this system with the $3d^5$ electronic configuration. Near the Fermi level, the states are mainly contributed by Te $5p$ orbitals hybridized with Mn $3d$ orbitals, resembling a charge-transfer system. Furthermore, the spin orientations of the ferrimagnetic (FiM) ground state display different conductive behaviors when along the $ab$ plane or out-of-plane directions: insulating vs metallic states. The energy difference between the FiM [110] insulating and FiM [001] metallic phases is very small $(\sim 0.71 \mathrm{meV}/\mathrm{Mn})$. Changing the angle $\theta$ of spin orientation from in-plane to out-of-plane directions, the band gaps of this system are gradually reduced, leading to an insulator-metal transition, resulting in an enhanced electrical conductivity, related to the colossal angular magnetoresistance (MR) effect. Although the three main magnetic couplings were found to be antiferromagnetic, overall the ground state is FiM. In addition, we also constructed the magnetic phase diagram using the classical $XY$ spin model studied with the MC method. Three magnetic phases were obtained, including antiferromagnetic order, noncollinear spin patterns, and FiM order. Moreover, we also investigated the Se and Ge doping into the ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ system: the FiM state has the lowest energy among the magnetic candidates for both Se- and Ge-doped cases. The magnetic anisotropy energy (MAE) decreases in the Se-doped case because the Mn orbital moment is reduced as the doping $x$ increases. Due to the small spin-orbit-coupling effect of Se, the insulator-metal transition caused by the spin orientation disappears in the Se-doped case, resulting in an insulating phase in the FiM [001] phase. This causes a reduced colossal angular MR. However, both the MAE and the band gap of the Ge-doped case do not change much with increasing doping $x$. Our results for ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ could provide guidance to experimentalists and theorists working on this system or related materials.

Hyperuniform disordered photonic materials have recently been shown to display large, complete photonic band gaps and isotropic optical properties, and are emerging as strong candidates for a plethora of optoelectronic applications, making them competitive with many of their periodic and quasiperiodic counterparts. In this work, high quality factor optical cavities in hyperuniform disordered architectures are fabricated through semiconductor slabs and experimentally addressed by scanning near-field optical microscopy. The wide range of confined cavity modes that we detect arise from carefully designed local modifications of the dielectric structure. Previous works on hyperuniform disordered photonic systems have previously identified several Anderson localized states spectrally located at the PBG edges with relatively high quality factors. In this work, by engineering the structural parameters of the cavity, we achieve an experimental quality factor of order 6000 (higher than the one of the Anderson states) and we demonstrate that three types of localized modes of different nature coexist within a small area and in a relatively narrow spectral window of the disordered correlated system. Their compatibility with general boundary constraints, in contrast with ordered architectures that suffer strict layout constraints imposed by photonic crystals’ axes orientation, makes optical cavities in disordered hyperuniform patterns a flexible optical insulator platform for planar optical circuits.

It is believed that the two-dimensional (2D) Anderson model exhibits localization for any nonzero disorder in the thermodynamic limit and it is also well known that the finite-size effects are considerable in the weak disorder limit. Here we numerically study the quantum chaos to localization transition in the finite 2D Anderson model using standard indicators used in the modern literature such as the level spacing ratio, spectral form factor, variances of observable matrix elements, participation entropy, and the eigenstate entanglement entropy. We show that many features of these indicators may indicate emergence of robust single-particle quantum chaos at weak disorder. However, we argue that a careful numerical analysis is consistent with the single-parameter scaling theory and predicts the breakdown of quantum chaos at any nonzero disorder value in the thermodynamic limit. Among the hallmarks of this breakdown are the universal behavior of the spectral form factor at weak disorder, and the universal scaling of various indicators as a function of the parameter $u={(WlnV)}^{-1}$ where $W$ is the disorder strength and $V$ is the number of lattice sites.

The investigation of the spectral and dynamical delocalization-localization (DL) transitions have revealed intriguing features in a wide expanse of non-Hermitian systems. The present study aims at exploring the spectral and the dynamical properties in a non-Hermitian quasiperiodic system with asymmetric hopping, in the presence of Rashba spin-orbit (RSO) interaction. In particular, in such systems, we have identified that the DL transition is associated with a concurrent change in the energy spectrum, where the eigenstates always break the time-reversal symmetry for all strengths of the quasiperiodic potential, contrary to the systems without RSO interaction. Furthermore, in this work, we have demonstrated that the open boundary energy spectrum in the prototypical 1D nonreciprocal lattice remains real up to a certain system size and forms complex spectral loops with an increase in the size of the lattice. We find that the skin effect remains unaltered irrespective of the nature of the spectrum. In addition, it is illustrated that the spin-flip term in the RSO interaction possesses a tendency to diminish the directionality of the skin effect. On scrutinizing the dynamical attributes in our non-Hermitian system, we unveil that in spite of the fact that the spectral DL transition accords with the dynamical phase transition, interestingly, the system acquires a nonzero transport behavior, and in fact comes across hyperdiffusive and negative diffusion dynamical regimes depending upon the strength of the RSO interaction, in the spectrally localized regime.

We present a joint experimental and computational study of second harmonic generation and Kerr effect induced by intense terahertz electric fields in the quantum paraelectric material ${\mathrm{KTaO}}_3$. The single-cycle terahertz (THz) electric pulses induce the free induction decay of the polar soft phonon mode and a ferroelectric polarization response that lasts for several picoseconds in both experiment and computation. We observe a much longer-lasting (43-ps) transient Kerr effect that follows the THz excitation. We explain the disparity in the relaxation times between the THz-induced second harmonic generation and the Kerr effect by the presence of the polar nanoregions intrinsic to ${\mathrm{KTaO}}_3$. The origin of the slow THz Kerr effect dynamics is not presently understood. We speculate that it may be associated with the rotational motion of polar nanoregions due to the material's proximity to the ferroelectric phase transition.

One of the core concepts from the non-Hermitian skin effect is the extended complex wave vectors (CW) in the generalized Brillouin zone (GBZ), while the origin of CW remains elusive, and further experimental demonstration of GBZ is still lacking. We show that the bulk states of an open system dynamically governed by the Lindblad master equation exhibit non-Bloch evolution which results in CW. Experimentally, we present temporal topolectrical circuits to serve as simulators for the dynamics of an open system. By reconstructing the correspondence between the bulk states of an open system and circuit voltage modes through gauge scale potentials in the circuit, the non-Bloch evolution is demonstrated. Facilitated by the simulators and proper approach to characterize the non-Bloch band proposed here, the GBZ is confirmed. Our work may advance the investigation of the dissipative topological modes and provide a versatile platform for exploring the unique evolution and topology for both closed and open systems.

We present the ground state properties of a new quantum antiferromagnet, ${\mathrm{YbBO}}_3$, in which the isotropic ${\mathrm{Yb}}^{3+}$ triangular layers are separated by a nonmagnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin $J_{\mathrm{eff}}=\frac{1}{2}$ state of ${\mathrm{Yb}}^{3+}$ ions at low temperatures, interacting antiferromagnetically with an intralayer coupling $J/k_{\mathrm{B}}\simeq 0.53$ K. The absence of oscillations and a $1/3$ tail in the zero-field muon asymmetries rules out the onset of magnetic long-range order as well as spin freezing down to 20 mK. An anomalous broad maximum in the temperature-dependent heat capacity with an unusually reduced value and a broad anomaly in the zero-field muon depolarization rate centered at $T^{*}\simeq 0.7J/k_{\mathrm{B}}$ provide compelling evidence for a wide fluctuating regime $(0.2\lesssim T/J\le 1.5)$ with slow relaxation. We infer that the fluctuating regime is a universal feature of highly frustrated triangular-lattice antiferromagnets while the absence of magnetic long-range order is due to perfect two-dimensionality of the spin lattice protected by nonmagnetic site disorder.

Antiferromagnetic (AFM) skyrmions are now becoming an intensively studied topic because they can overcome limitations of the skyrmion Hall effect and dipolar field-induced large size of skyrmions. However, most studies of the AFM skyrmions are focusing on the synthetic antiferromagnets consisting of metallic multilayers, but the investigation of AFM topological magnetism in synthetic vdW antiferromagnets is still rare as far as we know. Here, we propose and demonstrate from first-principles calculations and atomic spin model simulations that a variety of AFM topological spin textures, including skyrmion, chiral domain wall, and meron, can be induced in van der Waals (vdW) synthetic antiferromagnets, i.e., $\mathrm{Mn}{\mathrm{Bi}}_2\mathrm{Se}{\left(\mathrm{S}\right)}_2{\mathrm{Te}}_2$ bilayer and trilayer. We further show that these noncollinear spin textures are closely hinged with stacking properties and possess unique spin dynamics. Our findings provide not only specific material candidates but also a general approach for achieving and controlling AFM topological magnetism in vdW magnets.

Janus structures with the breaking of out-of-plane mirror symmetry have gained intensive attention. Here, on the basis of first principles, we not only investigate the recently discovered Janus 2H-MoSH monolayer [ACS Nano 15, 20319 (2021)], but also report the global minimum structure of a Janus 1T-MoSH monolayer. Meanwhile, 2H-MoSH monolayer can easily transform into the 1T phase, overcoming a relatively small barrier of 0.13 eV. Intriguingly, Janus 1T-MoSH is a charge-density wave (CDW) material and its CDW order can be regulated via external strains. When Janus 1T-MoSH is subjected to 3% compressive strain, its CDW is completely suppressed and a superconducting state with a superconducting transition temperature $\mathit{T}{}_c$ of 25.15 K emerges. Moreover, Janus 2H-MoSH monolayer is an intrinsic superconductor with a $\mathit{T}{}_c$ of $\sim 26.81$ K and its $\mathit{T}{}_c$ can be enhanced up to 36.69 K under a 1% tensile strain. Our work provides more in-depth insights for superconducting and CDW in low-dimensional transition metal dichalcogenides and related Janus systems.

We investigate how magnetic impurities may affect a system exhibiting a charge-density wave (CDW) in its ground state. We consider a disordered Hubbard-Holstein model with a homogeneous electron-phonon interaction but with a (randomly chosen) fraction of sites displaying a nonzero Coulomb repulsion $U$ and perform state-of-the-art finite-temperature quantum Monte Carlo simulations. For a single magnetic impurity, charge-charge correlations hamper the spin-spin ones around the repulsive site, thus requiring a strong enough value of $U$ to create nonnegligible antiferromagnetic (AFM) correlations. As the number of magnetic impurities increases, these AFM correlations become deleterious to CDW order and its features. First, the critical temperature is drastically reduced and seems to vanish $\sim 40\%$ of impurities (for fixed $U/\lambda =2$), which we correlate with the classical percolation threshold. We also notice that just a small amount of disorder suffices to create a bad insulating state, with the suppression of both Peierls and spin gaps, even within the charge-ordered phase. Finally, we have also found that pairing correlations are enhanced at large doping, driven by the competition between CDW and AFM tendencies.

Ballistic transport of a quantum system can be characterized by Drude weight, which quantifies the response of the system to a uniform electric field in the infinitely long timescale. The Drude weight is often discussed in terms of the Kohn formula, which gives the Drude weight by the derivative of the energy eigenvalue of a finite-size system with the periodic boundary condition in terms of the Aharonov-Bohm flux. Recently, the Kohn formula is generalized to nonlinear responses. However, the nonlinear Drude weight determined by the Kohn formula often diverges in the thermodynamic limit. In order to elucidate the issue, in this work we examine a simple example of a one-dimensional tight-binding model in the presence of a single defect at zero temperature. We find that its linear and nonlinear Drude weights given by the Kohn formula (i) depend on the Aharonov-Bohm flux and (ii) diverge proportionally to a power of the system size. We argue that the problem can be attributed to different order of limits. The Drude weight according to the Kohn formula (“Kohn-Drude weight”) indicates the response of a finite-size system to an adiabatic insertion of the Aharonov-Bohm flux. While it is a well-defined physical quantity for a finite-size system, its thermodynamic limit does not always describe the ballistic transport of the bulk. The latter should be rather characterized by a “bulk Drude weight” defined by taking the thermodynamic limit first before the zero-frequency limit. While the potential issue of the order of limits has been sometimes discussed within the linear response, the discrepancy between the two limits is amplified in nonlinear Drude weights. We demonstrate the importance of the low-energy excitations of $O(1/L)$, which are excluded from the Kohn-Drude weight, in regularizing the bulk Drude weight.

We analyze the global ground-state (quantum) phase diagram of the one-dimensional spinful Holstein model at half filling as a function of the strength of the electron-phonon coupling (represented by the strength of the phonon-induced attraction, $U$) and the phonon frequency ${\omega }_0$. In addition to reanalyzing the various asymptotic regimes, we carry out density-matrix renormalization group simulations to correct previous inferences concerning the antiadiabatic (large ${\omega }_0$) and strong-coupling (large $U$) regimes. There are two distinct phases—a fully gapped commensurate charge-density-wave phase and a spin-gapped Luther-Emery phase with a gapless charge mode—separated by a phase boundary, with a shape that reflects different microscopic physics in the weak- and strong-coupling limits.