We provide thermodynamic evidence for the presence of a magnetic-field-induced small phase pocket near the antiferroquadrupole phase boundary in the non-Kramers ${\mathrm{\Gamma }}_3$ doublet system ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$. In particular, we measured the specific heat as functions of temperature $T$, magnetic field $B$, and field angle ${\varphi }_B$, and found a second specific-heat anomaly in a relatively wide field-angle range near $B\parallel \left[001\right]$, although fine tuning of the field strength is required. We also investigated the rotational magnetocaloric effect and evaluated an entropy change in this phase pocket. The present findings demonstrate that multipole degrees of freedom give rise to a magnetic-field-induced exotic order in ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$, suggesting the possibility of switching between the order parameters or emergence of a multiple-$\mathit{q}$ order of quadrupoles.

We consider a one-dimensional fermionic lattice system with long-ranged power-law decaying hopping with exponent $\alpha$. The system is further subjected to dephasing noise in the bulk. We investigate two variants of the problem: (i) an open quantum system where the setup is further subjected to boundary reservoirs enabling the scenario of a nonequilibrium steady-state charge transport, and (ii) time dynamics of an initially localized single-particle excitation in the absence of boundary reservoirs. In both variants, anomalous superdiffusive behavior is observed for $1<\alpha <1.5$, and for $\alpha >1.5$ the setup is effectively short ranged and exhibits conventional diffusive transport. Our findings are supported by analytical calculations based on the multiple scale analysis technique that leads to the emergence of a fractional diffusion equation for the density profile. Our study unravels an interesting interplay between long-range interaction and dephasing mechanism that could result in the emergence of unconventional behavior in open quantum systems.

We calculate the energy current flowing in the bulk of a ($2+1$)-dimensional system of massive Dirac fermions and along a ($1+1$)-dimensional domain wall generated by flipping the sign of the particle mass. We show that, at low temperatures and in the long-wavelength limit, the system does not support a bulk thermal Hall current proportional to the temperature gradient. The only such contribution is due to states localized at the domain wall. This puts an end to a controversy existing in the literature and amends previous results obtained via first-order perturbation calculations.

The vibrational properties of monoclinic dirutile-type $\mathrm{CoTe}{\mathrm{O}}_4$ (space group $P{2}_1/c$) were investigated by means of Raman and infrared (IR) spectroscopy on stoichiometric single crystals and theoretical density-functional theory (DFT) calculations. Experimental in situ investigations under pressure and temperature variations up to 10.1 GPa and 750 K confirmed the singular phase stability of the monoclinic lattice and the absence of any structural transition. The IR spectra exhibit modes that are not observed in the Raman spectra, thus offering a comprehensive understanding of the vibrational behavior of the title compound. The symmetry and degeneracy of modes and their contributions to the overall vibrational spectrum of $\mathrm{CoTe}{\mathrm{O}}_4$ were analyzed using group theory. DFT calculations were performed both on a cluster level and based on the crystal structure of monoclinic $\mathrm{CoTe}{\mathrm{O}}_4$ in order to gain further insights into the vibrational properties. The calculated vibrational frequencies and modes were compared with the experimental data, allowing for a detailed investigation of the lattice dynamics and the nature of the vibrational modes in $\mathrm{CoTe}{\mathrm{O}}_4$, and are corroborated with the analytical Wilson's F-G matrix method. These findings provide fundamental insights into the crystal symmetry and lattice dynamics of $\mathrm{CoTe}{\mathrm{O}}_4$. The results are discussed and compared to those of isoformular systems.

Magnetic anisotropy is a fundamental property of magnetic materials that plays an essential role in the stability of magnetic domains and skyrmions. In this ferromagnetic resonance study, we report the evolution of magnetic anisotropy by substituting various $4d$ metals in the antiskyrmion host ${(\mathrm{Fe},\mathrm{Ni})}_3\mathrm{P}$ with $S_4$ tetragonal symmetry. In the undoped compound ${\left({\mathrm{Fe}}_{0.63}{\mathrm{Ni}}_{0.37}\right)}_3\mathrm{P}$ and in the Ru-doped ${\left({\mathrm{Fe}}_{0.59}{\mathrm{Ni}}_{0.32}{\mathrm{Ru}}_{0.09}\right)}_3\mathrm{P}$, the uniaxial magnetic anisotropy has an easy-plane character, while Pd doping turns the material to an easy-axis-type magnet, as observed in ${\left({\mathrm{Fe}}_{0.62}{\mathrm{Ni}}_{0.29}{\mathrm{Pd}}_{0.09}\right)}_3\mathrm{P}$. In addition to the dominant uniaxial anisotropy, we also quantify the fourfold anisotropy of the plane perpendicular to the tetragonal axis in ${\left({\mathrm{Fe}}_{0.63}{\mathrm{Ni}}_{0.30}{\mathrm{Pd}}_{0.07}\right)}_3\mathrm{P}$. Using analytical calculations and micromagnetic simulations, we discuss how this planar anisotropy competes with the anisotropic Dzyaloshinskii-Moriya interaction in determining the orientation of the magnetic stripes and the antiskyrmions.

Intermediate switching states are of great interest for developing advanced magnetic memories. In this paper, the magnetic switching process is studied in the multilayers that are fabricated using two distinct ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ building blocks, i.e., ${\mathrm{PbTiO}}_3$/${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$/${\mathrm{NdGaO}}_3$ (block A) and ${\mathrm{PbTiO}}_3$/${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$/${\mathrm{PbTiO}}_3$ (block B) with different intralayer ferromagnetic couplings. Accordingly, the multilayer that consists of two types of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ blocks is featured by two distinct Curie temperatures and coercive fields, as expected. On the other hand, the sample only composed of block B displays a single Curie temperature but two unexpected coercive fields. While the single Curie temperature indicates the same intralayer ferromagnetic coupling in all ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ sublayers, two coercive fields suggest the existence of an intermediate switching state. The detailed study on polarized neutron reflectometry reveals that this intermediate state exhibits a gradual change of magnetization, from parallel to antiparallel with respect to the external magnetic field, across ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ sublayers approaching the substrate. The competition between the interlayer exchange and magnetostatic interaction is highlighted to show the intermediate magnetization structure of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ multilayers with identical intralayer coupling. Our findings shed light on controlling switching states and may contribute to the design of nanoscale high-density magnetic memories.

The Yao-Lee model is an example of exactly solvable spin-orbital models that are generalizations of the original Kitaev honeycomb model with extra local orbital degrees of freedom. Similarly to the Kitaev model, both spin and orbital degrees of freedom are effectively represented using sets of three-flavored Majorana fermions. The Yao-Lee model exhibits a quantum spin liquid ground state with gapped and immobile $Z_2$ fluxes and threefold degenerate itinerant Majorana fermions. Our work demonstrated that by introducing different time-reversal-symmetry- (TRS) breaking fields one can split the degeneracy of Majorana fermions and close the gap for some of the bands, thus changing its topology. We calculated a comprehensive topological phase diagram for the Yao-Lee model by considering various combinations of TRS-breaking fields. This investigation revealed the emergence of distinct topological regions, each separated by nodal lines, signifying an evolution in the model's topological properties. We also investigated the impact of vacancies in the system. Our findings revealed that while vacancies modify the low-energy spectrum of the model, their presence has a limited impact on the topological properties of the model, at least for small-enough concentrations.

Linearly dispersive gapless quasiparticles can appear at general momentum points or on high-symmetry momentum lines of superconductors because of topological reasons such as K theory or symmetry indicators theory. However, the zero modes associated with these quasiparticles are generally “accidental” from symmetry point of view. In this paper, we apply projective representation (rep) theory to analyze the bulk gapless quasiparticles in BdG systems. Different from the description of semimetals, the particle-hole “symmetry” requires special treatment since it anticommutes with the BdG Hamiltonian. Accordingly, the notion of “simple irreducible reps (irreps)” and “composite irreps” are introduced to label the energy modes. Without charge conjugation symmetry (unitary symmetry that commutes with the Hamiltonian), no robust zero modes exist at any fixed momentum point in the bulk. However, robust zero modes at certain high-symmetry momentum points can be protected by (effective) charge conjugation symmetries, resulting in gapless quasiparticles with linear, quadratic, or higher-order dispersions. The low-energy properties of the quasiparticles, including the dispersions and responses to external probe fields, are dictated by the reps carried by these zero modes from the effective $k·p$ theory. Our theory provides a framework to classify nodal superconductors/superfluids/quantum spin liquids with specified (projective) symmetry group, and sheds light on the realization of Majorana-type massless quasiparticles in condensed matter physics.

Structural glasses display at low temperature a set of anomalies in thermodynamic observables. A prominent example is the linear-in-temperature scaling of the specific heat, at odds with the Debye cubic scaling found in crystals, due to acoustic phonons. Such an excess of specific heat in amorphous solids is thought of arising from phenomenological soft excitations dubbed tunneling two-level systems (TTLS). Their nature as well as their statistical properties remain elusive from a first-principle viewpoint. In this work, we investigate the canonically quantized version of the KHGPS model, a mean-field glass model of coupled anharmonic oscillators, across its phase diagram, with an emphasis on the specific heat. The thermodynamics is solved in a semiclassical expansion. We show that in the replica-symmetric region of the model, up to the marginal glass transition line where replica symmetry gets continuously broken, a disordered version of the Debye approximation holds: the specific heat is dominated by harmonic vibrational excitations inducing a power-law scaling at the transition, ruled by random matrix theory. This mechanism generalizes a previous semiclassical argument in the literature. We then study the marginal glass phase where the semiclassical expansion becomes nonperturbative due to the emergence of instantons that overcome disordered Debye behavior. Inside the glass phase, a variational solution to the instanton approach provides the prevailing excitations as TTLS, which generate a linear specific heat. This phase thus hosts a mix of TTLS and harmonic excitations generated by interactions. We finally suggest to go beyond the variational approximation through an analogy with the spin-boson model.

Adiabatic evolution is an emergent design principle for time modulated metamaterials, often inspired by insights from topological quantum computing such as braiding operations. However, the pursuit of classical adiabatic metamaterials is rooted in the assumption that classical and quantum adiabatic evolution are equivalent. We show that this is only true in the limit where the frequencies of all the bands are at infinite distance from 0 and some instances of quantum adiabatic evolution, such as those containing zero modes, cannot be reproduced in classical systems. This is because mode coupling is fundamentally different in classical mechanics. We derive classical conditions to ensure adiabaticity and demonstrate that only under these conditions—which are different from quantum adiabatic conditions—do the single band Berry phase and Wilczek-Zee matrix for everywhere degenerate bands emerge as meaningful quantities encoding the geometry of classical adiabatic evolution. Finally, for general multiband systems we uncover a correction term in the non-Abelian gauge potential for classical systems.

We present a neutron vibrational spectroscopy study to investigate the influence of ${\mathrm{NH}}_4^{+}$ motion on the magnetism in ${\left[{\left({\mathrm{NH}}_4\right)}_{1-x}{\mathrm{K}}_x\right]}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$. The parent compounds, ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right] (x=0$) and ${\mathrm{K}}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right](x=1)$ are isostructural at room temperature, yet displaying drastically different magnetic and multiferroic behavior. ${\mathrm{K}}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ is nonmultiferroic with type-A collinear antiferromagnetic structure below $T_N\approx 14.06\mathrm{K}$, whereas ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ is a type-II multiferroic with incommensurate cycloidal spin structure below $T_{\mathrm{FE}}\approx 6.8\mathrm{K}$. A recent study of the dielectric, structure, and magnetic properties in the mixed ${\left[{\left({\mathrm{NH}}_4\right)}_{1-x}{\mathrm{K}}_x\right]}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$ shows that a small amount of potassium substitution to replace ${\mathrm{NH}}_4^{+}$ transforms the spin structure from incommensurate cycloidal $(x\le$ 0.06) into commensurate collinear antiferromagnetic $(x\ge$ 0.15), indicating ${\mathrm{NH}}_4^{+}$ is essential to the emergent phenomena observed in this molecular multiferroic compound. Our vibrational spectroscopy study reveals that ${\mathrm{NH}}_4^{+}$ libration and torsion motion exhibit substantial temperature dependence at low temperatures. The intensity of ${\mathrm{NH}}_4^{+}$ libration and torsion modes increases slightly at 5 K in comparison with data at 25 K behaving like a magnon, indicating that they are coupled to the magnetism in ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$. Comparing data of $x=0$, 0.06, 0.09, and 0.15 samples further illustrates that the strength of the increased signal in ${\mathrm{NH}}_4^{+}$ libration mode is very sensitive to potassium concentration. The signal diminishes quickly with increasing potassium concentration and vanishes in the $x=0.15$ sample corresponding to the magnetic structure change for $x\ge$ 0.15. The results directly link the anomalous behavior in ${\mathrm{NH}}_4^{+}$ libration motion to the magnetism in ${\left[{\left({\mathrm{NH}}_4\right)}_{1-x}{\mathrm{K}}_x\right]}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right],$ providing new insights into the crucial role ${\mathrm{NH}}_4^{+}$ plays in the coupled phenomena in ${\left({\mathrm{NH}}_4\right)}_2\left[{\mathrm{FeCl}}_5\left({\mathrm{H}}_2\mathrm{O}\right)\right]$. The unique information opens a new door to go through in searching for new multifunctional materials by incorporation of ${\mathrm{NH}}_4$ via a material-by-design approach.

Tripartite antiferromagnets (AFMs) with a ${120}^{\circ }$ spin order, exhibiting unconventional transport properties, facilitate detecting AFM states. This inspires some studies on terahertz self-oscillations in these spin structures. However, self-oscillations under the influence of magnetic inertia, which comes out in the ultrafast dynamics, have not yet been unveiled. Here, resorting to the symmetry and numeric results, an effective analytic theory, which allows for a detailed analytic treatment, is developed to describe the self-oscillation triggered by a spin-orbit torque. Then, the phase boundaries of self-oscillation are determined, and it is found that the magnetic inertia wipes out the upper threshold, extending the useful frequency range. Furthermore, the precession holds almost in-plane with increasing current, resulting in a large-amplitude steady oscillation. Finally, the effect of Dzyaloshinsky-Moriya interaction on the tunable range of self-oscillation is discussed briefly. In virtue of the wide frequency tunability granted by the inertia and the unique features of the tripartite AFMs, this study may provide a significant contribution to the spintronic terahertz applications.

A large, or even infinite, local Hilbert space dimension poses a significant computational challenge for simulating quantum systems. In this work, we present a matrix product state (MPS) based method for simulating one-dimensional quantum systems with a large local Hilbert space dimension, an example being bosonic systems with a large on-site population. To this end, we split the local Hilbert space corresponding to one site into two sites, each with a smaller Hilbert space dimension. An advantage of this method is that it can be easily integrated into MPS based techniques such as time-dependent variational principle (TDVP) without changing their standard algorithmic structure. Here, we implement our method using the TDVP to simulate the dynamics of the spin-boson model, a prototypical model of a spin interacting with a large bath of bosonic modes. We benchmark our method against and find excellent agreement with previous studies.

Strongly correlated electron systems are challenging to calculate, and entanglement in such systems is not widely analyzed. We present an approach that can be used as a postprocessing step for calculating the two-site reduced density matrix and from it entanglement measures such as the mutual information and entanglement negativity. Input is only the one- and two-particle Green's function, which is the output of numerous many-body methods. As an illustration, we present results for a toy model, the Hubbard model on a $2\times 2$ cluster and a six-site ring.

Control of topological edge modes is desirable for encoding quantum information resiliently against external noise. Their implementation on quantum hardware, however, remains a long-standing problem due to current limitations of circuit depth and noise, which grows with the number of time steps. By utilizing recently developed constant-depth quantum circuits in which the circuit depth is independent of time, we demonstrate successful long-time dynamics simulation of bulk and surface modes in topological insulators on noisy intermediate-scale quantum (NISQ) processors, which exhibits robust signatures of localized topological modes. We further identify a class of one-dimensional topological Hamiltonians that can be readily simulated with NISQ hardware. Our results provide a pathway towards stable long-time implementation of topological quantum spin systems on present-day quantum processors.

Results are presented for the dynamics of edge modes in interacting Floquet Ising chains. It is shown that in addition to the quasistable 0 and $\pi$ edge modes, a third long lived edge mode arising from the operator product of the 0 and $\pi$ edge modes exists. Depending on the microscopic parameters, this Floquet product mode is shown to have a substantially longer lifetime than the individual 0 and $\pi$ modes. This is triggered by a scattering process which converts a 0 mode into a $\pi$ mode while scattering two bulk excitations. This process can lead to a rapid decay of both 0 and $\pi$ mode without affecting the product mode.

Exotic electronic bands, such as flat, linear crossing, and spontaneous valley- or spin-polarized bands, in two-dimensional materials have been a hot topic in condensed-matter physics. In this paper, we first proposed a general dispersion model for possible hatlike electronic bands. Then, we identified a single-spin wavy brim bucket hat in the valence-band edge of a stable ferromagnetic semiconducting electrene, the Janus GdIH monolayer, which can be well described by a simplified two-band Hamiltonian model. Specifically, the hat band exhibited a wavy brim with six valleys along the boundary of the first Brillouin zone and held a flat top close to the Fermi level. This resulted in the emergence of single-spin van Hove singularity divergence and Lifshitz transitions. Due to the breaking of both time-reversal and space-inversion symmetries, a sizable spontaneous valley polarization was observed between adjacent brim valleys. This provided the opportunity to realize the high-temperature anomalous valley Hall effect. Particularly, via modest strains and carrier doping, various conductive bipolar states (spin-up versus spin-down, K valley versus $-\text{K}$ valley, and ultra-low-speed versus ultra-high-speed) can be modulated from the distorted wavy brim bucket hat of GdIH monolayer.

We study the effects of photoirradiation with circularly polarized light on the Dirac half-metal state induced by the ferrimagnetic order in a triangular Kondo-lattice model. Our analysis based on the Floquet theory reveals that two types of Floquet Chern insulator phases appear as photoinduced nonequilibrium steady states and that these two phases can be experimentally detected and distinguished by measurements of the Hall conductivity. It is elucidated that these rich nonequilibrium topological phases come from higher-order terms in the high-frequency expansion called Brillouin-Wigner expansion, which is in striking contrast to usually discussed Floquet Chern insulator phases originating from the lowest-order terms of the expansion. So far, the lattice electron models on simple nonmultipartite lattices such as triangular lattices and square lattices have not been regarded as targets of the Floquet engineering because the lowest-order terms of the high-frequency expansion for Floquet effective Hamiltonians cancel each other to vanish in these systems. Our findings of the Floquet Chern insulator phases in a triangular Kondo-lattice model are expected to expand the range of potential models and even materials targeted by the Floquet engineering.

We numerically investigate photoresponses of the charge ordered state upon stimuli of pulsed laser light, especially paying attention to the differences in the pulse width, whose shortness has been a key to experimentally realize large photoinduced effects. As a model for charge ordering, we consider an interacting spinless fermion model on a one-dimensional chain coupled to classical phonons and numerically simulate its real-time dynamics. First, we demonstrate the case where spatially uniform dynamics take place, and show the delayed response of phonons after the photoirradiation causing destabilization of charge order. When a local impurity potential is introduced in the initial state, an inhomogeneous charge order melting occurs and the delay in the timescale also happens in the real-space melting process. Finally, by investigating wider parameter ranges using different pulse widths, we find that the use of ultrashort pulses considerably expands the frequency window for charge order melting; it is indispensable in realizing the photoinduced phase transition with off-resonant conditions.