When an electron tunnels between two metal contacts it temporarily induces an image charge (IC) in the electrodes which acts back on the tunneling electron. It is usually assumed that the IC forms instantaneously such that a static model for the image potential applies. Here we investigate how the finite IC formation time affects charge transport through a molecule suspended between two electrodes. For a single-level model, an analytical treatment shows that the conductance is suppressed by a factor $Z^2$, where $Z$ is the quasiparticle renormalization factor, compared to the static IC approximation. We show that $Z$ can be expressed either in terms of the plasma frequency of the electrode or as the overlap between electrode wave functions corresponding to an empty and filled level, respectively. First-principles GW calculations for benzene-diamine connected to gold electrodes show that the dynamical corrections can reduce the conductance by more than a factor of two when compared to static GW or density functional theory where the molecular energy levels have been shifted to match the exact quasiparticle levels.

When the thickness of ultrathin metal films approaches the nanometer scale comparable to the coherence length of the electrons, significant effects on the structure stability and the electronic properties of the metal films emerge due to electron confinement and quantization of the allowed electronic states in the direction perpendicular to the film. Using first-principles calculations, we showed that such quantum size effects can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even- or odd-layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment.

A complete theoretical characterization of dielectric, elastic, photoelastic, and piezoelectric tensors of the low-temperature rhombohedral phase of BaTiO${}_3$ was performed by accurate ab initio simulations within periodic boundary conditions, using one-electron Hamiltonians and atom-centered Gaussian-type-function basis sets as in the crystal program. Because this phase is stable only at very low temperature, experimental characterization is difficult, and none of such tensorial properties have been measured. For this reason, we validated our method by comparing structural, electronic, and vibrational properties of the other three phases of BaTiO${}_3$ (cubic, tetragonal, and orthorhombic) with available experimental data. The effect of the adopted one-electron Hamiltonian on the considered tensorial properties, beyond the simple local density approximation and the dependence on the electric field frequency of dielectric and photoelastic constants, is explicitly investigated.

The structure and ground state electronic structure of the recently synthesized SrPdO${}_3$ perovskite [A. Galal et al. J. Power Sources 195, 3806 (2010)] have been studied by means of screened hybrid functional and the GW approximation with the inclusion of electron-hole interaction within the test-charge/test-charge scheme. By conducting a structural search based on lattice dynamics and group theoretical method we identify the orthorhombic phase with $P_{nma}$ space group as the most stable crystal structure. The phase transition from the ideal cubic perovskite structure to the $P_{nma}$ one is explained in terms of the simultaneous stabilization of the antiferrodistortive phonon modes $R_4^{+}$ and $M_3^{+}$. Our results indicate that SrPdO${}_3$ exhibits an insulating ground state, substantiated by a GW${}_0$ gap of about 1.1 eV. Spin polarized calculations suggest that SrPdO${}_3$ adopts a low spin state ($t_{2g}^{\uparrow \downarrow \uparrow \downarrow \uparrow \downarrow }{e}_g^0$), and is expected to exhibit spin excitations and spin state crossovers at finite temperature, analogous to the case of 3$d$ isoelectronic LaCoO${}_3$. This would provide another playground for the study of spin state transitions in 4$d$ oxides and an opportunity to design multifunctional materials based on the 4$d$ $P_{nma}$ building block.

We study the topology and geometry of a fermionic model on the honeycomb lattice with spin-dependent hopping which breaks the time-reversal and charge-conjugation symmetries but preserves their composition. We show that in such a case the Zak phases are topological invariants that characterize the semimetallic state at half filling and determine the edge-state structure. As the strength of the spin-dependent hopping varies, the model shows several Lifshitz transitions corresponding to creation and merging of multiple Dirac points. We discuss the possible realization of this model in cold atom systems and propose experimental signals for detecting the Dirac points and Pancharathnam-Berry curvature of the bands.

The interplay between the electronic and structural subsystems has strong implications on the character of collective excitations in cooperative systems. Their detailed understanding can provide important information on the coupling mechanisms and coupling strengths in such systems. With the recent developments in femtosecond time-resolved optical probes, numerous advantages with respect to conventional time-integrated probes have been put forward. Owing to their high dynamic range, high-frequency resolution, fast data acquisition, and an inherent access to phases of coherent excitations, they provide direct access to the interplay between various degrees of freedom. In this paper, we present a detailed analysis of time-resolved optical data on blue bronzes (K${}_{0.3}$MoO${}_3$ and Rb${}_{0.3}$MoO${}_3$), prototype quasi-one-dimensional charge-density wave (CDW) systems. Numerous coherent (Raman active) modes appear upon the phase transition into the CDW state. We analyze the temperature dependence of mode frequencies, their damping times, as well as their oscillator strengths and phases using the time-dependent Ginzburg-Landau model. We demonstrate that these low-temperature modes are a result of linear coupling between the Fermi surface nesting driven modulation of the conduction electron density and the normal-state phonons at the CDW wave vector, and determine their coupling strengths. Moreover, we are able to identify the nature of excitation of these coupled modes, as well as the nature of the probing mechanisms in this type of experiments. We demonstrate that in incommensurate CDW systems, femtosecond optical excitation initially suppresses the electronic density modulation, while the reflectivity changes at frequencies far above the CDW induced gap in the single-particle excitation spectrum are governed by the modulation of interband transitions caused by lattice motion. This approach can be readily extended to more complex systems with spatially modulated ground states.

By an analysis of the local crystallinity based on model potential molecular dynamics simulations we investigated the effect of dissolved hydrogen on the thermally induced recrystallization of nanocrystalline silicon. By using the Kolmogorov-Johnson-Mehl-Avrami theory to analyze the atomistic data, we find that the recrystallization rate decreases exponentially with the hydrogen contamination. At low concentration, the kinetics is moderately affected by the H atoms that tend to migrate to the boundaries increasing their effective interface. At higher H content, we find an increasing number of Si${}_m$H${}_n$ hydrides that affect the crystalline order of the material and severely impede recrystallization. The analysis based on crystallinity is supported by the atomic scale study of the recrystallization mechanism, here identified as an inverted bond-switching process, and by the ability of hydrates to pin the amorphous-crystalline boundaries.

The results of spectroscopic studies of the diffraction anomalies (the so-called resonant Wood anomalies) in spatially periodic hybrid structures based on chalcogenide ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$-coated opal films of various thickness are presented. A theoretical analysis of spectral-angular dependencies of the Wood anomalies has been made by means of a phenomenological approach using the concept of the effective refractive index of the waveguiding surface layer.

We investigate the influence of the Hartree-Fock self-energy, acting as an effective magnetic field, on the anomalous D'yakonov-Perel' spin relaxation in InAs (110) quantum wells when the magnetic field in the Voigt configuration is much stronger than the spin-orbit-coupled field. The transverse and longitudinal spin relaxations are discussed both analytically and numerically. For the transverse configuration, it is found that the spin relaxation is very sensitive to the Hartree-Fock effective magnetic field, which is very different from the conventional D'yakonov-Perel' spin relaxation. Even an extremely small spin polarization ($P=0.1\%$) can significantly influence the behavior of the spin relaxation. It is further revealed that this comes from the unique form of the effective inhomogeneous broadening, originated from the mutually perpendicular spin-orbit-coupled field and strong magnetic field. It is shown that this effective inhomogeneous broadening is very small and hence very sensitive to the Hartree-Fock field. Moreover, we further find that in the spin polarization dependence, the transverse spin relaxation time decreases with the increase of the spin polarization in the intermediate spin polarization regime, which is also very different from the conventional situation, where the spin relaxation is always suppressed by the Hartree-Fock field. It is revealed that this opposite trend comes from the additional spin relaxation channel induced by the HF field. For the longitudinal configuration, we find that the spin relaxation can be either suppressed or enhanced by the Hartree-Fock field if the spin polarization is parallel or antiparallel to the magnetic field.

Two single-particle sources coupled in series to a chiral electronic waveguide can serve as a probabilistic source of two-particle excitations with tunable properties. The second-order correlation function, characterizing the state of emitted electrons in space-time, is expressed in terms of the Floquet scattering matrix of a source. It is shown that the Fourier transform of the correlation function, characterizing the emitted state in energy space, can be accessed with the help of an energy resolved shot-noise measurement. The two-electron state emitted adiabatically is discussed in detail. In particular, the two-electron wave function is represented via two different sets of single-particle wave functions accessible experimentally.

The local structure of SnTe has been studied using atomic pair distribution function analysis of x-ray and neutron data. Evidence is found for a locally distorted high-temperature state, which emerges on warming from an undistorted rocksalt structure. The structural distortion appears rapidly over a relatively narrow temperature range from 300 to 400 K. A similar effect has been reported in PbTe and PbS and dubbed emphanisis; here we report on emphanisis in a compound that does not contain Pb. The analysis suggests that this effect is unrelated to the low-temperature ferroelectric state in SnTe.

Raman scattering spectra of the new Fe-based $S=2$ spin-ladder compound BaFe${}_2$Se${}_2$O are measured in a temperature range between 15 and 623 K. All six $A_{1g}$ and two $B_{1g}$ Raman active modes of BaFe${}_2$Se${}_2$O, predicted by the factor-group analysis, have been experimentally observed at energies that are in a rather good agreement with the lattice dynamics calculation. The antiferromagnetic long-range spin ordering in BaFe${}_2$Se${}_2$O below $T_N=240$ K leaves a fingerprint both in the $A_{1g}$ and $B_{1g}$ phonon mode linewidth and energy. In the energy range between 400 and 650 cm${}^{-1}$ we have observed a magnetic excitation related structure in the form of a magnon continuum, with the peaks corresponding to the singularities in the one-dimensional density of magnon states. The onset value of magnetic continuum (2${\Delta }_S$) is found at about 437 cm${}^{-1}$ at 15 K. The magnetic continuum disappears at about 623 K, which led us to conclude that the short-range magnetic ordering in BaFe${}_2$Se${}_2$O exists apparently up to 2.6$T_N$.

We study the ground-state phase diagram of the Ashkin-Teller random quantum spin chain by means of a generalization of the strong-disorder renormalization group. In addition to the conventional paramagnetic and ferromagnetic (Baxter) phases, we find a partially ordered phase characterized by strong randomness and infinite coupling between the colors. This unusual phase acts, at the same time, as a Griffiths phase for two distinct quantum phase transitions, both of which are of infinite-randomness type. We also investigate the quantum multicritical point that separates the two-phase and three-phase regions, and we discuss generalizations of our results to higher dimensions and other systems.

The influence of the local environment on vacancy and self-interstitial formation energies has been investigated in a face-centered-cubic (fcc) Fe-10Ni-20Cr model alloy by analyzing an extensive set of first-principle calculations based on density functional theory. Chemical disorder has been considered by designing special quasirandom structures and four different collinear magnetic structures have been investigated in order to determine a relevant reference state to perform point defect calculations at 0 K. Two different convergence methods have also been used to characterize the importance of the method on the results. Although our fcc Fe-10Ni-20Cr would be better represented in terms of applications by the paramagnetic state, we found that the antiferromagnetic single-layer magnetic structure was the most stable at 0 K and we chose it as a reference state to determine the point defect properties. Point defects have been introduced in this reference state, i.e., vacancies and Fe-Fe, Fe-Ni, Fe-Cr, Cr-Cr, Ni-Ni, and Ni-Cr dumbbell interstitials oriented either parallel or perpendicular to the single layer antiferromagnetic planes. Each point defect studied was introduced at different lattice sites to consider a sufficient variety of local environments and analyze its influence on the formation energy values. We have estimated the point defect formation energies with linear regressions using variables which describe the local environment surrounding the point defects. The number and the position of Ni and Cr first nearest neighbors to the point defects were found to drive the evolution of the formation energies. In particular, Ni is found to decrease and Cr to increase the vacancy formation energy of the model alloy, while the opposite trends are found for the dumbbell interstitials. This study suggested that, to a first approximation, the first nearest atoms to point defects can provide reliable estimates of point defect formation energies.

Quantitative analysis was done of the x-ray photoelectron spectra structure in the binding energy (BE) range of 0 to ∼35 eV for neptunium dioxide (NpO${}_2$) valence electrons. The BEs and structure of the core electronic shells (∼35–1250 eV) as well as the relativistic discrete variation calculation results for the finite fragment of the NpO${}_2$ lattice and the data of other authors were taken into account. The experimental data show that the many-body effects and the multiplet splitting contribute to the spectral structure much less than the effects of formation of the outer (0–∼15 eV) and the inner (∼15–∼35 eV) valence molecular orbitals (OVMO and IVMO, respectively). The filled Np 5$f$ electronic states were shown to form in the NpO${}_2$ valence band. The Np 6$p$ electrons participate in formation of both the IVMO and the OVMO (bands). The filled Np 6$p_{3/2}$ and the O 2$s$ electronic shells were found to take the maximum part in the IVMO formation. The MO composition and the sequence order in the BE range 0–∼35 eV in NpO${}_2$ were established. The experimental and theoretical data allowed a quantitative MO scheme for NpO${}_2$, which is fundamental for both understanding the chemical bond nature in neptunium dioxide and the interpretation of other x-ray spectra of NpO${}_2$.

The polaron optical conductivity at zero temperature is derived within the strong-coupling approximation accounting for internal nonadiabaticity. The polaron optical conductivity spectrum is provided by the multiphonon optical transitions. The polaron optical conductivity spectra calculated within our analytic strong-coupling approach and the numerically accurate diagrammatic quantum Monte Carlo (DQMC) data are in a good agreement with each other at large $\alpha \gtrsim 9$.

In this paper the photon-assisted electron motion in a multiquantum well (MQW) semiconductor heterostructure in the presence of an electric field is investigated. The time-dependent Schrödinger equation is solved by using the split-operator technique to determine the photocurrent generated by the electron movement through the biased MQW system. An analysis of the energy shifts in the photocurrent spectra reveals interesting features coming from the contributions of localized and extended states on the MQW system. The photocurrent signal is found to increase for certain values of electric field, leading to the analog of the negative conductance in resonant tunneling diodes. The origin of this enhancement is traced to the mixing of localized states in the QWs with those in the continuum. This mixing appears as anticrossings between the localized and extended states and the enhanced photocurrent can be related to the dynamically induced Landau-Zener-Stückelberg-Majorana transition between two levels at the anticrossing.

We report on the doping-induced antiferromagnetic state and Fermi-liquid state that are connected by a superconducting region in a series of CeIrIn${}_{5-x}$Hg${}_x$, CeIrIn${}_{5-x}$Sn${}_x$, and CeIr${}_{1-x}$Pt${}_x$In${}_5$ single crystals. Measurements of the specific heat $C\left(T\right)$ and electrical resistivity $\rho \left(T\right)$ demonstrate that hole doping via Hg/In substitution gives rise to an antiferromagnetic ground state, but substitutions of In by Sn or Ir by Pt (electron doping) favor a paramagnetic Fermi-liquid state. A conelike non-Fermi-liquid region is observed near CeIrIn${}_5$, showing a diverging effective mass on the slightly Hg-doped side. The obtained temperature-doping phase diagram suggests that CeIrIn${}_5$ is in proximity to an antiferromagnetic quantum critical point, and heavy fermion superconductivity in this compound is mediated by magnetic quantum fluctuations rather than by valence fluctuations.

The nature of the pseudogap (PG) in high transition temperature (high $T_c$) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high $T_c$ superconductivity can be universally associated with the PG formation. Here we provide direct evidence of the existence of the PG phase via angle-resolved photoemission spectroscopy in another family of high $T_c$ superconductor, iron pnictides. Our results reveal a composition-dependent PG formation in the multiband electronic structure of BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. The PG develops well above the magnetostructural transition for low $x$, persists above the nonmagnetic superconducting dome for optimal $x$, and is destroyed for $x$ ∼ 0.6, thus showing a notable similarity with cuprates. In addition, the PG formation is accompanied by inequivalent energy shifts in the $zx/yz$ orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the fourfold rotational symmetry.

We elucidate the model parameters for a series of organic crystals called $\kappa$- and ${\beta }^{\prime }$-type salts by constructing the maximally localized Wannier orbitals which reproduce the bulk energy band of the first-principles methods based on the density functional theory (DFT). These materials host a dimer Mott insulator, localizing one hole per dimerized ET molecules due to strong on-dimer interaction, $U_{\mathrm{d}}$. For all these materials, we evaluate the parameters of the two representative effective lattice models in units of molecule and on dimer, and clarify two issues. First, the conventional relationships between the two models called “dimer approximation” does not hold. Second, contrary to the previous semiempirical estimates, the degree of dimerization (which approximates $U_{\mathrm{d}}$) does not depend much on materials, and that the overall ground state properties are controlled by the degree of anisotropy of the triangular lattice, denoted as $|t_c/t_a|$ in units of dimers. We update the DFT estimates $|t_c/t_a|$ of $\kappa$-ET${}_2$Cu${}_2$(CN)${}_3$, showing that it falls on a class of regular triangle with the strongest degree of frustration.