We present a perturbation method to compute the out-of-time-ordered correlator in the strongly disordered Heisenberg XXZ model in the deep many-body localized regime. We characterize the discrete structure of the information propagation across the eigenstates, revealing a highly structured light cone confined by the strictly logarithmic upper and lower bounds representing the slowest and fastest scrambling available in this system. We explain these bounds by deriving the closed-form expression of the effective interaction for the slowest scrambling and by constructing the effective model of a half length for the fastest scrambling. We extend our lowest-order perturbation formulations to the higher dimensions, proposing that the logarithmic upper and lower light cones may persist in a finite two-dimensional system in the limit of strong disorder and weak hopping.

Monolayer magnet of $\mathit{XY}$ type composes a pivotal part of the two-dimensional magnetism. As an important suspected $\mathit{XY}$-type antiferromagnet, whether $1L \mathrm{NiP}{\mathrm{S}}_3$ exhibits magnetic order remains elusive. Herein by helicity-resolved Raman and ultrafast spectroscopy of $\mathrm{NiP}{\mathrm{S}}_3$ from bulk to monolayer, we find that $1L \mathrm{NiP}{\mathrm{S}}_3$ is magnetically ordered with a Berezinskii-Kosterlitz-Thouless (BKT) transition at $T_{\mathrm{BKT}}\approx 140$ K. We have also performed large-scale density-matrix renormalization-group calculations to verify the ground state zigzag antiferromagnetic order and Monte Carlo simulation to confirm the BKT-transition temperature in $1L \mathrm{NiP}{\mathrm{S}}_3$. Our investigations establish $1L \mathrm{NiP}{\mathrm{S}}_3$ to be an $\mathit{XY}$ antiferromagnet with a relatively high-temperature BKT transition, providing an important platform for investigating complex couplings and topological excitations.

Measurement-induced phase transitions are the subject of intense current research, both from an experimental and a theoretical perspective. We explore the concept of implementing quantum measurements by coupling a many-body lattice system to an ancillary degree of freedom (implemented using two additional sites), on which projective measurements are performed. We analyze the effect of repeated (“stroboscopic”) measurements on the dynamical correlations of interacting hard-core bosons in a one-dimensional chain. An important distinctive ingredient of the protocol is the fact that the detector ancillae are not reinitialized after each measurement step. The detector thus maintains memory of the accumulated influence by the measured correlated system. Initially, we consider a model in which the ancilla is coupled to a single lattice site. This setup allows obtaining information about the system through Rabi oscillations in the ancillary degrees of freedom, modulated by the ancilla-system interaction. The statistics of quantum trajectories exhibits a “quantum-Zeno-valve effect” that occurs when the measurement becomes strong, with sharp branching between low and high entanglement. We proceed by extending numerical simulations to the case of two ancillae and, then, to measurements on all sites. With this realistic measurement apparatus, we find evidence of a disentangling-entangling measurement-induced transition as was previously observed in more abstract models. The dynamics features a broad distribution of the entanglement entropy.

The subtle interplay between low-dimensionality and spin correlations can lead to exotic ground states with unconventional excitations in two-dimensional honeycomb-lattice-based quantum magnets. Herein, we present the structural, magnetic, and heat capacity measurements; density functional theory $+$ Hubbard $U$ ($\mathrm{DFT}+U$) based electronic structure calculations; and quantum Monte Carlo simulations for $\mathrm{NaCuIn}{\left({\mathrm{PO}}_4\right)}_2$. The structure of $\mathrm{NaCuIn}{\left({\mathrm{PO}}_4\right)}_2$ consists of a well-separated, $\mathit{S}=\frac{1}{2}$ distorted ${\mathit{J}}_1\text{−}{\mathit{J}}_2$ honeycomb layer which is a combination of the magnetic couplings ${\mathit{J}}_1$ (forming spin dimers) and ${\mathit{J}}_2$ (constituting spin chains). At high temperatures, the magnetic susceptibility $\chi \left(T\right)$ follows paramagnetic behavior with a Curie-Weiss temperature ${\theta }_{\mathrm{CW}}\approx -16$ K, implying the presence of antiferromagnetic interactions. A broad maximum is observed at about $13\mathrm{K}$ in $\chi \left(T\right)$, indicating the presence of short-range spin correlations. The quantum Monte Carlo simulations using the $\mathit{S}=\frac{1}{2}{\mathit{J}}_1\text{−}{\mathit{J}}_2$ Heisenberg model on a distorted honeycomb lattice are in good agreement with the measured magnetic susceptibility data. The obtained ratio of the exchange couplings ($\frac{{\mathit{J}}_2}{{\mathit{J}}_1}$) is 2.63, which is consistent with the value obtained from our $\mathrm{DFT}+U$ calculations. The title material undergoes a magnetic long-range order at 0.4 K in the heat capacity, which is suppressed with an applied magnetic field of 10 kOe. The magnetic heat capacity data follow a linear temperature-dependent behavior well above the transition temperature, suggesting the presence of gapless excitations. The observed behavior can be attributed to the presence of low connectivity and weak magnetic frustration in this two-dimensional distorted honeycomb lattice.

The orthorhombic MnCoSi compounds have been found to present a large magnetoelastic coupling, which is regarded as the source for the magnetocaloric effect (MCE) and the magnetostrictive effect. As a result, these compounds are potential materials for caloric applications such as solid-state refrigeration. In the present study, we offer fundamental insights in the magnetoelastic coupling in these compounds based on their structural, metamagnetic, and MCE behavior. The directly measured adiabatic temperature change ($\mathrm{\Delta }{T}_{\mathrm{ad}}$) in different initial temperatures (down to 18 K) and pulsed magnetic fields (up to 40 T) presents a moderate MCE performance (the maximum $\mathrm{\Delta }{T}_{\mathrm{ad}}=–3.1\mathrm{K}$ for a field change of 13 T), which results from the metamagnetic behavior of these compounds. Furthermore, the magnetization measurements in pulsed (and static) magnetic fields indicate that the magnetoelastic coupling is significantly enhanced for increasing fields resulting in an improved saturation magnetization. The metamagnetic transition is continuously pushed to lower temperatures in higher fields. The phase diagram constructed from the experimental transition temperatures $T_t$ and the critical magnetic fields ${\mu }_0{H}_{\mathrm{cr}}$ indicate that the transition is terminated below 18 K and that ferromagnetism is stabilized for fields above 22.3 T. Our results provide unique insights into the strong magnetoelastic coupling under high pulsed magnetic fields, providing guidelines for the design of giant magnetocaloric materials for future caloric applications.

Simple shear and pure shear are two common methods for investigating the mechanical responses of materials using first-principles calculations; however, the stress-strain responses and deformation modes obtained are different. Taking HfC and TaC as examples, we first investigated their mechanical responses under the two methods, then proposed a semiconstrained method that combines the advantages of the two methods and can better describe their mechanical responses. Furthermore, we extended the Mohr-Coulomb relation to unify the shear stresses obtained in different ways, which can explain the enhancement of yield shear strength caused by pressure in terms of the friction between atomic layers.

The crystal and electronic structure of $\mathrm{YMn}{\mathrm{O}}_3$ thin films at the paraelectric-ferroelectric phase transition are monitored by scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) during in situ heating experiments. At the phase transition of a 14-unit-cell-thick film at $\sim 325{}^{\circ }\mathrm{C}$, we observe a clear transition from the polar $P{6}_{3}cm$ to the centrosymmetric $P{6}_3/mmc$ space group, manifested by the disappearance of the unit-cell tripling. Further, we reveal significant changes in the EELS data across the phase transition, and interpret them with the help of real-space multiple-scattering calculations. Upon crossing the Curie temperature, striking changes in the O-$K$ near-edge structure are identified, which are attributed to a modification in the Y $4d–\mathrm{O} 2p$ hybridization. In particular, a weakening of the Y $4d{}_z{}^2–\mathrm{O} 2p_z$ hybridization is detected for the paraelectric phase compared to the ferroelectric phase. Upon cooling below the Curie temperature, the ferroelectric phase is recovered and a stronger hybridization along the $c$-axis polarization direction is restored, stabilizing the zone-boundary ${\mathrm{K}}_3$ phonon mode. Our findings demonstrate the high sensitivity of the O-$K$ near-edge structure to structural distortions in $\mathrm{YMn}{\mathrm{O}}_3$ polymorphs and lay the groundwork for future studies on polymorphic phase transformations in improper ferroelectrics whose ferroelectricity emerges through coupling to a nonpolar lattice distortion.

A theoretical study is reported on the origin of an extremely high upper critical field $\sim 70$ T observed in ${\mathrm{UTe}}_2$ with the transition temperature $T_{\mathrm{c}}=1.6$–2 K, which exceeds the conventional orbital depairing limit set by the Fermi velocity and $T_{\mathrm{c}}$ for a superconductor (SC) in the clean limit. We investigate possible violation of the orbital limit in terms of a spin-triplet nonunitary state, which is effectively coupled to the underlying magnetization induced by an external field. This produces the reduced internal field by canceling it via magnetization. We formulate a theory within the Ginzburg-Landau framework to describe this orbital limit violation and analyze experimental data on the upper critical fields for various field orientations in ${\mathrm{UTe}}_2$. We show that the orbital limit violation for a spin-triplet SC, as well as the Pauli-Clogston limit violation for a spin-singlet SC, constitutes a complete and useful framework for examining the high field physics of superconductors in the clean limit.

Topological semimetal nanowires in proximity with $s$-wave superconductors are promising for Majorana-based topological quantum computers. To braid the Majorana modes, an interconnected nanowire network coupled to superconductor islands is required. Here, we have fabricated the nanostructures based on two Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanowires connected in parallel to form a symmetric superconducting quantum interference device (SQUID). A proximity-induced superconducting state is achieved in the SQUID, and its gate voltage and magnetic field dependence are investigated. It is found that the supercurrent can be switched on/off by modulating the gate voltage, behaving as a supercurrent field-effect transistor. Under an out-of-plane magnetic field, the SQUID shows an anomalous magnetic field -enhanced superconductivity behavior near the Dirac point. An in-plane magnetic field experiment shows a typical $\pi$ periodicity of a critical current with the rotation angle. Our work realizes the implementation and characterization of nanowire SQUID structures, which is a significant step toward integrating Dirac semimetal nanowires into scalable superconducting networks for braiding non-Abelian quantum states.

High-temperature superconducting paring is generally believed to be mediated by the magnetic fluctuations that mostly occur within the two-dimensional conducting layers (Cu-O, Fe-As, or Fe-Se) in copper oxides, iron pnictides, iron chalcogenides, etc. Here, on the basis of inelastic neutron scattering measurements on a superconducting iron pnictide ${\mathrm{Ca}}_{0.33}{\mathrm{Na}}_{0.67}{\mathrm{Fe}}_2{\mathrm{As}}_2$, we have discovered a highly three-dimensional spin resonance mode with upward V-shape dispersions both in the $ab$ plane and along the $c$ axis. The superconducting gaps exhibit strong $k_z$ modulations involved with significant changes on the size of one hole pocket and the density state of the $d_{z^2}$ orbital of Fe. The resonance dispersions don't always seem confined by the total gaps summed on those imperfectly nesting Fermi sheets. It is demonstrated that the $c$-axis dependence of both the resonance energy and its intensity in iron pnictides can be universally scaled with the distance between two adjacent Fe-As layers ($d$), and the $k_z$ modulation of the gap is also anticorrelated with $d$. These results highlight the role of interlayer coupling in iron-based superconductivity, suggesting that the interlayer pairing may also be driven by magnetic fluctuations under certain spin-orbit couplings.

Inspired by experiments on magic angle-twisted bilayer graphene, we present a lattice mean-field model for the quantum anomalous Hall effect in a moiré setting. Our hopping Hamiltonian provides a simple route to a moiré Chern insulator in commensurately twisted systems. We study our model in the ribbon geometry and demonstrate the presence of thick chiral edge states that have a transverse localization that scales with the moiré lattice spacing. We also study the electronic structure of a domain wall between opposite Chern insulators. Our model and results are relevant to experiments that will image or manipulate the moiré quantum anomalous Hall edge states.

The ubiquitous Rashba spin splitting occurring around time-reversal-invariant momenta (TRIM) reflects the fundamental interplay between spin-orbit coupling (SOC) and crystalline symmetry. Unexpectedly, recent measurements reveal novel Rashba spin splitting around the momenta lacking time-reversal symmetry (non-TRIM), whose mechanism remains elusive. Here, we theoretically elucidate the microscopic origin of Rashba spin splitting around non-TRIM and identify the design principles for its appearance. Conventionally, a Zeeman-like spin splitting dominates around non-TRIM due to the broken time-reversal symmetry and diminishes the Rashba-like features. Our results show that the Bloch wave function with specific orbital representation at certain non-TRIM could eliminate the first-order SOC effect. As a result, the corresponding little group hosts a type of quasisymmetry that commutes with the dominating SOC Hamiltonian, leading to a tiny SOC gap and helical spin texture. Furthermore, such a quasi-symmetry-induced tiny gap could manifest large Berry curvature, which is desirable for various topological applications. Our work expands the concept of Rashba physics beyond the conventional TRIM scenario, paving an avenue for designing novel spin-splitting materials through the cooperation of symmetry and orbital engineering.

To provide the best possible performance, modern infrared photodetector designs necessitate extremely precise modeling of the superlattice absorber region. We advance the Rode's method for the Boltzmann transport equation in conjunction with the $\mathbf{k}·\mathbf{p}$ band structure and the envelope function approximation for a detailed computation of the carrier mobility and conductivity of layered type-II superlattice structures, using which we unravel two crucial insights. First, the significance of both elastic- and inelastic-scattering mechanisms, particularly the influence of the interface roughness and polar optical phonon scattering mechanisms in technologically relevant superlattice structures. Second, that the structure-specific Hall mobility and Hall scattering factor reveal that temperature and carrier concentrations significantly affect the Hall scattering factor, which deviates significantly from unity even for small magnetic fields. This reinforces the caution that should be exercised when employing the Hall scattering factor in experimental estimations of drift mobilities and carrier concentrations. Our research hence offers a comprehensive microscopic understanding of carrier dynamics in such technologically relevant superlattices. Our models also provide highly accurate and precise transport parameters beyond the relaxation-time approximation and thereby paving the way to develop physics-based device modules for mid-wavelength infrared photodetectors.

Element-specified x-ray magnetic circular dichroism (XMCD) and the anomalous Hall effect (AHE) have been investigated to provide deep insights into the magnetic and electronic structural evolutions of different proton-doping spinel ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ (NCO) films. Our results show that the magnetic dichroism at the Co $L_{23}$-edge is mainly contributed by the spin magnetic moment at the tetrahedral ($T_d$)-site ${\mathrm{Co}}^{2+}$ in pristine NCO. The spin magnetic moments at Co and Ni sites have antiparallel alignments. The magnetic anisotropy is related to the considerable orbital magnetic moments therein. Upon proton-doping in ${\mathrm{NCO}}_{\text{int}}$ films (the intermediate state with the mixed phases of ${\mathrm{H}}_x\mathrm{NCO}$ and NCO), the magnetic dichroism at the Co $L_{23}$-edge still exists. The doping of one extra electron resides at the octahedral ($O_h$)-site ${\mathrm{Co}}^{3+}$ partially and changes ${\mathrm{Co}}^{3+}$ ($S=0)$ to ${\mathrm{Co}}^{2+}$ ($S=3/2$). This $O_h$-site ${\mathrm{Co}}^{2+}$ aligns in an antiparallel manner with $T_d$-site ${\mathrm{Co}}^{2+}$ through O-ions mediated antiferromagnetic superexchange interaction, resulting in a reduced spin magnetic moment in ${\mathrm{NCO}}_{\text{int}}$ compared to pristine NCO films. Moreover, the reduced magnetic anisotropy in ${\mathrm{NCO}}_{\text{int}}$ is due to the decreasing of the orbital magnetic moment, which can be related to the charge redistribution at Co $3d$ energy levels and the crystal-field effect. In the two-proton doped ${\mathrm{H}}_2\mathrm{NCO}$ films, the magnetic dichroism is absent where the antiferromagnetism arises from the antiparallel spin alignments of ${\mathrm{Co}}^{2+}$ in $T_d$ and $O_h$ sites. The intrinsic ferromagnetism is also verified by the AHE, which shows a square-shaped hysteresis as a function of the magnetic fields for pristine NCO and ${\mathrm{NCO}}_{\text{int}}$. Interestingly, the proton-doped ${\mathrm{NCO}}_{\text{int}}$ films exhibit an enhanced coercive field and the exchange bias effect with an asymmetric hysteresis loop at low temperatures, which can be ascribed to the antiferromagnetic phase stemming from partial Co valence state reduction by proton doping.