Strontium titanate (STO), apart from being a ubiquitous substrate for complex-oxide heterostructures, possesses a multitude of strongly coupled electronic and mechanical properties. Surface acoustic wave (SAW) generation and detection offers insight into electromechanical couplings that are sensitive to quantum paraelectricity and other structural phase transitions. Propagating SAWs can interact with STO-based electronic nanostructures, in particular ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO). Here, we report the generation and detection of SAW within LAO/STO heterointerfaces at cryogenic temperatures ($T\ge 2$ K) using superconducting interdigitated transducers. The temperature dependence shows an increase in the SAW quality factor that saturates at $T\approx 8$ K. The effect of backgate tuning on the SAW resonance frequency shows the possible acoustic coupling with the ferroelastic domain wall evolution. This method of generating SAWs provides a pathway towards the dynamic tuning of ferroelastic domain structures, which are expected to influence the electronic properties of complex-oxide nanostructures. Devices that incorporate SAWs may in turn help to elucidate the role of ferroelastic domain structures in mediating electronic behavior.

We show the role of strain in magnetoelectric effect through coupling between order parameters and their interplay using infrared/Raman-active optical phonons in hexagonal manganite ($h\text{−}\mathrm{ErMn}{\mathrm{O}}_3$). The magnetoelectric coupling is arbitrated through strain from infrared-active phonons, which by symmetry are also Raman active. The identification of the primitive order parameters by spin-phonon coupling opens a promising avenue to realize the strategy based on coupling of spins, optical phonons, and strain to create magnetoelectrics with strain-mediated interaction through spin-lattice coupling in bulk inducing a ferromagnetic-ferroelectric state in an antiferromagnetic-paraelectric phase.

We present a machine learning (ML) framework for large-scale dynamical simulations of charge density wave (CDW) states. The charge modulation in a CDW state is often accompanied by a concomitant structural distortion, and the adiabatic evolution of a CDW order is governed by the dynamics of the lattice distortion. Calculation of the electronic contribution to the driving forces for large systems, however, is computationally very expensive. Assuming the principle of locality for electron systems, a neural-network model is developed to accurately and efficiently predict local electronic forces with input from neighborhood configurations. Importantly, the ML model enables a linear complexity algorithm for dynamical simulations of CDWs. As a demonstration, we apply our approach to investigate the phase ordering dynamics of the Holstein model, a canonical system of CDW order. Our large-scale simulations reveal an intriguing growth of CDW domains that deviates significantly from the expected Allen-Cahn law for phase ordering of Ising-type order parameter field. This anomalous domain growth could be attributed to the complex structure of domain walls in this system. Our paper highlights the promising potential of ML-based force-field models for dynamical simulations of functional electronic materials.

Magnetic iron garnet films with perpendicular magnetic anisotropy (PMA) are highly desirable for spintronics and magnonics applications. In this study, we have grown single-phase epitaxial ${\mathrm{Bi}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ films on a ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ substrate with (111) orientation. The nondestructive ${sin}^2\psi$ technique of the x-ray diffraction measurement demonstrates the negative value of residual stress, suggesting the compressive nature of stress in the system. Ferromagnetic resonance and magnetic force microscopy (MFM) have been used to probe the magnetic anisotropies and magnetic domain texture, respectively. In-plane (IP) angular variation of the resonance field $H_r$ reveals a weak IP magnetic anisotropy with pronounced sixfold and fourfold symmetries in 15 and 10 nm thick films, respectively. For the 15 nm thick film, the out-of-plane angular variation of $H_r$ shows oblique magnetic anisotropy with the magnetic easy axis making an angle of ${25}^{\circ }$ with respect to the film plane. The oblique magnetic easy axis is further confirmed with $\theta$ mapping of the MFM image. However, for the 10 nm thick film there is complete reorientation of the magnetic easy axis perpendicular to the film plane. The cubic magnetocrystalline anisotropy along with the magnetoelastic anisotropy overcomes the shape anisotropy and leads to negative effective magnetic anisotropy $K_{\mathrm{eff}}$ and PMA. MFM images show a labyrinthine mazelike domain texture, typical of iron garnet films with PMA.

We demonstrate the existence and study in detail the features of chiral bimerons which are static solutions in an easy-plane magnet with the Dzyaloshinskii-Moriya interaction. These are skyrmionic textures with an integer topological charge, and they present essential analogies to the meron configurations introduced in the context of quark confinement in the O(3) nonlinear $\sigma$ model. We employ a Möbius transformation to show that, for weak chirality, bimeron configurations approach Belavin-Polyakov solutions characterized by tightly bound vortex and antivortex parts of the same size. Stronger chirality induces a larger difference in the vortex and antivortex sizes and also a detachment of merons, suggesting the possibility for a topological phase transition. Exploiting the fact that bimerons of opposite topological charges may exist in the same material, we demonstrate numerically a mechanism to generate meron pairs.

The magnetic structure is crucial in determining the physical properties inherent in magnetic compounds. We present an adequate descriptor for magnetic structure with proper magnetic symmetry and high discrimination performance, which does not depend on artificial choices for coordinate origin, axis, and magnetic unit cell in crystal. We extend the formalism called “smooth overlap of atomic positions” (SOAP), providing a numerical representation of atomic configurations to that of magnetic moment configurations. We introduce the descriptor in terms of the vector spherical harmonics to describe a magnetic moment configuration and partial spectra from the expansion coefficients. We discuss that the lowest-order partial spectrum is insufficient to discriminate the magnetic structures with different magnetic anisotropy, and a higher-order partial spectrum is required in general to differentiate detailed magnetic structures on the same atomic configuration. We then introduce the fourth-order partial spectrum and evaluate the discrimination performance for different magnetic structures, mainly focusing on the difference in magnetic symmetry. The modified partial spectra that are defined not to reflect the difference of magnetic anisotropy are also useful in evaluating magnetic structures obtained from the first-principles calculations performed without spin-orbit coupling. We apply the present method to the symmetry-classified magnetic structures for the crystals of ${\mathrm{Mn}}_3\mathrm{Ir}$ and ${\mathrm{Mn}}_3\mathrm{Sn}$, which are known to exhibit anomalous transport under the antiferromagnetic order, and examine the discrimination performance of the descriptor for different magnetic structures on the same crystal.

Artificial spin ices (ASIs) are designable arrays of interacting nanomagnets that span a wide range of magnetic phases. Here, we demonstrate that the phase of an artificial kagome spin ice can be identified through its initial magnetization curve. As a proof of concept, micromagnetic simulations of these curves were performed starting from representative microstates of different phases of the system. We show that the curves are characterized by phase-specific features in such a way that a supervised classification algorithm predicts the phase of the initial microstate with good reliability. Moreover, most curves associated with paramagnetic and spin ice 1 phases are recognizable simply by visual inspection. This achievement represents a different strategy for identifying phases in ASIs that is easier and more accessible than magnetic imaging techniques normally used for this task.

Magnetoresistance and ordinary Hall effects are studied depending on magnetic field, temperature, electron concentration, and the orientation between the an electric field, magnetic field, and the spin spiral propagation direction. The magnetic field is assumed to be lower than the pinning value $B_{\mathrm{pin}}$. Solving the Boltzmann equation for a nonequilibrium distribution function, we find the giant magnetoresistance in helimagnets by more than one order of magnitude higher than in a ferromagnetic phase. The giant magnetoresistance is only observed along the spin spiral propagation axis with a magnetic field directed perpendicular to this axis. The explanation of this effect is given in terms of the spin separation of electrons along the spiral axis specific to a helical state. We study ordinary Hall effect and find that the Hall constant remains almost unchanged compared to the free electron model at the same electron concentration in low and high magnetic fields. We also observe the dramatic increase in anisotropy in the longitudinal conductivity with an applied magnetic field. The temperature dependence reveals the increase or decrease of magnetoresistance depending on electron concentration. We provide the explanations of the found effects using the simple models.

We show the presence of a quantum critical point in $\mathrm{Ce}{\mathrm{Ru}}_{2\text{−}x}{\mathrm{Co}}_x{\mathrm{Ge}}_2$ $(0\le x\le 2)$ using detailed heat capacity and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements. The millikelvin heat capacity and $\mu \mathrm{SR}$ results for the critical composition $x_{\mathrm{C}}\sim 1.5$ are consistent with the spin fluctuation model of the quantum critical point. The time-field-temperature scaling of the heat capacity and $\mu \mathrm{SR}$ data further suggests the role of critical and cooperative spin fluctuations near the antiferromagnetic quantum critical point. In addition, the temperature-dependent x-ray absorption near edge structure reveals intermediate valence fluctuation down to the lowest temperature for the compositions $x\ge 1.5$. The noninteger valence for the critical composition $x_{\mathrm{C}}\sim 1.5$ is further demonstrated by room temperature x-ray photoelectron spectroscopy. Interestingly, the quantum criticality in $\mathrm{Ce}{\mathrm{Ru}}_{2\text{−}x}{\mathrm{Co}}_x{\mathrm{Ge}}_2$ is governed by the survival of valence and spin fluctuations down to the lowest temperature.

The dynamics of the geometric phase are studied in inhomogeneous quantum spin chains after a quench. Analytic expressions of the Pancharatnam geometric phase (PGP) are derived for both the period-2 quantum Ising chain (QIC) and the disordered QIC. In the period-2 QIC, due to the periodic modulation, the phase difference between the boundaries of the half Brillouin zone $(0,\pi ]$ changes with time, and consequently, the winding number ${\nu }_D\left(t\right)$ of the PGP is not quantized and thus is not topological anymore. Nevertheless, the PGP and its winding number show nonanalytic singularities at the critical times of the dynamical quantum phase transitions (DQPTs). This relation between the PGP and the DQPT is further confirmed in the disordered QIC. It is found that the critical time of the DQPT inherited from the homogeneous system and the additional one induced by weak disorder are also accompanied by the nonanalytic singularity of the PGP by decomposing the PGP into each quasiparticle mode. The connection between the nonanalytic behavior of the PGP at the critical time and the DQPT, regardless of whether the winding number is topological, can be explained by the fact that they both arise when the Loschmidt amplitude vanishes.

Using spin currents generated by fs laser pulses, we demonstrate excitation of GHz ferromagnetic resonance and THz ferrimagnetic exchange resonances in Co/Gd/Co/Gd multilayers by time-resolved magneto-optic Kerr effect measurements. Varying the Gd layer thickness allows for a tuning of the resonance spectrum by manipulating the total angular momentum and strength of effective exchange fields between the antiferromagnetically coupled layers. Close to the compensation point of angular momentum, a minimum in the frequency of the exchange-dominated mode and a maximum in the frequency of the ferromagnetic resonance mode is observed. Finally, to gain better understanding of the excitation mechanism, we analyze the anomalous variation in the measured exchange mode amplitude as a function of its frequency. A peak in this amplitude in the vicinity of the compensation point of angular momentum is explained using a macrospin model, taking nonlinear effects at finite precession amplitudes into account.

We study the surface criticality of a three-state antiferromagnetic Potts model on a simple cubic lattice, whose bulk critical behavior belongs to the universality class of the XY model because of emergent O(2) symmetry. We find that the surface antiferromagnetic next-nearest-neighboring interactions can drive the extraordinary-log phase to an ordinary phase, and the transition between the two phases belongs to the universality class of the well-known special transition of the XY model. Further enhancing the surface next-nearest-neighboring interactions, the extraordinary-log phase reappears, but the main critical behaviors are shown in the sublattices; the special point between the ordinary phase and the sublattice extraordinary-log phase belongs to a different universality class.

The electrical control of magnons opens up new ways to transport and process information for logic devices. In magnetoelectrical multiferroics, the Dzyaloshinskii-Moriya (DM) interaction directly allows for such control and hence is of major importance. We determine the origin and the strength of the (converse) spin-current DM interaction in the $R3c$ bulk phase of multiferroic ${\mathrm{BiFeO}}_3$ based on density functional theory. Our data support only the existence of one DM interaction contribution originating from the spin-current model. By exploring the magnon dispersion in the full Brillouin zone, we show that the exchange is isotropic, but the DM interaction and anisotropy prefer any propagation and any magnetization direction within the full (111) plane. Our work emphasizes the significance of the asymmetric potential induced by the spin current over the structural asymmetry induced by the anionic octahedron in multiferroics such as ${\mathrm{BiFeO}}_3$.

We study the boundary-dissipated transverse field Ising model described by a Lindblad master equation and exactly solve its Liouvillian spectrum in the whole parameter space. By mapping the Liouvillian into a Su-Schrieffer-Heeger model with imaginary boundary potentials under a parity constraint, we solve the rapidity spectrum analytically and thus construct the Liouvillian spectrum strictly with a parity constraint condition. Our results demonstrate that the Liouvillian spectrum displays four different structures, which are characterized by different numbers of segments. By analyzing the properties of rapidity spectrum, we can determine the phase boundaries between different spectrum structures analytically and prove the Liouvillian gap fulfilling a duality relation in the weak and strong dissipation region. Furthermore, we unveil the existence of a dynamical duality, i.e., the long-time relaxation dynamics exhibits almost the same dynamical behavior in the weak and strong dissipation region as long as the duality relation holds true.

The conventional Bardeen-Cooper-Schrieffer model of superconductivity assumes a frequency-independent order parameter, which allows a relatively simple description of the superconducting state. In particular, its excitation spectrum readily follows from the Bogoliubov–de Gennes (BdG) equations. A more realistic description of a superconductor is the Migdal-Eliashberg theory where the pairing interaction, the order parameter, and electronic self-energy are strongly frequency dependent. This paper combines these ingredients of phonon-mediated superconductivity with the standard BdG approach. Surprisingly, we find qualitatively new features, such as the emergence of a shadow superconducting gap in the quasiparticle spectrum at energies close to the Debye energy. We show how these features reveal themselves in standard tunneling experiments. Finally, we also predict the existence of additional high-energy bound states, which we dub “dark Andreev states.”

We investigate the formation mechanism of the recently proposed interlayer electronic superfluid state due to repulsive interaction in graphene double layers. Using the renormalization group argumentation we show how the emergence of a particular interlayer staggered order parameter wins the competition between several possible pairing mechanisms. We determine the effective action for the fluctuations of the order parameter and study its behavior in strong background magnetic fields. The filling factors obtained from the constraint conditions put on this action are in qualitative agreement with the previously experimentally observed results.

Characteristics of $\mathrm{SN}{\left(\mathrm{F}/\mathrm{N}\right)}_n\mathrm{IN}{\left(\mathrm{F}/\mathrm{N}\right)}_m\mathrm{S}$ Josephson junctions with $n,m=1$ to 3 are reported. Here, S is a superconductor (Nb), F is a magnetic material (nickel), N is a nonmagnetic metal (aluminum), and I is an insulator $\left(\mathrm{Al}/\mathrm{Al}{\mathrm{O}}_{\mathrm{x}}\right)$. The devices with $n=m=1$ and $n=1,m=2$ display critical current versus magnetic field $\left[I_{\mathrm{c}}\left(H\right)\right]$ dependences that imply the appearance of fractional magnetic vortices in the junction, corresponding to a half of the flux quantum. Stochastic switching between the two half-vortex polarities with emission of an integer vortex was observed. In the devices with $n=m=3$, a typical $I_{\mathrm{c}}\left(H\right)$ dependence consists of a background current with a large modulation period, and a Fraunhofer-like pattern with a small modulation period on top of the background current. The Fraunhofer-like pattern may be completely or partially inverted; in these states, there is a component of the Josephson current flowing against the bias current, indicating the presence of the $\pi$ phase in the junction. The experiments give evidence that switching between the zero and $\pi$ states can, potentially, be magnetically controlled.