Long-lived fluxon excitations can be trapped inside a superinductor ring, which is divided into an array of loops by a periodic sequence of Josephson junctions in the quantum regime, thereby allowing fluxons to tunnel between neighboring sites. By tuning the Josephson couplings and, implicitly, the fluxon tunneling probability amplitudes, a wide class of one-dimensional tight-binding lattice models may be implemented and populated with a stable number of fluxons. We illustrate the use of this quantum simulation platform by discussing the Su-Schrieffer-Heeger model in the one-fluxon subspace, which hosts a symmetry-protected topological phase with fractionally charged bound states at the edges. This pair of localized edge states could be used to implement a superconducting qubit increasingly decoupled from decoherence mechanisms.

We consider the quantum XX model in the presence of competing nearest-neighbor and global-range interactions, which is equivalent to a Bose-Hubbard model with cavity mediated global-range interactions in the hard-core boson limit. Using fermionic techniques the problem is solved exactly in one dimension in the thermodynamic limit. The ground state phase diagram consists of two ordered phases: ferromagnetic (F) and antiferromagnetic (AF), as well as an XY phase having quasi-long-range order. We have also studied quantum relaxation after sudden quenches. Quenching from the AF phase to the XY region remanent AF order is observed below a dynamical transition line. In the opposite quench, from the XY region to the AF phase beyond a static metastability line AF order arises on top of remanent XY quasi-long-range order, which corresponds to dynamically generated supersolid state in the equivalent Bose-Hubbard model with hard-core bosons.

We study theoretically entanglement and operator growth in a spin system coupled to an environment, which is modeled with classical dephasing noise. Using exact numerical simulations we show that the entanglement growth and its fluctuations are described by the Kardar-Parisi-Zhang equation. Moreover, we find that the wavefront in the out-of-time ordered correlator (OTOC), which is a measure for the operator growth, propagates linearly with the butterfly velocity, and broadens diffusively with a diffusion constant that is larger than the one of spin transport. The obtained entanglement velocity is smaller than the butterfly velocity for finite noise strength, yet both of them are strongly suppressed by the noise. We calculate perturbatively how the effective timescales depend on the noise strength, both for uncorrelated Markovian and for correlated non-Markovian noise.

Exchange bias (EB) as large as ∼5.5 kOe is observed in $\mathrm{SrLaC}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_4$, which is the highest ever found in any layered transition-metal oxides including Ruddlesden-Popper series. Neutron-diffraction measurement rules out long-range magnetic ordering and together with dc magnetic measurements suggests formation of short-range magnetic domains. Ac magnetic susceptibility, magnetic memory effect, and magnetic training effect confirm the system to be a cluster spin glass. By carrying out density functional calculations on several model configurations, we propose that EB is originated at the boundary between Mn-rich antiferromagnetic and Co-rich ferromagnetic domains at the subnanoscale. Reversal of magnetization axis on the Co side alters the magnetic coupling between the interfacial Mn and Co spins, which leads to EB. Our analysis infers that the presence of competing magnetic interactions is sufficient to induce exchange bias and thereby a wide range of materials exhibiting giant EB can be engineered for designing novel magnetic memory devices.

Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high-temperature superconductivity despite a very low carrier-density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking $2e$ fluctuations of the valence of the Tl dopants, but a direct measurement of the mixed valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photoemission spectroscopy measurements. It is shown via a quantitative analysis that the suppression of the carrier density in compositions exhibiting superconductivity and Kondo-like behavior can be accounted for by mixed valency, thus arguing against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of ${\mathrm{Tl}}^{+}$ and ${\mathrm{Tl}}^{3+}$ a possible third intermediate local charge-density is suggested by full fits to the data, the origins of which are also discussed but remain unclear.

Nuclear magnetic resonance and Knight shift measurements are critical tools in the identification of spin-triplet superconductors. We discuss the effects of spin-orbit coupling on the Knight shift and susceptibilities for a variety of spin triplet multi-orbital gap functions with orbital-singlet character and compare their responses to “traditional” single-band spin-triplet ($p_x+i{p}_y$) superconductors. We observe a non-negligible residual spin-susceptibility at low temperature.

Control is exerted over the exciton-polariton physics in metal-poly(9,9-dioctylfluorene)-metal microcavities via conformational changes to the polymer backbone. Using thin-film samples containing increasing fractions of $\beta$-phase chain segments, a systematic study is reported for the mode characteristics and resulting light emission properties of cavities containing two distinct exciton subpopulations within the same semiconductor. Ultrastrong coupling for disordered glassy-phase excitons is observed from angle-resolved reflectivity measurements, with Rabi splitting energies in excess of 1.05 eV (more than $30\%$ of the exciton transition energy) for both TE- and TM-polarized light. A splitting of the lower polariton branch is then induced via introduction of $\beta$-phase excitons and increases with their growing fraction. In all cases, the photoluminescence emanates from the lowermost polariton branch, allowing conformational control to be exerted over the emission energy and its angular variation. Dispersion-free cavities with highly saturated blue-violet emission are thus enabled. Experimental results are discussed in terms of the full Hopfield Hamiltonian generalized to the case of two exciton oscillators. The importance of taking account of the molecular characteristics of the semiconductor for an accurate description of its strong coupling behavior is directly considered, in specific relation to the role of the vibronic structure.

We investigate the influence of gauge fields induced by strain on the supercurrent passing through the graphene-based Josephson junctions. We show that, in the presence of a constant pseudomagnetic field ${\mathbf{B}}_S$ originating from an arc-shape elastic deformation, the Josephson current is monotonically enhanced. This is in contrast with the oscillatory behavior of supercurrent (known as Fraunhofer pattern) caused by real magnetic fields passing through the junction. The absence of oscillatory supercurrent originates from the fact that strain-induced gauge fields have opposite directions at the two valleys due to the time-reversal symmetry. Subsequently the Aharonov-Bohm phases due to ${\mathbf{B}}_S$ cancel out between electron and hole components of the current carrying Andreev bound states. Nevertheless, we find another phase factor coming from the pseudomagnetic fields which rotates the pseudospin and is in fact responsible for the enhancement of Josephson currents. When both magnetic and pseudomagnetic fields are present, Fraunhofer-like oscillations as a function of the real magnetic field flux are found. Intriguingly, the combination of two kinds of gauge fields results in two special fingerprints in the local supercurrent density: (i) strong localization of the Josephson current density with more intense maximum amplitudes; (ii) appearance of the inflated vortex cores—finite regions with almost diminishing Josephson currents—whose sizes increase by increasing ${\mathbf{B}}_S$. These findings reveal unexpected interference signatures of pseudomagnetic fields in graphene SNS junctions which can cause a twist in the investigations on strain-induced gauge fields.

A complex band structure describes the dispersion relation not only of propagating bulk states but also of evanescent ones, both of which are together referred to as generalized Bloch states and are important for understanding the electronic nature of solid surfaces and interfaces. On the basis of the real-space finite-difference formalism within the framework of the density functional theory, we formulate the Kohn-Sham equation for generalized Bloch wave functions as a generalized eigenvalue problem without using any Green's function matrix. By exploiting the sparseness of the coefficient matrices and using the Sakurai-Sugiura projection method, we efficiently solve the derived eigenvalue problem for the propagating and slowly decaying/growing evanescent waves, which are essential for describing the physics of surface/interface states. The accuracy of the generalized Bloch states and the computational efficiency of the present method in solving the eigenvalue problem obtained are compared with those by other methods using the Green's function matrix. In addition, we propose two computational techniques to be combined with the Sakurai-Sugiura projection method and achieve further improvement in the accuracy and efficiency. Complex band structures are calculated with the present method for single- and multiwall carbon nanotubes, and the interwall hybridization and branch points of evanescent electronic states observed in the imaginary parts of the band structures are also discussed.

We introduce $(3+1)$-dimensional models of short-range-interacting electrons that form a strongly correlated many-body state whose low-energy excitations are relativistic neutral fermions coupled to an emergent gauge field, ${\text{QED}}_4$. We discuss the properties of this critical state and its instabilities towards exotic phases such as a gapless “composite” Weyl semimetal and fully gapped topologically ordered phases that feature anyonic pointlike as well as linelike excitations. These fractionalized phases describe electronic insulators. They may be further enriched by symmetries which result in the formation of nontrivial surface states.

We present a first-principles study of the magnetotransport phenomena in $p$-doped diamond via the exact solution of the linearized Boltzmann transport equation, in which the materials' parameters, including electron-phonon and phonon-phonon interactions, are obtained from density functional theory. This approach gives results in very good agreement with a wide range of experimental data for Hall and drift mobilities, magnetoresistance, and Seebeck coefficient, including the phonon drag effect, at different temperatures and carrier concentrations. In particular, our results provide a detailed understanding of the inherent limits of the exceptionally high mobility and Seebeck coefficient, and predict a large magnetic-field-driven enhancement of the Seebeck coefficient, of up to 25% in a magnetic field of 50 kOe already at room temperature.

Elucidation of the energy-level alignment mechanism at a molecule/insulator/metal interface is a key to understanding the molecular and interfacial phenomena. Herein, we provide a detailed investigation into the electronic structures of a free-base phthalocyanine on NaCl films of various thicknesses using scanning tunneling microscopy/spectroscopy (STM/STS). The energy of the ionization and the affinity levels of the molecule were deduced from the STS spectra, and we determined their dependence on the NaCl-film thickness, which can be explained based on three effects: a voltage drop within the NaCl films, the degree of electric-field screening around the molecule, and a variation in the work function of the substrates. We further found that the energy levels relative to the vacuum level are independent of the work function of the substrate, and that the size of the energy gap increases with the thickness. Our results suggest that it is possible to predict the energy levels at the interfaces based on the energy levels of the molecules in a gas phase, the work function of the substrate, and the thickness of the insulating films.

Our first-principles calculation predicts a sizable transverse thermoelectric coefficient $N$ in a skyrmion crystal assumed on EuO monolayer. Such $N$ arises from the coexistence of large longitudinal thermoelectric coefficient and large Hall angle ratio, realized in the vicinity of a band gap with Chern number $\mathcal{C}=2$. This demonstrates a prototype of novel class of thermoelectric materials utilizing the topological texture of electronic spins, motivating further studies including relevant experiments.

Considerable excitement was generated by the observation of large and linear positive magnetoresistance in nonmagnetic silver chalcogenides. Renewed interest in these materials was kindled by the discovery that $\mathrm{A}{\mathrm{g}}_2\mathrm{Te}$ in particular is a topological insulator with gapless linear Dirac-type surface states. High-pressure x-ray-diffraction studies, combined with first-principles electronic structure calculations, have identified three phase transitions as the pressure is increased: an isostructural transition identified with an electronic topological transition followed by two structural phase transitions. These recent studies were carried out on nominally stoichiometric $\mathrm{A}{\mathrm{g}}_2\mathrm{Te}$. For the present work we have prepared single-phase self-doped $\mathrm{A}{\mathrm{g}}_{2\text{−}\delta }\mathrm{Te}$ samples with a well-characterized silver deficit ($\delta =2.0\times {10}^{-4}$) for structural and electrical transport measurements over extended ranges of pressure (0–43 GPa), temperature (2–300 K), and magnetic field (0–9 T). The temperature dependence of the resistivity exhibits anomalous behavior at 2.3 GPa, slightly above the isostructural transition, which we postulate is due to Fermi surface reconstruction associated with a charge density wave (CDW) phase. The anomaly is enhanced by the application of a 9-T magnetic field and shifted to higher temperature, implying that the electronic Zeeman energy is sufficient to alter the gapping of the Fermi surface. A peak in the pressure dependence of the resistivity and a sudden drop in the pressure dependence of the mobility, occurring at 2.3 GPa, provide additional evidence for a CDW phase at pressures slightly above the isostructural transition.

While the coherent control of two-level quantum systems, qubits, is now standard, their continuum electronic equivalents, flying qubits, are much less developed. The first step in this direction has been achieved in dc interferometry experiments. Here, we propose a simple setup to perform the second step, the spectroscopy of these flying qubits, by measuring the dc response to a high-frequency ac voltage drive. Using two different concurring approaches (Floquet theory and time-dependent simulations) and three different models (an analytical model, a simple microscopic model, and a realistic microscopic model), we predict the power-frequency map of the multiterminal device. We argue that this spectroscopy provides a direct measurement of the flying qubit characteristic frequencies and a key validation for more advanced quantum manipulations.

We explore the effect of an incommensurate potential on nodal-link semimetals. For the system with surface states, we show the nodal-link semimetal undergoes a series of phase transitions: first into a metallic phase, then to a loop semimetal phase, and back to a metallic phase with increasing the strength of incommensurate potential. The phase transitions are unveiled by analyzing the properties of energy spectra and wave functions. We also study the system without surface states and find that the Fermi surface evolves from lines to loops before the system enters into a metallic phase when increasing the incommensurate potential strength.

We measured absorption of THz radiation in ${\mathrm{YFeO}}_3$ single crystals at a temperature of 3 K in the magnetic field up to 17 T applied in all three crystallographic directions. Two spin-wave modes were observed at the $\mathrm{\Gamma }$ point with energies 1.2 meV (9.8 ${\mathrm{cm}}^{-1}$) and 2.4 meV (19.3 ${\mathrm{cm}}^{-1}$) in zero field. From the magnetic-field dependence of mode energies, we have refined the previously proposed model [S. E. Hahn et al., Phys. Rev. B 89, 014420 (2014)] and quantified the parameters of Dzyaloshinskii-Moriya interactions and single-ion anisotropies.

A simple two-level system is introduced to demonstrate the existence of the intermolecular mechanism of the appearance/disappearance of multiple maxima in molecular dynamic susceptibility at low temperature. With a minimum of two relaxation processes, quantum tunneling induced by the nuclear spin bath, and the direct process due to the spin-phonon interaction, we prove that the existence of a sufficiently wide dipolar field distribution in the experimental sample is the main cause of this phenomenon in zero or weak applied dc field. The correlations between the phenomenon and the applied dc field, temperature, or magnetic dilution of the sample are also investigated. Application to several experimental systems has shown a good agreement between the proposed theory and experiments. Cases with a more complex multiplet spectrum, more relaxation processes, and the possibility of more maxima in the molecular dynamic susceptibility are discussed.

We show that at the level of BCS mean-field theory, the superconducting $T_c$ is always increased in the presence of disorder, regardless of order parameter symmetry, disorder strength, and spatial dimension. This result reflects the physics of rare events—formally analogous to the problem of Lifshitz tails in disordered semiconductors—and arises from considerations of spatially inhomogeneous solutions of the gap equation. So long as the clean-limit superconducting coherence length, ${\xi }_0$, is large compared to disorder correlation length, $a$, when fluctuations about mean-field theory are considered, the effects of such rare events are small (typically exponentially in ${[{\xi }_0/a]}^d$); however, when this ratio is $\sim 1$, these considerations are important. The linearized gap equation is solved numerically for various disorder ensembles to illustrate this general principle.