In the Hermitian regime, the Wannier-Stark ladder characterizes the eigenstates of an electron in a periodic potential with an applied static electric field. In this work, we extend this concept to the complex regime for a periodic non-Hermitian system under a linear potential. We show that although the energy levels can be complex, they are still equally spaced by a real Bloch frequency. This ensures single-particle Bloch oscillations with a damping (or growing) rate. The system can also support standard two-particle Bloch oscillations under certain conditions. We propose two types of dimerized non-Hermitian systems to demonstrate our results. In addition, we also propose a scheme to demonstrate the results of particle-pair dynamics in a single-particle two-dimensional $\mathcal{PT}$-symmetric square lattice.

${\mathrm{Cr}}_2{\mathrm{O}}_3$ emerges as a prominent candidate material for spintronics and magnetoelectronic applications. However, a comprehensive understanding of its temperature-dependent spin dynamics remains elusive, impeding the engineering of novel spintronics based on this material. We delve into this through a combination of inelastic neutron scattering experiments and atomistic simulations. Our results unveiled the emergence of paramagnons above and below $T_N$. We demonstrated a significant softening of linear magnons upon heating in the antiferromagnetic state. Further analysis revealed that this softening primarily originated from four-magnon interactions, while thermal expansion played a minor role.

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of ${\mathrm{MnPSe}}_{3-x}{\mathrm{S}}_x$ $(x=0, 0.63, \text{and} 1.68)$ alloys, utilizing temperature- and polarization-dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing $x$. Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered ${\mathrm{MnPSe}}_{3-x}{\mathrm{S}}_x$ $(x=1.68)$ suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.

It was recently shown that a chiral topological phase emerges from the coupling of two twisted monolayers of superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ for certain twist angles. In this work, we reveal the behavior of such twisted superconducting bilayers with $d_{x^2-y^2}$ pairing symmetry in the presence of an applied magnetic field. Specifically, we show that the emergent vortex matter can serve as a smoking gun for the detection of topological superconductivity in such bilayers. Moreover, we report two distinct skyrmionic states that characterize the chiral topological phase and provide a full account of their experimental signatures and their evolution with the twist angle.

We present a mechanism of topological pumping in origami metamaterials with spatial modulation by tuning the rotation angles. Through coupling spatially modulated origami chains along an additional synthetic dimension, the pumping of waves from one topological edge state to another is achieved, where the Landau-Zener transition is demonstrated by varying the number of coupled origami chains. In addition, the inherent nonlinearity of origami metamaterials enables the excitation-dependent Landau-Zener tunneling probability. Furthermore, with the increase of nonlinearity, the topological states tend to localize in several regions in a way reminiscent of discrete breathers. Our findings pave the way towards interband transitions and associated topological pumping features in origami metamaterials.

In this work, we investigate the electronic structures, spin Hall effects, and topological properties of the superconductor ${\mathrm{YIr}}_2$, which crystallizes in the cubic Laves phase and contains two-dimensional (2D) kagome-lattice planes. We find it has an intrinsic 3D flat band originated from two intersecting kagome-lattice planes. This 3D flat band gives rise to large intrinsic spin Hall conductivity, which suggests ${\mathrm{YIr}}_2$ can be used for charge-spin conversion devices. On the (111) surface, it has Dirac-cone type topological surface states in close vicinity of the Fermi level. More importantly, they are distinct from the bulk states, indicating that ${\mathrm{YIr}}_2$ is a promising candidate topological superconductor to host Majorana zero modes. Our work provides an encouraging platform on which to study flat band physics, spin Hall effects, and topological superconductivity.

In the slave particle representation with $U\left(1\right)$ gauge symmetry, local constraints on physical states characterized by various mean field solutions belong to Dirac's second-class ones. Although constrained systems are extensively investigated, realistic methods to solve the gauge theory problem with second-class constraints are yet to be developed. We formulate a Becchi-Rouet-Stora-Tyutin (BRST) quantization theory, called consistent $U\left(1\right)$ gauge theory, that is consistent with both first- and second-class local constraints for strongly correlated condensed matter systems. In our consistent $U\left(1\right)$ gauge theory, the redundant gauge degrees of freedom are removed by proper gauge fixing conditions while the constraints are exactly retained and the gauge invariance is guaranteed by the BRST symmetry. Furthermore, the gauge fixing conditions endow the gauge field with dynamics. This turns the strongly correlated electron model into a weakly coupled slave boson model, so most of the system's physical properties can be calculated by the conventional quantum many-body perturbation method. We focus on the property of the strange metal phase in the $t\text{−}J$ model. The electron momentum distribution and the spectral function are calculated, and the non-Fermi-liquid behavior agrees with the angle-resolved photoemission spectroscopy measurements for cuprate materials. We also study the electromagnetic responses of the strange metal state. The observed non-Fermi-liquid anomalies are captured by our calculations. Especially, we find that the Hall resistivity decreases as temperature increases, and the sign of the Hall resistivity varies from negative to positive when the dopant concentration varies from optimal doping to underdoping in the strange metal regime.

The spin-boson model, describing a two-level system coupled to a bosonic bath, is extensively studied as a paradigmatic dissipative quantum system, exhibiting rich dynamical behavior and even a localization transition in the strong coupling regime. Here, we additionally consider dephasing as a source of Markovian dissipation on top of the non-Markovian dynamics due to an Ohmic bath, and investigate the dynamics of the spin. We show that the characteristic frequency of the spin dynamics, while strongly renormalized by the bosonic bath, changes in a simple fashion (or does not change at all) with dephasing. To obtain these results, we develop an exact nonperturbative method known as the stochastic Schrödinger equation, mimicking the Ohmic bath via a stochastic magnetic field combined with the Lindblad quantum master equation due to dephasing, which allows us to numerically compute the dynamics. Furthermore, we derive weak-coupling analytic results utilizing the well-known noninteracting blip approximation. Our findings are relevant to quantum simulation of the spin-boson model in the regime of strong coupling in trapped ions and circuit QED architectures among others.

We present a Raman scattering study of rocksalt ${\mathrm{Cd}}_{1-x}{\mathrm{Zn}}_x\mathrm{O}$ alloys in the compositional range between $x=0$ and $x=0.1$. We use the sharp feature in the second-order Raman spectra associated with the transverse acoustic (TA) modes at the $L$ point to monitor the effect of alloy disorder on the phonon lifetime. The $2\mathrm{TA}\left(L\right)$ peak shows a substantial broadening as the Zn content is increased up to 10%. The rate of increase is significantly higher than that expected from cationic mass fluctuation as predicted by the mean-field coherent potential approximation. Excellent agreement with the experiment is achieved by including the effect of the force-constant fluctuations using the itinerant coherent potential approximation, demonstrating the significant role of the force-constant disorder in limiting the phonon lifetime in the ${\mathrm{Cd}}_{1-x}{\mathrm{Zn}}_x\mathrm{O}$ alloys.

We apply density functional theory to investigate interactions between electronic and vibrational states in crystal defects with multimode dynamical Jahn-Teller (JT) systems. Our focus is on transitions between orbital singlet and degenerate orbital doublet characterized by $E\otimes (e\oplus e\oplus \cdots )$ JT coupling, which frequently occurs in crystal defects that are investigated for applications in quantum information science. We utilize a recently developed methodology to model the photoluminescence (PL) spectrum of the negatively charged split nickel-vacancy center $\left({\mathrm{NiV}}^{-}\right)$ in diamond, where JT-active modes significantly influence electron-phonon interactions. Our results validate the effectiveness of the methodology in accurately reproducing the observed 1.4 eV PL lineshape. The strong agreement between our theoretical predictions and experimental observations reinforces the identification of the 1.4 eV PL center with the ${\mathrm{NiV}}^{-}$ complex. This study highlights the critical role of JT-active modes in affecting optical lineshapes and demonstrates the power of advanced techniques for modeling optical properties in complex systems with multiple JT-active frequencies.

The search for ideal quantum materials with symmetry-protected topological states continues to intrigue intense research activities in condensed matter physics. Here, we present single crystal growth, Fermi surface studies, and topological state analysis of a binary boride ${\mathrm{YB}}_2$ that crystallizes in the hexagonal crystal structure with space group $P6/mmm$ and lattice constants $a=3.304\left(2\right)$ Å, $c=3.846\left(5\right)$ Å. The single crystal of ${\mathrm{YB}}_2$ was grown by the Czochralski method, in a tetra-arc furnace. The anisotropic physical properties are reported by performing the electrical transport and magnetization measurements. The electrical resistivity confirms the metallic nature of ${\mathrm{YB}}_2$ while the magnetization reveals that it is a diamagnetic material. Interestingly in the magnetoresistance and magnetization measurements, we observe quantum oscillations, namely, Shubnikov-de Haas (SdH) oscillations and de Haas van Alphen (dHvA) oscillations for the magnetic field direction parallel to [0001]. The quantum oscillations likely originates from a hole pocket centered at the $A$ point. Our first-principles results unveil that ${\mathrm{YB}}_2$ exhibits bulk nodal lines with interesting drumhead topological surface states.

The quantum Mpemba effect is the counterintuitive nonequilibrium phenomenon wherein the dynamic restoration of a broken symmetry occurs more rapidly when the initial state exhibits a higher degree of symmetry breaking. The effect has been recently discovered theoretically and observed experimentally in the framework of global quantum quenches, but so far it has only been investigated in one-dimensional systems. Here we focus on a two-dimensional free-fermion lattice employing the entanglement asymmetry as a measure of symmetry breaking. Our investigation begins with the ground-state analysis of a system featuring nearest-neighbor hoppings and superconducting pairings, the latter breaking explicitly the $\text{U}\left(1\right)$ particle-number symmetry. We compute analytically the entanglement asymmetry of a periodic strip using dimensional reduction, an approach that allows us to adjust the extent of the transverse size, achieving a smooth crossover between one and two dimensions. Further applying the same method, we study the time evolution of the entanglement asymmetry after a quench to a Hamiltonian with only nearest-neighbor hoppings, preserving the particle-number symmetry which is restored in the stationary state. We find that the quantum Mpemba effect is strongly affected by the size of the system in the transverse dimension, with the potential to either enhance or spoil the phenomenon depending on the initial states. We establish the conditions for its occurrence based on the properties of the initial configurations, extending the criteria found in the one-dimensional case.

We provide a new variational definition for the spread of an orbital under periodic boundary conditions (PBCs) that is continuous with respect to the gauge, consistent in the thermodynamic limit, well suited to diffuse orbitals, and systematically adaptable to schemes computing localized Wannier functions. Existing definitions do not satisfy all these desiderata, partly because they depend on an “orbital center”—an ill-defined concept under PBCs. Based on this theoretical development, we showcase a robust and efficient ($10\times$–$70\times$ fewer iterations) localization scheme across a range of materials.

Iron-based superconductors have sparked a large amount of research, but the comprehensive studies on the 112 family are still lacking. ${\mathrm{BaFeAs}}_2$ has been predicted as a promising candidate of an iron-based superconductor. Here we systematically study its stability, electronic structures, and magnetic orders from the first-principles calculations. According to the stacking manners and As distortion in the [${\mathrm{Ba}}_2{\mathrm{As}}_2$] layer, four possible ${\mathrm{BaFeAs}}_2$ structures with $P4/nmm$, ${P2}_1/m$, $I4/mmm$, and $I2mm$ symmetry are constructed. The calculations demonstrate that the most stable structure is determined to be $I2mm$ symmetry, with the structure and valence state in [${\mathrm{Fe}}_2{\mathrm{As}}_2$] layers closely resembling the characteristics of iron-based superconductors, as well as the electronic properties and stripe antiferromagnetic ground state. These findings are helpful for further research and comprehensive understanding of the 112-type iron-based superconductors.

We propose computationally cheap and accurate approximants to the noninteracting kinetic energy density functional $T_s\left[n\right]$ by leveraging the simplicity and computational efficiency of the Wang-Teter functional [L.-W. Wang and M. Teter, Phys. Rev. B 45, 13196 (1992)]. It depends on a single parameter, the average electron density ${\rho }_0$. We address limitations of the Wang-Teter functional, which include variational instabilities and inability to treat materials with finite band gaps. We introduce three physically motivated methods for determining ${\rho }_0$: DEN, minimizing the integrated difference of the self-consistent Wang-Teter electron density from the one from conventional Kohn-Sham density functional theory (DFT); KIN, minimizing the deviation between the Wang-Teter $T_s\left[n\right]$ and the exact value from conventional Kohn-Sham DFT; and finally ENE, minimizing the difference between the Wang-Teter and conventional Kohn-Sham DFT total energies. The crucial result of this work is that our approaches effectively mitigate the drawbacks of the Wang-Teter functional. We provide a thorough analysis of our methods and discuss their potential for large-scale simulations and as templates for density-dependent nonlocal functionals.

Transition-metal dichalcogenides (TMDs) host tightly bound electron-hole pairs—excitons—which can be either optically bright or dark based on spin and momentum selection rules. In tungsten-based TMDs, a momentum-forbidden dark exciton is the energy ground state, and therefore, it strongly affects the emission properties. In this work, we brighten the momentum-forbidden dark exciton by placing monolayer tungsten disulfide on top of nanotextured substrates, which imparts tensile strain, modifying its electronic band structure. This enables phonon-assisted exciton scattering between momentum valleys, thereby brightening momentum-forbidden dark excitons. In addition to offering a tuning knob for light-matter interactions in two-dimensional materials, our results pave the way for designing ultrasensitive strain-sensing devices based on TMDs.

Conventionally, the modulation of the intrinsic Weyl nodes in Weyl semimetals is challenging, due to topological protection. Here we report the structural dependence of the Weyl nodes in a ${\mathrm{Co}}_2\mathrm{MnGa}$ Heusler thin film via a temperature-dependent tetragonal distortion. The ability to manipulate these Weyl nodes allows for the control of the intrinsic electromagnetic properties. Temperature-dependent x-ray diffraction (XRD) measurements identify a compressive tetragonal distortion with decreasing temperature from 300 to 20 K. The calculated Weyl properties can be directly compared with experimental parameters through the temperature-dependent XRD measurements which show the intrinsic correlation between Weyl properties and important magnetic parameters. The microscopic momentum space properties of Weyl nodes such as the distance ($d_{\mathrm{W}}$), solid angle (${\mathrm{\Omega }}_{\mathrm{W}}$), tilt (${\phi }_{\mathrm{W}}$), and nodal point energy ($E_{\mathrm{W}}$) directly affect the macroscopic observable properties such as exchange stiffness ($A$), magnetization ($M$), and effective anisotropy field ($H_{\mathrm{K}}^{\mathrm{eff}}$), as shown via structure-dependent density functional theory calculations. These predictions are experimentally observed as large variations in the bulk magnetization and effective anisotropy field as a function of temperature. These results highlight a unique degree of freedom in the control of macroscopic magnetic properties via the modulation of the intrinsic properties of Weyl nodes through structural distortions.

Noncollinear antiferromagnets (nc-AFMs) have attracted increasing research attention in spintronics due to their unique spin structures and fascinating charge and spin transport properties. By using first-principles calculations, we comprehensively investigate the charge and spin Hall effects in representative noncollinear antiferromagnet $\mathrm{M}{\mathrm{n}}_3\mathrm{Pt}$. Our study reveals that the Hall effects in nc-AFMs are critically dependent on the crystal facet orientation and temperature. For (001)-oriented $\mathrm{M}{\mathrm{n}}_3\mathrm{Pt}$, each charge and spin Hall conductivity element comprises time-reversal odd ($T$-odd) and even ($T$-even) contribution, associated with longitudinal conductivity, which leads to sizable and highly anisotropic Hall conductivity. The temperature dependence of charge and spin Hall conductivity has been elucidated by considering both phonon and spin disorder scattering. The scaling relations between Hall conductivity and longitudinal conductivity have also been investigated. The existence of prominent spin Hall effect in nc-AFMs may generate spin current with $S_z$ spin polarization, which is advantageous for field-free switching of perpendicular magnetization. Our work may provide unambiguous understanding of the charge and spin transport in noncollinear antiferromagnets and pave the way for applications in antiferromagnetic spintronics.

Recently developed Josephson junction array transmission lines implement strong-coupling circuit electrodynamics compatible with a range of superconducting quantum devices. They provide both the high impedance which allows for strong quantum fluctuations, and photon modes with which to probe a quantum device, such as a small Josephson junction. In this high-impedance environment, current through the junction is accompanied by charge Bloch oscillations analogous to those in crystalline systems. However, the interplay between Bloch oscillations and environmental photon resonances remains largely unexplored. Here we describe the Bloch oscillations in a transmon-type qubit attached to high-impedance transmission lines with discrete photon spectra. Transmons are characterized by well-separated charge bands, favoring Bloch oscillations over Landau-Zener tunneling. We find resonances in the voltage-current relation and the spectrum of photons emitted by the Bloch oscillations. The transmon also scatters photons inelastically; we find the cross section for an anti-Stokes-like process whereby photons gain a Bloch oscillation quantum. Our results outline how Bloch oscillations leave fingerprints for experiments across the DC, MHz, and GHz ranges.