The temperature evolution of polar order in an $A$-site complex perovskite $(\mathrm{B}{\mathrm{i}}_{1/2}{\mathrm{K}}_{1/2})\mathrm{Ti}{\mathrm{O}}_3$ (BKT) has been investigated by measurements of dielectric permittivity, depolarization current, and stress-stain curves at elevated temperatures. Upon cooling from high temperatures, BKT first enters a relaxor state and then spontaneously transforms into a ferroelectric state. The analyses of temperature and frequency dependence of permittivity have revealed that polar nanoregions of the relaxor phase appear at temperatures higher than $560{}^{\circ }\mathrm{C}$, and also that their freezing at $296{}^{\circ }\mathrm{C}$ triggers the spontaneous relaxor-ferroelectric transition. We discuss the key factors determining the development of long-range polar order in $A$-site complex perovskites through a comparison with the relaxor $(\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{N}{\mathrm{a}}_{1/2})\mathrm{Ti}{\mathrm{O}}_3$. We also show that application of biasing electric fields and compressive stresses to BKT favors its ferroelectric phase, resulting in a significant shift of the relaxor-ferroelectric transition temperature towards higher temperatures. Based on the obtained results, electric field-temperature and stress-temperature phase diagrams are firstly determined for BKT.

Taking all the magnon modes into account, we derive the skyrmion dynamics in response to a weak external drive. A skyrmion has rotational symmetry, and the magnon modes can be characterized by an angular momentum. For a weak distortion of a skyrmion, only the magnon modes with an angular momentum $\left|m\right|=1$ govern the dynamics of skyrmion topological center. The skyrmion inertia is determined by the magnon modes in the continuum spectrum. For a skyrmion driven by a magnetic field gradient or by a spin transfer torque generated by a current, the dynamical response is practically instantaneous. This justifies the rigid skyrmion approximation used in Thiele's collective coordinate approach. For a skyrmion driven by a spin Hall torque, the torque couples to the skyrmion motion through the magnons in the continuum and damping; therefore the skyrmion dynamics shows sizable inertia in this case. The trajectory of a skyrmion is an ellipse for an ac drive of spin Hall torque.

We address the nature of spin-orbit torques at the magnetic surfaces of topological insulators using the linear-response theory. We find that the so-called Dirac torques in such systems possess a different symmetry compared to their Rashba counterpart, as well as a high anisotropy as a function of the magnetization direction. In particular, the damping torque vanishes when the magnetization lies in the plane of the topological-insulator surface. We also show that the Onsager reciprocal of the spin-orbit torque, the charge pumping, induces an enhanced anisotropic damping. Via a macrospin model, we numerically demonstrate that these features have important consequences in terms of magnetization switching.

We present atomistic spin dynamics modeling of thermally induced magnetization switching (TIMS) of disordered ferrimagnetic TbCo alloys varying the Tb concentration, laser pulse fluence, and its duration. Our results indicate that deterministic TIMS occurs in a wide range of Tb concentrations and at large laser fluences with a pulse duration of 50 fs. We furthermore demonstrate that the occurrence of the transient ferromagneticlike state is necessary, but after first reversal, the system may switch back. The presence of a magnetization compensation point or going through it is shown not to be required. With the increase of the laser pulse duration TIMS becomes stochastic so that for a 1 ps laser pulse width and beyond the deterministic heat-assisted AOS does not exist.

We report detailed neutron scattering studies on ${\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as ${\mathrm{Cu}}_{\mathrm{A}}$ and ${\mathrm{Cu}}_{\mathrm{B}}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the ${\mathrm{Cu}}_{\mathrm{A}}$ sublattice extending up to 300 meV and a much weaker dispersion of ${\mathrm{Cu}}_{\mathrm{B}}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the ${\mathrm{Cu}}_{\mathrm{B}}$ dispersion that cannot be explained by conventional spin-wave theory.

We present a microscopic theory for the Raman response of a clean multiband superconductor, with emphasis on the effects of vertex corrections and long-range Coulomb interaction. The measured Raman intensity, $R\left(\mathrm{\Omega }\right)$, is proportional to the imaginary part of the fully renormalized particle-hole correlator with Raman form factors $\gamma \left(\vec{k}\right)$. In a BCS superconductor, a bare Raman bubble is nonzero for any $\gamma \left(\vec{k}\right)$ and diverges at $\mathrm{\Omega }=2{\mathrm{\Delta }}_{\max }$, where ${\mathrm{\Delta }}_{\max }$ is the largest gap along the Fermi surface. However, for $\gamma \left(\vec{k}\right)$ = constant, the full $R\left(\mathrm{\Omega }\right)$ is expected to vanish due to particle number conservation. It was sometimes stated that this vanishing is due to the singular screening by long-range Coulomb interaction. In our general approach, we show diagrammatically that this vanishing actually holds due to vertex corrections from the same short-range interaction that gives rise to superconductivity. We further argue that long-range Coulomb interaction does not affect the Raman signal for any $\gamma \left(\vec{k}\right)$. We argue that vertex corrections eliminate the divergence at $2{\mathrm{\Delta }}_{\max }$. We also argue that vertex corrections give rise to sharp peaks in $R\left(\mathrm{\Omega }\right)$ at $\mathrm{\Omega }<2{\mathrm{\Delta }}_{\min }$ (the minimum gap along the Fermi surface), when $\mathrm{\Omega }$ coincides with the frequency of one of the collective modes in a superconductor, e.g., Leggett and Bardasis-Schrieffer modes in the particle-particle channel, and an excitonic mode in the particle-hole channel.

We report measurements of the London penetration depth $\left[\mathrm{\Delta }\lambda \right(T\left)\right]$ of the recently discovered iron-based superconductor (${\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x$)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, $\mathrm{\Delta }\lambda \left(T\right)$ displays nearly temperature-independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density ${\rho }_s\left(T\right)$, which cannot be well accounted for by a single-gap isotropic $s$-wave model but is consistent with either a two-gap model, a model for the orbital selective $s\times {\tau }_3$ state, or anisotropic $s$-wave superconductivity.

We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, which has a tetragonal-to-orthorhombic structural distortion at $T_s$ and antiferromagnetic (AF) phase transition at $T_N$ with ordered moments along the orthorhombic $a$ axis below $T_s\approx T_N\approx 136$ K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with $M_a=M_b=M_c$, where $M_a, M_b$, and $M_c$ are spin excitations polarized along the $a$-, $b$-, and $c$-axis directions of the orthorhombic lattice, respectively. On cooling towards $T_N$, significant spin excitation anisotropy with $M_a>M_b\approx M_c$ develops below 3 meV with a diverging $M_a$ at $T_N$. The in-plane spin excitation anisotropy in the tetragonal phase of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.

We study the classical antiferromagnetic Heisenberg model on the triangular lattice with Dzyaloshinskii-Moriya interactions in a magnetic field. We focus in particular on the emergence of a composite spin crystal phase, dubbed an antiferromagnetic skyrmion lattice, that was recently observed for intermediate fields. This complex phase can be made up from three interpenetrated skyrmion lattices, one for each sublattice of the original triangular one. Following these recent numerical results, in this paper we explicitly construct the low-energy effective action that reproduces the correct phenomenology and could serve as a starting point to study the coupling to charge carriers, lattice vibrations, structural disorder, and transport phenomena.

Recently, Q.-P. Ding et al. [Phys. Rev. B 95, 184404 (2017)] reported that their nuclear magnetic resonance (NMR) study on ${\mathrm{EuCo}}_2{\mathrm{As}}_2$ successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out ${}^{153}\mathrm{Eu}$, ${}^{31}\mathrm{P}$, and ${}^{59}\mathrm{Co}$ NMR measurements on the helical antiferromagnet ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ with an AFM ordering temperature $T_{\mathrm{N}}=66.5$ K. An incommensurate helical AFM structure was clearly confirmed by ${}^{153}\mathrm{Eu}$ and ${}^{31}\mathrm{P}$ NMR spectra on single-crystalline ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ in zero magnetic field at 1.6 K and its external magnetic field dependence. Furthermore, based on ${}^{59}\mathrm{Co}$ NMR data in both the paramagnetic and incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector $\mathbf{k}=(0,0,0.73\pm 0.09)2\pi /c$, where $c$ is the $c$-axis lattice parameter. The temperature dependence of k is also discussed.

The relaxation phenomena of spin-torque oscillators consisting of nanostructured ferromagnets are interesting research targets in magnetism. A theoretical study on the relaxation time of a spin-torque oscillator from one self-oscillation state to another is investigated. By solving the Landau-Lifshitz-Gilbert equation both analytically and numerically, it is shown that the oscillator relaxes to the self-oscillation state exponentially within a few nanoseconds, except when magnetization is close to a critical point. The relaxation rate, which is an inverse of relaxation time, is proportional to the current. On the other hand, a critical slowing down appears near the critical point, where relaxation is inversely proportional to time, and the relaxation time becomes on the order of hundreds of nanoseconds. These conclusions are primarily obtained for a spin-torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer, and are further developed for application to arbitrary types of spin-torque oscillators.

We have investigated codirectional and contradirectional couplings between spin wave and acoustic wave in a one-dimensional periodic structure (the so-called magphonic crystal). The system consists of two ferromagnetic layers alternating in space. We have taken into consideration materials commonly used in magnonics: yttrium iron garnet, CoFeB, permalloy, and cobalt. The coupled mode theory (CMT) formalism has been successfully implemented to describe the magnetoelastic interaction as a periodic perturbation in the magphonic crystal. The results of CMT calculations have been verified by more rigorous simulations with the frequency-domain plane-wave method and the time-domain finite-element method. The presented resonant coupling in the magphonic crystal is an active in-space mechanism which spatially transfers energy between propagating spin and acoustic modes, thus creating a propagating magnetoelastic wave. We have shown that CMT analysis of the magnetoelastic coupling is an useful tool to optimize and design a spin wave–acoustic wave transducer based on magphonic crystals. The effect of spin-wave damping has been included to the model to discuss the efficiency of such a device. Our model shows that it is possible to obtain forward conversion of the acoustic wave to the spin wave in case of codirectional coupling and backward conversion in case of contradirectional coupling. That energy transfer may be realized for broadband coupling and for generation of spin waves which are of different wavelength (in particular, shorter) than exciting acoustic waves.

By applying a coupling microwave field gated by random-telegraph-noise (RTN), we report the experimental demonstration of dynamically modulated Autler-Townes effect (ATE) in an artificial atom—a superconducting transmon qubit—and obtain the distinguished characteristics when a system is affected by RTN. By further applying another coupling microwave filed on the transmon, we use an intense bichromatic field to couple the energy levels. Depending on the detuning of the couplers, remarkable interference patterns emerge in the spectrum, which come from the dressed states induced by the modulated ATE. Numerical calculations agree well with the experimental data.

We have realized laser-doped all-silicon superconducting (S)/normal metal (N) bilayers of tunable thickness and dopant concentration. We observed a strong reduction of the bilayers' critical temperature when increasing the normal metal thickness, a signature of the highly transparent S/N interface associated to the epitaxial sharp laser doping profile. We extracted the interface resistance by fitting with the linearized Usadel equations, demonstrating a reduction of 1 order of magnitude from previous superconductor/doped Si interfaces. In this well-controlled crystalline system we exploited the low-resistance S/N interfaces to elaborate all-silicon lateral SNS junctions with long-range proximity effect. Their dc transport properties, such as the critical and retrapping currents, could be well understood in the diffusive regime. Furthermore, this work led to the estimation of important parameters in ultradoped superconducting Si, such as the Fermi velocity, the coherence length, or the electron-phonon coupling constant, fundamental to conceive all-silicon superconducting electronics.

Although single- and two-qubit gates are sufficient for universal quantum computation, single-shot three-qubit gates greatly simplify quantum error correction schemes and algorithms. We design fast, high-fidelity three-qubit entangling gates based on microwave pulses for transmon qubits coupled through a superconducting resonator. We show that when interqubit frequency differences are comparable to single-qubit anharmonicities, errors occur primarily through a single unwanted transition. This feature enables the design of fast three-qubit gates based on simple analytical pulse shapes that are engineered to minimize such errors. We show that a three-qubit ccz gate can be performed in 260 ns with fidelities exceeding $99.38\%$, or $99.99\%$ with numerical optimization.

A few approximate schemes to solve the Hedin equations self-consistently introduced in Phys. Rev. B 94, 155101 (2016) are explored and tested for the three-dimensional (3D) electron gas at metallic densities. We calculate one-electron spectra, dielectric properties, compressibility, and correlation energy. Considerable reduction in the calculated bandwidth (as compared to the self-consistent $GW$ result) has been found when vertex correction was used for both polarizability and self-energy. Generally, it is advantageous to obtain the diagrammatic representation of polarizability from the definition of this quantity as a functional derivative of the electronic density with respect to the total field (external plus induced). For self-energy, the first-order vertex correction seems to be sufficient for the range of densities considered. Whenever it is possible, we compare the accuracy of our vertex-corrected schemes with the accuracy of the self-consistent quasiparticle $GW$ approximation (QSGW), which is less expensive computationally. We show that the QSGW approach performs poorly and we relate this poor performance with an inaccurate description of the screening in the QSGW method (with an error comprising a factor 2–3 in the physically important range of momenta).

We calculate exact zero-temperature real-space properties of a substitutional magnetic impurity coupled to the edge of a zigzag silicenelike nanoribbon. Using a Lanczos transformation [A. Allerdt et al., Phys. Rev. B 91, 085101 (2015)] and the density-matrix renormalization-group method, we obtain a realistic description of stanene and germanene that includes the bulk and the edges as boundary one-dimensional helical metallic states. Our results for substitutional impurities indicate that the development of a Kondo state and the structure of the spin correlations between the impurity and the electron spins in the metallic edge state depend considerably on the location of the impurity. More specifically, our real-space resolution allows us to conclude that there is a sharp distinction between the impurity being located at a crest or a trough site at the zigzag edge. We also observe, as expected, that the spin correlations are anisotropic due to an emerging Dzyaloshinskii-Moriya interaction with the conduction electrons and that the edges scatter from the impurity and “snake” or circle around it. Our estimates for the Kondo temperature indicate that there is a very weak enhancement due to the presence of spin-orbit coupling.

We investigate theoretically the electronic properties of two-dimensional electron gases (2DEGs) with regular and distorted triangular antidot lattices. We show that the triangular antidot lattices embedded in 2DEGs behave like artificial graphene and host Dirac fermions. By introducing the Wannier representation, we obtain a tight-binding Hamiltonian including the second-nearest-neighboring hopping, which agrees well with the numerically exact solutions. Based on the tight-binding model, we find that spatially nonuniform distortions of the antidot lattices strongly modify the electronic structures, generate pseudomagnetic fields and the well-defined Landau levels. In contrast to graphene, we can design the nonuniform distortions to generate various configurations of pseudomagnetic fields. We show that the snake orbital states arise by designing the $\pm \mathbf{B}$ pseudomagnetic field configuration. We find that the disorders of antidot lattices during fabrication would not affect the basic feature of the Dirac electrons, but they lead to a reduction in conductance in strong disorder cases.