Electronic phase separation (EPS) is a common phenomenon in strongly correlated oxides. For colossal magnetoresistive (CMR) manganites, the EPS is so pronounced that not only does it govern the CMR behavior, but also raises a question whether EPS exists as a ground state for systems or a metastable state. While it has been well known that a magnetic field can drive the transition of the EPS state into a single-phase state in manganites, the reversibility of this transition is not well studied. In this work we use magnetic force microscopy (MFM) to directly visualize the reversibility of the field driven transition between the EPS state and the single-phase state at different temperatures. The MFM images correspond well with the global magnetic and transport property measurements, uncovering the underlying mechanism of the field driven transition between the EPS state and the single-phase state. We argue that EPS state is a consequence of system quenching whose response to an external magnetic field is governed by a local energy landscape.

We investigate the spinless Anderson-Holstein model routinely employed to describe the basic physics of phonon-assisted tunneling in molecular devices. Our focus is on small to intermediate electron-phonon coupling; we complement a recent strong coupling study [Phys. Rev. B 87, 075319 (2013)]. The entire crossover from the antiadiabatic regime to the adiabatic one is considered. Our analysis using the essentially analytical functional renormalization group approach backed up by numerical renormalization group calculations goes beyond lowest order perturbation theory in the electron-phonon coupling. In particular, we provide an analytic expression for the effective tunneling coupling at particle-hole symmetry valid for all ratios of the bare tunnel coupling and the phonon frequency. It contains the exponential polaronic as well as the power-law renormalization in the electron-phonon interaction; the latter can be traced back to x-ray edgelike physics. In the antiadiabatic and the adiabatic limit this expression agrees with the known ones obtained by mapping to an effective interacting resonant level model and lowest order perturbation theory, respectively. Away from particle-hole symmetry, we discuss and compare results from several approaches for the zero temperature electrical conductance of the model.

We investigate the effect of vibrational degrees of freedom on the linear thermoelectric transport through a single-level quantum dot described by the spinless Anderson-Holstein impurity model. To study the effects of strong electron-phonon coupling, we use the nonperturbative numerical renormalization group approach. We also compare our results, at weak to intermediate coupling, with those obtained by employing the functional renormalization group method, finding good agreement in this parameter regime. When applying a gate voltage at finite temperatures, the inelastic scattering processes, induced by phonon-assisted tunneling, result in an interesting interplay between electrical and thermal transport. We explore different parameter regimes and identify situations for which the thermoelectric power as well as the dimensionless figure of merit are significantly enhanced via a Mahan-Sofo type of mechanism. We show, in particular, that this occurs at strong electron-phonon coupling and in the antiadiabatic regime.

Recently, the existence of Dirac/Weyl cones in three-dimensional systems has been demonstrated experimentally. While in high-energy physics the isotropy of the Dirac/Weyl cones is guaranteed by relativistic invariance, in condensed-matter systems corrections to this can occur, with one possible type being a tilt. In this paper, we study the effect of tilted Weyl cones in collective effects. We study both the optical conductivity as well as the polarization function. We also investigate the perturbative effect of long-range Coulomb interactions using a renormalization-group calculation. We find that the tilt is perturbatively renormalized towards zero and at low energies the system flows to an effectively untilted theory.

We match analytic results to numerical calculations to provide a detailed picture of the metal-insulator and topological transitions found in density functional plus cluster dynamical mean-field calculations of pyrochlore iridates. We discuss the transition from Weyl metal to Weyl semimetal regimes, and then analyze in detail the properties of the Weyl semimetal phase and its evolution into the topologically trivial insulator. The energy scales in the Weyl semimetal phase are found to be very small, as are the anisotropy parameters. The electronic structure can, to a good approximation, be described as “Weyl rings” and one of the two branches that contributes to the Weyl bands is essentially flat, leading to enhanced susceptibilities. The optical longitudinal and Hall conductivities are determined; the frequency dependence includes pronounced features that reveal the basic energy scales of the Weyl semimetal phase.

In this work, we predict a band structure for the carbon-lithium (${\mathrm{C}}_4\mathrm{Li}$) compound using first principles calculations. We show that it exhibits two Dirac nodes near the Fermi level: (i) a line node protected by the nonsymmorphic symmetries of the compound and (ii) a type-II Dirac node with the highest tilt along the $\mathrm{\Gamma }-X$ line. The electron and hole pockets in the vicinity of the type-II Dirac node are separated in the stoichiometric compound, and they would touch each other when the Fermi level is doped up to the Dirac cone. The evolution of the Fermi surface with doping is also discussed. The bands crossing from $T$ to $W$ make a line node at the intersection of the $k_x=\pi$ and $k_y=\pi$ mirror planes. ${\mathrm{C}}_4\mathrm{Li}$ is a material with both a nonsymmorphic protected line node and a type-II Dirac cone near the Fermi level.

In this work we experimentally demonstrate a recently proposed criterion addressed to detect nonclassical behavior in the fluorescence emission of ensembles of single-photon emitters. In particular, we apply the method to study clusters of nitrogen-vacancy centers in diamond characterized with single-photon-sensitive confocal microscopy. Theoretical considerations on the behavior of the parameter at any arbitrary order in the presence of Poissonian noise are presented and, finally, the opportunity of detecting manifold coincidences is discussed.

A quantum mechanical model is developed for the Faraday effect in europium telluride, for photons of energy within the transparency gap. The model is based on the well known band edge electronic energy states in EuTe. A concise expression for the Verdet constant is obtained, determined by few parameters already available in the literature. The Verdet constant adopted here, defined by the ratio between the Faraday rotation angle and the magnetization, is in effect temperature independent. Its dependence on the photon energy and applied magnetic field is in excellent agreement with published results. Below 3 T the Verdet constant is also nearly independent on field, but above 3 T at low temperatures it increases due to the band gap redshift. The model is used to calculate the photoinduced Faraday rotation associated with photoinduced giant magnetic polarons in EuTe. The theoretical photoinduced Faraday rotation excitation describes quite well the main features seen experimentally. Due to the common band-edge electronic energy structure, the model reported here could be extended to all other europium chalcogenides.

We study the nonlocal transport property of Majorana quasiparticle states (MQPs) in a normal lead–topological superconducting wire–normal lead system. We find that the tunneling coefficient of the electron transmission process displays an interesting even-odd interference behavior. However, this even-odd interference behavior is difficult to observe in the tunneling coefficient of the crossed Andreev reflection process (CAR). We show that this even-odd interference behavior is directly related to the self-Hermitian property of MQPs. Due to the self-Hermitian property, the correction to the transport in the electron transmission process is in the first order while the correction to the CAR process is in the higher order. Thus, the interference behavior is more significant in the electron transmission process than in the CAR process. Such a unique transport property demonstrates the nonlocal and self-Hermitian characteristics of MQPs.

Stacking-dependent interlayer interactions are important for understanding the structural and electronic properties in incommensurable two-dimensional material assemblies where long-range moiré patterns arise due to small lattice constant mismatch or twist angles. Here we study the stacking-dependent interlayer coupling energies between graphene (G) and hexagonal boron nitride (BN) homo- and heterostructures using high-level random-phase approximation (RPA) ab initio calculations. Our results show that although total binding energies within LDA and RPA differ substantially by a factor of 200%–400%, the energy differences as a function of stacking configuration yield nearly constant values with variations smaller than 20%, meaning that LDA estimates are quite reliable. We produce phenomenological fits to these energy differences, which allows us to calculate various properties of interest including interlayer spacing, sliding energetics, pressure gradients, and elastic coefficients to high accuracy. The importance of long-range interactions (captured by RPA but not LDA) on various properties is also discussed. Parametrizations for all fits are provided.

The interaction between electrons and the vibrational degrees of freedom of a molecular quantum dot can lead to an exponential suppression of the conductance, an effect which is commonly termed Franck-Condon blockade. Here, we investigate this effect in a quantum dot driven by time-periodic gate voltages and tunneling amplitudes using nonequilibrium Green's functions and a Floquet expansion. Building on previous results showing that driving can lift the Franck-Condon blockade, we investigate driving protocols which can be used to pump charge across the quantum dot. In particular, we show that due to the strongly coupled nature of the system, the pump current at resonance is an exponential function of the drive strength.

High-energy surface-sensitive x-ray diffraction (HESXRD) is a powerful high-energy photon technique (E > 70 keV) that has in recent years proven to allow a fast data acquisition for the 3D structure determination of surfaces and nanoparticles under in situ and operando conditions. The use of a large-area detector facilitates the direct collection of nearly distortion-free diffraction patterns over a wide $q$ range, including crystal truncation rods perpendicular to the surface and large-area reciprocal space maps from epitaxial nanoparticles, which is not possible in the conventional low-photon energy approach ($E=10-20\mathrm{keV}$). Here, we present a comprehensive mathematical approach, explaining the working principle of HESXRD for both single-crystal surfaces and epitaxial nanostructures on single-crystal supports. The angular calculations used in conventional crystal truncation rod measurements at low-photon energies are adopted for the high-photon-energy regime, illustrating why and to which extent large reciprocal-space areas can be probed in stationary geometry with fixed sample rotation. We discuss how imperfections such as mosaicity and finite domain size aid in sampling a substantial part of reciprocal space without the need of rotating the sample. An exact account is given of the area probed in reciprocal space using such a stationary mode, which is essential for in situ or operando time-resolved experiments on surfaces and nanostructures.

We present a description of vacuum polarization in a circular Dirac quantum dot in two spatial dimensions assuming $\alpha$—the relative strength of the Coulomb interaction small enough to render an approximation with a single electron (hole) lowest energy level relevant. Applying this approximation, we find that for ${\alpha }_c\approx 1.05$ the lowest level is half filled irrespective of the number of flavors that are present. The ground state can be represented as a superposition of particular (even number) excitonic states which constitute an excitonic cloud that evolves in a crossover manner. The ground state is degenerate with an intervalley excitonic state at ${\alpha }_c\approx 1.05$, a critical strength, that in our approximation marks a point with single electron and exciton resonances.

One of the hallmarks of time-reversal-symmetric topological insulators in three dimensions is the topological magnetoelectric effect (TME). So far, a time-reversal breaking variant of this effect has attracted much attention, in the sense that the induced electric charge changes sign when the direction of an externally applied magnetic field is reversed. Theoretically, this effect is described by the so-called axion term. Here, we discuss a time-reversal-symmetric TME, where the electric charge depends only on the magnitude of the magnetic field but is independent of its sign. We obtain this nonperturbative result both analytically and numerically, and suggest a mesoscopic setup to demonstrate it experimentally.

We analyze the unusual slow dynamics that emerges in the bad metal delocalized phase preceding the many-body localization transition by using single-particle Anderson localization on the Bethe lattice as a toy model of many-body dynamics in Fock space. We probe the dynamical evolution by measuring observables such as the imbalance and equilibrium correlation functions, which display slow dynamics and power laws strikingly similar to those observed in recent simulations and experiments. We relate this unusual behavior to the nonergodic spectral statistics found on Bethe lattices. We discuss different scenarios, such as a true intermediate phase which persists in the thermodynamic limit versus a glassy regime established on finite but very large time and length scales only, and their implications for real-space dynamical properties. In the latter, slow dynamics and power laws extend on a very large time window but are eventually cut off on a time scale that diverges at the many-body localization transition.

We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal $\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti $2p$ and $3p$ spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the $3p$ spectrum, and very good for the $2p$ spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.