Motivated by recent theoretical and experimental progress in the heavy-fermion system UCoGe, we study ferromagnetic chiral superconductors in the presence of magnetic domains. Within mean-field approximations, it is shown that chiral superconducting domains are naturally induced by the ferromagnetic domains. The domain wall current flows in the opposite direction to the naively expected one as in the ${}^3\mathrm{He}$-A phase due to contributions from “unpaired electrons.” Consequently, the domain wall current flows in the same direction as the surface currents when the magnetic domain wall lies parallel to the sample surface, and therefore it contributes to the net current along the whole sample. We find that because of the noncancellation between the domain wall current and surface current, a Fulde-Ferrell-like superconducting state can be stabilized in an anisotropic sample for all the temperatures below the superconducting transition temperature.

We examined the proximity effect in granular films made of Pb, a superconductor, and Ni, a ferromagnet, with various compositions. Slow decay of the critical temperature as a function of the relative volume concentration of Ni per sample was demonstrated by our measurements, followed by a saturation of $T_c$. Using an approximate theoretical description of our granular system in terms of a layered one, we show that our data can only be reasonably fitted by a trilayer model. This indicates the importance of the interplay between different ferromagnetic grains, which should lead to triplet Cooper pairing.

Using a proper cooling procedure, a controllable amount of nonmagnetic structural disorder can be introduced at low temperature in ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$. Here we performed simultaneous measurements of transport and magnetic properties of ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$ in its normal and superconducting states, while finely covering three orders of magnitude of the cooling rate around the anion ordering temperature. Our result reveals, with increasing density of disorder, the existence of a crossover between homogeneous defect-controlled $d$-wave superconductivity and granular superconductivity. At slow cooling rates, with small amount of disorder, the evolution of superconducting properties is well described with the Abrikosov-Gorkov theory, providing further confirmation of non-$s$-wave pairing in this compound. In contrast, at fast cooling rates, zero resistance and diamagnetic shielding are achieved through a randomly distributed network of superconducting puddles embedded in a normal conducting background and interconnected by proximity effect coupling. The temperature dependence of the ac complex susceptibility reveals features typical for a network of granular superconductors. This makes ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$ a model system for granular superconductivity where the grain size and their concentration are tunable within the same sample.

Using time-resolved scanning electron microscopy with polarization analysis (TR-SEMPA) the dynamics of magnetic vortices and antivortices in a cross-tie wall is investigated. Under quasistatic external field drive both quasiparticles oscillate at opposite phase to each other (in analogy to an optical mode). An additional common motion, which stems from the finite boundary conditions in a patterned FeCoSiB rectangle, is observed (in analogy to an acoustic mode), leading in total to a stronger field-dependent displacement for the vortices compared to the antivortices. Both types of magnetic solitons are mutually coupled via the minimization of the total magnetic energy of the structure. It is shown that the TR-SEMPA results can be explained via coupling in the framework of micromagnetic simulations and that the coupling constants can be extracted. The motion is composed from the contributions of the two terminating vortices and a mutual vortex-antivortex coupling along the chain.

We study the spin-phonon relaxation rate of both Kramers and non-Kramers molecular magnets in strongly diluted samples at low temperature. Using the “rotational” contribution to the spin-phonon Hamiltonian, universal formulas for the relaxation rate are obtained. Intriguingly, these formulas are all entirely expressed via measurable or ab initio computable physical quantities. Moreover, they are also independent of the energy gaps to excited states involved in the relaxation process. These obtained expressions for direct and Raman processes offer an easy way to determine the lowest limit of the spin-phonon relaxation of any spin system based on magnetic properties of the ground doublet only. In addition, some intriguing properties of Raman process are also found. Particularly, Raman process in Kramers system is found dependent on the magnetic field's orientation but independent of its magnitude, meanwhile, the same process in non-Kramers system is significantly reduced out of resonance, i.e., for an applied external field. Interestingly, Raman process is demonstrated to vary as $T^9$ for both systems. Application of the theory to a recently investigated cobalt(II) complex shows that it can provide a reasonably good description for the relaxation. Based on these findings, a strategy in developing efficient single-molecule magnets by enhancing the mechanical rigidity of the molecular unit is proposed.

We show that polycrystalline $\mathrm{GeS}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor-insulator transition versus pressure at low temperatures reveals a second-order quantum phase transition with linear dependence of the transition temperature on the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also becomes superconducting. We also find that in the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and we suggest an explanation for this anomaly.

The fully relativistic spin-polarized multiple-scattering theory is developed for inhomogeneous superconductors, including superconducting/normal-metal/ferromagnet heterostructures. The method allows the solution of the first-principles Dirac–Bogoliubov–de Gennes equations combined with a semiphenomenological parametrization of the exchange-correlation functional. Simple conditions are derived for the case when the right-hand-side and left-hand-side solutions must be treated separately when setting up the corresponding Green's function. As an application of the theory, we calculate the order parameters of Nb/Fe and Nb/Au/Fe systems. We find Fulde-Ferrell-Larkin-Ovchinnikov-like oscillations in the iron layers, but more interestingly an oscillatory behavior is observed in the gold layers as well. The band-structure calculations suggest that this is the consequence of an interplay between the quantum-well states and ferromagnetism.

The dodecaboride ${\mathrm{LuB}}_{12}$ with cage-glass state and rattling modes has been studied to clarify the nature of the large amplitude vibrations of Lu ions. Discovered anisotropy of charge transport in conjunction with distortions of the conventional fcc symmetry of the crystal lattice may be attributed to coherent motion of Lu ions along a singular direction in the lattice. Arguments are presented in favor of cooperative dynamic Jahn-Teller effect in the boron sublattice to be the reason of the rattling mode, lattice distortion, and formation of the filamentary structure of the higher conducting channels—dynamic charge stripes.

We study $N$-component interacting particles (hardcore bosons and fermions) loaded in topological lattice models with $\mathrm{SU}\left(N\right)$-invariant interactions based on exact diagonalization and density matrix renormalization group method. By tuning the interplay of interspecies and intraspecies interactions, we demonstrate that a class of $\mathrm{SU}\left(N\right)$ fractional quantum Hall states can emerge at fractional filling factors $\nu =N/(N+1)$ for bosons [$\nu =N/(2N+1)$ for fermions] in the lowest Chern band, characterized by the nontrivial fractional Hall responses and the fractional charge pumping. Moreover, we establish a topological characterization based on the $\mathbf{K}$ matrix and discuss the close relationship to the fractional quantum Hall physics in topological flat bands with Chern number $N$.

We discuss the scattering of graphene surface plasmon polaritons (SPPs) at an interface between two semi-infinite graphene sheets with different doping levels and/or different underlying dielectric substrates. We take into account retardation effects and the emission of free radiation in the scattering process. We derive approximate analytic expressions for the reflection and the transmission coefficients of the SPPs as well as the same quantities for the emitted free radiation. We show that the scattering problem can be recast as a Fredholm equation of the second kind. Such an equation can then be solved by a series expansion, with the first term of the series corresponding to our approximated analytical solution for the reflection and transmission amplitudes. We have found that almost no free radiation is emitted in the scattering process and that under typical experimental conditions the back-scattered SPP transports very little energy. This paper provides a theoretical description of graphene plasmon scattering at an interface between distinct Fermi levels, which could be relevant for the realization of plasmonic circuitry elements such as plasmonic lenses or reflectors, and for controlling plasmon propagation by modulating the potential landscape of graphene.

We report the observation of efficient charge-to-spin conversion in the three-dimensional topological insulator (TI) $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and Ag bilayer by the spin-torque ferromagnetic resonance technique. The spin-orbit-torque ratio in the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{Ag}/\mathrm{CoFeB}$ heterostructure shows a significant enhancement as the Ag thickness increases to ∼2 nm and reaches a value of 0.5 for 5 nm Ag, which is ∼3 times higher than that of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{CoFeB}$ at room temperature. The observation reveals the interfacial effect of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{Ag}$ exceeds that of the topological surface states (TSSs) in the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ layer and plays a dominant role in the charge-to-spin conversion in the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{Ag}/\mathrm{CoFeB}$ system. Based on first-principles calculations, we attribute our observation to the large Rashba splitting bands which wrap the TSS band and have the same net spin polarization direction as the TSS of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. Subsequently, we demonstrate Rashba-induced magnetization switching in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{Ag}/\mathrm{Py}$ with a low current density of $5.8\times {10}^5\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$.

Topological nodal superconductors with protected gapless points in momentum space are generally realized based on unconventional pairings. In this work we propose a minimal model to realize these topological nodal phases with only $s$-wave interaction. In our model the linear and quadratic spin-orbit couplings along the two orthogonal directions introduce anisotropic effective unconventional pairings in momentum space. This model may support different nodal superconducting phases characterized by either an integer winding number in BDI class or a ${\mathbb{Z}}_2$ index in D class at the particle-hole invariant axes. In the vicinity of the nodal points the effective Hamiltonian can be described by either type-I or type-II Dirac equations, and the Lifshitz transition from type-I nodal phases to type-II nodal phases can be driven by external in-plane magnetic fields. We show that these nodal phases are robust against weak impurities, which only slightly renormalizes the momentum-independent parameters in the impurity-averaged Hamiltonian, thus these phases are possible to be realized in experiments with real semi-Dirac materials. The smoking-gun evidences to verify these phases based on scanning tunneling spectroscopy method are also briefly discussed.

When a ${\mathrm{Co}}^{2+}$ impurity is embedded in a semiconductor structure, crystal strain strongly influences the zero-field splitting between ${\mathrm{Co}}^{2+}$ states with spin projection $S_z=\pm 3/2$ and $S_z=\pm 1/2$. Experimental evidence of this effect has been given in previous studies; however, direct measurement of the strain-induced zero-field splitting has been inaccessible so far. Here this splitting is determined thanks to magneto-optical studies of an individual ${\mathrm{Co}}^{2+}$ ion in an epitaxial CdTe quantum dot in a ZnTe barrier. Using partially allowed optical transitions, we measure the strain-induced zero-field splitting of the ${\mathrm{Co}}^{2+}$ ion directly in the excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of $S_z=+3/2$ and $S_z=-1/2{\mathrm{Co}}^{2+}$ spin states in a magnetic field, we determine the axial and in-plane components of the crystal field acting on the ${\mathrm{Co}}^{2+}$. The proposed technique can be applied to optical determination of the zero-field splitting of other transition-metal ions in quantum dots.

Orbital modeling of two electron spins confined in a double quantum dot is revisited. We develop an extended Hund-Mulliken approach that includes excited orbitals, allowing for a triplet configuration with both electrons residing in a single dot. This model improves the reliability and applicability of the standard Hund-Mulliken approximation, while remaining largely analytical, thus it enables us to identify the mechanisms behind the exchange coupling dynamics that we find. In particular, our calculations are in close agreement with exchange values that were recently measured at a high interdot bias regime, where the double occupancy triplet configuration is energetically accessible, demonstrating reduced sensitivity to bias fluctuations, while maintaining the large exchange needed for fast gating.

Materials with properties that are modulated in time are known to display wave phenomena showing energy increasing with time, with the rate mediated by the modulation. Until now there has been no accounting for material dissipation, which clearly counteracts energy growth. This paper provides an exact expression for the amplitude of elastic or acoustic waves propagating in lossy materials with properties that are periodically modulated in time. It is found that these materials can support a special propagation regime in which waves travel at constant amplitude, with temporal modulation compensating for the normal energy dissipation. We derive a general condition under which amplification due to time-dependent properties offsets the material dissipation. This identity relates band-gap properties associated with the temporal modulation and the average of the viscosity coefficient, thereby providing a simple recipe for the design of loss-compensated mechanical metamaterials.

A quantum solid (solid ${}^4\mathrm{He}$) in contact with a classical solid defines a new class of interfaces. In addition to its quantum nature, solid ${}^4\mathrm{He}$ is indeed a very plastic medium. We examine the thermal interface resistance upon solidification of superfluid ${}^4\mathrm{He}$ in contact with a silicon crystal surface (111) and show that dislocations play a crucial role in the thermal interface transport. The growth of solid ${}^4\mathrm{He}$ and the measurements are conducted at the minimum of the melting curve of helium (0.778 K and $\sim 25\mathrm{bar}$). The results display a first-order transition in the Kapitza resistance from a value of $R_{K,L}=(80\pm 8)\mathrm{c}{\mathrm{m}}^2\mathrm{K}/\mathrm{W}$ at a pressure of 24.5 bar to a value of $R_{K,S}=(41.7\pm 8)\mathrm{c}{\mathrm{m}}^2\mathrm{K}/\mathrm{W}$ after the formation of solid helium at $\sim 25.2\mathrm{bar}$. The drop in $R_{K,S}$ is only of a factor of $\sim 2$, although transverse phonon modes in solid ${}^4\mathrm{He}$ now participate in heat transmission at the interface. We provide an explanation for the measured $R_{K,S}$ by considering the interaction of thermal phonons with vibrating dislocations in solid ${}^4\mathrm{He}$. We demonstrate that this mechanism, also called fluttering, induces a thermal resistance $R_{Fl}\propto N_d{T}^{-6}$, where $T$ is the temperature and $N_d$ is the density of dislocations. We estimate that for dislocation densities on the order of $\sim {10}^7\mathrm{c}{\mathrm{m}}^{-2}$, $R_{Fl}$ predominates over the boundary resistance $R_{K,S}$. These fundamental findings shed light on the role of dislocations and provide a quantitative explanation for previous experiments which showed no measurable change in the Kapitza resistance between Cu and superfluid ${}^4\mathrm{He}$ upon solidification of the latter. This demonstrates the possibility of using dislocations as an additional means to tailor thermal resistances at interfaces, formed especially with a plastic material.

We study ramp and periodic dynamics of ultracold bosons in an one-dimensional (1D) optical lattice which supports quantum critical points separating a uniform and a $Z_3$ or $Z_4$ symmetry broken density-wave ground state. Our protocol involves both linear and periodic drives which takes the system from the uniform state to the quantum critical point (for linear drive protocol) or to the ordered state and back (for periodic drive protocols) via controlled variation of a parameter of the system Hamiltonian. We provide exact numerical computation, for finite-size boson chains with $L\le 24$ using exact diagonalization (ED), of the excitation density $D$, the wave function overlap $F$, and the excess energy $Q$ at the end of the drive protocol. For the linear ramp protocol, we identify the range of ramp speeds for which $D$ and $Q$ show Kibble-Zurek scaling. We find, based on numerical analysis with $L\le 24$, that such scaling is consistent with that expected from critical exponents of the $q$-state Potts universality class with $q=3,4$. For the periodic protocol, we show that the model displays near-perfect dynamical freezing at specific frequencies; at these frequencies $D,Q\to 0$ and $\left|F\right|\to 1$. We provide a semi-analytic explanation of such freezing behavior and relate this phenomenon to a many-body version of Stuckelberg interference. We suggest experiments which can test our theory.

An idea of controlling hyperbolic dispersion of dielectric materials by an ultrashort laser pulse is proposed. Taking the diamond as a concrete example and using time-dependent density functional theory calculations, we show that the permittivity tensor of the material can be effectively tuned by an ultrashort laser pulse, serving as a transient hyperbolic medium with wide working frequency window. With easily tunable laser parameters, the material can even be switched by reversal of both elliptic and hyperbolic for a particular light frequency. Our result points out a route toward transient hyperbolic materials, and it offers methods to achieve tunable hyperbolic dispersion with great potential for ultrafast device applications.

We compute the real-space spin correlations and frequency and wave-vector resolved dynamic structure factors $S(\vec{q},\omega )$ for the nearest-neighbor kagome-lattice Heisenberg model (KLHM) at finite temperatures using the numerical linked cluster expansion (NLCE) method. A triangle-based NLCE is used to calculate frequency moments of the dynamic structure factors in the thermodynamic limit, which show excellent convergence for $T>J/4$. A Gaussian approximation and the fluctuation-dissipation relation are used to reconstruct the frequency dependence. We find that some features of the low temperature KLHM structure factors begin to set in at temperatures of order $J$. Our results are in very good agreement with powder diffraction measurements reported earlier on the Herbertsmithite materials ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$. However, the calculated properties differ from the low temperature $\left(T\approx J/100\right)$ experimental measurements in one important regard. In line with the experimental observations, the spectral weight has a diffuse nature, which is predominantly spread along the extended Brillouin-zone boundary. However, the maximum intensity is found in our calculations to be at the $K$ point of the extended Brillouin zone in contrast to the low temperature experiments, where it is at the $M$ point. We suggest that experiments should be done at various temperatures to look for such a crossover of the maximum from the $K$ point to the $M$ point. In the absence of such a crossover, the Herbertsmithite materials must differ from KLHM in a significant manner.