As a new magnetoelectric material, honeycomb-based antiferromagnet ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ has attracted a great deal of attention due to its prominent magnetoelectric (ME) coupling and high Néel temperature, while the physics of magnetoelectricity is far from understood. In the present study, we present our systematic investigations of the anisotropic ME effect, electric polarization reversal, and nonlinear ME effect of ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ single crystals, thus highlighting the phase diagram extended down to 10 K. Our results provide clear evidence for electric polarization reversal driven by magnetic field ($H$) along the [110] and [1-10] directions, respectively, while no such polarization reversal occurs as $H$ is applied along the [001] direction. The nonlinear ME effects and electric control of magnetism are unambiguously demonstrated. In addition, the angular-dependent probing reveals a 2θ rotation of the induced electric polarization around the $c$ axis upon the rotation of magnetic field by an angle θ. The electric polarization responses and concomitant ME coupling are well explained by means of the metal-ligand hybridization $p\text{−}d$ mechanism. This work represents an essential step forward in the understanding of ME coupling not only in this $A_4{M}_2{\mathrm{O}}_9$ honeycomb magnet.

Nonstabilizerness, also known as magic, quantifies the number of non-Clifford operations needed to prepare a quantum state. As typical measures either involve minimization procedures or a computational cost exponential in the number of qubits $N$, it is notoriously hard to characterize for many-body states. In this paper, we show that nonstabilizerness, as quantified by the recently introduced stabilizer Rényi entropies (SREs), can be computed efficiently for matrix product states (MPSs). Specifically, given an MPS of bond dimension $\chi$ and integer Rényi index $n>1$, we show that the SRE can be expressed in terms of the norm of an MPS with bond dimension ${\chi }^{2n}$. For translation-invariant states, this allows us to extract it from a single tensor, the transfer matrix, while for generic MPSs this construction yields a computational cost linear in $N$ and polynomial in $\chi$. We exploit this observation to revisit the study of ground-state nonstabilizerness in the quantum Ising chain, providing accurate numerical results up to large system sizes. We analyze the SRE near criticality and investigate its dependence on the local computational basis, showing that it is, in general, not maximal at the critical point.

The quasi-one-dimensional metal ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ shows a metal-to-insulator transition upon laser-induced pumping of the d-d exciton at 1.1 eV. The oxygen-chain-related phonon modes are highly susceptible to this metal-to-insulator transition due to their sensitivity to structural and electronic changes. We employ angle-dependent UV-resonance Raman spectroscopy to probe the nonequilibrium oxygen-chain-related phonon dynamics and the concomitant changes in the angle-dependent midinfrared reflectance. The latter shows the expected reduction of charge carrier density upon pumping, whereas the central chain mode at 680 $\mathrm{cm}{}^{-1}$ can be selectively quenched upon pumping the d-d excitons. We find that the underlying quenching mechanism for this phonon is electronically driven and related to the photo-induced bleaching of the charge-transfer transition. First-principles calculations show the connection between phonon dispersion and electronic band structure leading to the selective quenching of the oxygen-chain mode. Our results contribute to a deeper understanding of the interplay between lattice and charge degrees of freedom for exciton-based optoelectronics.

The simulation of open many-body quantum systems is challenging, requiring methods to both handle exponentially large Hilbert spaces and represent the influence of (infinite) particle and energy reservoirs. These two requirements are at odds with each other: larger collections of modes can increase the fidelity of the reservoir representation but come at a substantial computational cost when included in numerical many-body techniques. An increasingly utilized and natural approach to control the growth of the reservoir is to cast a finite set of reservoir modes themselves as an open quantum system. There are, though, many routes to do so. Here, we introduce an accumulative reservoir construction—an ARC—that employs a series of partial refreshes of the extended reservoirs. Through this series, the representation accumulates the character of an infinite reservoir. This provides a unified framework for both continuous (Lindblad) relaxation and a recently introduced periodically refresh approach (i.e., discrete resets of the reservoir modes to equilibrium). In the context of quantum transport, we show that the phase space for physical behavior separates into discrete and continuous relaxation regimes with the boundary between them set by natural, physical timescales. Both of these regimes “turnover” into regions of over- and underdamped coherence in a way reminiscent of Kramers' crossover. We examine how the range of behavior impacts errors and the computational cost, including within tensor networks. These results provide the first comparison of distinct extended reservoir approaches, showing that they have different scaling of error versus cost (with a bridging ARC regime decaying fastest). Exploiting the enhanced scaling, though, will be challenging, as it comes with a substantial increase in (operator space) entanglement entropy.

Planar Josephson junctions comprising semiconductors with strong spin-orbit interaction (SOI) are promising platforms to host Majorana bound states (MBSs). Previous works on MBSs in planar Josephson junctions have focused on electron gases, where SOI is linear in momentum. In contrast, a two-dimensional hole gas in planar germanium (Ge) exhibits SOI that is cubic in momentum. Nevertheless, we show here that due to the particularly large SOI, Ge is a favorable material. Using a discretized model, we numerically simulate a Ge planar Josephson junction and demonstrate that also cubic SOI can lead to the emergence of MBSs. Interestingly, we find that the cubic SOI yields an asymmetric phase diagram as a function of the superconducting phase difference across the junction. We also find that trivial Andreev bound states can imitate the signatures of MBSs in a Ge planar Josephson junction, therefore making the experimental detection of MBSs difficult. We use experimentally realistic parameters to assess if the topological phase is accessible within experimental limitations. Our analysis shows that two-dimensional Ge is an auspicious candidate for topological phases.

Proliferation of defects is a mechanism that allows for topological phase transitions. Such a phase transition is found in two dimensions for the XY model, which lies in the Berezinskii-Kosterlitz-Thouless (BKT) universality class. The transition point can be found using renormalization group analysis. We apply renormalization group arguments to determine the nature of BKT transitions for the three-dimensional plaquette-dimer model, which is a model that exhibits fractonic mobility constraints. We show that an important part of this analysis demands a modified dimensional analysis that changes the interpretation of scaling dimensions upon coarse-graining. Using this modified dimensional analysis, we compute the beta functions of the model and predict a finite critical value above which the fractonic phase melts, proliferating dipoles. Importantly, the transition point is found through a renormalization group analysis that accounts for the phenomenon of UV/IR mixing, characteristic of fractonic models.

We report the Pockels electro-optic properties of ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$ orthorhombic $Pbc{2}_1$ and rhombohedral $R3m$ ferroelectric phases, and we compare them to the well-known rhombohedral $R3c$ ${\mathrm{LiNbO}}_3$ Pockels material from density functional theory calculations using the crystal suite of programs. Specifically, three essential processes are explicitly investigated: The electronic, the ionic (or vibrational), and the piezoelectric contributions. Our calculations reveal that the ionic part coming from the low-frequency phonon modes contributes the most to the electro-optic coefficients of rhombohedral ${\mathrm{LiNbO}}_3$ and of orthorhombic ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$. Additionally, the low-frequency phonon modes exhibit zero contribution to the Pockels coefficients of ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$ rhombohedral $R3m$ phase.

The layered chalcogenide ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ has recently attracted much interest as a strong candidate for a long-sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure ($P$), it is important to clarify the $P$ evolution of a microscopic electronic state in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. Here we report the optical conductivity $\left[\sigma \right(\omega )$] of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ measured at high $P$ to 10 GPa and at low temperatures to 8 K. With cooling at $P=0$, $\sigma \left(\omega \right)$ develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV as reported previously. With increasing $P$, the energy gap becomes narrower and the excitonic peak is diminished. Above a structural transition at $P_s\simeq 3$ GPa, the energy gap becomes partially filled, indicating that ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is a semimetal after the EI state is suppressed by $P$. At higher $P$, $\sigma \left(\omega \right)$ exhibits metallic characteristics with no energy gap. The detailed $P$ evolution of the energy gap and $\sigma \left(\omega \right)$ is presented, and discussed mainly in terms of a weakening of excitonic correlation with $P$.

We experimentally demonstrate a simple graphene/ferrolectric device, termed ferrotronic (electronic effect from ferroelectric) device in which the band structure of single-layer graphene is modified. The device architecture consists of graphene deposited on a ferroelectric substrate which encodes a periodic surface potential achieved through domain engineering. This structure takes advantage of the nature of conduction through graphene to modulate the Fermi velocity of the charge carriers by the variations in surface potential, leading to the emergence of energy minibands and a band gap at the superlattice Brillouin zone boundary. Our work represents a simple route to building circuits whose functionality is controlled by the underlying substrate.

We perform a numerical investigation of Anderson metal-insulator transition (MIT) in a two-dimensional system of chiral symmetry class AIII by combining finite-size scaling, transport, density of states, and multifractality studies. The results are in agreement with the $\sigma$-model renormalization-group theory where MIT is driven by proliferation of vortices. We determine the phase diagram and find an apparent nonuniversality of several parameters on the critical line of MIT, which is consistent with the analytically predicted slow renormalization towards the ultimate fixed point of the MIT. The localization-length exponent $\nu$ is estimated as $\nu =1.55\pm 0.1$.

We have computationally studied the properties of higher-order magnetic anisotropy constants in an $L{1}_0/A1\text{−}\mathrm{FePt}$ core-shell system which is characterized by a strong second-order two-ion Fe-Pt anisotropy component. We show that the core-shell structure induces an unexpected fourth-order anisotropy constant $K_2$, the magnitude of which varies nonmonotonically with the core-size ratio $R$ reaching a peak at $R\approx 0.50$. Furthermore, we find that $K_2$ scales with the normalized magnetization by ${(M/M_s)}^{2.2}$ at temperatures below the Curie temperature, a remarkable deviation from the established Callen-Callen theory which instead predicts a scaling exponent of 10. We construct an analytic model which demonstrates $K_2$ arises from the canting of the core and shell magnetization, and successfully reproduces and justifies the scaling exponent obtained from numerical simulation.

Magnetization and specific heat measurements were conducted on single-crystalline ${\mathrm{Gd}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$, in which magnetic Gd–Al layers with a distorted Kagome lattice structure and nonmagnetic Ru–Al layers are stacked alternately along the $c$ axis. A previous study indicated that the distorted Kagome lattice structure of Gd–Al layers effectively translates into an antiferromagnetic (AFM) triangular lattice in association with ferromagnetic (FM) spin trimerization at low temperatures. The present results indicate that the spin system of ${\mathrm{Gd}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ has two types of anisotropies: easy-plane-type and easy-axis-type anisotropies. The trimers carry magnetic quadrupole moments when the FM directivity of the component spins is imperfect. The origin of the unusual magnetic anisotropies that were observed can be explained by the magnetic quadrupole interactions that take place between the trimers. The stability of a helical structure in the crystal with inversion-symmetry is discussed.

The spin ladder system ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ presents puzzling magnetic behaviors, such as a spin-glass-like phase below $\sim 50$ K. In this paper, an exchange bias effect with a large vertical shift in the field-cooled hysteresis loop is observed below 50 K. We also evidence the existence of uncompensated spins by susceptibility, magnetic remanence, and hysteresis loop measurements. The thermoremanent and isothermoremanent magnetization curves evidence a two-dimensional diluted antiferromagnet (DAFF) nature. Moreover, a nanometer-sized layered structure is observed by scanning electron microscope (SEM) and confirmed by scanning transmission electron microscope (STEM) coupled with electron energy loss spectroscopy (EELS). The discovery of exchange bias in antiferromagnetic ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ crystals adds a new dimension to the research of its superconducting and multiferroic properties.

X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analyzing such spectra are therefore in high demand for complementing experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled many-body XAS (MBXAS) [Y. Liang et al., Phys. Rev. Lett. 118, 096402 (2017)], which approximates the final (initial) state as a Slater determinant constructed from Kohn-Sham (KS) orbitals optimized in the absence (presence) of the relevant core electron has shown promising prospects in evaluating the transition amplitudes. In this article, we show that the MBXAS approach can be rederived using a transition operator expressed entirely in the basis of core-excited state KS orbitals and that this reformulation offers substantial practical and conceptual advantages. In addition to circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals, the aforementioned representation reduces the computational expense by rendering the calculation of such orbitals unnecessary altogether. The reformulated approach also provides a direct pathway for comparing the many-body approximation with the so-called single-particle treatment and indicates the relative importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation. Finally, using the core-excited state basis, we define auxiliary orbitals for x-ray absorption and demonstrate their utility in explaining the spectral intensity by contrasting them with single-particle approximations to the excited state.