We demonstrate that the entanglement entropy area law for free fermion ground states and the corresponding volume law for highly excited states are related by a position-momentum duality, and thus are of the same origin. For a typical excited state in the thermodynamic limit, we further show that the reduced density matrix of a subsystem approaches the thermal density matrix, provided the subsystem's linear size is small compared to that of the whole system in all directions, a property we dub eigenstate typicality. This provides an explicit example of thermalization via entanglement, and reveals how statistical physics emerges from a single eigenstate by tracing out a large number of degrees of freedom.

Magnetization is induced in an ensemble of quantum dots, each charged with a single electron, when it is illuminated with a short circularly polarized light pulse that is resonant with the fundamental energy gap of the quantum dots. In this investigation, a quantum-mechanical model for the light-induced magnetization is presented. The phase of the magnetization precession as a function of the strength of the magnetic field in a Voigt geometry is in excellent agreement with experimental data measured on (In,Ga)As singly charged quantum dot ensembles. It is demonstrated that the precession of the hole in the trion plays a vital role because it determines the amplitude and phase of the magnetization precession. The model could also be easily extended to describe positively charged quantum dots. We also suggest that our theory, combined with measurements of the phase as a function of magnetic field, can be used as a technique to measure the resonant trion lifetime as a function of QD emission energy.

We develop a gauge-independent perturbation theory for the grand potential of itinerant electrons in two-dimensional tight-binding models in the presence of a perpendicular magnetic field. At first order in the field, we recover the result of the so-called modern theory of orbital magnetization and, at second order, deduce a new general formula for the orbital susceptibility. In the special case of two coupled bands, we relate the susceptibility to geometrical quantities such as the Berry curvature. Our results are applied to several two-band systems—either gapless or gapped. We point out some surprising features in the orbital susceptibility—such as in-gap diamagnetism and parabolic band-edge paramagnetism—coming from interband coupling. From this we draw general conclusions on the orbital magnetism of itinerant electrons in multiband tight-binding models.

Periodic driving can create topological phases of matter absent in static systems. In terms of the displacement of the position expectation value of a time-evolving wave packet in a closed system, a type of adiabatic dynamics in periodically driven systems is studied for general initial states possessing coherence between different Floquet bands. Under one symmetry assumption, the displacement of the wave-packet center over one adiabatic cycle is found to consist of two components independent of the time scale of the adiabatic cycle: a weighted integral of the Berry curvature summed over all Floquet bands, plus an interband coherence induced correction. The found correction is beyond a naive application of the quantum adiabatic theorem but survives in the adiabatic limit due to interband coherence. Our theoretical results are hence of general interest towards an improved understanding of the quantum adiabatic theorem. Our theory is checked using a periodically driven superlattice model with nontrivial topological phases. In addition to probing topological phase transitions, the adiabatic dynamics studied in this work is now also anticipated to be useful in manifesting coherence and decoherence effects in the representation of Floquet bands.

We study a transmission line resonator which is driven by electrons tunneling through a voltage-biased tunnel junction. Using the Born-Markovian quantum master equation in the polaron basis we investigate the nonequilibrium photon state and emission spectrum of the resonator as well as properties of the transport current across the tunnel junction and its noise spectrum. The electroluminescence is optimized, with maximum peak height and narrow linewidth, when the back-action of the tunnel junction on the resonator and the damping of the resonator are similar in strength. For strong coupling between the resonator and tunnel junction, multiphoton effects create signatures in the transport current and current noise spectrum.

Concerted single-nanowire (NW) absorption and emission spectroscopies have been used to measure Stokes shifts in the optical response of individual CdSe NWs. Obtained spectra are free of inhomogeneous broadening inherent to ensemble measurements. They reveal apparent size-dependent NW Stokes shifts with magnitudes on the order of 30 meV. Given that an effective mass model previously used to explain CdSe NW excited state progressions predicts no sizable emission Stokes shift, we have investigated modifications to the theory to rationalize their existence. This has entailed better accounting for the effects of crystal field splitting on NW band edge states. What results are important changes to the spectroscopic assignment of NW band edge transitions that arise from the crossing of hole levels. Furthermore, these modifications simultaneously predict Stokes shifts with size-dependent magnitudes up to 20 meV. However, quantitative agreement with experiment is only achieved by accounting for the role of exciton trap states. Consequently, we conclude that CdSe NW Stokes shifts contain both intrinsic and extrinsic contributions—the latter arising from band edge exciton potential energy fluctuations. At a broader level, these concerted absorption and emission measurements have provided detailed insight into the electronic structure of CdSe NWs, beyond what could be obtained using either single-particle absorption or emission spectroscopies alone.

Recently, phosphorene, a monolayer honeycomb structure of black phosphorus, was experimentally manufactured and has attracted rapidly growing interest. Motivated by phosphorene, here we investigate the stability and electronic properties of the honeycomb structure of the arsenic system based on first-principles calculations. Two types of honeycomb structures, buckled and puckered, are found to be stable. We call them arsenenes, as in the case of phosphorene. We find that both buckled and puckered arsenenes possess indirect gaps. We show that the band gap of puckered and buckled arsenenes can be tuned by applying strain. The gap closing occurs at 6% strain for puckered arsenene, where the bond angles between the nearest neighbors become nearly equal. An indirect-to-direct gap transition occurs by applying strain. Specifically, 1% strain is enough to transform puckered arsenene into a direct-gap semiconductor. We note that a bulk form of arsenic called gray arsenic exists which can be used as a precursor for buckled arsenene. Our results will pave the way for applications to light-emitting diodes and solar cells.

We study bond order parameters for generalized $t\text{−}J$ models on a square lattice. Using the plane-wave limit the considered order parameters form basis functions for irreducible representations of the symmetry transformations of the point group and of time reversal. We show that, for instability wave vectors along the diagonals, all possible basis functions either are fine tuned (i.e., obey restrictions beyond the requirements of symmetry) or break time-reversal symmetry and thus describe flux states. For instability wave vectors along the crystalline axes, corresponding to the observed case in underdoped cuprates, there are only three representations with $A_1,B_1$, and $E$ symmetry which do not break time-reversal symmetry in the general case. We suggest that one of them has recently been observed in resonant elastic x-ray scattering.

We present a quantitative comparison between the measurements of the complex conductance at low [kilohertz (kHz)] and high [gigahertz (GHz)] frequency in a thin superconducting film of NbN and the theoretical predictions of the dynamical Berezinksii-Kosterlitz-Thouless theory. While the data in the GHz regime can be well reproduced by extending the standard approach to the realistic case of a inhomogeneous sample, the low-frequency measurements present an anomalously large dissipative response around $T_c$. This anomaly can only be accounted for by assuming a strong slowing down of the vortex diffusion in the kHz regime, or analogously a strong reduction of the length scale probed by the incoming finite-frequency field. This effect suggests the emergence of an intrinsic length scale for the vortex motion that coincides with the typical size of inhomogeneity probed by STM measurements in disordered NbN films.

The mineral malachite, $\mathrm{Cu}{}_2\left(\mathrm{OD}\right){}_2\mathrm{CO}{}_3$, has a quantum spin-liquid ground state and no long-range magnetic order down to at least $T=0.4$ K. Inelastic neutron scattering measurements show that the excitation spectrum consists of dispersive gapped singlet-triplet excitations, characteristic of spin-1/2 dimer-forming Heisenberg antiferromagnets. We identify a distinct two-dimensional dimerized coupling scheme with strong interdimer coupling $J^{\prime }/J_1\approx 0.3$ that places malachite between strongly coupled alternating chains, square lattice antiferromagnets, and infinite-legged ladders. The geometry of the interaction scheme resembles the staggered dimer lattice, which may allow unconventional quantum criticality.

The effects of microwave radiation on the transport properties of atomically thin ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ films were studied in the 0.1–13 GHz frequency range. Resistance changes induced by microwaves were investigated at different temperatures near the superconducting transition. The nonlinear response decreases by several orders of magnitude within a few GHz of a cutoff frequency ${\nu }_{\text{cut}}\approx 2\text{GHz}$. Numerical simulations that assume an ac response to follow the dc $V-I$ characteristics of the films reproduce well the low-frequency behavior, but fail above ${\nu }_{\text{cut}}$. The results indicate that two-dimensional (2D) superconductivity is resilient against high-frequency microwave radiation, because vortex-antivortex dissociation is dramatically suppressed in 2D superconducting condensates oscillating at high frequencies.

As the first experimentally confirmed ferroelectric metal, ${\mathrm{LiOsO}}_3$ has received extensive research attention recently. Using density-functional calculations, we perform a systematic study on the origin of the metallic ferroelectricity in ${\mathrm{LiOsO}}_3$. We confirm that the ferroelectric transition in this compound is order-disorder-like. By doing electron screening analysis, we unambiguously demonstrate that the long-range ferroelectric order in ${\mathrm{LiOsO}}_3$ results from the incomplete screening of the dipole-dipole interaction along the nearest-neighboring Li-Li chain direction. We conclude that highly anisotropic screening and local dipole-dipole interactions are the two most important keys to form ${\mathrm{LiOsO}}_3$-type metallic ferroelectricity.

Type, degree, and evolution of structural order are important aspects for understanding and controlling the properties of highly spin-polarized Heusler compounds, in particular, with respect to the optimal film growth procedure. In this work, we compare the structural order and the local magnetic properties revealed by nuclear magnetic resonance (NMR) spectroscopy with the macroscopic properties of thin ${\mathrm{Co}}_2\mathrm{FeAl}{}_{0.5}\mathrm{Si}{}_{0.5}$ Heusler films with varying thickness. A detailed analysis of the measured NMR spectra presented in this paper enables us to find a very high degree of $L{2}_1$-type ordering up to 81% concomitantly with excess Fe of 8%–13% at the expense of Al and Si. We show that the formation of certain types of order depends not only on the thermodynamic phase diagrams as in bulk samples, but also that the kinetic control may contribute to the phase formation in thin films. It is an exciting finding that ${\mathrm{Co}}_2\mathrm{FeAl}{}_{0.5}\mathrm{Si}{}_{0.5}$ can form an almost ideal $L{2}_1$ structure in films, though with a considerable amount of Fe-Al/Si off stoichiometry. Moreover, the very good quality of the films as demonstrated by our NMR study suggests that the technique of off-axis sputtering used to grow the films sets the stage for the optimized performance of ${\mathrm{Co}}_2\mathrm{FeAl}{}_{0.5}\mathrm{Si}{}_{0.5}$ in spintronic devices.

By using first-principles electronic and magnetic as well as ferroelectric property calculations on the ferroelectric compound ${\mathrm{PbBaFe}}_2{\mathrm{O}}_5$, we find that the ground states of the two ${\mathrm{PbBaFe}}_2{\mathrm{O}}_5$ phases, the space group $Imma$ and $Pnma$ phases, exhibit an antiferromagnetic structure with a propagation vector $[0,\frac{1}{2},\frac{1}{2}]$ state and the magnetic moment of $3.99 {\mu }_B$ on Fe atoms, which are in good agreement with neutron diffraction experiments. In addition, we find that this magnetic ordering state is an insulating state with an energy gap of 2.24 (2.62) eV for the $Imma \left(Pnma\right)$ phase. These findings can be tested by future optical measurements. Furthermore, the parameters of magnetic exchange coupling are also evaluated to illustrate the nature of frustrated magnetic behaviors based on the localized Heisenberg model. Finally, we discuss whether the presence of an improper geometric spin frustration causes the appearance of ferroelectricity.

Bad metals display transport behavior that differs from what is commonly seen in ordinary metals. One of the most significant differences is a resistivity that is linear in temperature and rises to well above the Ioffe-Regel limit (where the mean-free path is equal to the lattice spacing). Using an exact Kubo formula, we show that a linear resistivity naturally occurs for many systems when they are in an incoherent intermediate-temperature state. First, we provide a simple analytic model to give intuition for this phenomenology. Then, we verify the analytic arguments with numerical calculations for a simplified version of the Hubbard model which is solved with dynamical mean-field theory. Similar features have also been seen in Hubbard models, where they can begin at even lower temperatures due to the formation of resilient quasiparticles.

Within the scheme of the large-scale atomic effective pseudopotential program (LATEPP), the Schrödinger equation of an electronic system is solved within an effective single-particle approach. Although not limited to, it focuses on the recently introduced atomic effective pseudopotentials derived from screened local effective crystal potentials as obtained from self-consistent density functional theory calculations. The problem can be solved in both real (real-space grid) and reciprocal space (plane-wave basis functions). Following the idea of atomic effective pseudopotentials, the density, and hence a self-consistent cycle, is not required and not implemented. An iterative solver is implemented to deliver the eigenstates close to a selected reference energy, e.g., around the band gap of a semiconductor. This approach is particularly well suited for theoretical investigations of the electronic structure of semiconductor nanostructures and we demonstrate linear scaling with the system size up to around $100000$ atoms on a single standard compute node. Moreover, an efficient real-space treatment of spin-orbit coupling within the pseudopotential framework is proposed in this work allowing for a fully relativistic description.

We report the crystal growth of a new compound, $\mathrm{Y}{\mathrm{b}}_3\mathrm{O}{\mathrm{s}}_4\mathrm{G}{\mathrm{e}}_{13}$, by using a Bi flux method. X-ray diffraction measurement reveals that it crystallizes in the quasiskutterudite-type caged structure with a cubic space group of $Pm\overline{3}n$ (No. 223). Magnetic measurements reveal almost fully localized Yb $4f$-moments above 120 K. The resistivity exhibits a logarithmic increase below ∼40 K. The specific heat coefficient shows a rapid upturn below $\sim 5\mathrm{K}$ and exceeds $2\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$ at 2 K. Our experimental analysis and electronic band structure calculations demonstrate that $\mathrm{Y}{\mathrm{b}}_3\mathrm{O}{\mathrm{s}}_4\mathrm{G}{\mathrm{e}}_{13}$ exhibits the Kondo effect due to strong hybridization of the localized Yb $f$ moments with the $p$ electrons of the surrounding Ge cages.

Density functional perturbation theory is used to calculate the phonons at the zone center for monoclinic ${\mathrm{CsSnCl}}_3$. We report the calculated normal mode frequencies classified according to irreducible representations for both infrared-active and Raman-active modes. We also report the dielectric constants and Born-effective charges and investigate the anisotropy of the nonanalyticities near the zone center. The first-principles calculated Raman-active phonon frequencies and simulated Raman spectra are compared with experimental results from literature. We find that aside from the first-order allowed Raman modes of $A_g$ and $B_g$ symmetries, forbidden LO Raman modes appear prominently in the measured spectra. Several of these modes are close to or coincide with allowed modes and alter their intensity, while others appear as separate otherwise unexplained peaks. With this interpretation, good agreement is obtained with experimental mode frequencies to within a few ${\mathrm{cm}}^{-1}$.

We report on a nitride interface structure formed between epitaxial graphene and SiC (0001). The nitride interface can be obtained by the pretreatment of SiC in an $\mathrm{Ar}/{\mathrm{N}}_2$ atmosphere at 1600 °C, followed by graphene growth in Ar at 1700 °C. Our detailed high-resolution transmission electron microscopy revealed a nitride atomic layer between the 0th carbon layer and the SiC substrate. Due to the nitride interface, the interface carrier scattering was reduced, which resulted in an improvement of the room temperature mobility of graphene, indicating that this is another technique to modify the electronic properties of graphene.