In a microring of a superconductor with a spin-triplet equal-spin pairing state, a fluxoid, a combined object of magnetic flux and circulating supercurrent, can penetrate as half-integer multiples of the flux quantum. A candidate material to investigate such half-quantum fluxoids is ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We fabricated ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ microrings using single crystals and measured their resistance behavior under magnetic fields controlled with a three-axis vector magnet. Proper Little-Parks oscillations in the magnetovoltage as a function of an axially applied field, associated with fluxoid quantization, are clearly observed using bulk single-crystalline superconductors. We then performed magnetovoltage measurements with additional in-plane magnetic fields. By carefully analyzing both the voltages $V_{+}$ ($V_{-}$) measured at positive (negative) current, we find that, above an in-plane threshold field of about 10 mT, the magnetovoltage maxima convert to minima. We interpret this behavior as the peak splitting expected for the half-quantum fluxoid states.

Pressure-induced polarization switching in ferroelectric thin films has emerged as a powerful method for domain patterning, allowing us to create predefined domain patterns on free surfaces and under thin conductive top electrodes. However, the mechanisms for pressure-induced polarization switching in ferroelectrics remain highly controversial, with flexoelectricity, polarization rotation and suppression, and bulk and surface electrochemical processes all being potentially relevant. Here we classify possible pressure-induced switching mechanisms, perform elementary estimates, and study in depth using phase-field modeling. We show that magnitudes of these effects are remarkably close and give rise to complex switching diagrams as a function of pressure and film thickness with nontrivial topology or switchable and nonswitchable regions.

The dynamics of the phase transition in antiferroelectric ${\mathrm{PbZrO}}_3$  which is the subject of a decades-long debate, is examined using first-principles-based simulations. This is achieved through the development of a computational approach that allows calculations of generalized complex susceptibilities at an arbitrary point of the Brillouin zone. Application of this approach to the case of ${\mathrm{PbZrO}}_3$  predicts the temperature evolution of many of its lattice modes, some of which remain elusive or even “invisible” in experiments. The computational data suggest that two lattice modes are primarily responsible for the antiferroelectric phase transition in this material: the one associated with oxygen octahedra tilts dynamics and the other due to lead ions antipolar vibrations.

This article reports on the experimental realization of a flow-free, pseudospin-based acoustic topological insulator designed using the strategy of zone folding. Robust sound one-way propagation is demonstrated with the presence of non-spin-mixing defects. On the other hand, it is shown that spin-mixing defects, which break the geometric symmetry and therefore the pseudo-time-reversal symmetry, can open up nontrivial band gaps within the edge state frequency band, and their width can be tailored by the extent of the defect. This provides a possible route for realizing tunable acoustic topological insulators.

Phonon spectra of CdSe nanoplatelets (2–6 ML) with the zinc-blende structure were calculated from first principles within the density-functional theory. It turned out that the Lamb modes in nanoplatelets are in fact optical rather than acoustic vibrations. Phonon spectra of the nanoplatelets show the appearance of a large number of low-frequency modes inherited from TA phonons in bulk CdSe. Calculations of the Raman spectra indicate a need to revise the interpretation of available experimental data. The largest contribution to the Raman spectra is provided by the quasi-Lamb modes with the $A_1$ symmetry. The $B_2$ modes whose frequencies depend on the environment of nanoplatelets and whose properties are closest to the properties of LO phonons explain the results obtained in the “nanoparticle-on-mirror” geometry. The features in Raman spectra previously attributed to surface optical (SO) modes should be interpreted as a manifestation of lower-order quasi-Lamb $A_1$ modes. Calculations of the infrared spectra find, in addition to the TO phonon line, the appearance of intense lines from surface modes originating from terminating F(Cl) atoms on the surface of nanoplatelets and true SO modes.

We provide numerical evidence that a Kelvin-Helmholtz instability occurs in the Dirac fluid of electrons in graphene and can be detected in current experiments. This instability appears for electrons in the viscous regime passing though a micrometer-scale obstacle and affects measurements on the time scale of nanoseconds. A possible realization with a needle-shaped obstacle is proposed to produce and detect this instability by measuring the electric potential difference between contact points located before and after the obstacle. We also show that, for our setup, the Kelvin-Helmholtz instability leads to the formation of whirlpools similar to the ones reported in Bandurin et al. [Science 351, 1055 (2016)]. To perform the simulations, we develop a lattice Boltzmann method able to recover the full dissipation in a fluid of massless particles.

We report significant correlation of the multiferroic order and ferroelectric polarization to the orthorhombic structural distortion for ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{BaCo}}_4{\mathrm{O}}_7$ ($x\le$ 0.05). Analysis of the synchrotron diffraction studies reveal that La doping increases considerable structural distortion, which is associated with the increase of multiferroic ordering temperature and electric polarization. Intriguingly, the value of polarization increases remarkably to $\approx 385\mu \mathrm{C}/{\mathrm{m}}^2$ ($x=0.05$) from $\approx 150\mu \mathrm{C}/{\mathrm{m}}^2$ ($x=0$) for a 3 kV/cm poling field. Synchrotron diffraction studies in magnetic field provides an important clue, where structural distortion provides more impact on the polarization value than the contribution from the change in unit cell volume. Geometric magnetic frustration holds the key for the occurrence of the structural distortions, around which multiferroic ordering takes place for ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$. Our work thus highlights crystal structural distortion as a rich playground for tuning multiferroic order as well as polarization value.

We identify the phase boundary between spiral spin and ferromagnetic phases in ${\mathrm{Au}}_2\mathrm{Mn}$ at a critical pressure of 16.4 kbar, as determined by neutron diffraction, magnetization, and magnetoresistance measurements. The temperature-dependent critical field at a given pressure is accompanied by a peak in magnetoresistance and a step in magnetization. The critical field decreases with increasing temperature and pressure. The critical pressure separating the spiral phase and ferromagnetism coincides with the disappearance of the magnetoresistance peak, where the critical field goes to zero. The notable absence of an anomalous Hall effect in the the ferromagnetic phase is attributable to the high conductivity of this material.

Gallium arsenide (GaAs) is the most widely used second-generation semiconductor with a direct band gap, and it is being increasingly used as nanofilms. However, the magnetic properties of GaAs nanofilms have never been studied. Here we find by comprehensive density-functional-theory calculations that GaAs nanofilms cleaved along the $\langle 111\rangle$ and $\langle 100\rangle$ directions become intrinsically metallic films with strong surface magnetism and the magnetoelectric effect. Surface magnetism and electrical conductivity are realized via a combined effect of charge transfer induced by spontaneous electric polarization through the film thickness and spin-polarized surface states. The surface magnetism of $\langle 111\rangle$ nanofilms can be significantly and linearly tuned by a vertically applied electric field, endowing the nanofilms with unexpectedly high magnetoelectric coefficients, which are tens of times higher than those of ferromagnetic metals and transition-metal oxides.

This work shows an unconventional route for spin-driven ferroelectricity originating from a metastable magnetic field-induced canting of the chromium sublattice in the presence of gadolinium moments in $\mathrm{GdCrTi}{\mathrm{O}}_5$ at low temperatures. Compared to the isostructural neodymium compound, significant differences of magnetism and magnetoelectric effects are seen. We present the results of thorough investigations of temperature and magnetic field dependent magnetization as well as ac and dc magnetic susceptibility. These bulk measurements are complemented by local-probe spectroscopy utilizing electron-spin resonance and muon-spin rotation/relaxation for probing the chromium moments. Ferroelectric order is inferred from pyro- and magnetocurrent measurements. $\mathrm{GdCrTi}{\mathrm{O}}_5$ shows a pyrocurrent signal around 10 K, only if the system is cooled in an applied magnetic field exceeding 10 kOe. A distinct spin-driven ferroelectric order is revealed in this state for temperatures below 10 K, which can be switched by changing the magnetic-field direction and the polarity of the electric field. The magnetic measurements reveal no clear signature of long-range magnetic ordering. The presence of such ‘‘meta-magnetoelectric-type’’ behavior in the absence of any ‘‘meta-magnetic’’ behavior is rare in literature. Our microscopic spectroscopy results indicate significant changes of the magnetic properties around ∼10 K. Probably there exists exchange frustration between Gd and Cr moments, which prevents long-range magnetic ordering at higher temperatures. Below 10 K, weak ferromagnetic magnetic order occurs by minimizing frustration due to lattice distortion, which supports magnetoelectric coupling. However, nonpolar distortions attain appreciable values after application of magnetic fields above 10 kOe, obviously breaking spatial inversion symmetry and creating ferroelectricity.

We propose a surprisingly efficient procedure to map a sophisticated treatment of electronic correlations to a few-parameter model Hamiltonian. By pushing first-principles coupled-cluster methods to the limit we get a lucid picture of the electronic excitations responsible for the magnetic interactions in a paradigm ${\mathrm{Ni}}_4$ cluster. Using an extended Hubbard model, we demonstrate that the magnetism in this system cannot be explained solely on the basis of spin models but additionally requires consideration of the indirect mechanism involving hopping and on-site repulsion. We present a protocol to extract the corresponding parameters from experiment or calculations.

We theoretically investigate spin-wave propagation through a magnetic metamaterial with spatially modulated Dzyaloshinskii-Moriya interaction. We establish an effective Schrödinger equation for spin waves and derive boundary conditions for spin waves passing through the boundary between two regions having different Dzyaloshinskii-Moriya interactions. Based on these boundary conditions, we find that the spin wave can be amplified at the boundary and the spin-wave band gap is tunable either by an external magnetic field or the strength of Dzyaloshinskii-Moriya interaction, which offers a spin-wave analog of the field-effect transistor in traditional electronics.

Electrical (longitudinal) resistivity ${\rho }_{xx}$, at $H=0$ and $H=80$ kOe, anomalous Hall resistivity ${\rho }_{xy}^{AH}$, and magnetization $M$, have been measured at different temperatures in the range 5–300 K on the ${\mathrm{Co}}_2\mathrm{FeSi}$ (CFS) Heusler-alloy thin films, grown on Si(111) substrate, with thickness ranging from 12 to 100 nm. At fixed fields $H=0$ and $H=80$ kOe, ${\rho }_{xx}\left(T\right)$ goes through a minimum at $T=T_{\min }$ (which depends on the film thickness) in all the CFS thin films. In sharp contrast, both the anomalous Hall coefficient $R_A$ and ${\rho }_{xy}^{AH}$ monotonously increase with temperature without exhibiting a minimum. Elaborate analyses of ${\rho }_{xx}$, $R_A$, and ${\rho }_{xy}^{AH}$ establishes the following. (i) The enhanced electron-electron Coulomb interaction (EEI) quantum correction (QC) is solely responsible for the upturn in “zero-field” and “in-field” ${\rho }_{xx}\left(T\right)$ at $T<T_{\min }$ whereas the interband spin-flip electron-magnon ($e\text{−}m$) and intraband non-spin-flip electron-phonon ($e\text{−}p$) inelastic-scattering processes, present even at $T<T_{\min }$, overwhelm the EEI QC at temperatures above $T_{\min }$ and thereby cause ${\rho }_{xx}$ to increase with temperature. At all temperatures, $e\text{−}m$ scattering contribution dominates over the $e\text{−}p$ scattering contribution. (ii) Compared to the intrinsic ($I$) Berry-phase curvature mechanism, skew scattering (Sk) and side-jump (Sj) both make sizable contributions to ${\rho }_{xy}^{AH}\left(T\right)$ or $R_A\left(T\right)$ but, at any temperature in the range 5–300 K, the contribution of Sj is considerably larger than that of Sk. The observations (i) and (ii) provide a conclusive experimental evidence for the theoretical prediction that the EEI quantum corrections to anomalous Hall resistivity/coefficient vanish for both skew scattering and side-jump mechanisms even though the EEI corrections to longitudinal resistivity are finite. A strong suppression (by an order of magnitude) is observed in the anomalous Hall conductivity from the resonantly enhanced intrinsic value of ${10}^3$ S/cm when the Fermi level is located near the anticrossing of band dispersions split by spin-orbit interaction. This suppression is shown to correspond to one or both of the possibilities: the resonance condition is not satisfied, or the interband spin-flip inelastic electron-magnon scattering enhances the side-jump contribution at the expense of the intrinsic contribution. We demonstrate that $R_A$, or equivalently ${\rho }_{xy}^{AH}$, scales with ${\rho }_{xxT}$, the temperature-dependent part of ${\rho }_{xx}$, over a considerably wide temperature range (5–300 K) only when ${\rho }_{xy}^{AH}$ is corrected for the observed temperature dependence of spontaneous magnetization and ${\rho }_{xx}$ for the EEI effects.

In the present work a superconducting Co/${\mathrm{CoO}}_x$/${\text{Cu}}_{41}{\text{Ni}}_{59}$/Nb/${\text{Cu}}_{41}{\text{Ni}}_{59}$ nanoscale thin film heterostructure is investigated, which exhibits a superconducting transition temperature, $T_c$, depending on the history of magnetic field applied parallel to the film plane. In more detail, around zero applied field, $T_c$ is lower when the field is changed from negative to positive polarity (with respect to the cooling field), compared to the opposite case. We interpret this finding as the result of the generation of the odd-in-frequency triplet component of superconductivity arising at noncollinear orientation of the magnetizations in the ${\text{Cu}}_{41}{\text{Ni}}_{59}$ layer adjacent to the ${\mathrm{CoO}}_x$ layer. This interpretation is supported by superconducting quantum interference device magnetometry, which revealed a correlation between details of the magnetic structure and the observed superconducting spin-valve effects. Readout of information is possible at zero applied field and, thus, no permanent field is required to stabilize both states. Consequently, this system represents a superconducting magnetic random access memory element for superconducting electronics. By applying increased transport currents, the system can be driven to the full switching mode between the completely superconducting and the normal state.

A systematic temperature-dependent study of the femtosecond optical superconducting (SC) state destruction and recovery in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ cuprate superconductor by means of the all-optical polarization-sensitive multipulse spectroscopy is presented. At low temperatures and a partial SC state suppression, an anisotropic SC-gap recovery time scale is suggested by the data. The SC state destruction and recovery dynamics are compared to the recent TR-ARPES-inferred SC-gap dynamics and a qualitative agreement is found. Using a phenomenological response function, the experimental data are also compared to time-dependent Ginzburg-Landau model simulations.

The internal chirality of Cooper pairs is shown to modify strongly the response of a superconductor to the local heating by a laser beam. The suppression of the chiral order parameter inside the hot spot appears to induce the supercurrents flowing around the spot region. The chirality also affects the sequential stage of thermal quench developing according to the Kibble-Zurek scenario: besides the generation of vortex-antivortex pairs the quench facilitates the formation of superconducting domains with different chirality. These fingerprints of the chiral superconducting state can be probed by any experimental techniques sensitive to the local magnetic field. The supercurrents encircling the hot spot originate from the inhomogeneity of the state with the broken time-reversal symmetry, and their detection would provide a convenient alternative to the search for spontaneous edge currents sensitive to the boundary properties. Thus, the suggested setup can help to resolve the long-standing problem of unambiguous detection of type of pairing in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, which is considered a good candidate for chiral superconductivity.

The commensurate charge-density wave (CDW) in the layered compound $1T-{\mathrm{TaS}}_2$ has hitherto mostly been treated as a quasi-two-dimensional phenomenon. Recent band structure calculations have, however, predicted that the CDW coexists with a nearly one-dimensional metallic dispersion perpendicular to the crystal planes. Using synchrotron radiation-based angle-resolved photoemission spectroscopy, we show that this metallic band does in fact exist. Its occupied band width is in excellent agreement with predictions for a simple ${\tau }_c$ stacking order of the CDW between adjacent layers, and its periodicity in the $c$ direction is $2\pi /c$.

Kitaev's quantum double models, including the toric code, are canonical examples of quantum topological models on a two-dimensional spin lattice. Their Hamiltonian defines the ground space by imposing an energy penalty to any nontrivial flux or charge, but does not distinguish among those. We generalize this construction by introducing a family of Hamiltonians made of commuting four-body projectors that provide an intricate splitting of the Hilbert space by discriminating among nontrivial charges and fluxes. Our construction highlights that anyons are not in one-to-one correspondence with energy eigenspaces, a feature already present in Kitaev's construction. This discrepancy is due to the presence of local degrees of freedom in addition to topological ones on a lattice.

We study the dynamics of an isotropic spin-$\frac{1}{2}$ Heisenberg chain starting in a domain-wall initial condition where the spins are initially up on the left half-line and down on the right half-line. We focus on the long-time behavior of the magnetization profile. We perform extensive time-dependent density-matrix renormalization-group simulations (up to $t=350$) and find that the data are compatible with a diffusive behavior. Subleading corrections decay slowly blurring the emergence of the diffusive behavior. We also compare our results with two alternative scenarios: superdiffusive behavior and enhanced diffusion with a logarithmic correction. We finally discuss the evolution of the entanglement entropy.

We present a characterization of the many-body lattice wave functions obtained from the conformal blocks (CBs) of the Ising conformal field theory (CFT). The formalism is interpreted as a matrix product state using continuous ancillary degrees of freedom. We provide analytic and numerical evidence that the resulting states can be written as BCS states. We give a complete proof that the translationally invariant 1D configurations have a BCS form and we find suitable parent Hamiltonians. In particular, we prove that the ground state of the finite-size critical Ising transverse field (ITF) Hamiltonian can be obtained with this construction. Finally, we study 2D configurations using an operator product expansion (OPE) approximation. We associate these states to the weak pairing phase of the $p+ip$ superconductor via the scaling of the pairing function and the entanglement spectrum.