We have systematically grown large single crystals of the layered compounds $\beta -\mathrm{PdB}{\mathrm{i}}_2$, and both the hole-doped $\mathrm{PdB}{\mathrm{i}}_{2-x}\mathrm{P}{\mathrm{b}}_x$ and the electron-doped $\mathrm{N}{\mathrm{a}}_x\mathrm{PdB}{\mathrm{i}}_2$, and studied their magnetic and transport properties. Hall effect measurements on $\mathrm{PdB}{\mathrm{i}}_2$, $\mathrm{PdB}{\mathrm{i}}_{1.8}\mathrm{P}{\mathrm{b}}_{0.2}$, and $\mathrm{N}{\mathrm{a}}_{0.057}\mathrm{PdB}{\mathrm{i}}_2$ show that the charge transport is dominated by electrons in all of the samples. The electron concentration is substantially reduced upon Pb doping in $\mathrm{PdB}{\mathrm{i}}_{2-x}\mathrm{P}{\mathrm{b}}_x$ and increased upon Na intercalation in $\mathrm{N}{\mathrm{a}}_x\mathrm{PdB}{\mathrm{i}}_2$, indicating effective hole doping by Pb and electron doping by Na. We observed a monotonic decrease of the superconducting transition temperature $\left(T_c\right)$ from 5.4 K in undoped $\mathrm{PdB}{\mathrm{i}}_2$ to less than 2 K for $x>0.35$ in hole-doped $\mathrm{PdB}{\mathrm{i}}_{2-x}\mathrm{P}{\mathrm{b}}_x$. Meanwhile, a rapid decrease of $T_c$ with Na intercalation is also observed in the electron-doped $\mathrm{N}{\mathrm{a}}_x\mathrm{PdB}{\mathrm{i}}_2$, which is in disagreement with the theoretical expectation. In addition, both the magnetoresistance and Hall resistance further reveal evidence for a possible spin excitation associated with Fermi surface reconstruction at ∼50 K in the Na-intercalated $\mathrm{PdB}{\mathrm{i}}_2$ sample. The complete phase diagram is thus established from hole doping to electron doping. Meanwhile, a high-pressure study of the undoped $\mathrm{PdB}{\mathrm{i}}_2$ shows that the $T_c$ is linearly suppressed under pressure with a $d{T}_c/dP$ coefficient of −0.28 K/GPa.

We study a quasi-two-dimensional classical Landau-Ginzburg-Wilson effective field theory in the presence of quenched disorder in which incommensurate charge-density wave and superconducting orders are intertwined. The disorder precludes long-range charge-density wave order, but not superconducting or nematic order. We select three representative sets of input parameters and compute the corresponding charge-density wave structure factors using both large-$N$ techniques and classical Monte Carlo simulations. Where nematicity and superconductivity coexist at low temperature, the peak height of the charge-density wave structure factor decreases monotonically as a function of increasing temperature, unlike what is seen in x-ray experiments on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. Conversely, where the thermal evolution of the charge-density wave structure factor qualitatively agrees with experiments, the nematic correlation length, computed to one-loop order, is shorter than the charge-density wave correlation length.

We study experimentally photon-assisted tunneling in $\mathrm{Nb}/{\mathrm{AlO}}_x/\mathrm{Nb}$ Josephson junctions. We perform a quantitative calibration of the microwave field inside the junction. This allows direct verification of the quantum efficiency of microwave photon detection, which corresponds to tunneling of one electron per one absorbed microwave photon. We observe that voltages of photon-assisted tunneling steps vary both with the microwave power and the tunneling current. However, this variation is not monotonous but staircaselike. The phenomenon is caused by mutual locking of positive and negative step series. A similar locking is observed with Shapiro steps. As a result, the superconducting gap assumes quantized values equal to multiples of the quarter of the photon energy. The quantization is a manifestation of nonequilibrium tuning (suppression or enhancement) of superconductivity by the microwave field.

We examine the effects of disorder on dimerized quantum antiferromagnets in a magnetic field, using the mapping to a lattice gas of hard-core bosons with finite-range interactions. Combining a strong-coupling expansion, the replica method, and a one-loop renormalization-group analysis, we investigate the nature of the glass phases formed. We find that away from the tips of the Mott lobes, the transition is from a Mott insulator to a compressible Bose glass, however the compressibility at the tips is strongly suppressed. We identify this finding with the presence of a rare Mott glass phase and demonstrate that the inclusion of replica symmetry breaking is vital to correctly describe the glassy phases. This result suggests that the formation of Bose and Mott glass phases is not simply a weak localization phenomenon but is indicative of much richer physics. We discuss our results in the context of both ultracold atomic gases and spin-dimer materials.

We have carried out a detailed first-principles study of a $d^{4.5}$ quaternary iridate ${\mathrm{Ba}}_3{\mathrm{YIr}}_2{\mathrm{O}}_9$ both in its 6H-perovskite-type ambient pressure (AP) phase and also for the high pressure (HP) cubic phase. Our analysis reveals that the AP phase belongs to the intermediate spin-orbit coupling (SOC) regime. This is further supported by the identification of the spin moment as the primary order parameter (POP) obtained from a magnetic multipolar analysis. The large $t_{2g}$ bandwidth renormalizes the strength of SOC and the Ir intersite exchange interaction dominates resulting in long-range magnetic order in the AP phase. In addition to SOC and Hubbard $U$, strong intradimer coupling is found to be crucial for the realization of the insulating state. At high pressure (HP) the system undergoes a structural transformation to the disordered cubic phase. In sharp contrast to the AP phase, the calculated exchange interactions in the HP phase are found to be much weaker and SOC dominates leading to a quantum spin-orbital liquid (SOL) state.

We study the attractive Hubbard model within the dynamical mean-field theory, to elucidate how the pseudogap and superconductivity at strong attractive interaction are related to those found in the repulsive Hubbard model, and thereby to bridge cold fermions and cuprate high-temperature superconductors from a microscopic point of view. We propose that a unified understanding is obtained by investigating single-particle excitation dynamics, in which emergent and hidden fermions coupled to quasiparticles consistently account for the numerical results in both attractive and repulsive cases. In the attractive case, the quasiparticle dynamics is observable by virtually breaking a tightly bound pair, where we find two qualitatively different regions crossing over each other within the strong-coupling superconductivity phase. Among them, the region close to the critical temperature shares characteristic dynamics with the repulsive interaction case, where the normal and anomalous parts of the self-energy show strong low-energy peaks while they are hidden in the quasiparticle spectral weight. These prominent self-energy peaks are understood by the coupling of the quasiparticle to the hidden fermionic excitation, emergent from a strong-coupling effect. The pseudogap above the critical temperature is also accounted for by the same hidden fermion.

We report on studies of quantum turbulence with second sound in superfluid ${}^4\mathrm{He}$ in which the turbulence is generated by the flow of the superfluid component through a wide square channel, the ends of which are plugged with sintered silver superleaks, the flow being generated by compression of a bellows. The superleaks ensure that there is no net flow of the normal fluid. In an earlier paper [S. Babuin et al., Phys. Rev. B 86, 134515 (2012)] we have shown that steady flow of this kind generates a density of vortex lines that is essentially identical to that generated by thermal counterflow, when the average relative velocity between the two fluids is the same. In this paper we report on studies of the temporal decay of the vortex-line density, observed when the bellows is stopped, and we compare the results with those obtained from the temporal decay of thermal counterflow remeasured in the same channel and under the same conditions. In both cases there is an initial fast decay which, for low enough initial line density, approaches for a short time the form $t^{-1}$ characteristic of the decay of a random vortex tangle. This is followed at late times by a slower $t^{-3/2}$ decay, characteristic of the decay of large “quasiclassical” eddies. However, in the range of investigated parameters, we observe always in the case of thermal counterflow, and only in a few cases of high steady-state velocity in superflow, an intermediate regime in which the decay either does not proceed monotonically with time or passes through a point of inflexion. This difference, established firmly by our experiments, might represent one essential ingredient for the full theoretical understanding of counterflow turbulence.

Strongly correlated analogs of topological insulators have been explored in systems with purely on-site symmetries, such as time-reversal or charge conservation. Here, we use recently developed tensor network tools to study a quantum state of interacting bosons which is featureless in the bulk, but distinguished from an atomic insulator in that it exhibits entanglement which is protected by its spatial symmetries. These properties are encoded in a model many-body wave function that describes a fully symmetric insulator of bosons on the honeycomb lattice at half filling per site. While the resulting integer unit cell filling allows the state to bypass “no-go” theorems that trigger fractionalization at fractional filling, it nevertheless has nontrivial entanglement, protected by symmetry. We demonstrate this by computing the boundary entanglement spectra, finding a gapless entanglement edge described by a conformal field theory as well as degeneracies protected by the nontrivial action of combined charge conservation and spatial symmetries on the edge. Here, the tight-binding representation of the space group symmetries plays a particular role in allowing certain entanglement cuts that are not allowed on other lattices of the same symmetry, suggesting that the lattice representation can serve as an additional symmetry ingredient in protecting an interacting topological phase. Our results extend to a related insulating state of electrons, with short-ranged entanglement and no band insulator analog.

We analyze the spatial correlation structure of the spin density of an electron gas in the vicinity of an antiferromagnetically coupled Kondo impurity. Our analysis extends to the regime of spin-anisotropic couplings, where there are no quantitative results for spatial correlations in the literature. We use an original and numerically exact method, based on a systematic coherent-state expansion of the ground state of the underlying spin-boson Hamiltonian. It has not yet been applied to the computation of observables that are specific to the fermionic Kondo model. We also present an important technical improvement to the method that obviates the need to discretize modes of the Fermi sea, and allows one to tackle the problem in the thermodynamic limit. As a result, one can obtain excellent spatial resolution over arbitrary length scales, for a relatively low computational cost, a feature that gives the method an advantage over popular techniques such as the numerical and density-matrix renormalization groups. We find that the anisotropic Kondo model shows rich universal scaling behavior in the spatial structure of the entanglement cloud. First, SU(2) spin-symmetry is dynamically restored in a finite domain in the parameter space in the vicinity of the isotropic line, as expected from poor man's scaling. More surprisingly, we are able to obtain in closed analytical form a set of different, yet universal, scaling curves for strong exchange asymmetry, which are parametrized by the longitudinal exchange coupling. Deep inside the cloud, i.e., for distances smaller than the Kondo length, the correlation between the electron spin density and the impurity spin oscillates between ferromagnetic and antiferromagnetic values at the scale of the Fermi wavelength, an effect that is drastically enhanced at strongly anisotropic couplings. Our results also provide further numerical checks and alternative analytical approximations for the Kondo overlaps that were recently computed by Lukyanov, Saleur, Jacobsen, and Vasseur [Phys. Rev. Lett. 114, 080601 (2015)].

We investigate the resilience of symmetry-protected topological edge states at the boundaries of Kitaev chains in the presence of a bath which explicitly introduces symmetry-breaking terms. Specifically, we focus on single-particle losses and gains, violating the protecting parity symmetry, which could generically occur in realistic scenarios. For homogeneous systems we show that the Majorana mode decays exponentially fast. By the inclusion of strong disorder, where the closed system enters a many-body localized phase, we find that the Majorana mode can be stabilized substantially. The decay of the Majorana converts into a stretched exponential form for particle losses or gains occurring in the bulk. In particular, for pure loss dynamics we find a universal exponent $\alpha \simeq 2/3$. We show that this holds both in the Anderson and many-body localized regimes. Our results thus provide a first step to stabilize edge states even in the presence of symmetry-breaking environments.

We theoretically study the symmetry properties of the single-band Hubbard model with general spin-orbit coupling (SOC) on the kagome lattice. We show that the global U(1) spin-rotational symmetry is present in the Hubbard Hamiltonian owing to the inversion symmetry centered at the sites. The corresponding spin Hamiltonian has, therefore, SO(2) spin-rotational symmetry, which can be captured by including SOC nonperturbatively. The exact classical ground states, which we obtain for arbitrary SOC, are governed by the SU(2) fluxes associated with SOC threading the constituent triangles. The ground states break the SO(2) symmetry, and the associated Berezinsky-Kosterlitz-Thouless transition temperature is determined by the SU(2) fluxes through the triangles, which we confirm by a finite temperature classical Monte Carlo simulation.

We report the physical properties and electronic structure calculations of a layered chromium oxypnictide, ${\mathrm{Sr}}_2{\mathrm{Cr}}_3{\mathrm{As}}_2{\mathrm{O}}_2$, which crystallizes in a ${\mathrm{Sr}}_2{\mathrm{Mn}}_3{\mathrm{As}}_2{\mathrm{O}}_2$-type structure containing both ${\mathrm{CrO}}_2$ planes and ${\mathrm{Cr}}_2{\mathrm{As}}_2$ layers. The newly synthesized material exhibits a metallic conduction with a dominant electron-magnon scattering. Magnetic and specific-heat measurements indicate at least two intrinsic magnetic transitions below room temperature. One is an antiferromagnetic transition at 291 K, probably associated with a spin ordering in the ${\mathrm{Cr}}_2{\mathrm{As}}_2$ layers. Another transition is broad, occurring at around 38 K, and possibly due to a short-range spin order in the ${\mathrm{CrO}}_2$ planes. Our first-principles calculations indicate predominant two-dimensional antiferromagnetic exchange couplings, and suggest a KG-type [i.e., ${\mathrm{K}}_2{\mathrm{NiF}}_4$ (abbreviated as “K”) type in the ${\mathrm{CrO}}_2$ sublattice and Néel (usually denoted by “G”) type in the ${\mathrm{Cr}}_2{\mathrm{As}}_2$ sublattice] magnetic structure, with reduced moments for both Cr sublattices. The corresponding electronic states near the Fermi energy are mostly contributed from $\mathrm{Cr}-3d$ orbitals which weakly (modestly) hybridize with the $\mathrm{O}-2p$ ($\mathrm{As}-4p$) orbitals in the ${\mathrm{CrO}}_2$ (${\mathrm{Cr}}_2{\mathrm{As}}_2$) layers. The bare band structure density of states at the Fermi level is only $\sim 1/4$ of the experimental value derived from the low-temperature specific-heat data, consistent with the remarkable electron-magnon coupling. The title compound is argued to be a possible candidate to host superconductivity.

The nodal points in a Weyl semimetal are generally considered as the causes of the chiral anomaly and the chiral magnetic effect (CME). Employing a linear-response analysis of a two-band lattice model, we show that the Weyl nodes and thus the chirality are not required for the CME, while they remain crucial for the chiral anomaly. Similar to the anomalous Hall effect, the CME results directly from the Berry curvature of energy bands, even when there is no monopole source from the Weyl nodes. Therefore, the phenomenon of the CME could be observed in a wider class of materials. Motivated by this result, we suggest that the nodeless CME may appear in three-dimensional quantum anomalous Hall insulators, but after they become metallic due to the band deformation caused by inversion symmetry breaking.

We investigate the infrared response of graphene dimers with various doping and polarization configurations. The interaction between the plasmonic resonances of graphene nanodisks leads to a rich, tunable behavior. The hybridization of the nanodisk modes enables the excitation of resonances that would be invisible or dark in a single disk. The simulation results show various anticrossings that depend on dark-bright or bright-bright mode coupling, which we can describe via a simple Hamiltonian model. In addition, we determine the response of a dimer bridged by a tunable graphene junction. This structure leads to charge transfer plasmons, with an even higher absorption efficiency and tunability than nonbridged dimers.

Electron-phonon coupling in semiconductor quantum dots plays a significant role in determining the optical properties of excited excitons, especially the spectral nature of emitted photons. This paper presents a comprehensive theory and analysis of emission spectra from artificial atoms or quantum dots coupled to structured photon reservoirs and acoustic phonons, when excited with incoherent pump fields. As specific examples of structured reservoirs, we chose a Lorentzian cavity and a slow-light coupled-cavity waveguide, which have both been explored experimentally. For the case of optical cavities, we directly compare and contrast the spectra from three well-known and distinct theoretical approaches to treat electron-phonon coupling, including a Markovian polaron master equation, a non-Markovian phonon correlation expansion technique, and a semiclassical linear susceptibility approach, and we point out the limitations of these models. For the cavity-QED polaron master equation, which treats the cavity-mode operator at the level of a system operator, we give closed form analytical solutions to the phonon-assisted scattering rates in the weak excitation approximation, fully accounting for temperature, cavity-exciton detuning, and cavity-dot coupling. We also show explicitly why the semiclassical linear susceptibility approach fails to correctly account for phonon-mediated cavity feeding. For weakly coupled cavities, we calculate the optical spectra using a more general photon reservoir polaron master-equation approach, and explain its differences from the above approaches in the low-$Q$ limit of a Lorentzian cavity. We subsequently use this general reservoir approach to calculate the emission spectra from quantum dots coupled to slow-light photonic crystal waveguides, which demonstrate a number of striking photon-phonon coupling effects.

Electronic many-body correlation effects in one-dimensional (1D) systems such as carbon nanotubes have been predicted to strongly modify the nature of photoexcited states. Here we directly probe this effect using broadband elastic light scattering from individual suspended carbon nanotubes under electrostatic gating conditions. We observe significant shifts in optical transition energies, as well as line broadening, as the carrier density is increased. The results demonstrate the role of screening of many-body electronic interactions on the different length scales, a feature inherent to quasi-1D systems. Our findings further demonstrate the possibility of electrical tuning of optical transitions and provide a basis for understanding of various optical phenomena in carbon nanotubes and other quasi-1D systems in the presence of charge carrier doping.

Phase transformations induced in the cubic C-type lanthanide sesquioxides, ${\mathrm{Ln}}_2{\mathrm{O}}_3$ (Ln = Sm, Gd, Ho, Tm, and Lu), by dense electronic excitation are investigated. The structural modifications resulting from exposure to beams of 185 MeV Xe and 2246 MeV Au ions are characterized using synchrotron x-ray diffraction and Raman spectroscopy. The formation of a B-type polymorph, an X-type nonequilibrium phase, and an amorphous phase are observed. The specific phase formed and the transformation rate show dependence on the material composition, as well as the ion beam mass and energy. Atomistic mechanisms for these transformations are determined, indicating that formation of the B-type phase results from the production of anti-Frenkel defects and the aggregation of anion vacancies into planar clusters, whereas formation of the X-type and amorphous phases requires extensive displacement of both anions and cations. The observed variations in phase behavior with changing lanthanide ionic radius and deposited electronic energy density are related to the energetics of these transformation mechanisms.

We investigate the properties in finite magnetic field of an extended anisotropic $XXZ$ spin-1/2 model on the kagome lattice, originally introduced by Balents, Fisher, and Girvin [Phys. Rev. B 65, 224412 (2002)]. The magnetization curve displays plateaus at magnetization $m=1/6$ and $1/3$ when the anisotropy is large. Using low-energy effective constrained models (quantum loop and quantum dimer models), we discuss the nature of the plateau phases, found to be crystals that break discrete rotation and/or translation symmetries. Large-scale quantum Monte Carlo simulations were carried out in particular for the $m=1/6$ plateau. We first map out the phase diagram of the effective quantum loop model with an additional loop-loop interaction to find stripe order around the point relevant for the original model as well as a topological ${\mathbb{Z}}_2$ spin liquid. The existence of a stripe crystalline phase is further evidenced by measuring both standard structure factor and entanglement entropy of the original microscopic model.