The $N$-flavor interacting Su-Schrieffer-Heeger (i-SSH) model realizable in cold atoms in an optical lattice is studied. We clarify the relationship between the i-SSH model and the Chiral-Gross-Neveu-Wilson (CGNW) model. Following the previous study of the CGNW model in the high-energy physics community, the ground-state phases of the i-SSH model are investigated and interpreted from the viewpoint of the phases of the CGNW model. The interaction effect on the i-SSH model, belonging to the topological BDI class, is grasped by following the view of the dynamical breakdown of chiral symmetry in the CGNW model. Furthermore, we compare the large-$N$ ground-state phase diagram with that of the $N=1$ case obtained by exact diagonalization, and then we propose a tabletop cold-atom quantum simulator to test the model.

The dependence of the spin accumulation voltage on the electric bias current for a nondegenerate Si-based spin valve was quantitatively investigated using both experiments and calculations. We attempted to clarify the reason for the disagreement between the experimentally measured values and those calculated using the spin drift-diffusion equation that takes into account the effect of the spin-dependent interfacial resistance of tunneling barriers, which is an important question in semiconductor spintronics. Unlike the case of metallic spin valves, it was found that the bias dependence of the resistance-area product for the ferromagnet/MgO/Si interface causes a conductance mismatch, and this plays a central role in producing the deviation between the experimental and numerical results.

Highly Co-doped ZnO shows uncompensated antiferromagnetic order and a vertical exchange bias shift even in the absence of a ferromagnet. Therefore, it is an ideal model system to study the behavior of uncompensated antiferromagnetic moments, which play a crucial role in the description of conventional exchange bias. Temperature- and cooling-field-dependent magnetometry measurements provide further information on the compensated and uncompensated antiferromagnetic configurations in Co-doped ZnO, revealing that observed effects of both vertical exchange shift and open hysteresis stem from similar magnetic configurations. This transition is evidenced by the increase of the vertical exchange shift on the expense of the hysteresis opening by lowering the temperature.

Third sound measurements of superfluid ${}^4\mathrm{He}$ thin films adsorbed on 10-nm-diameter multiwall carbon nanotubes are used to probe the superfluid onset temperature as a function of the film thickness, and to study the temperature dependence of the film compressibility. The nanotubes provide a highly ordered carbon surface, with layer-by-layer growth of the adsorbed film as shown by oscillation peaks in the third sound velocity at the completion of the third, fourth, and fifth atomic layers, arising from oscillations in the compressibility. In temperature sweeps the third sound velocity at very low temperatures is found to be linear with temperature, but oscillating between positive and negative slope depending on the film thickness. Analysis shows that this can be attributed to a linearly decreasing compressibility of the film with temperature that appears to hold even near zero temperature. The superfluid onset temperature is found to be linear in the film thickness, as predicted by the Kosterlitz-Thouless theory, but the slope is anomalous, a factor of 3 smaller than the predicted universal value.

Motivated by the recent observations of possible $p$-wave superconductivity in a bismuth-nickel (Bi/Ni) bilayer, we explore theoretically the possibilities of realizing $p$-wave superconductivity in this bilayer. We begin with a literature survey on this system and related materials that have similar superconducting transition temperature. From the survey, the superconducting mechanism in this bilayer system is suggested to be phonon mediated type-II superconductivity. A simple model is proposed to explain why the $p$-wave-like Andreev reflection signals are likely to be observed in the surface probe, assuming that the strong spin-orbit coupled surface state of Bi thin film is not completely destroyed by the formation of alloys.

We present a class of three-dimensional (3D) two-band Floquet topological insulators constructed from two-dimensional Floquet topological insulators with a $Z$ topological index. It is shown that the 3D two-band Floquet topological insulator has a $Z_2$ topological index, whose value can be obtained by numerical calculations or by using a relation to the winding number. The classification of the 3D $Z_2$ Floquet topological insulator, however, cannot be attributed to the stable homotopy groups. Thus, it is an example outside the proposed $K$-theory classifications of Floquet topological insulators. We also analyze the edge modes of the 3D $Z_2$ Floquet topological insulator and find the parity of the number of edge modes reflects the bulk $Z_2$ index.

The coupling of monochromatic light fields and solids introduces nonequilibrium Floquet states, offering opportunities to create and explore new topological phenomena. Using combined first-principles and Floquet analysis we show that one can freely engineer Floquet-Dirac fermions (FDFs) in graphene by tuning the frequency and intensity of linearly polarized light. Not only type-II FDFs are created, but they also coexist with type-I FDFs near the Fermi level. Intriguingly, topologically nontrivial edge states connecting type-I and type-II Floquet-Dirac points emerge in photodriven graphene, providing an ideal channel to realize electron transport between the two types of Dirac states. Simulating time- and angle-resolved photoelectron spectroscopy suggests that the coexisting state of type-I and type-II fermions is experimentally accessible. This work implies that a rich FDF phenomenon can be engineered in atomically thin graphene, hinting for developments of optoelectronic and quantum computing devices.

At the Mott transition, electron-electron interaction changes a metal, in which electrons are itinerant, to an insulator, in which electrons are localized. This phenomenon is central to quantum materials. Here we contribute to its understanding by studying the two-dimensional Hubbard model at finite temperature with plaquette cellular dynamical mean-field theory. We provide an exhaustive thermodynamic description of the correlation-driven Mott transition of the half-filled model by calculating pressure, charge compressibility, entropy, kinetic energy, potential energy, and free energy across the first-order Mott transition and its high-temperature crossover (Widom line). The entropy is extracted from the Gibbs-Duhem relation and shows complex behavior near the transition, marked by discontinuous jumps at the first-order boundary, singular behavior at the Mott endpoint, and inflections marking sharp variations in the supercritical region. The free energy allows us to identify the thermodynamic phase boundary, to discuss phases stability and metastability, and to touch upon nucleation and spinodal decomposition mechanisms for the transition. We complement this thermodynamic description of the Mott transition by an information-theoretic description. We achieve this by calculating the local entropy, which is a measure of entanglement, and the single-site total mutual information, which quantifies quantum and classical correlations. These information-theoretic measures exhibit characteristic behaviors that allow us to identify the first-order coexistence regions, the Mott critical endpoint and the crossovers along the Widom line in the supercritical region.

We recently showed [Phys. Rev. Lett. 121, 220602 (2018)] that the average bipartite entanglement entropy of all energy eigenstates of the quantum Ising chain exhibits a universal (for translationally invariant quadratic fermionic models) leading term that scales linearly with the subsystem's volume, while in the thermodynamic limit the first subleading correction does not vanish at the critical field (it only depends on the ratio $f$ between the volume of the subsystem and volume of the system) and vanishes otherwise. Here we show, analytically for bounds and numerically for averages, that the same remains true for the spin-1/2 XY chain in a transverse magnetic field. We then tighten the bounds for the coefficient of the universal volume-law term, which is a concave function of $f$. We develop a systematic approach to compute upper and lower bounds, and we provide explicit analytic expressions for up to the fourth-order bounds.

In this work we analyze the electronic structure at the junction between a ${\mathrm{SrTiO}}_3$ (001) single crystal and a thin tetragonal CuO layer, grown by off-axis rf sputtering. A detailed characterization of the film growth, based on atomic force microscopy and x-ray photoelectron diffraction measurements, demonstrates the epitaxial growth. We report several markers of a thickness-dependent modification of the film gap, found on both Cu $2p$ and valence band spectra; through spectroscopic ellipsometry analysis, we provide direct proof of a band gap increase in the tetragonal CuO layer (1.57 eV) with respect to the thicker monoclinic CuO layer (1.35 eV). This phenomenon is further discussed in light of cluster calculations and density functional theory $+U$ simulations. Finally, we report the full experimental band junction diagram, showing a staggered configuration suitable for charge-separation applications, such as photovoltaics and photocatalysis; this configuration is observed up to very low ($<3$ nm) film thickness due to the gap broadening effect.

The ground-state orbital occupancy of the ${\mathrm{Ru}}^{4+}$ ion in ${\mathrm{Ca}}_{2-x}{\mathrm{La}}_x{\mathrm{RuO}}_4[x=0$, 0.05(1), 0.07(1), and 0.12(1)] was investigated by performing x-ray absorption spectroscopy (XAS) in the vicinity of the O $K$ edge as a function of the angle between the incident beam and the surface of the single-crystal samples. A minimal model of the hybridization between the O $2p$ states probed at the $K$ edge and the Ru $4d$ orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies $n_{xy}/n_{yz,zx}$ to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground state ($x\le 0.07$), $n_{xy}/n_{yz,zx}$ is found to increase significantly with increasing doping, with increasing temperature acting to further enhance $n_{xy}/n_{yz,zx}$. For the $x=0.12$ sample, which has a metallic ground state, the XAS spectra are found to be independent of temperature and not to be describable by the minimal hybridization model, while being qualitatively similar to the spectra displayed by the $x\le 0.07$ samples above their insulating to metallic transitions. To understand the origin of the evolution of the electronic structure of ${\mathrm{Ca}}_{2-x}{\mathrm{La}}_x{\mathrm{RuO}}_4$ across its phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field $\mathrm{\Delta }$, and spin-orbit coupling $\lambda$, of a Ru-O-Ru cluster, with realistic values used to parametrize the various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of $\mathrm{\Delta }/\lambda$ provide an excellent description of the general trends displayed by the data. In particular they establish that the enhancement of $n_{xy}/n_{yz,zx}$ is driven by significant modifications to the crystal field as the tetragonal distortion of the ${\mathrm{RuO}}_6$ octahedral changes from compressive to tensile with La doping. We have also used our model to show that the hole occupancy of the O $2p$ and Ru $4d$ orbitals displays the same general trend as a function of $\mathrm{\Delta }/\lambda$, thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La-doped ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ is the structurally induced redistribution of holes within the $t_{2g}$ orbitals, rather than the injection of free carriers.

In this work we show that the composite fermion construction for the torus geometry is modular covariant. We show that this is the case both before and after projection and that modular covariance properties are preserved under both exact projection and under JK projection which was recently introduced by Pu, Wu, and Jain [Phys. Rev. B 96, 195302 (2017)]. It is crucial for the modular properties to hold that the composite fermion state is a proper state, i.e., that there are no vacant $\mathrm{\Lambda }$-level orbitals directly underneath any $\mathrm{\Lambda }$-level orbital occupied by a composite fermion.

Two-photon Rabi oscillations hold potential for quantum computing and quantum information processing because during a Rabi cycle a pair of entangled photons may be created. We theoretically investigate the onset of this phenomenon in a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. Two-photon Rabi oscillations in this system occur due to a coherent two-photon process involving the ground-to-biexciton transition in the quantum dot. The presence of a metal nanoparticle near the quantum dot results in a self-action of the quantum dot via the metal nanoparticle because the polarization state of the latter depends on the quantum state of the former. The interparticle interaction gives rise to two principal effects: (i) enhancement of the external field amplitude and (ii) renormalization of the quantum dot's resonance frequencies and relaxation rates of the off-diagonal density matrix elements, both depending on the populations of the quantum dot's levels. Here, we focus on the first effect, which results in interesting features, in particular, an increased number of Rabi cycles per pulse compared to an isolated quantum dot and subsequent growth of the number of entangled photon pairs per pulse. We also discuss the destructive role of radiative decay of the excitonic states on two-photon Rabi oscillations for both an isolated quantum dot and a heterodimer.

In this study we have measured the magnetoresistance response of inverted HgTe quantum wells in the presence of a large parallel magnetic field up to 33 T. We show that in quantum wells with inverted band structure a monotonically decreasing magnetoresistance is observed when a magnetic field up to the order of 10 T is applied parallel to the quantum well plane. This feature is accompanied by a vanishing of nonlocality and is consistent with a predicted modification of the energy spectrum that becomes gapless at a critical in-plane field $B_c$. Magnetic fields in excess of $B_c$ allow us to investigate the evolution of the magnetoresistance in this field-induced semimetallic region beyond the known regime. After the longitudinal resistance reaches a minimum in the presumably gapless phase, we observe a strong upturn of the longitudinal resistance. A small residual Hall signal picked up in nonlocal measurements suggests that this feature is likely a bulk phenomenon and is caused by the semimetallicity of the sample. Theoretical calculations indeed support that the origin of these features is classical and a power law upturn of the resistance can be expected due to the specifics of two-carrier transport in thin (semi)metallic samples subjected to large magnetic fields.

The intriguing physics of exceptional points (EPs) and spectral singularities in open (non-Hermitian) active photonic systems has recently sparked increased interest among the research community. These spectral degeneracies have been obtained in asymmetric active and passive photonic configurations but their demonstration with symmetric active plasmonic structures still remains elusive. In this paper, we present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several functionalities. The proposed symmetric active system operates near its cutoff wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an EP that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to superscattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission line model, and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices.

First-principles transport modeling of disordered nanostructures commonly resorts to certain effective medium theory constructed with single-site methods. These methods are essentially approximations that account for the diffusive aspect of the problem, but missing the interference-induced long-range effects such as localization, which also plays an important role in the mesoscopic regime. In this work, we report a real-space implementation of the dual-fermion method in the nonequilibrium Keldysh formalism for correcting the transport coefficients given by single-site methods. A mapping between the Keldysh Green's function and its dual counterpart is established, and a diagrammatic perturbation technique is used in the dual space, whereby the long-range Cooperon is taken into account. When treated at the zeroth order, this theory reproduces the nonequilibrium coherent potential approximation. We require self-consistency on two aspects: the dual Green's function is solved consistently with its self-energy, and the medium Green's function must have its real-space diagonal equal to that of the local impurity. The method is applied to a quasi-one-dimensional hopping lattice which mimics a disordered transport structure. The numerically computed transmission coefficient shows quantitative agreement with the exact solution in the weak-localization regime, significantly correcting the single-site results. In addition, we find that the dual-fermion method leads to a power-law dependency of resistance on the channel length, instead of the classical Ohm's law, and the exponent is insensitive to disorder strengths. We also show that the negative magnetoresistance effect, a phenomenon associated with weak localization, is obtained by our numerical model when a perpendicular magnetic field is introduced. The method presented here paves the way for an ab initio atomistic implementation to simulate disordered quantum transport in real nanostructures.

We introduce highly local basis sets for electronic structure which are very efficient for correlation calculations near the complete basis set limit. Our approach is based on gausslets, recently introduced waveletlike smooth orthogonal functions. We adapt the gausslets to particular systems using one-dimensional (1D) coordinate transformations, putting more basis functions near the nuclei, while maintaining orthogonality. Three-dimensional basis functions are composed out of products of the 1D functions in an efficient way called multislicing. We demonstrate these bases with both Hartree-Fock and density matrix renormalization group calculations on hydrogen chain systems. With both methods, we can go to higher accuracy in the complete basis set limit than is practical for conventional Gaussian basis sets, with errors near 0.1 mH per atom.

The phase transition phenomenon in a semimetallic $1T-\mathrm{TiS}{\mathrm{e}}_2$ has attracted attention as an excitonic insulator. However, as the phase transition accompanying superlattice peaks has the $q$ vector connecting the Fermi surfaces of the three-dimensional shape of hole and electron pockets, it also assumes the charge density wave (CDW) state owing to the electron–phonon interaction. To understand the electronic state at the low temperature, control of the chemical potential was attempted by electronic doping through $\mathrm{C}{\mathrm{u}}^{+}$ intercalation. Physical properties measurements and synchrotron x-ray diffraction experiments in $\mathrm{C}{\mathrm{u}}_x\mathrm{TiS}{\mathrm{e}}_2$ ($x=0–0.35$) were performed. The phase transition was determined to occur as a cooperative phenomenon between the honeycomb lattice type CDW corresponding to the nesting vector and the ordered state of the $\mathrm{C}{\mathrm{u}}^{+}$ ions between $\mathrm{TiS}{\mathrm{e}}_2$ layers at a specific doping amount ($x=1/3$). The behavior of $\mathrm{C}{\mathrm{u}}^{+}$ ions in highly doped regions suggests the occurrence of a two-dimensional liquid-solid state transition based on the temperature dependence of the x-ray diffuse scattering.

One bottleneck of quantum Monte Carlo (QMC) simulation of strongly correlated electron systems lies at the scaling relation of computational complexity with respect to the system sizes. For generic lattice models of interacting fermions, the best methodology at hand still scales with $\beta N^3$ where $\beta$ is the inverse temperature and $N$ is the system size. Such scaling behavior has greatly hampered the accessibility of the universal infrared (IR) physics of many interesting correlated electron models at (2+1)D, let alone (3+1)D. To reduce the computational complexity, we develop a new QMC method with inhomogeneous momentum-space mesh, dubbed elective-momentum ultrasize quantum Monte Carlo (EQMC) method. Instead of treating all fermionic excitations on an equal footing as in conventional QMC methods, by converting the fermion determinant into the momentum space, our method focuses on fermion modes that are directly associated with low-energy (IR) physics in the vicinity of the so-called hot spots, while other fermion modes irrelevant for universal properties are ignored. As shown in the paper, for any cutoff-independent quantities, e.g., scaling exponents, this method can achieve the same level of accuracy with orders of magnitude increase in computational efficiency. We demonstrate this method with a model of antiferromagnetic itinerant quantum critical point, realized via coupling itinerant fermions with a frustrated transverse-field Ising model on a triangle lattice. The system size of $48\times 48\times 32$ ($L\times L\times \beta$, almost 3 times of previous investigations) are comfortably accessed with EQMC. With much larger system sizes, the scaling exponents are unveiled with unprecedentedly high accuracy, and this result sheds new light on the open debate about the nature and the universality class of itinerant quantum critical points.

We construct a many-body quantized invariant that sharply distinguishes among two-dimensional nonequilibrium driven phases of interacting fermions. This is an interacting generalization of a band-structure Floquet quasienergy winding number and describes chiral pumping of quantum information along the edge. In particular, our invariant sharply distinguishes between a trivial and anomalous Floquet Anderson insulator in the interacting, many-body localized setting. It also applies more generally to models where only fermion parity is conserved, where it differentiates between trivial models and ones that pump Kitaev Majorana chains to the boundary, such as ones recently introduced in the context of emergent fermions arising from eigenstate ${\mathbb{Z}}_2$ topological order. We evaluate our invariant for the edge of such a system with eigenstate ${\mathbb{Z}}_2$ topological order, and show that it is necessarily nonzero when the Floquet unitary exchanges electric and magnetic excitations, proving a connection between bulk anyonic symmetry and edge chirality that was recently conjectured.