We report the observation of multigap superconductivity in ${\mathrm{TaRh}}_2{\mathrm{B}}_2$. We show ${\mathrm{TaRh}}_2{\mathrm{B}}_2$ is a bulk type-II superconductor with a transition temperature $T_{\mathrm{c}}=6.00\left(5\right)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same $\left(s+s\right)$ model, providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that ${\mathrm{TaRh}}_2{\mathrm{B}}_2$ has an upper critical field of 15.2(1) T, which is significantly higher than previously reported and exceeds the Pauli limit.

We clarify unconventional forms of the scaling functions of conductance, critical conductance distribution, and localization length in a disorder-driven quantum phase transition between band insulator and Weyl semimetal phases. Quantum criticality of the phase transition is controlled by a clean-limit fixed point with spatially anisotropic scale invariance. We argue that the anisotropic scale invariance is reflected on the scaling function forms in the quantum phase transition. We verify the proposed scaling function forms in terms of transfer-matrix calculations of conductance and localization length in a tight-binding model.

The steady-state behavior of thermal transport in bulk and nanostructured semiconductors has been widely studied, both theoretically and experimentally. On the other hand, fast transients and frequency dynamics of thermal conduction has been given less attention. The frequency response of thermal conductivity has become more crucial in recent years, especially in light of the constant rise in the clock frequencies in microprocessors and terahertz sensing applications. Thermal conductivity in response to a time-varying temperature field starts decaying when the frequency exceeds a cutoff frequency ${\mathrm{\Omega }}_c$, which is related to the inverse of phonon relaxation time $\tau$, on the order of 2–10 ps in most bulk semiconductors. Phonons in graphene have much longer phonon relaxation times, which we show leads to far lower ${\mathrm{\Omega }}_c$. Our calculations, based on the phonon Boltzmann equation coupled with first-principles dispersion, show that dynamical thermal conductivity of graphene resembles a low-pass filter that decays beyond an ${\mathrm{\Omega }}_c$ ranging from 100 MHz to 10 GHz, controlled by temperature and ribbon width. The response parallels the Drude model of electrons, but with far lower cutoff. Moreover, the presence of strong normal processes in graphene results in a complex-valued conductivity and gradual transition around ${\mathrm{\Omega }}_c$, with the resistive contribution to the heat flux having higher cutoff frequency and smaller phase lag than the hydrodynamic part. The dynamical conductivity will impact dissipation in high-frequency applications of graphene. Our findings also provide a platform for future studies of hydrodynamic transport and wavelike, or second sound, heat transfer by tuning the frequency of the applied temperature field.

We investigated the effect of pressure on the magnetic and thermoelectric properties of ${\mathrm{Sr}}_{3.1}{\mathrm{Y}}_{0.9}{\mathrm{Co}}_4{\mathrm{O}}_{10+\delta }$. The magnetization is reduced with the application of pressure, reflecting the spin-state modification of the ${\mathrm{Co}}^{3+}$ ions into the nonmagnetic low-spin state. Accordingly, with increasing pressure, the Seebeck coefficient is enhanced, especially at low temperatures, at which the effect of pressure on the spin state becomes significant. These results indicate that the spin-orbital entropy is a key valuable for the thermoelectric properties of the strongly correlated cobalt oxides.

Quasiparticles are an intrinsic source of relaxation and decoherence for superconducting qubits. Recent works have shown that normal-metal traps may be used to evacuate quasiparticles, and potentially improve the qubit lifetime. Here, we investigate how far the normal metals themselves may introduce qubit relaxation. We identify the Ohmic losses inside the normal metal and the tunneling current through the normal-metal–superconductor interface as the relevant relaxation mechanisms. We show that the Ohmic loss contribution depends strongly on the device and trap geometry, as a result of the inhomogeneous electric fields in the qubit. The correction of the quality factor due to the tunneling current on the other hand is highly sensitive to the nonequilibrium distribution function of the quasiparticles. Overall, we show that even when choosing less than optimal parameters, the presence of normal-metal traps does not affect the quality factor of state-of-the-art qubits.

The determination of the Landau free energy (the grand thermodynamic potential) by a perturbation theory is advanced to arbitrary order for the specific case of noninteracting fermionic or bosonic systems perturbed by a one-particle potential. Peculiar features of the formalism are highlighted. The results are employed to develop a more explicit approach describing exchange interactions between spins of Anderson's magnetic impurities in metals, semiconductors, and insulators. Within the fourth order our theory provides a unified basis on which a single formula including the Ruderman-Kittel-Kasuya-Yosida, Bloembergen-Rowland, superexchange, and two-electron exchange integrals at nonzero temperature is derived.

We put forward a tight-binding model for rhombohedral topological insulator materials with the space group $D_{3d}^5\left(R\overline{3}m\right)$. The model describes the bulk band structure of these materials over the whole Brillouin zone. Within this framework, we also describe the topological nature of surface states, characterized by a Dirac conelike dispersion and the emergence of surface projected bulk states near to the Dirac point in energy. We find that the breaking of the $R_3$ symmetry as one moves away from the $\mathrm{\Gamma }$ point has an important role in the hybridization of the $p_x, p_y$, and $p_z$ atomic orbitals. In our tight-binding model, the latter leads to a band mixing matrix element ruled by a single parameter. We show that our model gives a good description of the strategies/mechanisms proposed in the literature to eliminate and/or energy shift the bulk states away from the Dirac point, such as stacking faults and the introduction of an external applied electric field.

We present numerical results for the $J_1\text{−}{J}_2$ Heisenberg model on a triangular lattice at finite temperatures $T>0$. In contrast to unfrustrated lattices we reach much lower $T\sim 0.15J_1$. In static quantities the novel feature is a quite sharp low-$T$ maximum in the specific heat. Dynamical spin structure factor $S(\mathbf{q},\omega )$ allows for the extraction of the effective spin-wave energies ${\omega }_{\mathbf{q}}\left(T\right)$ and their damping ${\gamma }_{\mathbf{q}}\left(T\right)$. While for $J_2=0$ our results are consistent with $T=0$ spin ordering, $J_2/J_1\sim 0.1$ induces additional frustration with a signature of spin-liquid ground state. In the latter case, the results for the spin-lattice relaxation rate indicate the low-$T$ accessible regime on $1/T_1\propto T^{\alpha }$ with $\alpha \ge 1$, as observed in recent spin-liquid materials on a triangular lattice.

We theoretically investigate the role of intraband transitions in laser-induced carrier generation for different photon energy regimes: (i) strongly off resonant, (ii) multiphoton resonant, and (iii) resonant conditions. Based on the analysis for the strongly off resonant and multiphoton resonant cases, we find that intraband transitions strongly enhance photocarrier generation in both multiphoton absorption and tunneling excitation regimes, and thus, they are indispensable for describing the nonlinear photocarrier generation processes. Furthermore, we find that intraband transitions enhance photocarrier generation even in the resonant condition, opening additional multiphoton excitation channels once the laser irradiation becomes sufficiently strong. The above findings suggest a potential for efficient control of photocarrier generation via multicolor laser pulses through optimization of the contributions from intraband transitions.

We theoretically investigate the D'yakonov-Perel' spin-relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant $k$-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small $k$-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real space that is modulated by the spin-precession length induced by the $k$-linear terms. For this reason, the experimentally detected spin relaxation largely depends on the ratio between the nanowire radius and the spin-precession length as well as the type of measurement. In particular, it is shown that while a variation of the radius hardly affects the magnetoconductance correction, which is governed by the long-lived spin states, the change in the spin lifetime observed in optical experiments can be dramatic. We compare our results with recent experimental studies on wurtzite InAs nanowires.

The chirality of electronic Bloch bands is responsible for many intriguing properties of layered two-dimensional materials. We show that in bilayers of transition metal dichalcogenides (TMDCs), unlike in few-layer graphene and monolayer TMDCs, both intralayer and interlayer couplings give important contributions to the Berry curvature in the $K$ and $-K$ valleys of the Brillouin zone. The interlayer contribution leads to the stacking dependence of the Berry curvature and we point out the differences between the commonly available 3R type and 2H type bilayers. Due to the interlayer contribution, the Berry curvature becomes highly tunable in double gated devices. We study the dependence of the valley Hall and spin Hall effects on the stacking type and external electric field. Although the valley and spin Hall conductivities are not quantized, in ${\mathrm{MoS}}_{2}2\text{H}$ bilayers, they may change sign as a function of the external electric field, which is reminiscent of the behavior of lattice Chern insulators.

The surface scattering of free electrons strongly modifies the electromagnetic response near the interface. Due to the inherent anisotropy of the surface scattering that necessarily reverses the normal to the interface component of the electron velocity while its tangential component may remain the same, a thin layer near a high-quality interface shows strong dielectric anisotropy. The formation of the resulting hyperbolic dispersion layers near the metal-dielectric interface strongly modifies the local density of states, and leads to orders of magnitude changes in all associated phenomena.

We studied the in-plane dynamic and static charge conductivity of electron doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott gap of 0.55 eV. Upon substitution of $2\%$ La per formula unit the Mott gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or Drude peak) we observe a soft collective mode (SCM) with a broad maximum at $40\mathrm{meV}$. Doping to $10\%$ increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons.

We present a systematic study of the quasiparticle band structures of transition-metal halides CuCl, CuBr, AgCl, and AgBr. We show that GW calculations for cuprous halides are significantly more challenging computationally than ZnO, a much-discussed extreme case. The local-density approximation (LDA) within density functional theory severely underestimates the band gaps of CuCl and CuBr due to the inaccurate treatment of the semicore $d$ electrons. As a result, many-body perturbation calculations within the ${\mathrm{G}}^0{\mathrm{W}}^0$ approach fail to give accurate quasiparticle properties starting from the LDA mean-field solution. The $\mathrm{LDA}+U$ method (with the screened Coulomb and exchange parameters calculated using a constrained random-phase-approximation approach), on the other hand, provides a much better starting point for subsequent ${\mathrm{G}}^0{\mathrm{W}}^0$ calculations. When properly converged, the ${\mathrm{G}}^0{\mathrm{W}}^0/\mathrm{LDA}+U$ approach is able to reproduce the experimental minimum band gaps of all four compounds to within 0.1 eV. These results, however, can be achieved only by applying extremely high cutoff parameters, which would be very difficult without using our recently developed accelerated GW approach. Our work demonstrates the applicability and accuracy of the ${\mathrm{G}}^0{\mathrm{W}}^0/\mathrm{LDA}+U$ method in predicting the quasiparticle band structure of these materials and other systems involving localized semicore states.

Most works on pyrochlore magnets deal with the interacting spin-1/2 local moments. We here study the spin-one local moments on the pyrochlore lattice and propose a generic interacting spin model on a pyrochlore lattice. Our spin model includes the antiferromagnetic Heisenberg interaction, the Dzyaloshinskii-Moriya interaction, and the single-ion spin anisotropy. We develop a flavor wave theory and combine with a mean-field approach to study the global phase diagram of this model and establish the relation between different phases in the phase diagram. We find the regime of the quantum paramagnetic phase where a degenerate line of the magnetic excitations emerges in the momentum space. We further predict the critical properties of the transition out of the quantum paramagnet to the proximate orders. The presence of quantum order by disorder in the parts of the ordered phases is then suggested. We point out the existence of degenerate and topological excitations in various phases. We discuss the relevance with fluoride pyrochlore material ${\mathrm{NaCaNi}}_2{\mathrm{F}}_7$ and explain the role of the spin-orbit coupling and the magnetic structures of the Ru-based pyrochlore ${\mathrm{A}}_2{\mathrm{Ru}}_2{\mathrm{O}}_7$ and the Mo-based pyrochlore ${\mathrm{A}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$.

We formulate a general theory to study the time-dependent charge and energy transport of an adiabatically driven quantum dot in contact with normal and superconducting reservoirs at $T=0$. This setup is a generalization of a quantum $RC$ circuit, with capacitive components due to Andreev processes and induced pairing fluctuations, in addition to the conventional normal charge fluctuations. The dynamics for the dissipation of energy is ruled by a Joule law for four channels in parallel with the universal Büttiker resistance $R_0=e^2/2h$ per channel. Two transport channels are associated with the two spin components of the usual charge fluctuations, while the other two are associated with electrons and holes due to pairing fluctuations. The latter leads to an “anomalous” component of the Joule law and takes place with a vanishing net current due to the opposite flows of electrons and holes.

Linear parametric amplification is a key operation in information processing. Our interest here is quantum-limited parametric amplification, i.e., amplification of quantum signals while adding the minimum amount of noise allowed by quantum mechanics, which is essential for any viable implementation of quantum information processing. We describe parametric amplifiers based on the dispersive nonlinearity of Josephson junctions driven with appropriate tones playing the role of pumps. We discuss two defining characteristics in the architecture of these amplifiers: the number of modes occupied by the signal, idler, and pump waves and the number of independent ports through which these waves enter into the circuit. The scattering properties of these amplifiers are also reviewed. The main focus of this work are the computations of the dynamic range and phase-space distributions of the fluctuations of the modes of the amplifiers.

We study the nematic isotropic phase transition by applying the functional renormalization group to the Landau–de Gennes model. We derive the flow equations for the effective potential as well as the “couplings” constants and the anomalous dimension. We then solve the coupled flow equations on a grid using the Newton-Raphson method. A first-order phase transition is observed. We also investigate the nematic isotropic puzzle (the NI puzzle) in this paper. We obtain the NI transition temperature difference $T_c-T^{*}=5.85\text{K}$.