We report point-contact Andreev reflection spectroscopy studies on possible topological superconductor candidates Cl- (Br-) doped (${\mathrm{PbSe})}_{1.12}\left({\mathrm{TaSe}}_2\right)$ with superconducting transition temperature $T_c\sim$ 1.2 K. A common double-peak feature is observed for the conductance curves at low temperature in the absence of a zero-bias conductance peak. We analyze conductance curves with the Blonder-Tinkham-Klapwijk model and the extracted superconducting gap follows a typical Bardeen-Cooper-Schrieffer temperature- and field-dependent behavior, with $2{\mathrm{\Delta }}_0/k_B{T}_c\approx 3.7$ and 3.8 for Cl and Br doping, respectively. Our results strongly suggest that both Cl- and Br-doped $({\mathrm{PbSe})}_{1.12}\left({\mathrm{TaSe}}_2\right)$ are fully gapped superconductors in an intermediate-coupling regime.

We investigate torsional chiral magnetic effect (TCME) induced by skyrmion-vortex textures in the A phase of the superfluid ${}^3\mathrm{He}$. In ${}^3\mathrm{He}\text{−}\mathrm{A}$, Bogoliubov quasiparticles around point nodes behave as Weyl fermions, and the nodal direction represented by the $\ell$ vector may form a spatially modulated texture. $\ell$ textures generate a chiral gauge field and a torsion field directly acting on the chirality of Weyl-Bogoliubov quasiparticles. It has been clarified by Volovik [Pi'sma Zh. Eksp. Teor. Fiz. 43, 428 (1986)] that, if the $\ell$ vector is twisted as $\widehat{\mathbf{\ell }}·\left(\mathrm{curl}\widehat{\mathbf{\ell }}\right)\ne 0$, the chiral gauge field is responsible for the chiral anomaly, leading to an anomalous current along $\mathbf{\ell }$. Here we show that, even if $\widehat{\mathbf{\ell }}·\left(\mathrm{curl}\widehat{\mathbf{\ell }}\right)=0$, a torsion arising from $\ell$ textures brings about contributions to the equilibrium currents of Weyl-Bogoliubov quasiparticles along $\mathrm{curl}\mathbf{\ell }$. This implies that while the anomalous current appears only for the twisted (Bloch-type) skyrmion of the $\ell$ vector, the extra mass current due to TCME always exists regardless of the skyrmion type. Solving the Bogoliubov–de Gennes equation, we demonstrate that both Bloch-type and Néel-type skyrmions induce chiral fermion states with spectral asymmetry, and possess spatially inhomogeneous structures of Weyl bands in the real coordinate space. Furthermore, we discuss the contributions of Weyl-Bogoliubov quasiparticles and continuum states to the mass current density in the vicinity of the topological phase transition. In the weak-coupling limit, continuum states give rise to backflow to the mass current generated by Weyl-Bogoliubov quasiparticles, which makes a non-negligible contribution to the orbital angular momentum. As the topological transition is approached, the mass current density is governed by the contribution of continuum states.

We have measured the superconducting penetration depth $\mathrm{\Lambda }\left(T\right)$ in the heavy-fermion/intermediate-valent superconducting alloy series ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{CoIn}}_5$ using a transverse-field muon spin relaxation to study the effect of intermediate-valent Yb doping on Fermi-liquid renormalization. From $\mathrm{\Lambda }\left(T\right)$ we determine the superfluid density ${\rho }_s\left(T\right)$ and find that it decreases continuously with increasing nominal Yb concentration $x$, i.e., with increasing intermediate valence. The temperature-dependent renormalization of the “normal” fluid density ${\rho }_N\left(T\right)={\rho }_s\left(0\right)-{\rho }_s\left(T\right)$ in both the heavy-fermion and intermediate valence limits is proportional to the temperature-dependent renormalization of the specific heat. This indicates that the temperature-dependent Fermi-liquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavy-fermion phenomena in superconductors.

We provide a description of a diagrammatic Monte Carlo algorithm for the resonant Fermi gas in the normal phase. Details are given on the diagrammatic framework, Monte Carlo moves, and incorporation of ultraviolet asymptotics. Apart from the self-consistent bold scheme, we also describe a non-self-consistent scheme, for which the ultraviolet treatment is more involved.

We exploit topological semimetallic phases resulting from the Kondo screening in Anderson lattice models. It is shown that by including spin-orbit interactions both in the bulk electrons and in the hybridization between the conduction electrons and electrons in the $f$ orbit, all types of topological semimetallic phases can be realized in Anderson lattice models. Specifically, upon either broken time-reversal symmetry or broken inversion symmetry, we find that either the Weyl semimetallic phase, Dirac semimetallic phase, or nodal-ring semimetallic phases always emerge between insulating phases and can be accessed by tuning either temperature or spin-orbit interaction. For Anderson lattice models with general three-dimensional spin-orbit hybridization between the conduction electrons and electrons in the $f$ orbit, we find that Weyl nodal-ring semimetallic phases emerge between strong and weak topological insulating phases. Furthermore, in the presence of an exchange field, Weyl semimetallic phases form after two Weyl points of charge $\pm 1$ split off from a Dirac point at time-reversal momenta. On the other hand, when the spin-orbit interaction is included in the conduction electron, we find that upon the rotation symmetry being broken with anisotropic hopping amplitudes, a Weyl semimetallic phase emerges with a double Weyl node with charges of $\pm 2$. Furthermore, the Weyl semimetallic phases with charges of $\pm 2$ can be tuned into Weyl semimetallic phases with charges of $\pm 1$ through the inclusion of the Rashba spin-orbit interaction. Our analyses indicate that Anderson lattices with appropriate spin-orbit interactions provide a platform for realizing all types of topological semimetallic phases.

Extremely large nonsaturating magnetoresistance has recently been reported for a large number of both topologically trivial and nontrivial materials. Different mechanisms have been proposed to explain the observed magnetotransport properties, yet without arriving to definitive conclusions or portraying a global picture. In this work, we investigate the transverse magnetoresistance of materials by combining the Fermi surfaces calculated from first principles with the Boltzmann transport theory approach relying on the semiclassical model and the relaxation time approximation. We first consider a series of simple model Fermi surfaces to provide a didactic introduction into the charge-carrier compensation and open-orbit mechanisms leading to nonsaturating magnetoresistance. We then address in detail magnetotransport in three representative materials: (i) copper, a prototypical nearly free-electron metal characterized by the open Fermi surface that results in an intricate angular magnetoresistance, (ii) bismuth, a topologically trivial semimetal in which very large magnetoresistance is known to result from charge-carrier compensation, and (iii) tungsten diphosphide ${\mathrm{WP}}_2$, a recently discovered type-II Weyl semimetal that holds the record of magnetoresistance in compounds. In all three cases our calculations show excellent agreement with both the field dependence of magnetoresistance and its anisotropy measured at low temperatures. Furthermore, the calculations allow for a full interpretation of the observed features in terms of the Fermi surface topology. Our study thus establishes guidelines to clarifying the physical mechanisms underlying the magnetotransport properties in a broad range of materials. These results will help addressing a number of outstanding questions, such as the role of the topological phase in the pronounced large nonsaturating magnetoresistance observed in topological materials.

We address the question of whether transport coefficients obtained from a unitary closed system setting, i.e., the standard equilibrium Green-Kubo formula, are the same as the ones obtained from a weakly driven nonequilibrium steady-state calculation. We first derive a nonequilibrium Kubo-like expression for the steady-state diffusion constant expressed as a time integral of either a current or a conserved density nonequilibrium correlation function. This expression has certain advantages over the equilibrium Green-Kubo formula, but it is not clear if it gives the same value of the diffusion constant. We then rigorously show that if the unitary dynamics is diffusive, the nonequilibrium formula indeed gives exactly the same transport coefficient. The form of finite-size correction is also predicted. Theoretical results are verified by an explicit calculation of the diffusion constant in several interacting many-body models.

Interband photoexcitation of electron states with the twisted photons in GaAs, a direct band-gap bulk semiconductor, is considered theoretically. Assuming linearity of the quantum transition amplitudes and applying Wigner-Eckart theorem, we derive a plane-wave expansion of twisted-photon amplitudes. We also obtain relative probabilities for magnetic sublevel population of the photoelectrons in conduction band. The approach for calculating the position-dependent electron polarization, resulting from photoabsorption of twisted light, is described for vertical transitions in the $\mathrm{\Gamma }$ point. Theoretical predictions for GaAs show modification of the magnitude and the sign of photoelectron polarization in the region near the photon's phase singularity.

We propose a topological qubit in which braiding and readout are mediated by the $4\pi$ Majorana-Josephson effect. The braidonium device consists of three Majorana nanowires that come together to make a trijunction. In order to control the superconducting phase differences at the trijunction, the nanowires are enclosed in a ring made of a conventional superconductor. In order to perform initialization and readout, one of the nanowires is coupled to a fluxonium qubit through a topological Josephson junction. We analyze how flux-based control and readout protocols can be used to demonstrate braiding and qubit operation for realistic materials and circuit parameters.

We develop a time-dependent three-dimensional coupled-wave theory (3D-CWT) for the transient analysis of photonic-crystal surface-emitting lasers (PCSELs). Our model takes into account the temporal evolution of both the photon and carrier distribution inside PCSELs, which enable the analysis of various above-threshold lasing characteristics including the relaxation oscillation, spatial hole burning, and multimode lasing. With the developed time-dependent 3D-CWT, we perform transient analysis of the high-power, high-beam-quality PCSELs with double-lattice photonic crystals and reproduce the experimental results of near-field patterns and lasing spectra under high current injection. Our theory enables the comprehensive understanding of the device physics of PCSELs toward the realization of higher-power continuous-wave lasing and short-pulse lasing.

Two-photon photoemission measurements reveal a near-zero-dispersion empty electronic state, approximately 2.6 eV above the Fermi energy and near the Brillouin zone center, induced by oxygen intercalation at the graphene-Ir(111) interface. While oxygen intercalation leads to quasi-free-standing graphene, electron diffraction shows $2\times 2$ periodicity due to the patterned intercalant. Near the zone center, large-wave-vector zone folding, driven by this $2\times 2$ periodicity, replicates states from near the Dirac cone that have little dispersion due to trigonal warping, explaining the nearly flat band. The zone-folding mechanism is supported by results from angle-resolved photoemission measurements and from density-functional-theory-based calculations of the unfolded energy bands. These results demonstrate zone-folding effects in graphene on a wave vector and energy scale that has largely been unexplored, and may open new opportunities to engineer the graphene electronic states.

Understanding the dynamics of excitons in two-dimensional semiconductors requires a theory that incorporates the essential physics distinct from their three-dimensional counterparts. In addition to the modified dielectric environment, single-particle states with strongly nonparabolic dispersion appear in many two-dimensional band structures, so that “effective mass” is ill-defined. Focusing on electrostatically biased bilayer graphene as an example where quartic (and higher) dispersion terms are necessary, we present a semianalytic theory used to investigate the properties of ground and excited excitonic states. This includes determination of relative oscillator strengths and magnetic moments (valley $g$-factors) which can be directly compared to recent experimental measurements.

The sensitivity of charge, heat, or momentum transport to the sample geometry is a hallmark of viscous electron flow. Therefore hydrodynamic electronics requires a detailed understanding of electron flow in finite geometries. The solution of the corresponding generalized Navier-Stokes equations depends sensitively on the nature of boundary conditions. The latter can be characterized by a slip length $\zeta$ with extreme cases being no-slip $\left(\zeta \to 0\right)$ and no-stress $\left(\zeta \to \infty \right)$ conditions. We develop a kinetic theory that determines the temperature dependent slip length at a rough interface for Dirac liquids, e.g., graphene, and for Fermi liquids. For strongly disordered edges that scatter electrons in a fully diffuse way, we find that the slip length is of the order of the momentum conserving mean free path $l_{ee}$ that determines the electron viscosity. For boundaries with nearly specular scattering, $\zeta$ is parametrically large compared to $l_{ee}$. Since for all quantum fluids $l_{ee}$ diverges as $T\to 0$, the ultimate low-temperature flow is always in the no-stress regime. Only at intermediate $T$ and for sufficiently large sample sizes can the slip lengths be short enough such that no-slip conditions are appropriate. We discuss numerical examples for several experimentally investigated systems. To identify hydrodynamic flow governed by no-stress boundary conditions, we propose the transport through an infinitely long strip containing an impenetrable circular obstacle.

Optical spectroscopy of resonant reflection has been used for both experimental and theoretical studies of the exciton-light interaction in wide, high-quality quantum-well structures with the widths exceeding the exciton Bohr radius by an order of magnitude or more. The light mixes the low-lying confined exciton states which are captured by the generalized model developed by M. M. Voronov et al. [Phys. Solid State 49, 1792 (2007)]. We demonstrate that the high-energy confined states in the wide QWs can still be described by the standard model in which the amplitude reflection coefficient from the QW is a sum of individual size-quantized exciton resonances. The excitonic parameters extracted from fitting the experimental spectrum to the standard model agree with those obtained by the numerical solution of the two-particle Schrödinger equation in a rectangular quantum well. The measured and microscopically calculated spectra are compared with those found with the widely used model of the center-of-mass exciton quantization and the polaritonic model. The comparison shows that the two approximate models considerably underestimate the interaction of confined excitons with light because they ignore the strong modification of the exciton wave function near the QW interfaces.

Measurements of electronic structure and theoretical models indicate that the Fermi surface of ${\mathrm{LaNiO}}_3$ (LNO) is predominantly holelike, with a small electron pocket. However, measurements of the Hall and Seebeck effects yield nominally opposite signs for the dominant charge carrier type, making charge transport in LNO puzzling. Here, we combine measurements of the Hall, Seebeck, and Nernst coefficients in high-mobility epitaxial LNO thin films, and resolve this puzzle by demonstrating that the negative Seebeck coefficient is generated by the diffusion of holes from cold to hot regions of LNO. We further examine this counter-thermal flow of holes by measuring the evolution of the Nernst coefficient from the diffusive to the ballistic regime, where the suppression of energy-dependent scattering leads to a reversal of the flow of holes.

To fathom the mechanism of high-temperature ($T_c$) superconductivity, the dynamical vertex approximation is evoked for the two-dimensional repulsive Hubbard model. After showing that our results reproduce well the cuprate phase diagram with a reasonable $T_c$ and dome structure, we keep track of the scattering processes that primarily affect $T_c$. We find that local particle-particle diagrams significantly screen the bare interaction at low frequencies, which in turn suppresses antiferromagnetic spin fluctuations and hence the pairing interaction. Thus we identify dynamical vertex corrections as one of the main oppressors of $T_c$, which may provide a hint toward higher $T_c$'s.

We consider the impact of disorder on the spectrum of three-dimensional nodal-line semimetals. We show that the combination of disorder and a tilted spectrum naturally leads to a non-Hermitian self-energy contribution that can split a nodal line into a pair of exceptional lines. These exceptional lines form the boundary of an open and orientable bulk Fermi ribbon in reciprocal space on which the energy gap vanishes. We find that the orientation and shape of such a disorder-induced bulk Fermi ribbon is controlled by the tilt direction and the disorder properties, which can also be exploited to realize a twisted bulk Fermi ribbon with nontrivial winding number. Our results put forward a paradigm for the exploration of non-Hermitian topological phases of matter.

A two-dimensional carbon allotrope, Stone-Wales graphene, is identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. Stone-Wales graphene consists of well-arranged Stone-Wales defects, and it can be constructed through a ${90}^{\circ }$ bond rotation in a $\sqrt{8}\times \sqrt{8}$ supercell of graphene. Its calculated energy relative to graphene, +149 meV/atom, makes it more stable than the most competitive previously suggested graphene allotropes We find that Stone-Wales graphene (SW-graphene) based on a $\sqrt{8}$ supercell is more stable than those based on $\sqrt{9}\times \sqrt{9},\sqrt{12}\times \sqrt{12}$, and $\sqrt{13}\times \sqrt{13}$ supercells, and is a “magic size” that can be further understood through a simple “energy splitting and inversion” model. The calculated vibrational properties and molecular dynamics of SW-graphene confirm that it is dynamically stable. The electronic structure shows SW-graphene is a semimetal with distorted, strongly anisotropic Dirac cones.

We present an angle-resolved photoemission spectroscopy study of single-layer $\mathrm{Mo}{\mathrm{S}}_2$ where interaction with a supporting highly ordered pyrolytic graphite substrate was controlled by temperature change, consistent with related modifications in the layer-substrate distance. The impact of interface potential landscape changes on the electronic properties and charge dynamics on the $\mathrm{Mo}{\mathrm{S}}_2$ layer was evaluated by valence-band dispersion and photoemission line-shape analysis. Our results indicate that the hole states at the $K$-valley point are essentially unaffected by interface potential, reflecting the strong-in plane localization of the electronic wave function.

We study theoretically the relaxation of hot quantum-Hall edge-channel electrons under the emission of both acoustic and optical phonons. Aiming to model recent experiments with single-electron sources, we describe simulations that provide the distribution of electron energies and arrival times at a detector a fixed distance from the source. From these simulations we extract an effective rate of emission of optical phonons that contains contributions from both a direct emission process as well as one involving inter-edge-channel transitions that are driven by the sequential emission of first an acoustic and then an optical phonon. Furthermore, we consider the mean energy loss due to acoustic phonon emission and resultant broadening of the electron energy distribution and derive an effective drift-diffusion model for this process.