Motivated by the recent experimental observations of quantum oscillations in the Kondo insulators ${\mathrm{SmB}}_6$ and ${\mathrm{YbB}}_{12}$, we study magneto-optical conductivity of Kondo insulators. We show that there exist nontrivial cyclotron resonance peaks in a frequency domain below the hybridization gap as a result of the characteristic Landau level structure of the system. Furthermore, these low-energy peaks are enhanced near the insulator-metal phase transition driven by an applied magnetic field and stable even at low temperatures. They are smoothly connected via the Kondo effect to resonance peaks for unhybridized conduction and $f$ electrons at high temperature. The nontrivial cyclotron resonance is a dynamical counterpart of quantum oscillations in static quantities, and through this intimate connection it will provide a useful insight for the latter.

We report on a dynamical mean-field theoretical analysis of emerging low-temperature phases in multicomponent gases of fermionic alkaline-earth(-like) atoms in state-dependent optical lattices. Using the example of ${}^{173}\mathrm{Yb}$ atoms, we show that a two-orbital mixture with two nuclear spin components is a promising candidate for studies of not only magnetic but also staggered orbital ordering peculiar to certain solid-state materials. We calculate and study the phase diagram of the full Hamiltonian with parameters similar to existing experiments and reveal an antiferro-orbital phase. This long-range-ordered phase is inherently stable, and we analyze the change of local and global observables across the corresponding transition lines, paving the way for experimental observations. Furthermore, we suggest a realistic extension of the system to include and probe a Jahn-Teller source field playing one of the key roles in real crystals.

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with particular emphasis on the low-energy region below 10 meV that has been difficult to resolve experimentally. By applying inelastic helium atom scattering (HAS), the entire phonon dispersion was determined and compared with density functional perturbation theory (DFPT) calculations. The intensity of the phonon modes is dominated by a strong Rayleigh mode, in contrast to previous experimental works. Moreover, also at variance with recent reports, no Kohn anomaly is observed. These observations are in excellent agreement with DFPT calculations. Besides these results, the experimental data reveal—via bound-state resonance enhancement—two additional dispersion curves in the gap below the Rayleigh mode. They are possibly associated with an excitation of a surface electron density superstructure that we observe in HAS diffraction patterns. The electron-phonon coupling parameter $\lambda =0.23$, derived from our temperature-dependent Debye-Waller measurements, compares well with values determined by angular resolved photoemission or Landau level spectroscopy. Our work opens up a new perspective for terahertz (THz) measurements on two-dimensional (2D) materials as well as the investigation of subtle details (band bending, the presence of a 2D electron gas) with respect to the electron-phonon coupling.

A topological phase transition from a trivial insulator to a ${\mathbb{Z}}_2$ topological insulator requires the bulk band gap to vanish. In the case of noncentrosymmetric materials, these phases are separated by a gapless Weyl semimetal phase. However, at finite temperature, the gap is affected by atomic motion, through electron-phonon interaction, and by thermal expansion of the lattice. As a consequence, the phase space of topologically nontrivial phases is affected by temperature. In this paper, the pressure and temperature dependence of the indirect band gap of BiTeI is investigated from first principles. We evaluate the contribution from both electron-phonon interaction and thermal expansion, and show that their combined effect drives the topological phase transition towards higher pressures with increasing temperature. Notably, we find that the sensitivity of both band extrema to pressure and topology for electron-phonon interaction differs significantly according to their leading orbital character. Our results indicate that the Weyl semimetal phase width is increased by temperature, having almost doubled by 100 K when compared to the static lattice results. Our findings thus provide a guideline for experimental detection of the nontrivial phases of BiTeI and illustrate how the phase space of the Weyl semimetal phase in noncentrosymmetric materials can be significantly affected by temperature.

Full-shell nanowires (semiconducting nanowires fully coated with a superconducting shell) have been recently presented as an alternative means to create Majorana zero modes. In contrast to partially coated nanowires, it has been argued that full-shell nanowires do not require high magnetic fields and low densities to reach a putative topological regime. Here we present a theoretical study of these devices taking into account all the basic ingredients, including a charge distribution spread across the section of the nanowire, required to qualitatively explain recent experimental results (Vaitiekėnas et al., arXiv:1809.05513). We derive a criterion, dependent on the even-odd occupation of the radial subbands with zero angular momentum, for the appearance of Majorana zero modes. In the absence of angular subband mixing, these give rise to strong zero-bias anomalies in tunneling transport in roughly half of the system's parameter space under an odd number of flux quanta. Due to their coexistence with gapless subbands, the zero modes do not enjoy generic topological protection. Depending on the details of subband mixing in realistic devices, they can develop a topological minigap, acquire a finite lifetime, or even be destroyed.

Allotropes of carbon, including one-dimensional carbon nanotubes and two-dimensional graphene sheets, continue to draw attention as promising platforms for probing the physics of electrons in lower dimensions. Recent research has shown that the electronic properties of graphene multilayers are exquisitely sensitive to the relative orientation between sheets and in the bilayer case exhibit strong electronic correlations when close to a magic twist angle. Here we investigate the electronic properties of a carbon nanotube deposited on a graphene sheet by deriving a low-energy theory that accounts for both rotations and rigid displacements of the nanotube with respect to the underlying graphene layer. We show that this heterostructure is described by a translationally invariant, a periodic, or a quasiperiodic Hamiltonian, depending on the orientation and the chirality of the nanotube. Furthermore, we find that, even for a vanishing twist angle, rigid displacements of a nanotube with respect to a graphene substrate can alter its electronic structure qualitatively. Our results identify a promising direction for strong correlation physics in low dimensions.

We propose a new parton theory of the hole-doped cuprates, describing the evolution from the pseudogap metal with small Fermi surfaces to the conventional Fermi liquid with a large Fermi surface. We introduce two ancilla qubits per square lattice site, and employ them to obtain a variational wave function of a fractionalized Fermi liquid for the pseudogap metal state. We propose a multilayer Hamiltonian for the cuprates, with the electrons residing in the physical layer, and the ancilla qubits in two hidden layers: the hidden layers can be decoupled from the physical layer by a canonical transformation, which leaves the hidden layers in a trivial gapped state. This Hamiltonian yields an emergent gauge theory, which describes not only the fractionalized Fermi liquid, but also the conventional Fermi liquid, and possible exotic intermediate phases and critical points. The fractionalized Fermi liquid has hole pockets with quasiparticle weight which is large only on Fermi arcs, and fermionic spinon excitations, which carry charges of the emergent gauge fields.

Marcasite class of compounds provide a facile platform to explore novel phenomena of fundamental and technological importance, such as unconventional superconductivity or high-performance electrocatalyst. We report the synthesis and experimental investigation of a marcasite CoSeAs in this paper. Experimental investigation of this material using neutron scattering measurements reveals weak magnetic correlation of cobalt ions below $T=36.2$ K. The modest isotropic exchange interaction between cobalt moments, inferred from random phase approximation analysis, hints of a magnetically unstable environment. It is a desirable characteristic to induce unconventional superconductivity via chemical pressure application.

We discuss the response of an rf-SQUID formed by anomalous Josephson junctions embedded in a superconducting ring with a non-negligible inductance. We demonstrate that a properly sweeping in-plane magnetic field can cause both the total flux and the current circulating in the device to modulate and to behave hysteretically. The bistable response of the system is analyzed as a function of the anomalous phase shift at different values of the screening parameter, in order to highlight the parameter range within which a hysteretic behavior can be observed. The magnetic flux piercing the SQUID ring is demonstrated to further modulate the hysteretical response of the system. Moreover, we show that the anomalous phase shift can be conveniently determined through the measurement of the out-of-plane magnetic field at which the device switches to the voltage state and the number of trapped flux quanta changes. Finally, we compare the response of two different device configurations, namely, a SQUID including only one or two anomalous junctions. In view of these results, the proposed device can be effectively used to detect and measure the anomalous Josephson effect.

We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite $\mathrm{N}{\mathrm{d}}_2\mathrm{ZnIr}{\mathrm{O}}_6$ in a temperature range of 4–330 K, spanning a broad spectral range from 20 to $5500\mathrm{c}{\mathrm{m}}^{-1}$. The paper focuses on lattice vibrations and electronic transitions involving Kramers doublets of the rare-earth $\mathrm{N}{\mathrm{d}}^{3+}$ ion with local $C_1$ site symmetry. Temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling between lattice and electronic degrees of freedom in $\mathrm{N}{\mathrm{d}}_2\mathrm{ZnIr}{\mathrm{O}}_6$. Strong coupling between phonons and crystal-field excitation is observed via renormalization of the self-energy parameter of the phonons, i.e., peak frequency and linewidth. The phonon frequency shows abrupt hardening and linewidth narrowing below ∼100 K for the majority of the observed first-order phonons. We observed splitting of the lowest Kramers doublets of ground state (${}^4{I}_{9/2}$) multiplets, i.e., lifting of the Kramers degeneracy, prominently at low temperature (below ∼100 K), attributed to the Nd-Nd/Ir exchange interactions and the intricate coupling with the lattice degrees of freedom. The observed splitting is of the order of ∼2–3 meV and is consistent with the estimated value. We also observed a large number of high-energy modes, 46 in total, attributed to the intraconfigurational transitions between $4f^3$ levels of $\mathrm{N}{\mathrm{d}}^{3+}$ coupled to the phonons reflected in their anomalous temperature evolution.

The Anderson model with decoherence features a temporal evolution from localized eigenstates to a uniform spatial distribution bar any interference features. We discuss the growth and decay of pronounced interference peaks on transient timescales and develop an analytic understanding for the emergence of these peaks.

Optical control of spins in condensed matter systems such as semiconductor nanostructures requires precise knowledge of the polarization properties of the associated optical transitions subject to an external magnetic field. Here, we demonstrate that coherent optical spectroscopy in the form of photon echoes can be successfully used to evaluate the magnetic anisotropies of valence-band states. It manifests in drastic changes of the transient signals when the orientation of the magnetic field with respect to the crystallographic axes is varied. In particular, we use the two-pulse spin-dependent photon echo to study the in-plane hole spin anisotropy in a 20-nm-thick CdTe/${\mathrm{Cd}}_{0.76}{\mathrm{Mg}}_{0.24}\mathrm{Te}$ single quantum well by exciting the donor-bound exciton resonance. We take advantage of the photon echo sensitivity to the relative phase of the electron and hole spin precession in the ground and excited states, respectively, and study various interactions contributing to the hole in-plane spin properties. The main contribution is found to arise from the crystal cubic symmetry described by the Luttinger parameter $q=0.095\pm 0.005$, which is substantially larger than the one theoretically expected for CdTe or found in other quantum well structures. Another contribution is induced by the strain within the quantum well. These two contributions lead to different harmonics of the spin precession frequencies in the photon echo experiment, when the strength and orientation of the Voigt magnetic field are varied. The magnitude of the effective in-plane hole $g$ factor is found to vary in the range $|\widetilde{g_h}|$=0.125–0.160 in the well plane.

Quantum systems that violate the eigenstate thermalization hypothesis, thereby falling outside the paradigm of conventional statistical mechanics, are of both intellectual and practical interest. We show that such a breaking of ergodicity may arise purely due to local constraints on random many-body Hamiltonians. As an example, we study an ergodic quantum spin-1/2 model which acquires a localized phase upon addition of East-type constraints. We establish its phenomenology using spectral and dynamical properties obtained by exact diagonalization. Mapping the Hamiltonian to a disordered hopping problem on the Fock space graph we find that potentially nonresonant bottlenecks in the Fock-space dynamics, caused by spatially local segments of frozen spins, lie at the root of localization. We support this picture by introducing and solving numerically a class of random matrix models that retain the bottlenecks. Finally, we obtain analytical insight into the origins of localization using the forward-scattering approximation. A numerical treatment of the forward-scattering approximation yields critical points which agree quantitatively with the exact diagonalization results.

Heterostructures involving graphene and bismuth, with their ability to absorb light over a very wide energy range, are of interest for engineering next-generation optoelectronics. Critical to the technological application of such heterostructures is an understanding of the underlying physics governing their properties. Here, using first-principles calculations, we study the interfacial interactions between graphene and bismuth thin films. Our study reveals nonintuitive phenomena associated with the moiré physics of these superlattices. We show a preservation of graphene-derived Dirac cones in spite of proximity to a substrate with large spin-orbit coupling, a greater influence of graphene on the electronic structure properties of bismuth, and the surprising presence of a magnetic solution, only slightly higher in energy (by several meV) than the nonmagnetic structure, possibly validating experiments. Such subtle and unanticipated phenomena associated with the moiré physics are expected to play key roles in the practical applications of heterogeneous assemblies of two-dimensional quantum systems.

Electrostatic confinement in bilayer graphene van der Waals heterostructures provides a versatile platform for studying electronic transport in bilayer graphene nanostructures. We study bilayer graphene cavities which we interpret in terms of quantum dots that are strongly coupled to the leads. We investigate how the transport signatures evolve when changing the size of the cavity for both electron and hole occupation. In addition, we analyze the interplay of single and double quantum dot physics in the regime where transport through the quantum dot system is almost pinched off.

We study superconductivity in two-band models where one of the bands does or does not intersect the Fermi level depending on the parameter values. Applying a many-variable variational Monte Carlo method for a Hubbard model on two-leg ladder and bilayer square lattices, we show that superconductivity can be enhanced in a parameter regime where the edge of one of the bands is near the Fermi energy, that is, when the band is incipient. The resemblance of the present results to those obtained by a weak-coupling method in a recent study suggests that, even in the large-$U$ regime, the suppression of the near-zero-energy spin fluctuations and the development of finite-energy spin fluctuations are the key factors for the enhancement of superconductivity by an incipient band.

We explore the time evolution of two impurities in a trapped one-dimensional Bose gas that follows a change of the boson-impurity interaction. We study the induced impurity-impurity interactions and their effect on the quench dynamics. In particular, we report on the size of the impurity cloud, the impurity-impurity entanglement, and the impurity-impurity correlation function. The presented numerical simulations are based upon the variational multilayer multiconfiguration time-dependent Hartree method for bosons. To analyze and quantify induced impurity-impurity correlations, we employ an effective two-body Hamiltonian with a contact interaction. We show that the effective model consistent with the mean-field attraction of two heavy impurities explains qualitatively our results for weak interactions. Our findings suggest that the quench dynamics in cold-atom systems can be a tool for studying impurity-impurity correlations.

We investigate a mesoscopic spin current for strongly interacting Fermi gases through a quantum point contact. Under the situation in which spin polarizations in the left and right reservoirs are the same in magnitude but opposite in sign, we calculate the contribution of quasiparticles to the current by means of the linear response theory and many-body $T$-matrix approximation. For a small spin-bias regime, the current in the vicinity of the superfluid transition temperature is strongly suppressed due to the formation of pseudogaps. For a large spin-bias regime where the gases become highly polarized, on the other hand, the current is affected by the enhancement of a minority density of states due to Fermi polarons. We also discuss the broadening of a quasiparticle peak associated with an attractive polaron at a large momentum, which is relevant to the enhancement.

Particles have been used for more than a decade to visualize and study the dynamics of quantum vortices in superfluid helium. In this work we study how the dynamics of a collection of particles set inside a vortex reflects the motion of the vortex. We use a self-consistent model based on the Gross-Pitaevskii equation coupled with classical particle dynamics. We find that each particle oscillates with a natural frequency proportional to the number of vortices attached to it. We then study the dynamics of an array of particles trapped in a quantum vortex and use particle trajectories to measure the frequency spectrum of the vortex excitations. Surprisingly, due to the discreetness of the array, the vortex excitations measured by the particles exhibit bands, gaps, and Brillouin zones, analogous to the ones of electrons moving in crystals. We then establish a mathematical analogy where vortex excitations play the role of electrons and particles that of the potential barriers constituting the crystal. We find that the height of the effective potential barriers is proportional to the particle mass and the frequency of the incoming waves. We conclude that large-scale vortex excitations could be in principle directly measured by particles and novel physics could emerge from particle-vortex interaction.

Based on the matrix product states method, we investigate numerically the ground-state properties of one-dimensional mixtures of repulsive bosons and spin-imbalanced attractive fermions, the latter being in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, where Cooper pairs condense at a finite momentum $k=k_{\mathrm{FFLO}}$. We find that the visibility of such a state is dramatically enhanced as the repulsive Bose-Fermi mixture is brought close to the phase-separation point. In particular, large-amplitude self-induced oscillations with wave vector $2k_{\mathrm{FFLO}}$ appear in both the fermion total density and the boson density profiles, leaving sharp fingerprints in the corresponding static structure factors. We show that these features remain quite visible in cold-atom systems trapped longitudinally by a smooth flat-bottom potential. Hence bosons can be used to directly reveal the modulated Fermi superfluid in experiments.