Bloch oscillations in nanoscale Josephson junctions with a Coulomb charging energy comparable to the Josephson coupling energy are explored within the context of a model previously considered by Geigenmüller and Schön that includes Zener tunneling and treats quasiparticle tunneling as an explicit shot-noise process. The dynamics of the junction quasicharge are investigated numerically using both Monte Carlo and ensemble approaches to calculate voltage-current characteristics in the presence of microwaves. We examine in detail the origin of harmonic and subharmonic Bloch steps at dc biases $I=(n/m)2ef$ induced by microwaves of frequency $f$ and consider the optimum parameters for the observation of harmonic ($m=1$) steps. We also demonstrate that the GS model allows a detailed semiquantitative fit to experimental voltage-current characteristics previously obtained at the Chalmers University of Technology, confirming and strengthening the interpretation of the observed microwave-induced steps in terms of Bloch oscillations.

A nonequilibrium theory of optical conductivity of dirty-limit superconductors and commensurate charge density wave is presented. We discuss the current response to different experimentally relevant light-field probe pulses and show that a single frequency definition of the optical conductivity $\sigma \left(\omega \right)\equiv j\left(\omega \right)/E\left(\omega \right)$ is difficult to interpret out of the adiabatic limit. We identify characteristic time-domain signatures distinguishing between superconducting, normal-metal, and charge density wave states. We also suggest a route to directly address the instantaneous superfluid stiffness of a superconductor by shaping the probe light field.

Structural and thermoelectric properties are reported for a specially designed class of $A$-site substituted perovskite titanates, $\left(\mathrm{S}{\mathrm{r}}_{1-x-y}\mathrm{C}{\mathrm{a}}_x\mathrm{N}{\mathrm{d}}_y\right)\mathrm{Ti}{\mathrm{O}}_3$. Two series synthesized with various $A$-site Sr-rich or Ca-rich (Sr-poor) concentrations were investigated using high-resolution neutron powder diffraction as a function of temperature and Nd doping. Each series was designed to have a nominally constant tolerance factor at room temperature. We determine the room temperature structures as tetragonal $I4/mcm$ and orthorhombic $Pbnm$ for the Sr-rich and Ca-rich series, respectively. Three low-temperature orthorhombic structures, $Pbnm, Ibmm$, and $Pbcm$ were also observed for the Sr-rich series, whereas the symmetry of the Ca-rich series remains unchanged throughout the full measured temperature range. Thermoelectric properties of $\left(\mathrm{S}{\mathrm{r}}_{1-x-y}\mathrm{C}{\mathrm{a}}_x\mathrm{N}{\mathrm{d}}_y\right)\mathrm{Ti}{\mathrm{O}}_3$ were investigated and correlated with the structural variables. We succeeded in achieving a relatively high figure of merit $ZT=0.07$ at $\sim 400\mathrm{K}$ in the Sr-rich $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_3$ composition which is comparable to that of the best $n$-type TE $\mathrm{SrT}{\mathrm{i}}_{0.80}\mathrm{N}{\mathrm{b}}_{0.20}{\mathrm{O}}_3$ oxide material reported to date. For a fixed tolerance factor, the Nd doping enhances the carrier density and effective mass at the expense of the Seebeck coefficient. Thermal conductivity greatly reduces upon Nd doping in the Ca-rich series. With an enhanced Seebeck coefficient at elevated temperatures and reduced thermal conductivity, we predict that $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_3$ and similar compositions have the potential to become some of the best materials in their class of thermoelectric oxides.

We present an experimental investigation of the fragility and heat capacity of metallic glass ${\mathrm{Zr}}_{77}{\mathrm{Ni}}_{23}$. The ribbon sample was produced by melt-spinning technique. Glass transition temperature $T_g$ obtained by differential scanning calorimetry with various heating rates was used to estimate fragility parameter $m$. Heat capacity measurements were performed in a wide temperature interval, ranging from 1.8 K up to room temperature, for as-cast and relaxed samples in different magnetic field strengths. Our results do not show any excess of vibrational density of states over the Debye contribution corresponding to the boson peak. Relaxation of the sample causes a slight decrease of Debye contribution consistent with the measured increase of Young modulus. The fact that no boson peak is observed in heat capacity, together with the obtained intermediate fragility of $m=53$, positions ${\mathrm{Zr}}_{77}{\mathrm{Ni}}_{23}$ well outside established correlations between fragility, boson peak strength, and glass forming ability.

We investigate the conditional average and the conditional variance of dissipated energy considering, as a prototypical example, a driven spin-boson system. We follow a measurement protocol in which the spin is prepared in a certain initial state before undergoing a periodic driving. Subsequently, the spin is projected onto a postselected final state. We compare the conditional average of dissipated energy to the lower bound, which directly follows from the well-known fluctuation relations. We further report that a special selection of the initial (preselected) and final (postselected) spin states leads to an enhanced energy emission with simultaneous noise suppression at driving times of order of the relaxation time.

We report thermal control of mode hybridization between the ferromagnetic resonance and a planar resonator (notch filter) working at 4.74 GHz. The chosen magnetic material is a ferrimagnetic insulator (yttrium iron garnet: YIG) covered by 6 nm of platinum (Pt). A current-induced heating method has been used in order to enhance the temperature of the YIG/Pt system. The device permits us to control the transmission spectra and the magnon-photon coupling strength at room temperature. These experimental findings reveal a potentially applicable tunable microwave filtering function.

We theoretically examine a cross effect of magnetic field and charge current on antiferromagnetic domain wall dynamics. Since antiferromagnetic materials are largely insensitive to external magnetic fields in general, charge current has been shown recently as an alternative and efficient way to manipulate antiferromagnets. We find a new role of the magnetic field in the antiferromagnetic dynamics that appears when it is combined with charge current, demonstrating a domain wall motion in the presence of both field and current. We show that a spatially varying magnetic field can shift the current-driven domain-wall velocity, depending on the domain-wall structure and the direction of the field gradient. Our result suggests a novel concept of field control of current-driven antiferromagnetic dynamics.

Spin ice materials represent an intriguing class of frustrated magnetic systems which, through their geometry, admit an exponential number of approximately degenerate configurations. In this paper, the relaxation properties of a thermally active artificial kagome spin ice system are studied. Through application of an external magnetic field, an out-of-equilibrium vertex charge ordered configuration is selected and relaxed under approximate zero-field conditions. Using x-ray photo-emission electron microscopy, the magnetic moment and vertex charge degrees of freedom are followed in space and time, revealing different dynamics to that seen in past athermal equilibration protocols, and a relaxation which is well described by a point-dipolar model system. Furthermore the charge correlations are found to relax with a time scale several times smaller than that of the moment correlations.

The temperature dependence of spin-orbit torques (SOTs) and spin-dependent transport parameters is measured in bilayer Ta/TbCo ferrimagnetic alloy films with bulk perpendicular magnetic anisotropy. We find that the dampinglike (DL)-SOT effective field diverges as temperature is swept through the magnetic compensation temperature ($T_{\mathrm{M}}$), where the net magnetization vanishes due to the opposing contributions from the Tb and Co sublattices. We show that DL-SOT scales with the inverse of the saturation magnetization ($M_{\mathrm{s}}$), whereas the spin-torque efficiency is independent of the temperature-dependent $M_{\mathrm{s}}$. Our findings provide insight into spin transport mechanisms in ferrimagnets and highlight low-$M_{\mathrm{s}}$ rare-earth/transition-metal alloys as promising candidates for SOT device applications.

We report that special care is needed when longitudinal magnetic susceptibility is computed in a magnetically ordered phase, especially in metals. We demonstrate this by studying static susceptibility in both a ferromagnetic and an antiferromagnetic state in the random phase approximation to the two-dimensional Hubbard model on a square lattice. In contrast to the case in the disordered phase, a first derivative of the chemical potential (or the density) with respect to a magnetic field does not vanish in a magnetically ordered phase when the field is applied parallel to the magnetic moment. This effect is crucial and should be included when computing magnetic susceptibility in the ordered phase, otherwise an unphysical result would be obtained. In addition, consequently the magnetic susceptibility becomes different when computed at a fixed density and a fixed chemical potential in the ordered phase. In particular, we cannot employ magnetic susceptibility at a fixed chemical potential to describe a system with a fixed density even if the chemical potential is tuned to reproduce the correct density.

We use a numerical solution of the deterministic time-dependent Ginzburg-Landau (TDGL) equations to determine the response induced by a probe field in a material quenched into a superconducting state. We characterize differences in response according to whether the probe is applied before, during, or after the phase stiffness has built up to its final steady-state value. We put an emphasis on the extent to which superfluid response requires a non-negligible phase stiffness, which for the considered quench has to build up dynamically. A key finding is that the time-dependent phase stiffness controls the likelihood of phase slips as well as the magnitude of the electromagnetic response. Additionally, we address the electromagnetic response expected if the probe itself is strong enough to activate phase slip processes. If the probe is applied before phase stiffness is sufficiently built up, we find that phase slips occur so that the vector potential is compensated and no long-term supercurrent is induced, while if it is applied at sufficient phase stiffness a weak probe pulse will induce a state with a long-lived supercurrent. If the probe is strong enough to activate the phase slip process, the supercurrent state will only be metastable with a lifetime that scales logarithmically with the amplitude of fluctuations in the magnitude of the order parameter. Finally, we study the response to experimentally motivated probe fields (electric field that integrates to zero). Interestingly, depending on the relative time difference of the probe field to the buildup of superconductivity, long-lived supercurrents can be induced even though the net change in vector potential is zero.

We calculate the optical spectra of a charge-transfer crystal modulated by a terahertz pulse, accounting for electron-vibration coupling. The model Hamiltonian is parametrized against first principle calculations and adiabatic results are validated against a fully non-adiabatic calculation where relaxation phenomena are introduced via the coupling of the quantum system to a dissipative bath of classic anharmonic oscillators. The experiment is well reproduced by the proposed model with no need to introduce any ad hoc assumption on the temporal dependence of model parameters, but just accounting for the quadratic dependence of the Hubbard $U$ on non-totally symmetric molecular coordinates.

Superconducting proximity effect in topological insulator and superconductor hybrid structure has attracted intense attention in recent years in an effort to search for Majorana fermions in condensed matter systems. Here we report on the superconducting proximity effect in a ${\mathrm{Bi}}_2{\mathrm{Se}}_3/{\mathrm{NbSe}}_2$ junction fabricated with an all-dry transfer method. Experimentally measured differential conductance spectra exhibit a bias-independent conductance plateau (BICP) in the vicinity of zero bias below 7 K and a zero-bias conductance peak (ZBCP) appears below 2 K. We show that the BICP is due to the strong superconducting proximity effect between the superconductor and the topological surface states while the ZBCP is due to the bulk states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our study gives direct evidence that the topological surface states can be strongly coupled to a superconductor and clarifies the different roles of surface states and bulk states in superconducting proximity effect.

We analyze the momentum and temperature dependences of the magnetic susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by a combination of density functional theory and dynamical mean-field theory (DFT+DMFT). By considering a general derivation of the orbital-resolved effective model for spin degrees of freedom for Hund's metals, we relate momentum-dependent susceptibilities in the paramagnetic phase to the magnetic exchange. We then calculate nonuniform orbital-resolved susceptibilities at high-symmetry wave vectors by constructing appropriate supercells in the DMFT approach. Extracting the irreducible parts of susceptibilities with respect to Hund's exchange interaction, we determine the corresponding orbital-resolved exchange interactions, which are then interpolated to the whole Brillouin zone. Using the spherical model we estimate the temperature dependence of the resulting exchange between local moments.

Recent experiments on the thermal expansion of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, a prototypical example of strong three-dimensional topological insulators, have shown an intriguing anomaly in the thermal expansion coefficient along the hexagonal axis $\left({\alpha }_{{}_{\parallel }}\right)$, which drops sharply to almost zero in a narrow range of temperature around $\sim 225$ K. With no accompanying signatures in other properties, the origin of this anomaly is not understood. We present here femtosecond pump-probe differential reflectivity measurements on single crystals of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ as a function of temperature from 3 to 300 K to determine the temperature dependence of coherent optical and acoustic phonons along with the dynamics of the photoexcited carriers. We find clearly anomalous temperature dependence of the parameters associated with vibrational and electronic relaxation in the narrow temperature range of 200–250 K. Within first-principles density functional theoretical analysis, we show that the observed anomalies can be explained with a mechanism of formation of stacking faults stabilized by vibrational entropy above 200 K. As a similar anomaly in the thermal expansion is also observed in other chalcogenides in the same family, the proposed mechanism may also be applicable to these layered strong topological insulators.

It was recently shown that nonsymmorphic space-group symmetries can protect novel surface states with hourglasslike dispersions. In this paper, we show that such dispersions can also appear in the bulk of three-dimensional (3D) systems which respect nonsymmorphic symmetries. Specifically, we construct 3D lattice models featuring hourglasslike dispersions in the bulk, which are protected by nonsymmorphic and time-reversal symmetries. We call such systems hourglass semimetals, as they have point or line nodes associated with hourglasslike dispersions. Hourglass nodal lines appear in glide-invariant planes, while hourglass Weyl points can occur on screw-invariant axes. The Weyl points and surface Fermi arcs in hourglass Weyl semimetals are stable against weak perturbations breaking those nonsymmorphic symmetries. Our results may shed light on searching for exotic Weyl semimetals in nonsymmorphic materials.

We present a study of the mode degeneracy in non-Hermitian photonic crystals (PC) with TM-like polarization and $C_{4v}$ symmetry from the perspective of the coupled-wave theory (CWT). The CWT framework is extended to include TE-TM coupling terms which are critical for modeling the accidental triple degeneracy within non-Hermitian PC systems. We derive the analytical form of the wave function and the condition of Dirac-like-cone dispersion when radiation loss is relatively small. We find that, similar to a real Dirac cone, the Dirac-like cone in non-Hermitian PCs possesses good linearity and isotropy, even with a ring of exceptional points (EPs) inevitably existing in the vicinity of the second-order $\mathrm{\Gamma }$ point. However, the Berry phase remains zero at the $\mathrm{\Gamma }$ point, indicating the cone does not obey the Dirac equation and is only a Dirac-like cone. The topological modal interchange phenomenon and nonzero Berry phase of the EPs are also discussed.

We demonstrate enhanced second-harmonic generation in arrays of nanowaveguides satisfying modal-phase-matching condition, both theoretically and experimentally. The overlap of interacting fields defined by the fundamental mode of the pump and the second-order mode of the second-harmonic wave is enhanced by the longitudinal component of the nonlinear polarization density. For guided modes with significant longitudinal electric field components, the overlap of fields is comparable to that obtained in the quasi-phase-matching technique leading to higher conversion efficiencies. Thus, the presented method is preferable to achieve higher conversion efficiency in second-order nonlinear processes in nanowaveguides.