The spin dynamics in the block magnetic phase of the iron-based ladder compound ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ has been studied by means of single-crystal inelastic neutron scattering. Using linear spin-wave theory and Monte Carlo simulations, our analysis points to a magnetic Heisenberg model with effective frustrated antiferromagnetic couplings only, able to describe both the exotic block order and its dynamics. This new and purely antiferromagnetic picture offers a fruitful perspective to describe multiferroic properties, but also understand the origin of the stripelike magnetic instability observed under pressure as well as in other parent compounds with similar crystalline structure.

We study the effects of infinite-range (IR) interaction on the localization properties of one-dimensional disordered harmonic chains. Two kinds of disordered models are considered: one is with disordered masses, but constant spring coefficient (mass model), and the other is with disordered spring coefficient, but constant mass (spring model). It is found that the IR interaction induces a gap between the $\omega =0$ and the excited frequency band for both types of disordered models, and their gaps are both proportional to the IR interaction coefficient $\gamma$ for larger $\gamma$. In addition, the width of the excited frequency band of the mass model is also proportional to $\gamma$ for larger $\gamma$, while that of the spring model is independent of $\gamma$. By employing the normalized participation ratio, we find that the low-frequency modes of the mass model except for the lowest frequency change from extended to localized states, while the localized properties of the spring model modes remain unchanged. We also discuss the effects of IR interaction on the dynamic behaviors of the two kinds of disordered models.

We study magnon excitations and their interactions in honeycomb antiferromagnets with the Dzyaloshinskii-Moriya interaction. When an applied magnetic field is turned on, the classical ground state of the system is in noncollinear antiferromagnetic order. Therefore, it is crucial to consider the interactions between magnons. We find an exact and well-formed solution of the bosonic Bogoliubov–de Gennes Hamiltonian, which allows us to analytically study the topological properties and damping effects of the magnon bands. In particular, the formulas of the Berry phase and Berry curvature for the magnon bands are worked out analytically. The Mn-based trichalcogenide ${\mathrm{MnPS}}_3$ is taken as a candidate material realization to estimate the order of magnitude of the thermal Hall conductivity and its evolution with the temperature and the Dzyaloshinskii-Moriya interaction strength. We develop a nonlinear spin wave theory to study the damping effects of the topological magnons and find the different damping behaviors of the upper and lower topological magnon bands when tuning the applied magnetic field. In some parameter regions, the damping effects are so strong that the free magnon description breaks down. Finally, we discuss how to observe these effects in experiments.

The emergence of nonrelativistic band spin splitting in fully compensated collinear antiferromagnets provides an intriguing platform for investigating nontrivial spin-dependent phenomena. Here, we explore the relationship between magnetic symmetry and electronic states of two-dimensional (2D) antiferromagnets and demonstrate that nonrelativistic and giant band spin splitting and topological electronic features emerge in room-temperature antiferromagnetic metal ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ that is sufficiently stable to be exfoliated from the layered compound. More interestingly, the anisotropic spin-momentum coupling of monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ gives rise to the spin-Hall effect where the spin-charge conversion ratio is expected to be over 30%, and two fully spin-polarized and separated conduction channels with opposite spin polarization make it an ideal candidate for an electrode that is applied in the antiferromagnetic tunneling junction with large magnetoresistance. Our work elucidates the unexplored potential of 2D antiferromagnetic metals with nonrelativistic band spin splitting for their practical applications in spintronics.

Materials that can host macroscopic persistent current are important because they are useful for energy storage. However, there are very few examples of such materials in nature. Superconductors are known as an example in which flow of supercurrent can persist up to 100 000 years. The chiral magnetic current is possibly the second example predicted by the chiral magnetic effect. It was proposed to be realized in recently discovered Weyl semimetals. However, a no-go theorem negates the chiral magnetic effect and shows that the chiral magnetic current is generally absent in any equilibrium condensed-matter system. Here we show how to break the no-go theorem by resorting to dynamical transitions in time-frequency space. By driving an insulator using a time-periodic potential and coupling it to a phonon heat bath that provides suitable dissipation, we show that a Floquet-Weyl semimetallic phase with Fermi-Dirac–like distribution emerges. Furthermore, we show that, even in the presence of a static magnetic field, the resulting steady Floquet-Weyl semimetal supports nonvanishing chiral magnetic current. Our dynamical model provides a systematic way to fully realize the chiral magnetic effect in condensed-matter systems.

In this work, we propose an exactly solvable two-dimensional lattice model of strongly correlated electrons that realizes a quantum anomalous Hall insulator with Chern number $\mathcal{C}=1$. First, we show that the interplay of ionic potential, Rashba spin-orbit coupling, and Zeeman splitting leads to the appearance of quantum anomalous Hall effect. Next, we calculate in an exact manner the Chern number for the correlated system where electron-electron interactions are introduced in the spirit of the Hatsugai-Kohmoto model using two complementary methods, one relying on the properties of many-body ground state and the other utilizing single-particle Green's function, and subsequently we determine stability regions. By leveraging the presence of inversion symmetry, we find boundaries between topological and trivial phases on the analytical ground. Notably, we show that in the presence of correlations, onset of topological phase is no longer signaled by a spectral gap closing consistently with phenomenon called first-order topological transition in literature. We provide a clear microscopic understanding of this inherently many-body feature by pinpointing that the lowest energy excited states in the correlated system are no longer of the single-particle nature and thus, are not captured by a spectral function.

We applied time-resolved transient absorption spectroscopy to systematically investigate the ultrafast optical responses of condensed matter systems. Under an intense pump pulse, absorption spectra indicate that the noninteracting electrons of band insulators produce a field-induced redshift, known as the dynamical Franz-Keldysh effect, as commonly expected. In contrast to band insulators, in Mott insulators, unconventional spectra are observed which do not fully reflect the dynamical Franz-Keldysh effect. While the spectra still exhibit fishbonelike structures mimicking the dynamical Franz-Keldysh effect, they show a negative difference absorption below the band edge, rendering a blueshift. In addition, the decomposed calculation reveals that the negative difference absorption is mainly contributed from the creation and the annihilation of transient double occupancy, implying that the unconventional spectra are purely driven by the electron correlations. These demonstrated unconventional responses can guide us to better understanding of the correlation-originated ultrafast electron dynamics in condensed matter systems.

We perform a numerical study of Floquet topological insulators with temporal disorder to investigate the existence of quantized charge transport without Anderson localization. We first argue that in setups with temporal imperfections, Anderson localization cannot be expected but bulk transport is diffusive in the long-time limit. In a second step we compute the corrections to the cumulative averaged pumped charge due to the temporal disorder and show that transport is characterized by two regimes: the transient regime, represented by a plateau for uncorrelated disorder, and the long-time behavior with a common scaling law for both uncorrelated and correlated disorder. Most notably, our numerical results indicate that the dynamic corrections vanish in the long-time limit, such that quantized charge transport and diffusive bulk motion can coexist in temporally disordered Floquet topological insulators.

Focusing on examples of Majorana zero modes on the corners of a two-dimensional lattice, we introduce a method to find parameter regions where the Majorana modes are perfectly localized on a single site. Such a limit allows us to study the dimerization structure of the sparse bulk Hamiltonian that results in the higher-order topology of the system. Furthermore, such limits typically provide an analytical understanding of the system energy scales. Based on the dimerization structure we extract from the two-dimensional model, we identify a more general stacking procedure to construct Majorana zero modes in arbitrary corners of a $d$-dimensional hypercube, which we demonstrate explicitly in $d\le 3$.

High-frequency transport in the edge states of the quantum spin Hall (QSH) effect has rarely been explored, though it could cast light on the scattering mechanisms taking place therein. Here we report on the measurement of the plasmon velocity in topological HgTe quantum wells both in the QSH and quantum Hall (QH) regimes, using harmonic GHz excitations and phase-resolved detection. We observe low plasmon velocities in both regimes, with, in particular, large transverse widths in the QH regime despite a sharp edge confinement profile. We ascribe these observations to the prominent influence of charge puddles forming in the vicinity of edge channels. Together with other recent works, it suggests that puddles play an essential role in the edge state physics and probably constitute a main hurdle on the way to clean and robust edge transport.

Useful in the enhancement of light-matter interaction, localization of light is at the heart of photonics studies. Different approaches have been proposed to localize light, including those based on dynamical localization, topological trivial or nontrivial defects in the band gap of photonic crystals, and bound states in the continuum. Recent studies on the non-Hermitian skin effect have provided us new means to localize waves. Inspired by two recent studies [Li et al., Phys. Rev. Lett. 128, 223903 (2022); Zhu and Gong, Phys. Rev. B 106, 035425 (2022)], we propose an innovative method towards localized light in continuous systems via photonic corner skin modes, which are hybrid skin-topological modes as a special kind of second-order non-Hermitian skin effect with symmetrically distributed gain and loss. Specifically, we propose to make use of weak pseudoinversion symmetric gain and loss, which does not close the band gap of the system, to realize a non-Hermitian photonic Chern insulator with chiral edge states. The chiral edge states then accumulate at certain corners of the system. Intriguing phenomena such as corner skin modes arising from an underlying bipolar second-order non-Hermitian skin effect and multiple-corner skin modes are predicted in continuous systems.

We investigate single crystals of the trigonal antiferromagnet ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ ($P\overline{3}m1$) by means of electrical transport, magnetization measurements, x-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES), and ab initio band structure calculations ($\mathrm{DFT}+U$). We find that the electrical resistivity of ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (colossal magnetoresistance effect). Resonant magnetic scattering reveals a magnetic ordering vector of $q=\left(00\frac{1}{2}\right)$, corresponding to an $A$-type antiferromagnetic order, below $T_{\mathrm{N}}=23.7\mathrm{K}$. We find that the moments are canted out of the $a\text{−}a$ plane by an angle of about ${40}^{\circ }\pm {10}^{\circ }$ and aligned along the [100] direction in the $a\text{−}a$ plane. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below $\approx 150\mathrm{K}$, the temperature below which also the field dependence of the material's resistivity starts to increase. An analysis of the infrared reflectivity spectrum at $T=295\mathrm{K}$ allows us to resolve the main phonon bands and intraband and interband transitions, and estimate indirect and direct band gaps of $E_i^{\mathrm{opt}}=0.09$ and $E_d^{\mathrm{opt}}=0.33\mathrm{eV}$, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly $T$ independent above and below $T_{\mathrm{N}}$. The comparison of the theoretical and experimental data shows a weak intermixing of the Eu $4f$ states close to the $\mathrm{\Gamma }$ point with the bands formed by the phosphorous $3p$ orbitals leading to an induction of a small magnetic moment at the P sites.

Twisted double bilayer graphene is a compensated semimetal near the charge neutrality point with the presence of small electron and hole pockets in its band structure. We show that strong Coulomb attraction between the electrons and holes can lead to the formation of indirect excitons. Condensation of these excitons at low temperature creates an excitonic metal with charge density wave order for an appropriate range of interaction strength. This has interesting implications for low-temperature transport in the system as a function of carrier density and temperature. The reorganization of the single-particle excitations and their density of states in the excitonic metal can lead to peaks in resistivity as a function of carrier density, recently seen in experiments at low temperatures. The fluctuations of the Landau damped order parameter in the quantum critical metal lead to non-Fermi liquid behavior, which can explain the sublinear $T^{2/3}$ dependence of the resistance near the charge neutrality point.

The ternary chalcogenide superconductor ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{Se}}_2$ was discovered to have charge density wave (CDW) order and electron-phonon coupling (EPC). However, it is still debated whether the phase transition at $\sim 240\mathrm{K}$ is a CDW one. Here, we systematically performed Raman scattering investigations on single-crystal ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{Se}}_2$. Angle-resolved polarized Raman spectroscopic measurements reveal the in-plane anisotropic EPC in ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{Se}}_2$, which inhibits the Raman intensity. Temperature-dependent x-ray diffraction and Raman measurements indicate the existence of the incommensurate (ICCDW) and commensurate (CCDW) CDW orders below $\sim 250$ and $\sim 170$ K, respectively, in ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{Se}}_2$. The appearance of zone-folded phonon modes at $\sim 170$ K confirms the CCDW-induced $2\times 2$ reconstruction of the Brillouin zone. Semiquantitative analysis based on Landau theory of second-order phase transitions of CDW-correlated and uncorrelated phonon modes reflects extensive coupling between the phonons and CDW orders. Our findings elucidate the existence of CCDW and ICCDW and confirm the critical role of EPC in CDW transitions in ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{Se}}_2$.

The recently discovered kagome metals $A{\text{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ offer an exciting route to study exotic phases arising due to interplay between electronic correlations and topology. In addition to superconductivity, these materials exhibit a charge-density-wave (CDW) phase occurring at $\sim 100$ K, whose origin still remains elusive. The robust multicomponent $2\times 2$ CDW phase in these systems is of great interest due to the presence of an unusually large anomalous Hall effect. In quasi-two-dimensional systems with weak interlayer coupling, fluctuation-driven exotic phases may appear. In systems with multicomponent order parameters, fluctuations may lead to establishment of composite order when only products of individual order parameters condense while the individual ones themselves remain disordered. We argue that such a fluctuation-driven regime of composite CDW order may exist in thin films of kagome metals above the CDW transition temperature. It is suggested that the melting of the trihexagonal state in the material doped away from the Van Hove singularities gives rise to a pseudogap regime where the spectral weight is concentrated in small pockets and most of the original Fermi surface is gapped. Our findings suggest the possible presence of exotic phases in the weakly coupled layered kagome metals, more so in the recently synthesized thin films of kagome metals.

We apply first-principles calculations to investigate $V_{\mathrm{B}}\text{−}{\mathrm{C}}_{\mathrm{B}}$ and $V_{\mathrm{B}}\text{−}{\mathrm{Si}}_{\mathrm{B}}$ complexes in cubic boron nitride as potential quantum defects. We find that these centers possess a triplet ground-state spin, analogous to that of the prototype quantum defect, the NV center in diamond. In contrast, the main optical transition of these complexes occurs in the telecom O-band, making them appealing for quantum networking applications. Furthermore, the coupling to phonons is weaker than in the NV center, resulting in a much larger fraction of photons (22%) being emitted in the zero-phonon line. One inherent drawback of the longer emission wavelength is stronger nonradiative recombination; however, the resulting lower quantum efficiency can be mitigated by cavity coupling.

The intensely studied measurement-induced entanglement phase transition has become a hallmark of nonunitary quantum many-body dynamics. Usually, such a transition only appears at the level of each individual quantum trajectory, and is absent for the density matrix averaged over measurement outcomes. In this work, we introduce a class of adaptive random circuit models with feedback that exhibit transitions in both settings. After each measurement, a unitary operation is either applied or not depending on the measurement outcome, which steers the averaged density matrix towards a unique state above a certain measurement threshold. Interestingly, the transition for the density matrix and the entanglement transition in the individual quantum trajectory in general happen at different critical measurement rates. We demonstrate that the former transition belongs to the parity-conserving universality class by explicitly mapping to a classical branching-annihilating random-walk process.