We explore the role of proximity-induced odd-frequency pairing in the thermoelectricity of a ferromagnet when coupled to a conventional $s$-wave spin-singlet superconductor through a spin-active interface. By varying both the polarization and its direction in the ferromagnet and the interfacial spin-orbit interaction strength, we analyze the behavior of all proximity-induced pair amplitudes in the ferromagnet and their contributions to the thermoelectric coefficients. Based on our results for the Seebeck coefficient, we predict that odd-frequency spin-triplet Cooper pairs are much more efficient than the conventional spin-singlet even-frequency pairs in enhancing thermoelectricity of the junction, and especially mixed-spin triplet pairing is favorable. Our results on the thermoelectric figure of merit show that ferromagnet/superconductor junctions are very good thermoelectric systems when superconductivity is dominated by odd-frequency pairing.

We study the competition of homogeneous SC with pair density wave states in a spinless formalism focusing on systems with band structures that exhibit commensurate nesting features. We show that a specific triplet commensurate pair density wave superconducting (SC) state, the staggered $d$-wave triplet state (triplet dPDW), may coexist with homogeneous triplet SC states and even dominate, eliminating them for small deviations from nesting. We point out qualitative characteristics of the tunneling density of states and specific heat that identify this triplet dPDW state. Our results are qualitatively extendable to similar systems with nested band structures.

Sound transmission across an air/water interface is known to be ineffective due to a large acoustic impedance mismatch. Efficient sound transmission across an interface between acoustically different media can improve acoustic systems. Here, we demonstrate efficient sound transmission and radiation across an air/water interface using resonant circular bubbles close to the interface. We find that perfect sound transmission is realized under the critical coupling condition, where radiation leakages into air and water are equal. In addition, without incident waves, a resonant bubble as a radiation source exhibits very high sound radiation to air, exceeding sound radiation to water for shallow depths. Moreover, we observe multiresonance sound transmission based on high-order resonance modes in bubbles with a thin elastic shell. Our results provide a critical understanding of the roles of resonant bubbles for sound transmission across an air/water interface.

We investigated the $5d$ element substitution effect on the thermal conductivity in $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattice thin films epitaxially grown on a MgO (100) substrate. We found unique crossover behavior in the period dependences of thermal conductivity for Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$ and $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices. The $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices with periods more than 40 nm appeared to have much lower thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattices due to the substitution of V by Ta. Unexpectedly, at a shorter periodic length, less than 20 nm, $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices appeared to have higher thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattices despite such a heavy element substitution by Ta. This surprising experimental fact was well accounted for with theoretical calculations, which predicted the dominant contribution of phonons having a shorter mean free path in the Ta substituted samples.

We present synthesis and characterization of ${\mathrm{Ba}}_2{\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{P}}_2{\mathrm{O}}_{13}$ (BCTPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements. BCTPO crystallizes in a monoclinic structure with ${\mathrm{Cu}}^{2+}$ ions forming coupled chains. In magnetization $M\left(T\right)$ measurements, a broad maximum is observed around 53 K. Upon the application of magnetic fields, a sharp anomaly evolves around 4 K in the heat capacity $C_P\left(T\right)$ data, indicative of long-range order, while it is absent in zero field. The ${}^{31}\mathrm{P}$ NMR line shift at about 93.954 kOe tracks the spin susceptibility, whereas the line broadening evidences the onset of magnetic order at about 4 K or lower. The inferred isotropic and axial components of the hyperfine coupling constant are $A_{hf}^{\mathrm{iso}}\simeq$ 3635 $\text{Oe}/{\mu }_B$ and $A_{hf}^{ax}\simeq$ 479 $\text{Oe}/{\mu }_B$, respectively. The ${}^{31}\mathrm{P}$ NMR spin-lattice relaxation rate $1/T_1$ shows a pronounced divergence for $T⟶0$. We assign this behavior to critical fluctuations on the verge of order.

We report numerical calculations of a dynamic pair-breaking current density $J_d$ and a critical superfluid velocity $v_d$ in a nonequilibrium superconductor carrying a uniform, large-amplitude AC current density $J\left(t\right)=J_{a}sin\mathrm{\Omega }t$ with $\mathrm{\Omega }$ well below the gap frequency $\mathrm{\Omega }\ll {\mathrm{\Delta }}_0/\hslash$. The dependencies $J_d(\mathrm{\Omega },T)$ and $v_d(\mathrm{\Omega },T)$ near the critical temperature $T_c$ were calculated from either the full time-dependent nonequilibrium equations for a dirty $s$-wave superconductor or the time-dependent Ginzburg-Landau (TDGL) equations for a gapped superconductor, taking into account the GL relaxation time of the order parameter ${\tau }_{\text{GL}}$ and the inelastic electron-phonon relaxation time of quasiparticles ${\tau }_E$. We show that both approaches give similar frequency dependencies of $J_d\left(\mathrm{\Omega }\right)$ and $v_d\left(\mathrm{\Omega }\right)$ which gradually increase from their static pair-breaking GL values $J_c$ and $v_c$ at $\mathrm{\Omega }{\tau }_E\ll 1$ to $\sqrt{2}{J}_c$ and $\sqrt{2}{v}_c$ at $\mathrm{\Omega }{\tau }_E\gg 1$. Here $J_d$, $v_d$ and a dynamic superheating field at which the Meissner state becomes unstable were calculated in two different regimes of a fixed AC current and a fixed AC superfluid velocity induced by the applied AC magnetic field $H=H_{a}sin\mathrm{\Omega }t$ in a thin superconducting filament or a type-II superconductor with a large GL parameter. We also calculated a nonlinear electromagnetic response of a nonequilibrium superconducting state, particularly a dynamic kinetic inductance and a dissipative quasiparticle conductivity, taking into account the oscillatory dynamics of superconducting condensate and the kinetics of quasiparticles driven by a strong AC current. It is shown that an AC current density produces multiple harmonics of the electric field, the amplitudes of the higher-order harmonics diminishing as ${\tau }_E$ increases.

In two dimensions, topological phases of free Majorana fermions coupled to a ${\mathbb{Z}}_2$ gauge field are known to be classified according to the Chern number $\nu \in \mathbb{Z}$. Its value mod 16 specifies the type of anyonic excitations. In this paper, we investigate triangular vortex configurations (and their dual) in the Kitaev honeycomb model and show that fourteen of these sixteen phases can be obtained by adding a time-reversal symmetry-breaking term. Missing phases are $\nu =\pm 7$. More generally, we prove that any periodic vortex configuration with an odd number of vortices per geometric unit cell can only host even Chern numbers whereas odd Chern numbers can be found in other cases.

Exotic multifold topological excitations have been predicted and were recently observed in transition metal silicides like $\beta$-RhSi. Herein, we report that interesting topological features of RhSi are also observed in its orthorhombic $\alpha$ phase, which displays multiple types of Dirac nodes very close to the Fermi level ${\varepsilon }_F$. We discuss the symmetry analysis, band connectivity along high-symmetry lines using group representations, band structure, the nature of the Dirac points and of a nodal line occurring near ${\varepsilon }_F$ which is protected by the crystalline symmetry. The de Haas–van Alphen effect indicates a Fermi surface in agreement with the calculations. We find an elliptically shaped nodal line very close to ${\varepsilon }_F$ around and near the $S$ point on the $k_y\text{−}{k}_z$ plane that results from the intersection of two upside-down Dirac cones. Both Dirac points of the participating Kramers degenerate bands are only 5 meV apart; hence, an accessible magnetic field might induce a crossing between the spin-up partner of the upper Dirac cone and the spin-down partner of the lower Dirac cone, possibly explaining the anomalies observed in the magnetic torque. $\alpha$-RhSi is a unique system since all bands crossing ${\varepsilon }_F$ emerge from Dirac nodes.

In this paper, a series of $\nu =2/5$ fractional quantum Hall wave functions are constructed from conformal field theory(CFT). They share the same topological properties with states constructed by Jain's composite fermion approach. Upon exact lowest Landau level (LLL) projection, some of Jain's composite fermion states would not survive if constraints on Landau level indices given in the appendices of this paper were not satisfied. By contrast, states constructed from CFT are always in LLL. These states are characterized by different topological shifts and multibody relative angular momenta. As a by-product, in the appendices we prove the necessary conditions for general $\nu =p/(2p+1)$ composite fermion states to have nonvanishing LLL projection.

We generalize the dynamical large-$N$ multichannel Schwinger-boson approach to study the quantum phase transition of a two-dimensional Kondo-Heisenberg model, relevant for heavy electron systems, close to an antiferromagnetic Kondo destruction quantum critical point. By breaking up a Kondo singlet into a charge fermion (holon) and a bosonic spinon, we identify and characterize the quantum phase transition from an antiferromagnetically ordered state to a Kondo-dominated paramagnetic state, and attribute a jump in the holon phase shift to Kondo breakdown. We calculate transport and thermodynamic quantities. The global finite-temperature phase diagram and the critical behavior of various physical observables therein show a close resemblance to experimental observations.

Recent experimental discoveries in axis-dependent conduction polarity, or goniopolarity, have observed that the charge carriers can conduct like either electrons or holes depending on the crystallographic direction they travel along in layered compounds such as $\mathrm{Na}{\mathrm{Sn}}_2{\mathrm{As}}_2$. The original theoretical proposal is based on the opposite signs of the carrier effective mass, or the curvature of the Fermi surface, without examining the effect of electron lifetimes, thus leaving a crucial question to address. To elucidate this unusual transport behavior, we present an ab initio study of electron scattering in such systems. We study different microscopic scattering mechanisms in $\mathrm{Na}{\mathrm{Sn}}_2{\mathrm{As}}_2$, and we present the electron-phonon scattering time distribution on its Fermi surface in momentum space, the open concave shape of which is proposed to be the origin of the axis-dependent conduction polarity. Further, we obtain the overall anisotropic lifetime tensors in real space at different electron chemical potentials and temperatures, and we discuss how they contribute to the macroscopic thermopower. While we find that the contribution of the in-plane and cross-plane lifetimes exhibits a similar trend, the concave portion of the Fermi surface alters the electron motion significantly in the presence of a magnetic field, thus flipping the conduction polarity as measured via the Hall effect. Our calculations and analysis of $\mathrm{Na}{\mathrm{Sn}}_2{\mathrm{As}}_2$, in comparison with similar systems, also suggest the strong possibility of hydrodynamic electron flow in the system. Finally, our work has implications for anisotropic electron lifetimes in a broad class of goniopolar materials and provides key, general insights into electron scattering on open Fermi surfaces.

Dirac semimetals (DSMs) are the focus of study as new candidates for topological superconductivity and spintronics. Although the Dirac points are not necessarily associated with crystalline symmetries, the experimentally realized robust DSMs such as $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ have fourfold-degenerate Dirac points at generic wave vectors on high symmetry line enabled by a fourfold crystalline symmetry. The magnetic counterpart of these robust DSMs remains unknown. On the other hand, the theoretically proposed magnetic DSMs in potential antiferromagnetic (AFM) spintronics materials where both time-reversal $\left(\mathcal{T}\right)$ and inversion $\left(\mathcal{P}\right)$ symmetry are broken but their combination $\mathcal{P}\mathcal{T}$ is preserved have Dirac points on the boundary of the Brillouin zone protected by a low-order twofold crystalline symmetry. Here, combined with first-principles calculations and symmetry analysis, we find the Dirac fermion at generic wave vector on high symmetry line with associated nontrivial surface states in $\mathcal{P}\mathcal{T}$-symmetric tetragonal CuMnAs, which is the prototype AFM spintronics material observed in experiments. Furthermore, we reveal the hidden spin textures in this $\mathcal{P}\mathcal{T}$-symmetric magnetic system, demonstrating interesting vortex-like spin textures. Our study opens the way for designing robust magnetic DSMs that can serve as an ideal platform to study the interplay of magnetism, Dirac fermions, and spin-orbit interactions, and could stimulate a series of research in the field of topological antiferromagnetic spintronics.

We provide systematic analysis on a non-Hermitian $\mathcal{PT}$-symmetric quantum impurity system both in and out of equilibrium, based on exact computations. In order to understand the interplay between non-Hermiticity and Kondo physics, we focus on a prototypical noninteracting impurity system, the resonant level model, with complex coupling constants. Explicitly constructing biorthogonal basis, we study its thermodynamic properties as well as the Loschmidt echo starting from the initially disconnected two free fermion chains. Remarkably, we observe the universal crossover physics in the Loschmidt echo, both in the $\mathcal{PT}$ broken and unbroken regimes. We also find that the ground state quantities we compute in the $\mathcal{PT}$ broken regime can be obtained by analytic continuation. It turns out that Kondo screening ceases to exist in the $\mathcal{PT}$ broken regime, which was also previously predicted in the non-Hermitian Kondo model. All the analytical results are corroborated against biorthogonal free fermion numerics.

Spin relaxation was studied in a two-dimensional electron gas confined in a wide GaAs quantum well. Recently, the control of the spin relaxation anisotropy by diffusive motion was first shown in D. Iizasa et al., arXiv:2006.08253. Here, we demonstrate electrical control by drift transport in a system with two subbands occupied. The combined effect of in-plane and gate voltages was investigated using time-resolved Kerr rotation. The measured relaxation time presents strong anisotropy with respect to the transport direction. For an in-plane accelerating electric field along $\left[110\right]$, the lifetime was strongly suppressed irrespective of the applied gate voltage. Remarkably, for transport along $\left[1\overline{1}0\right]$, the data shows spin lifetime that was gate dependent and longer than in the $\left[110\right]$ direction regardless of the in-plane voltage. In agreement, independent results of anisotropic spin precession frequencies are also presented. Nevertheless, the long spin lifetime, strong anisotropy and drift response seen in the data are beyond the existing models for spin drift and diffusion.

We investigate the optical orientation, polarization pinning, and depolarization of optically confined semiconductor exciton-polariton condensates. We perform a complete mapping of the condensate polarization as a function of incident nonresonant excitation polarization and power. We utilize a ring-shaped excitation pattern to generate an exciton-induced potential that spatially confines polariton condensates into a single mode. We observe that formation of circular polarization in the condensate persists even for a weakly cocircularly polarized pump. By varying the excitation ring diameter we realize a transition from the condensate polarization being pinned along the coordinate-dependent cavity-strain axes, to a regime of zero degree of condensate polarization. Analysis through the driven-dissipative stochastic Gross-Pitaevskii equation reveals that this depolarization stems from a competition between sample-induced in-plane polarization splitting and the condensate-reservoir overlap. An increase in the role of the latter results in weakening of the condensate fixed-point phase space attractors, and enhanced random phase space walk and appearance of limit cycle trajectories, reducing the degree of time-integrated polarization.

We propose a state of excitonic solid for bilayer two-dimensional electron-hole systems in transition metal dicalcogenides stacked on opposite sides of thin layers of BN. Properties of the exciton lattice are studied. We found that for the experimental parameters of interest, the solid by itself will melt. However, the solid can be stabilized by the potential due to the BN. We studied the possible manifestation of supersolid behavior due to quautum transport of defects and found that it is manifested as an approximately quantized conductance in Coulomb drag measurements.

Interference phenomena allow tailoring propagation of electromagnetic waves by controlling phases of several scattering channels. Huygens element, being a representative example of this approach, enables enhancement of the scattering from an object in a forward direction, while the reflection is suppressed. However, a typical resonant realization of Huygens element employs constructive interference between electric and magnetic dipolar resonances that makes it relatively narrowband. Here we develop the concept of a broadband resonant Huygens element, based on a circular array of vertically aligned near-field coupled metal wires. Accurate management of multipole interference in an electrically small structure results in directional scattering over a large bandwidth, acceding 10% of the carrier frequency. Being constructed from nonmagnetic materials, this structure demonstrates a strong magnetic response appearing in dominating magnetic multipoles over electric counterparts. Moreover, we predict and observe higher-order magnetic multipoles, including hexadecapole (M16-pole) and magnetic triakontadipole (M32-pole) with quality factors, approaching 6000. The experimental demonstration is performed at the low GHz spectral range. Broadband Huygens elements can be employed in a set of practical applications, where compact electromagnetic devices for tailoring wave propagation are needed, i.e., antenna devices, directional reflectors, and even solar cells, given that the concept is scaled to the optical frequency range.

Combining the technologies of quantum optics and magnonics, we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and spins generates the optical Barnett field, and its direction is controllable by switching the laser chirality. We show that the optical Barnett field develops the total magnetization in insulating ferrimagnets with reversing the local magnetization, which leads to the quasiequilibrium magnon BEC. This laser-induced magnon BEC transition through optical Barnett effect, dubbed the optomagnonic Barnett effect, provides an access to coherent magnons in the high-frequency regime of the order of terahertz. We also propose a realistic experimental setup to observe the optomagnonic Barnett effect using current device and measurement technologies as well as the laser chirping. The optomagnonic Barnett effect is a key ingredient for the application to ultrafast spin transport.

We study the superconducting properties of the bulk states of a doped topological insulator. We obtain that hexagonal warping stabilizes the nematic spin-triplet superconducting phase with $E_u$ pairing and the direction of the nematic order parameter which opens the full gap is the ground state. This order parameter exhibits non-BCS behavior. The ratio of the order parameter to the critical temperature of $\mathrm{\Delta }\left(0\right)/T_c$ differs from the BCS ratio. It depends on the chemical potential and the value of the hexagonal warping. We discuss the relevance of the obtained results for the explanation of the experimental observations.

Experimental investigations of ultrafast magnetization dynamics increasingly employ resonant magnetic spectroscopy in the ultraviolet (XUV) spectral range. Besides allowing one to disentangle the element-specific transient response of functional magnetic systems, these techniques also promise to access attosecond to few-femtosecond dynamics of spin excitations. Here, we report on a study of transient magnetic circular dichroism (MCD) on the transition metals Fe, Co, and Ni as well as on a FeNi and GdFe alloy after optical excitation and reveal a delayed onset between the electronic and the MCD response for measurements with narrow bandwidth XUV pulses tuned to a specific energy around the respective elemental $M_{2,3}$ resonances. We argue that for this experimental implementation, the MCD observable does not necessarily reflect the initial femtosecond magnetization dynamics of the sample, but instead—supported by ab initio calculations—an effective delay can be attributed to a transient energy shift of the MCD spectra due to nonequilibrium changes of electron occupations.