Applying the linearized Usadel equations, we consider the nucleation of superconductivity in multiply connected mesoscopic superconductor/ferromagnet (S/F) hybrids such as a thin superconducting ring on a ferromagnet with a uniform in-plane magnetization $M$ and a spin-active S/F interface. We demonstrate that the exchange field in F provokes a switching between superconducting states with different vorticities which may increase the critical temperature($T_c$) of the superconductor in a magnetic field. We study the interplay between oscillations in $T_c$ due to the Little-Parks effect and oscillations in $T_c$ induced by the exchange field. Furthermore, we analyze the influence of long-range spin-triplet correlations on the switching between different vorticities.

Motivated by a recent experimental observation of superconductivity in the Al-Zn-Mg quasicrystal, we study the low-temperature behavior of electrons moving in the quasiperiodic potential of the Ammann–Beenker tiling in the presence of a local attraction. We employ the Bogoliubov–de Gennes approach for approximants of different sizes and determine the local pairing amplitude ${\mathrm{\Delta }}_i$ as well its spatial average, ${\mathrm{\Delta }}_0$, the superconducting order parameter. Due to the lack of periodicity of the octagonal tiling, the resulting superconducting state is inhomogeneous, but we find no evidence of the superconductivity islands, as observed in disordered systems, with ${\mathrm{\Delta }}_i\to 0$ at $T_c$ for all sites. In the weak-coupling regime, we find that the superconducting order parameter depends appreciably on the approximant size only if the Fermi energy sits at a pseudogap in the noninteracting density of states, with ${\mathrm{\Delta }}_0$ decreasing as the system size increases. These results are in line with the experimental observations for the Al-Zn-Mg quasicrystal, and they suggest that, despite their electronic structure, quasicrystals are prone to display conventional BCS-like superconductivity.

Neutron and x-ray total scattering measurements have been performed on powder samples of the iron chalcogenide superconductor FeSe. Using pair distribution function analysis of the total scattering data to investigate short-range atomic correlations, we establish the existence of an instantaneous, local orthorhombic structural distortion attributable to nematic fluctuations that persists well into the high-temperature tetragonal phase, at least up to 300 K and likely to significantly higher temperatures. This short-range orthorhombic distortion is correlated over a length scale of about 1 nm at 300 K and grows to several nm as the temperature is lowered toward the long-range structural transition temperature. In the low-temperature nematic state, the local instantaneous structure exhibits an enhanced orthorhombic distortion relative to the average structure with a typical relaxation length of 3 nm. The quantitative characterization of these orthorhombic fluctuations sheds light on nematicity in this canonical iron-based superconductor.

The role of nematic fluctuations in the pairing mechanism of iron-based superconductors is frequently debated. Here, we present a method to reveal such fluctuations by identifying the energy and momentum of the corresponding nematic boson through the detection of a boson-assisted resonant amplification of Friedel oscillations. Using Fourier-transform scanning tunneling spectroscopy, we observe for the unconventional superconductor LiFeAs strong signatures of bosonic states at momentum $q\sim 0$ and energy $\mathrm{\Omega }\approx 8$ meV. We show that these bosonic states survive in the normal conducting state, and, moreover, that they are in perfect agreement with well-known strong above-gap anomalies in the tunneling spectra. Attributing these small-$q$ boson modes to nematic fluctuations, we provide a spectroscopic approach to the nematic boson in an unconventional superconductor.

We study electron pumping in the strong coupling and non-Markovian regime. Our model is a single quantum dot with periodically modulated energy and tunneling amplitudes. We identify four parameters to control the direction of the current: the driving phase, the coupling strength, the driving frequency, and the location of the maxima of the spectral density. In the high-frequency regime, we use a Markovian embedding strategy to map our model to three serial quantum dots weakly coupled to the reservoirs allowing us to use a Floquet master equation. We observe a rectification effect of the pumped charge that is exclusive to the non-Markovian character of our model. In the low-frequency regime, we apply an additional transformation to see our model as three independent transport channels. With the use of full counting statistics, we study charge fluctuations and validate that our model behaves as a single-electron source.

Trigonal selenium and tellurium crystalize in helical chainlike structures and thus possess interesting properties such as nontrivial band topology, gyrotropic effects, and nonlinear optical responses. By performing systematic density-functional-theory calculations with the generalized gradient approximation plus scissors correction, we study their linear and nonlinear optical (NLO) properties. We find that both materials exhibit large second-harmonic generation (SHG) and linear electro-optic (LEO) effect. In particular, tellurium has the huge SHG coefficient $\left({\chi }_{xxx}^{\left(2\right)}\right)$ in the photon energy range of $0\sim 3\mathrm{eV}$ with the maximum magnitude being about 16 times larger than that of GaN, a widely used NLO material. Tellurium is also found to possess the gigantic static SHG coefficient ${\chi }_{xyz}^{\left(2\right)}$, which is up to 100 times larger than that of GaN. On the other hand, selenium exhibits the large LEO coefficient $r_{xxx}\left(0\right)$, which is more than six times larger than that of GaN. Thus, tellurium and selenium may find valuable applications in NLO and LEO devices such as frequency conversion, electro-optical switches, and light signal modulators. Interestingly, our calculations also reveal that for each material, the ${\chi }_{xxx}^{\left(2\right)}$ values for the two helical structures are equal but the ${\chi }_{xyz}^{\left(2\right)}$ values differ in sign, suggesting that the SHG spectroscopy is a useful probe of their chirality. The calculated static and optical dielectric constants as well as SHG coefficients at the ${\mathrm{CO}}_2$ laser frequency are in good agreement with the available experiments. Finally, much stronger NLO responses of selenium and tellurium compared with the semiconductors with similar band gaps are attributed to their quasi-one-dimensional structures with directional covalent bonding and lone-pair electrons. These findings will help the search for new materials with large NLO coefficients.

Near-field radiative heat transfer (NFRHT) is strongly related with many applications such as near-field imaging, thermo-photovoltaics and thermal circuit devices. The active control of NFRHT is of great interest since it provides a degree of tunability by external means. In this work, a magnetically tunable multiband NFRHT is revealed in a system of two suspended graphene sheets at room temperature. It is found that the single-band spectra for $B=0$ split into multiband spectra under an external magnetic field. Dual-band spectra can be realized for a modest magnetic field (e.g., $B=4\mathrm{T}$). One band is determined by intraband transitions in the classical regime, which undergoes a blue shift as the chemical potential increases. Meanwhile, the other band is contributed by inter-Landau-level transitions in the quantum regime, which is robust against the change of chemical potentials. For a strong magnetic field (e.g., $B=15\mathrm{T}$), there is an additional band with the resonant peak appearing at near-zero frequency (microwave regime), stemming from the magnetoplasmon zero modes. The great enhancement of NFRHT at such low frequency has been little reported. This work may pave a way for multiband thermal information transfer based on atomically thin graphene sheets.

We derive and calculate thermal transport coefficients for a quantum Hall system in the linear response regime, and show that they are exponentially small in the bulk, in contrast to the quantized value of the charge Hall coefficient, thus violating the Wiedemann-Franz law. This corroborates earlier reports about the essential difference between the charge and thermal quantum Hall effect, that originates from the different behavior of the corresponding $U\left(1\right)$ and gravitational anomalies. We explicitly calculate the bulk currents when a temperature profile is applied within the bulk, and show that they are proportional to the second derivative of the respective gravitational potential (tidal force), and nonuniversal, in contrast to the charge current which is proportional to the first derivative of the electrochemical potential.

We investigate how tilting affects the off-diagonal, dissipationless response of a pair of chirally imbalanced Weyl cones to various external perturbations. The pair of chirally imbalanced Weyl cones can be described as a chiral electron fluid, that can flow with a velocity field that contains vorticity. Upon applying an external magnetic field, we obtain the so-called magnetovortical linear-response matrix that relates electric and heat currents to the magnetic field (chiral magnetic effect) and the vorticity (chiral vortical effect). We show how this response matrix becomes anisotropic upon tilting the cones and determine its nonanalytic long-wavelength behavior, as well as the corresponding ac response. In addition, we discuss how the tilt dependence of the electronic (or density-density) susceptibility introduces anisotropy in the dispersion relation of the soundlike excitations in the fluid of chiral fermions, which are known as chiral magnetic waves. In the case of an externally applied electric field and a temperature gradient, we find a Hall-like response in the electric and heat current density that is perpendicular to both the tilting direction and the perturbations. As the tilting direction forms a time-reversal symmetry breaking vector, a nonzero (heat) orbital magnetization manifests itself. We calculate the magnetization currents microscopically and elucidate how to subtract these contributions to obtain the transport currents.

We report measurements of photon-assisted transport and noise in a tunnel junction in the regime of dynamical Coulomb blockade. We have measured both dc nonlinear transport and low frequency noise in the presence of an ac excitation at frequencies up to 33 GHz. In both experiments we observe replicas at finite voltage of the zero bias features, a phenomenon characteristic of photon emission/absorption. However, the ac voltage necessary to explain our data is notably different for transport and noise, indicating that usual theory of photon-assisted phenomena fails to account for our observations.

We show that Majorana edge modes appear in a strongly correlated phase of semiconducting nanowires with discrete rotational symmetry in the cross section. These modes exist in the absence of spin-orbit coupling, magnetic fields, and superconductivity. They appear purely due to the combination of the three-dimensional Coulomb interaction and orbital physics, which generates a topological gap in one sector of the excitation spectrum as well as a topological ground-state degeneracy. The gap can be comparable in magnitude to the topological superconducting gap in other solid-state candidate systems for Majorana edge modes and may similarly be probed via tunnel spectroscopy.

We study the behavior of nonequilibrium spin density and spin-orbit torque in a topological insulator/antiferromagnet heterostructure. Unlike ferromagnetic heterostructures where the Dirac cone is gapped due to time-reversal symmetry breaking, here the Dirac cone is preserved. We demonstrate the existence of a staggered spin density corresponding to a dampinglike torque which is quite robust against scalar impurities when the transport energy is such that the transport is confined to the topological insulator surface. We show the contribution to the nonequilibrium spin density due to both surface and bulk topological insulator bands. Finally, we show that the torques in topological insulator/antiferromagnet heterostructures exhibit an angular dependence that is consistent with the standard spin-orbit torque obtained in Rashba system with some additional structure arising from the interfacial coupling.

The high-temperature ferroelectric order and a remarkable magnetoelectric effect driven by the magnetic field cooling are reported in $R_2{\mathrm{BaCuO}}_5(R$ = Er, Dy, Sm) series. The ferroelectric (FE) orders are observed at much higher temperatures than their magnetic orders for all three members. The value of FE Curie temperature $\left(T_{\mathrm{FE}}\right)$ is considerably high as $\sim 235$ K with the polarization value $\left(P\right)$ of $\sim 1410\mu \mathrm{C}/{\mathrm{m}}^2$ for a 4 kV/cm poling field in the case of ${\mathrm{Er}}_2{\mathrm{BaCuO}}_5$, whereas the values of $T_{\mathrm{FE}}$ and $P$ are also promising as $\sim 232$ and $\sim 992\mu \mathrm{C}/{\mathrm{m}}^2$ for ${\mathrm{Dy}}_2{\mathrm{BaCuO}}_5$, and $\sim 184$ K and $\sim 980\mu \mathrm{C}/{\mathrm{m}}^2$ for ${\mathrm{Sm}}_2{\mathrm{BaCuO}}_5$. The synchrotron diffraction studies of ${\mathrm{Dy}}_2{\mathrm{BaCuO}}_5$ confirm a structural transition at $T_{\mathrm{FE}}$ to a polar $Pna{2}_1$ structure, which correlates the FE order. An unusual magnetoelectric coupling is observed below the $R$ order for Er and Dy compounds and below the Cu order for Sm compound, when the pyroelectric current is recorded only with the magnetic field both in heating and cooling cycles, i.e., the typical magnetic field cooled effect. The magnetic field cooled effect driven emergence of polarization is ferroelectric in nature, as it reverses due to the opposite poling field.

Ytterbium dihydride (${\mathrm{YbH}}_2$) shows a well-known transition at $\approx 16\mathrm{GPa}$ from a ${\mathrm{CaH}}_2$-type structure to a high-pressure (high-$P$) phase with Yb at hcp sites and unknown H positions. Here, we report its complete structure determination by neutron diffraction at 34 GPa. Hydrogen (deuterium) is located at $2a$ and $2d$ positions of space group $P{6}_3/mmc$, thus forming a high-symmetry “collapsed” close-packed lattice. The transition is sluggish and can be seen as a transfer of 1/2 of the hydrogen atoms from strongly corrugated H layers to interstitial sites of the Yb lattice. We demonstrate by first-principles calculations that the transition is related to a change from a completely filled $f$-electron configuration to a fractional $f$-hole ($\approx 0.25\mathrm{h}$) occupation in the high-$P$ phase. The $f\to d$ charge transfer closes the gap at the transition and leads to a metallic ground state with a sizable electron-phonon interaction involving out-of-plane vibrational modes of interstitial hydrogen.

We report nonequilibrium magnetodynamics in the Rashba-superconductor GeTe, which lacks inversion symmetry in the bulk. We find that at low temperature the system exhibits a nonequilibrium state, which decays on timescales that exceed conventional electronic scattering times by many orders of magnitude. This reveals a nonequilibrium magnetoresponse that is asymmetric under magnetic-field reversal and, strikingly, induces a nonequilibrium superconducting state distinct from the equilibrium one. We develop a model of a Rashba system in which nonequilibrium configurations relax on a finite timescale that captures the qualitative features of the data. We also obtain evidence for the slow dynamics in another nonsuperconducting Rashba system. Our work provides insights into the dynamics of noncentrosymmetric superconductors and Rashba systems in general.

We derive boundary conditions that allow a three-dimensional periodic array of superfluid vortices to be modeled in a Cartesian domain. The method is applicable to vortices in the Gross-Pitaevskii description of a superfluid and to fluxtubes in the Ginzburg-Landau description of a superconductor. Unlike standard methods for modeling infinite arrays of vortices, the boundary conditions can be used to study the three-dimensional tangling and reconnection of vortex lines expected in superfluid turbulence. In the two-dimensional case, the boundary conditions include two parameters that determine the lattice offset, which for a single superfluid is essentially arbitrary. In the three-dimensional case the boundary conditions include three parameters that must satisfy a particular linear relationship. We present an algorithm for finding all vortex lattice states within a given domain. We demonstrate the utility of the boundary conditions in two specific problems with imperfect or tangled lattices.

The semiclassical approach introduced by Sachdev and collaborators proved to be extremely successful in the study of quantum quenches in massive field theories, both in homogeneous and inhomogeneous settings. While conceptually very simple, this method allows one to obtain analytic predictions for several observables when the density of excitations produced by the quench is small. At the same time, a novel generalized hydrodynamic (GHD) approach, which captures exactly many asymptotic features of the integrable dynamics, has recently been introduced. Interestingly, also this theory has a natural interpretation in terms of semiclassical particles and it is then natural to compare the two approaches. This is the objective of this work: we carry out a systematic comparison between the two methods in the prototypical example of the sine-Gordon field theory. In particular, we study the “bipartitioning protocol” where the two halves of a system initially prepared at different temperatures are joined together and then left to evolve unitarily with the same Hamiltonian. We identify two different limits in which the semiclassical predictions are analytically recovered from GHD: a particular nonrelativistic limit and the low-temperature regime. Interestingly, the transport of topological charge becomes subballistic in these cases. Away from these limits we find that the semiclassical predictions are only approximate and, in contrast to the latter, the transport is always ballistic. This statement seems to hold true even for the so-called “hybrid” semiclassical approach, where finite time DMRG simulations are used to describe the evolution in the internal space.

A semiconductor single-atom micromaser consists of a microwave cavity coupled to a gain medium, a double quantum dot driven out of equilibrium by a bias voltage. The masing threshold of this system was recently probed by measuring photon statistics in the cavity [Y-Y. Liu et al., Phys. Rev. Lett. 119, 097702 (2017)]. In this paper, we develop an in-depth, rigorous understanding of this experiment and related works. First, we use a semiclassical theory and study transmission spectroscopy. This approach allows us to derive the masing threshold condition for arbitrary temperature and voltage bias, and expose microscopic principles required for realizing photon gain and thereby a photon amplifier. Next, by employing the quantum master equation approach we extend the Scully-Lamb quantum theory of a laser to the present setup, and investigate the statistics of emitted photons below and above the masing threshold as a function of experimentally tunable parameters. Although our focus is primarily on hybrid quantum dot circuit - quantum electrodynamics systems, our approach is adaptable to other light-matter systems where the gain medium consists of a mesoscopic structure.

We introduce and study a minimum two-orbital Hubbard model on a triangular lattice, which captures the key features of both the trilayer ABC-stacked graphene-boron nitride heterostructure and twisted transition metal dichalcogenides in a broad parameter range. Our model comprises first- and second-nearest neighbor hoppings with valley-contrasting flux that accounts for trigonal warping in the band structure. For the strong-coupling regime with one electron per site, we derive a spin-orbital exchange Hamiltonian and find the semiclassical ground state to be a spin-valley density wave. We show that a relatively small second-neighbor exchange interaction is sufficient to stabilize the ordered state against quantum fluctuations. Effects of spin- and valley Zeeman fields as well as thermal fluctuations are also examined.