We study the effect of laser driving on a minimal model for a hexagonal two-dimensional material with broken inversion symmetry. Through the application of circularly polarized light and coupling to a thermal free electron bath, the system is driven into a nonequilibrium steady state with asymmetric, nonthermal carrier populations in the two valleys. We show that, in this steady state, interband superconducting correlations between electrons can develop independent of the sign of the electronic interactions. We discuss how our results apply, for example, to transition metal dichalcogenides. This work opens the door to technological applications of superconductivity in a range of materials that were hitherto precluded from it.

Using time-dependent density matrix renormalization group calculations we study the transport properties ($I-V$ curves and shot noise) of the interacting resonant level model (IRLM) in a large range of the interaction parameter $U$, in the scaling limit. We find that these properties can be described remarkably well by those of the boundary sine-Gordon model (BSG), which are known analytically [Fendley et al., Phys. Rev. B 52, 8934 (1995).]. We argue that the two models are nevertheless in different universality classes out-of-equilibrium: this requires a delicate discussion of their infrared (IR) properties (i.e., at low bias), where we prove in particular that the effective tunneling charge is $e$ in the infrared regime of the IRLM (except at the self-dual point where it jumps to $2e$), while it is known to be a continuously varying function of $U$ in the BSG. This behavior is confirmed by careful analysis of the numerical data in the IR. The remarkable agreement of the transport properties, especially in the crossover region, remains however unexplained.

Multiorbital interactions have the capacity to produce an interesting kind of doublon-holon bound state that consists of a single-hole state in one band and a doubly occupied state in another band. Interband doublon-holon pair excitations in the two-orbital Hubbard model are studied by using dynamical mean-field theory with the Lanczos method as the impurity solver. We find that the interband bound states may provide several in-gap quasiparticle peaks in the density of states of the narrow band in the orbital-selective Mott phase with a small Hund's rule coupling ($J$). There exists a corresponding energy relation between the in-gap states of the narrow band and the peaks in the excitation spectrum of the doublon for the wide band. We also find that the spin-flip and pair-hopping Hund interactions can divide one quasiparticle peak into two peaks, where the splitting energy increases linearly with increasing $J$. Strong Hund's rule coupling can move the interband doublon-holon pair excitations outside the Mott gap and restrict the bound states by suppressing the orbital selectivity of the doubly occupied and single-hole states.

We report on the interaction of multiply charged ions at keV energies with free Ag nanoparticles with diameters of $\sim 6$ nm (containing about 6600 Ag atoms). As the ionization energy increases only very slowly with the degree of ionization, multielectron capture processes are very likely to occur in peripheral collisions with large cluster sizes. However, due to the large particle size, the produced highly charged Ag nanoparticles are mostly stable. For projectile charge states below $q=8$ the geometrical cross section overcomes the cross section for peripheral electron transfer, and penetrating collisions become dominant. Therefore, these collisions can be described as ion collision with a nanosurface, where the distribution of small-size fragments with $n<20$ is due to sputtering events from the nanoparticle surface. This is in contrast to nanoparticles with a smaller diameter ($<2$ nm), where small fragments are produced by fission of multiply charged clusters.

Oxygen vacancies are a common source of excess electrons in complex oxides. In Mott insulators, these additional electrons can induce a metal-insulator transition (MIT), fundamentally altering the electronic properties of the system. Here we study the effect of oxygen vacancies in ${\mathrm{LaTiO}}_3$, a prototypical Mott insulator close to the MIT. We show that the introduction of oxygen vacancies creates a vacancy-related band immediately below the partially filled $\mathrm{Ti}-t_{2g}$ bands. We study the effect of this additional band on the Mott MIT using a combination of density functional theory and dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$), employing a minimal correlated subspace consisting of effective $\mathrm{Ti}-t_{2g}$ orbitals plus an additional Wannier function centered on the vacancy site. We find that the Mott insulating state in ${\mathrm{LaTiO}}_3$ is robust to the presence of the vacancy band, which remains fully occupied even in the presence of a local Coulomb repulsion, and therefore does not cause a doping of the Mott insulator.

In orthorhombic $\mathrm{SmM}{\mathrm{n}}_2{\mathrm{O}}_5$ single crystals, $\mathrm{S}{\mathrm{m}}^{3+}$ crystal-field (CF) excitations are studied by infrared transmission as a function of temperature and under applied magnetic field up to 10 T. These measurements are complemented with the study of Raman-active phonon frequency shifts as a function of temperature between 300 and 5 K. The frequencies of all ${}^{6}H_j$ crystal-field levels of $\mathrm{S}{\mathrm{m}}^{3+}$ were determined as well as those of ${}^{6}F_j$. At high temperatures, the evolutions of $\mathrm{S}{\mathrm{m}}^{3+}$ CF excitations exhibit anomalies around the characteristic temperatures, $T^{*}\sim 60\mathrm{K}$ and $T^{\mathrm{S}}\sim 120\mathrm{K}$ and reflect the thermal disorder induced by splitting of the Sm–O bonds in $\mathrm{SmM}{\mathrm{n}}_2{\mathrm{O}}_5$ that contribute to the frequency and linewidth phonon shifting. At low temperatures, the degeneracy of the ground-state Kramers doublet is lifted (${\mathrm{\Delta }}_0\sim 36\mathrm{c}{\mathrm{m}}^{-1}$) due to the $\mathrm{S}{\mathrm{m}}^{3+}-\mathrm{M}{\mathrm{n}}^{3+}$ interaction in the ferroelectric phase and strongly enhanced below $T_{\mathrm{C}}\sim 26\mathrm{K}$. The Sm-Mn exchange interaction $J_6$ is determined and compared to that of Gd-Mn interaction in $\mathrm{GdM}{\mathrm{n}}_2{\mathrm{O}}_5$. The Sm magnetic moment $m_{\mathrm{Sm}}\left(T\right)$ and the Sm contribution to the magnetic susceptibility are also evaluated from ${\mathrm{\Delta }}_0\left(T\right)$, indicating that the Sm-Mn interaction is strongly implicated in the magnetic and the ferroelectric orderings below $\sim 26\mathrm{K}$. The study of the $\mathrm{S}{\mathrm{m}}^{3+}$ CF excitations as a function of magnetic field reveals twinning in $\mathrm{SmM}{\mathrm{n}}_2{\mathrm{O}}_5$. This twinning could affect its electric polarization behavior versus magnetic field.

The topological charge of the spin texture in a quantum dot with spin-orbit couplings is shown analytically here to be stable against the ellipticity of the dot. It is directly tunable by a single magnetic field and is related to the sign of the Landé $g$ factor. In a quantum-dot helium, the overall winding number could have a different property from that of the single-electron case (quantum-dot hydrogen), since tuning the number of electrons affects the winding number by the Coulomb interaction and the $z$ component angular momentum $\langle L_z^{}\rangle$. The density profile and the spin texture influence each other when the Coulomb interaction is present. When $\langle L_z^{}\rangle$ is biased away from an integer by the spin-orbit couplings, the rotational symmetry is broken which induces strong density deformation. The sign of the topological charge may also be reversed with increasing magnetic field. These findings are of major significance since the applied magnetic field alone now provides a direct route to control the topological properties of quantum dots.

Black phosphorus has attracted great research interest due to its numerous applications in electronic devices, optoelectronic devices, energy storage, and so on. Compared with the majority of two-dimensional materials, black phosphorus possesses a unique property—its strong in-plane anisotropy. All the properties reported so far, including its effective mass, electron mobility, light absorption, thermal conductivity, and so on, have shown great anisotropy in the basal plane. This property renders black phosphorus applications not achievable with other two-dimensional materials. In this work, however, we discover a remarkable isotropic behavior in the strongly anisotropic black phosphorus—its electrostatic screening of point charges. We use the tip-induced band bending of a scanning tunneling microscope to map out the Coulomb field of ionized potassium adatoms on black phosphorus and reveal its isotropic charge screening. This discovery is important for understanding electron scattering and transport in black phosphorus.

The superconducting condensate in a bilayer normal-metal–superconducting film has a nonuniform dependence throughout the layer. For sufficiently thin normal metallic films, the superconducting correlations induced by the contacting superconducting layer result in the formation of a minigap and the characteristic length scales governing the response of the condensate will be different. In this work we use scanning tunneling spectroscopy to visualize the vortex states as a function of the applied magnetic field in a Au-MoGe nanostructure. By comparing the obtained zero-bias conductance maps with time-dependent Ginzburg-Landau simulations, we directly confirm that the observed vortex distributions can only be explained by taking into account the impact of the normal metallic layer. We illustrate this impact on the vortex state for two lithographically fabricated mesoscopic bilayer structures containing an identical antidot array but having different lateral sizes.

We introduce an approach to compute the spectra of reduced density matrices for local quantum unitary circuits of finite depth and infinite width. Suppose the time-evolved state under the circuit is a matrix-product state with bond dimension $D$; then the reduced density matrix of a half-infinite system has the same spectrum as an appropriate $D\times D$ matrix acting on an ancilla space. We show that reduced density matrices at different spatial cuts are related by quantum channels acting on the ancilla space. This quantum channel approach allows for efficient numerical evaluation of the entanglement spectrum and Rényi entropies and their spatial fluctuations at finite times in an infinite system. We benchmark our numerical method on random unitary circuits, where many analytic results are available, and also show how our approach analytically recovers the behavior of the kicked Ising model at the self-dual point. We study various properties of the spectra of the reduced density matrices and their spatial fluctuations in both the random and translation-invariant cases.

We investigate the effects of quenched disorder on a noninteracting tilted Dirac semimetal in two dimensions. Depending on the magnitude of the tilting parameter, the system can have either Fermi points (type I) or Fermi lines (type II). In general, there are three different types of disorders for Dirac fermions in two dimensions, namely, the random scalar potential, the random vector potentials along and perpendicular to the tilting direction, and the random mass. We study the effects of weak disorder in terms of the renormalization group, which is performed by integrating out the modes with large energies, instead of large momenta. Since the parametrization of the low-energy degrees of freedom depends on the structure of the Fermi surface, the resulting one-loop renormalization-group equations depend on the type of tilted Dirac fermions. Whenever the disorder is a marginal perturbation, we examine its role on low-energy physics by a mean-field approximation of the replica field theory or the first-order Born approximation. Based on our analysis, we suggest the phase diagrams of a two-dimensional tilted Dirac fermion in the presence of different types of disorder.

The Su-Schrieffer-Heeger (SSH) model on a two-dimensional square lattice exhibits a topological phase transition which is related to the Zak phase determined by bulk band topology. The strong modulation of electron hopping causes nontrivial charge polarization even in the presence of inversion symmetry. The energy band structures and topological edge states have been calculated numerically in previous studies. Here, however, the full energy spectrum and explicit form of wave functions for two-dimensional bulk and one-dimensional ribbon geometries of the SSH model are analytically derived using the wave mechanics approach. Explicit analytic representations of wave functions provide the information of parity for each subband, localization length, and critical point of the topological phase transition in the SSH ribbon. IThe dimensional crossover of the topological transition point for the SSH model from one to two dimensions is also shown.

We study the conductivity of two-dimensional graphene-type materials with tilted cones as well as their three-dimensional Weyl counterparts and show that a covariant quantum Boltzmann equation is capable of providing an accurate description of these materials' transport properties. The validity of the covariant Boltzmann approach is corroborated by calculations within the Kubo formula. We find a strong anisotropy in the conductivities parallel and perpendicular to the tilt direction upon an increase of the tilt parameter $\eta$, which can be interpreted as the boost parameter of a Lorentz transformation. While the ratio between the two conductivities is $\sqrt{1-{\eta }^2}$ in the two-dimensional case where only the conductivity perpendicular to the tilt direction diverges for $\eta \to 1$, both conductivities diverge in three-dimensional Weyl semimetals, where $\eta =1$ separates a type-I (for $\eta <1$) from a type-II Weyl semimetal (for $\eta >1$).

The recent realization of a coherent interface between a single electron in a silicon quantum dot and a single photon trapped in a superconducting cavity opens the way for implementing photon-mediated two-qubit entangling gates. In order to couple a spin to the cavity electric field, some type of spin-charge hybridization is needed, which impacts spin control and coherence. In this work we propose a cavity-mediated two-qubit gate and calculate cavity-mediated entangling gate fidelities in the dispersive regime, accounting for errors due to the spin-charge hybridization, as well as photon- and phonon-induced decays. By optimizing the degree of spin-charge hybridization, we show that two-qubit gates mediated by cavity photons are capable of reaching fidelities exceeding $90\%$ in present-day device architectures. High iswap gate fidelities are achievable even in the presence of charge noise at the level of $2\mu \text{eV}$.

Doped perovskite oxides are promising “material building blocks” for achieving tunable and intrinsically low-loss plasmonic features in the visible- to near-infrared range, which are unlikely to obtain from conventional plasmonic materials. Here, using a combination of spectroscopic ellipsometry, soft x-ray absorption, x-ray photoemission, and ultraviolet photoemission spectroscopic measurements we observe an unconventional tunable low-loss quasilocal plasmon in the IR energy (<1 eV) range in the Nb-substituted perovskite oxide $\mathrm{EuTi}{\mathrm{O}}_3$ ($\mathrm{EuT}{\mathrm{i}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$). This low-loss plasmon oscillation is formed by many-body interactions, an interplay of quasifree $d^1$ electrons and hybridizations of Eu 4f-Ti $3d$ and O 2p-Ti $3d$. We find that the Nb substitution at Ti site modifies the $\mathrm{Ti}{\mathrm{O}}_6$ octahedral tilting and, consequently, alters the electronic structure. Our study reveals the importance of charge, orbital, and lattice to generate plasmons in $\mathrm{EuT}{\mathrm{i}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$.

We present the derivation and application of an analytical effective potential that is able to describe, in a simple way, the interaction of many electrons confined in low-dimensional structures of realistic size. The effective potential takes into account the contribution of both the electron-electron interaction inside the nanostructure and the quantum confinement by a surrounding material. With this model, we explore the electronic distribution in quantum wells, wires, and dots in the full range from doped to high-doped concentrations. We also use this effective potential to explicitly determine the parameters that trigger the formation of the Wigner molecule in quantum wires. The comparison with experimental data reported in the literature shows the accuracy and reliability of this potential model.

We present a detailed cavity quantum electrodynamic model of a nanospherical dimer and use it to analyze bright hybrid spasing modes there. Using an approximate numerical scheme, we model the complete spasing system as an open quantum system under the Lindblad dissipator formalism. We show that while in general the longitudinal dimer setups display higher intensity spasing as compared with transverse dimers, the latter actually consistently lead to output with higher coherence. Intriguingly, and somewhat counterintuitively, we find that transverse dimers only reach peak output at an intermediate dimer separation at which field confinement is not the strongest. We show that this is due to low radiative decay rates of transverse dimers with significant dimer gaps. We also find that transverse dimers outperform longitudinal dimers in terms of output intensity in a weakly pumped sparse gain medium made of dimers with relatively large separations. Moreover, in all the configurations considered, we find that the second-order coherence of the spasing output shows a peaked behavior just before the threshold, suggesting that the coherence is a useful indicator of spasing. Even though the scheme we describe is focused on dimers, owing to the generic form of the analysis presented, it can be easily extended to investigate spasing in the bright modes of multiple coupled plasmon sources.