Distinguishing between topological Majorana zero modes and quasi Majorana modes—trivial low-energy Andreev bound states—is an important step towards realizing topological hybrid nanowire devices. We propose that this distinction can be made by connecting a hybrid nanowire to a dissipative resonant level realized by a quantum dot weakly coupled to high resistance leads. We show that the qualitative temperature scaling of the width of the resonant level zero-bias conductance peak provides a signature unique to true Majorana zero modes. The degeneracy induced by a true Majorana zero mode is further shown to stabilize this conductance peak against particle-hole asymmetry.

Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer ${\mathrm{MoS}}_2$ are studied. Two distinct exit charge state distributions, characterized by high and low charge states, are observed. They are accompanied by smaller and larger kinetic energy losses, as well as scattering angles, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independent of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for ${\mathrm{MoS}}_2$ and graphene. A common dependence of the kinetic energy loss on the charge exchange for ${\mathrm{MoS}}_2$ as well as graphene is also observed. Considering the similarities of the zero band-gap material graphene and the 1.9 eV band-gap material ${\mathrm{MoS}}_2$, we suggest that electron transport on the femtosecond timescale is dominated by the strong influence of the ion's Coulomb potential in contrast to the dispersion defined by the material's band structure.

Low-frequency quasilocalized modes of amorphous glass appear to exhibit universal density of states, depending on the frequencies as $D\left(\omega \right)\sim {\omega }^4$. To date, various models of glass formers with short-range binary interaction and network glass with both binary and ternary interactions were shown to conform with this law. In this paper, we examine granular amorphous solids with long-range electrostatic interactions and find that they exhibit the same law. To rationalize this wide universality class, we return to a model proposed by Gurevich, Parshin, and Schober (GPS) [Phys. Rev. B 67, 094203 (2003)] and analyze its predictions for interaction laws with varying spatial decay, exploring this wider-than-expected universality class. Numerical and analytic results are provided for both the actual system with long-range interaction and for the GPS model.

Recently, it was established that there exists a direct relation between the non-Hermitian skin effects, strong dependence of spectra on boundary conditions for non-Hermitian Hamiltonians, and boundary zero modes for Hermitian topological insulators. On the other hand, in terms of the spectral theory, the skin effects can also be interpreted as instability of spectra for nonnormal (non-Hermitian) Hamiltonians. Applying the latter interpretation to the former relation, we develop a theory of zero modes with quantum anomaly for general Hermitian lattice systems. Our theory is applicable to a wide range of systems: Majorana chains, nonperiodic lattices, and long-range hopping systems. We relate exact zero modes and quasi-zero modes of a Hermitian system to spectra and pseudospectra of a non-Hermitian system, respectively. These zero and quasi-zero modes of a Hermitian system are robust against a class of perturbations even if there is no topological protection. The robustness is measured by nonnormality of the corresponding non-Hermitian system. We also present explicit construction of such zero modes by using a graphical representation of lattice systems. Our theory reveals the presence of nonnormality-protected zero modes, as well as the usefulness of the nonnormality and pseudospectra as tools for topological and/or non-Hermitian physics.

By means of the discrete truncated Wigner approximation, we study dynamical phase transitions arising in the steady state of transverse-field Ising models after a quantum quench. Starting from a fully polarized ferromagnetic initial condition, these transitions separate a phase with nonvanishing magnetization along the ordering direction from a disordered symmetric phase upon increasing the transverse field. We consider two paradigmatic cases, a one-dimensional long-range model with power-law interactions $\propto 1/r^{\alpha }$ decaying algebraically as a function of distance $r$ and a two-dimensional system with short-range nearest-neighbor interactions. In the former case, we identify dynamical phase transitions for $\alpha \lesssim 2$ and we extract the critical exponents from a data collapse of the steady-state magnetization for up to 1200 lattice sites. We find identical exponents for $\alpha \lesssim 0.5$, suggesting that the dynamical transitions in this regime fall into the same universality class as the nonergodic mean-field limit. The two-dimensional Ising model is believed to be thermalizing, which we also confirm using exact diagonalization for small system sizes. Thus, the dynamical transition is expected to correspond to the thermal phase transition, which is consistent with our data upon comparing to equilibrium quantum Monte Carlo simulations. We further test the accuracy of the discrete truncated Wigner approximation by comparing against numerically exact methods such as exact diagonalization, tensor network, as well as artificial neural network states and we find good quantitative agreement on the accessible time scales. Finally, our work provides an additional contribution to the understanding of the range and the limitations of qualitative and quantitative applicability of the discrete truncated Wigner approximation.

Recent research has shown that a proper spatiotemporal modulation of material properties (i.e., mass density or stiffness) can break reciprocity for elastic waves; however, such concepts are challenging to implement experimentally in mechanical waveguides. Piezoelectric metamaterials offer the possibility of parameter modification via electrical circuitry, constituting a convenient platform for spatiotemporal modulation. We introduce a fully coupled electromechanical framework and circuit strategies to enable nonreciprocal piezoelectric metamaterials using various schemes that include capacitive and inductive shunts as well as smooth modulation and nonsmooth switching. The high-fidelity framework presented herein does not rely on the assumption of simplified effective material property representation, and can be used to explore and predict the complete dynamics of the modulated system, accounting for full-system stability, circuit limitations, and more complex modulation schemes. A set of results is presented based on nonreciprocal configurations that illustrate potential implementation schemes, and demonstrate the versatility of piezoelectric materials for the design of truly integrated acoustic/elastic wave devices featuring nonreciprocal transmission.

We report on our evaluation of field-induced magnetic moments in the paramagnetic state of the $\text{spin-}\frac{1}{2}$ antiferromagnetic trimerized ($J_1-J_2-J_2$) chain compound ${\mathrm{Cu}}_3{\left({\mathrm{P}}_2{\mathrm{O}}_6\mathrm{OD}\right)}_2$ with $J_1=111\mathrm{K}$ and $J_2=30\mathrm{K}$ [M. Hase et al., Phys. Rev. B 76, 064431 (2007)]. Magnetic reflections with integer indices, generated by field-induced magnetic moments, were observed at neutron-diffraction experiments performed in an applied magnetic field, and we evaluated the magnitudes of the moments to be $M_1=0.43\left(2\right){\mu }_{\mathrm{B}}/\mathrm{Cu}$ and $M_2=0.013\left(10\right){\mu }_{\mathrm{B}}/\mathrm{Cu}$ on the two crystallographic ${\mathrm{Cu}}^{2+}$ (Cu1 and Cu2) sites, respectively, at 6 T and 1.6 K. The resulting ratio $M_2/M_1=0.03\left(2\right)$ is in good agreement with the theoretical value for the ratio of the magnetization at the two sites. We thus conclude that for this well-understood spin system (which has the advantage of being amenable to detailed theoretical calculations) the extracted field-induced magnetic moments reproduce the correct information on the magnetization. Our result leads us to believe that we can precisely evaluate exchange interactions in paramagnets with multiple exchange interactions and multiple crystallographic magnetic-ion sites by using field-induced magnetic moments in combination with macroscopic physical quantities. This idea is expected to be applicable to a wide variety of quantum and frustrated magnets without long-range order.

While the electrical current manipulation of antiferromagnets (AFMs) has been demonstrated, the extent of the studied AFM materials has been limited with few systematic experiments and a poor understanding. We compare the electrical current switching of the exchange-bias field ($H_{\text{ex}}$) in AFM-${\mathrm{Mn}}_{3}A\mathrm{N}$/ferromagnet-${\mathrm{Co}}_3\mathrm{FeN}$ bilayers. An applied pulse current can manipulate $H_{\text{ex}}$ with respect to the current density and FM layer magnetization, which shifts exponentially as a function of the current density. We found that the saturation current density and exponential decay constant $\tau$ increase with the local moment of AFM Mn atoms. Our results highlight the effect of the AFM local moment to electrical current switching of $H_{\text{ex}}$, although it has a near-zero net magnetization, and may provide a facile way to explore the electrical current manipulation of AFM materials.

Spin-transfer-driven oscillations of a transverse domain wall confined to a perpendicular spin-valve structure are investigated using a one-dimensional model. The stack consists of a polarizer, nonmagnetic spacer, soft free layer, and pinned magnetic layer. It is found that the domain-wall oscillation frequency is a nonmonotonic, highly asymmetrical function of applied electric current, showing a strong dependence on the current direction and the relative strengths of the interfacial and bulk spin-transfer torques. Micromagnetic analysis reveals that the surprising and atypical oscillator response is due to an interplay between the interfacial spin-transfer torque, the bulk spin-transfer torque, the exchange torque, and the damping torque. The underlying physical and material responses are examined, including the important role of the domain-wall twist. The competitions between the involved torques under different operating conditions suggest that the oscillator could serve as a model system to investigate magnetic and spintronic phenomena at the nanoscale. The observed current-dependent twisting of the free-layer magnetization about the axis of precession may further be found interesting for investigations of the interaction between spin-polarized current and chiral spin structures.

We report measurements on ${\mathrm{Ti}}_4{\mathrm{MnBi}}_2$, where a crystal structure involving linear chains of Mn ions suggests one-dimensional magnetic character. The electrical resistivity is metallic, consistent with the results of electronic-structure calculations that find a robust Fermi surface albeit with moderate electronic correlations. A Curie-Weiss fit to the magnetic susceptibility suggests that the Mn moments are in the low-spin $S=1/2$ configuration. Neutron diffraction measurements detect weak antiferromagnetic order within the Mn chains, with further evidence for the small staggered moment coming from the entropy associated with the ordering peak in the specific heat as well as from the results of spin-polarized electronic-structure calculations. The antiferromagnetic moments are apparently associated with the $d_{x^2-y^2}$ and $d_{xy}$ orbitals of Mn while the remaining Mn orbitals are delocalized and nonmagnetic. Strong quantum fluctuations, possibly related to an electronic instability that forms the Mn moment or to the one-dimensional character of ${\mathrm{Ti}}_4{\mathrm{MnBi}}_2$, nearly overcome magnetic order.

We have investigated the magnetic correlations in the candidate Weyl semimetals ${\mathrm{EuCd}}_{2}P{n}_2$ ($Pn=\mathrm{As}$, Sb) by resonant elastic x-ray scattering at the ${\mathrm{Eu}}^{2+}$ $M_5$ edge. The temperature and field dependence of the diffuse scattering of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ provide direct evidence that the Eu moments exhibit slow ferromagnetic (FM) correlations well above the Néel temperature. By contrast, the diffuse scattering in the paramagnetic phase of isostructural ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ is at least an order of magnitude weaker. The FM correlations present in the paramagnetic phase of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ could create short-lived Weyl nodes.

We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Au}}_x\right)}_2{\mathrm{As}}_2$ system by polarization-resolved Raman spectroscopy and neutron diffraction. In this system, there are two phase transitions: a tetragonal-to-orthorhombic structural phase transition at temperature $T_S$ and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature $T_N$ ($\le T_S$). $T_S$ and $T_N$ almost coincide in the pristine compound, whereas they differ by as much as 8 K for compounds with dilute gold substitution for iron. Raman coupling to the $A_g$(As) phonon is forbidden for the $XY$ scattering geometry in the tetragonal phase above $T_S$, whereas it becomes allowed in the orthorhombic phase below $T_S$: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below $T_S$, first, weak $A_g$(As) phonon mode intensity appears in the $XY$ scattering geometry spectra; however, the mode's intensity is significantly enhanced in the magnetic phase below $T_N$. The $A_g$(As) phonon also shows an asymmetric line shape below $T_N$ and an anomalous linewidth broadening upon Au doping. We describe the anomalous behavior of the $A_g$(As) mode in the $XY$ scattering geometry using a Fano model involving the $A_g$(As) phonon interacting with the $B_{2g}(D_{4h}$)-symmetry-like electron-hole continuum. We conclude that the temperature dependence of light coupling amplitude to the $A_g$(As) phonon follows the evolution of the magnetic order parameter $M\left(T\right)$. We propose that the intensity enhancement of the $A_g$(As) phonon in the $XY$ scattering geometry below $T_N$ is due to electronic anisotropy induced by the collinear SDW order parameter.

We present a novel method for embedding spin and charge fluctuations in an anisotropic, multiband, and full-bandwidth Eliashberg treatment of superconductivity. Our analytical framework, based on the random phase approximation, allows for a self-consistent calculation of material-specific characteristics in the interacting, and more specifically, the superconducting state. We apply this approach to bulk FeSe as a representative for the iron-based superconductors and successfully solve for the superconducting transition temperature $T_c$, the gap symmetry, and the gap magnitude. We obtain $T_c\approx 6$ K, consistent with experiment ($T_c\approx 8$ K), as well as other quantities in good agreement with experimental observations, thus supporting spin fluctuations mediated pairing in bulk FeSe. On the contrary, applying our approach to monolayer FeSe on ${\mathrm{SrTiO}}_3$ we find that spin fluctuations within the full Eliashberg framework give a $d$-wave gap with $T_c\le 11$ K and therefore cannot provide an explanation for a critical temperature as high as observed experimentally ($T_c\approx 70$ K). Our results hence point towards interfacial electron-phonon coupling as the dominant Cooper pairing mediator in this system.

We present analytic results for a special dimer model on the nonbipartite and nonplanar checkerboard lattice that does not allow for parallel dimers surrounding diagonal links. We exactly calculate the number of closed packed dimer coverings on finite checkerboard lattices under periodic boundary conditions and determine all dimer-dimer correlations. The latter are found to vanish beyond a certain distance. We find that this solvable model, despite being nonplanar, is in close kinship with well-known paradigm-setting planar counterparts that allow exact mappings to ${\mathbb{Z}}_2$ lattice gauge theory.

Layered magnets have recently received tremendous attention, however spin-exchange coupling mechanism across their interlayer regions is yet to be revealed. Here, we report a Bethe-Slater-curve (BSC)-like behavior in nine transition metal dichalcogenide bilayers ($M{X}_2$, $M=\mathrm{V}$, Cr, Mn; $X=\mathrm{S}$, Se, Te) and established interlayer spin-exchange coupling mechanisms at their van der Waals gaps using first-principle calculations. The BSC-like behavior offers a distance-dependent interlayer antiferromagnetic (AFM) to ferromagnetic (FM) transition. This phenomenon is explained with the spin-exchange coupling mechanisms established using bilayer $\mathrm{CrS}{\mathrm{e}}_2$ as a prototype in this work. The Se $p_{\mathrm{z}}$ wave functions from two adjacent interfacial Se sublayers overlap at the interlayer region. The spin alignment of the region determines interlayer magnetic coupling. At a shorter interlayer distance, Pauli repulsion at the overlapped region dominates and thus favors antiparallel oriented spins leading to interlayer AFM. For a longer distance, kinetic-energy gain of polarized electrons across the bilayer balances the Pauli repulsion and the bilayer thus prefers an interlayer FM state. In light of this, the AFM-FM transition is a result of competition between Pauli and Coulomb repulsions and kinetic-energy gain. All these results open a route to tune interlayer magnetism and the revealed spin-exchange coupling mechanisms are paramount additions to those previously established ones.

The I-Mn-V antiferromagnet, NaMnBi, develops a very large positive magnetoresistance (MR) up to 10 000% at 2 K and 9 T in crystals showing a semiconductor-to-metal transition (SMT). In the absence of an SMT, a modest (20%) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears giving rise to new Bragg peaks due to charge and defect ordering in a checkerboard pattern, with two kinds of modulation vectors, $q_1=(\frac{2}{3},0,1)$ and $q_2=(\frac{2}{3},\frac{1}{3},\frac{1}{2})$. This constitutes a superlattice transition ($T_s$) that lowers the symmetry from the high-temperature centrosymmetric $P4/nmm$ to the noncentrosymmetric $P\overline{4}m2$. In crystals with a large MR, a close to room temperature $T_s$ is observed with $q_1$ appearing first, followed by $q_2$. In crystals with low MR, however, $T_s$ is much lower and only $q_1$ is observed. The charge modulation and spin fluctuations may both contribute to the enhancement of MR.

The recovery of metastable structures formed at high pressure has been a long-standing goal in the field of condensed matter physics. While laser-driven compression has been used as a method to generate novel structures at high pressure, to date no high-pressure phases have been quenched to ambient conditions. Here we demonstrate, using in situ x-ray diffraction and recovery methods, the successful quench of a high-pressure phase which was formed under laser-driven shock compression. We show that tailoring the pressure release path from a shock-compressed state to eliminate sample spall, and therefore excess heating, increases the recovery yield of the high-pressure $\omega$ phase of zirconium from 0% to 48%. Our results have important implications for the quenchability of novel phases of matter demonstrated to occur at extreme pressures using nanosecond laser-driven compression.

Simple elastic models of spin-crossover compounds are known empirically to exhibit classical critical behavior. We demonstrate how the long-range interactions responsible for this behavior arise naturally upon integrating out mechanical fluctuations of such a model. A mean-field theory applied to the resulting effective Hamiltonian quantitatively accounts for both thermodynamics and kinetics observed in computer simulations, including a barrier to magnetization reversal that grows extensively with system size. For nanocrystals, which break translational symmetry, a straightforward extension of mean-field theory yields similarly accurate results.