We study the electronic structure of well-characterized epitaxial films of FeO (wustite), $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ (maghemite), and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (magnetite) using hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption near-edge spectroscopy (XANES), and electron energy loss spectroscopy (EELS). We carry out HAXPES with incident photon energies of 12 and 15 keV in order to probe the bulk-sensitive Fe $1s$ and Fe $2p$ core level spectra. Fe $K$-edge XANES is used to characterize and confirm the Fe valence states of FeO, $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$, and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films. EELS is used to identify the bulk plasmon loss features. A comparison of HAXPES results with model calculations for an $M{\mathrm{O}}_6$ cluster provides us with microscopic electronic structure parameters such as the onsite Coulomb energy $U_{dd}$, the charge-transfer energy $\mathrm{\Delta }$, and the metal-ligand hybridization strength $V$. The results also provide estimates for the ground-state and final-state contributions in terms of the $d^n, d^{n+1}{\underline{L}}^1$, and $d^{n+2}{\underline{L}}^2$ configurations. Both FeO and $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with $U_{dd}>\mathrm{\Delta }$, consistent with earlier work. However, the $M{\mathrm{O}}_6$ cluster calculations do not reproduce an extra satellite observed in Fe $1s$ spectra of $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Based on simplified calculations using an $M_2{\mathrm{O}}_7$ cluster with renormalized parameters, it is suggested that nonlocal screening plays an important role in explaining the two satellites observed in the Fe $1s$ core level HAXPES spectra of $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4$.

In this work, the difficulties inherent to perturbative calculations in the velocity gauge are addressed. In particular, it is shown how calculations of nonlinear optical responses in the independent particle approximation can be done to any order and for any finite band model. The procedure and advantages of the velocity gauge in such calculations are described. The addition of a phenomenological relaxation parameter is also discussed. As an illustration, the nonlinear optical response of monolayer graphene is numerically calculated using the velocity gauge.

The electronic gap of bulk NaCl and the band-edge levels of the NaCl(100) surface are calculated using GW methods and self-consistent dielectric dependent hybrid functionals. Optical spectra are calculated by solving the Bethe-Salpeter equations (GW-BSE). For the calculation of the electronic structure of the surface, a procedure based on dielectric dependent range-separated hybrid (sc-DDRSH) functionals is proposed where the $\omega$ parameter is evaluated self-consistently from the surface polarizability tensor. The resulting band-edge energies deviate less than 0.1 eV from the GW results. Both GW and sc-DDRSH results closely match the most recent experimental values of the valence band maximum and the conduction band minimum. The GW-BSE results obtained for the NaCl(100) surface indicate that there are a small number of surface states 0.4–0.5 eV below the first absorption maximum at 7.6–7.7 eV. The calculated optical band gaps of the bulk are 7.7–7.9 eV, close to the range of experimental values.

We report time domain observation of vacuum Rabi oscillations in a single quantum dot strongly coupled to a nanocavity under incoherent optical carrier injection. We realize a photonic crystal nanocavity with a very high quality factor of $>80000$ and employ it to clearly resolve the ultrafast vacuum Rabi oscillations by simple photoluminescence-based experiments. We found that the time domain vacuum Rabi oscillations were largely modified when changing the pump wavelength and intensity, even when marginal changes were detected in the corresponding photoluminescence spectra. We analyze the measured time domain oscillations by fitting to simulation curves obtained with a cavity quantum electrodynamics model. The observed modifications of the oscillation curves were mainly induced by the change in the carrier capture and dephasing dynamics in the quantum dot, as well as the change in bare-cavity emission. This result suggests that vacuum Rabi oscillations can be utilized as a highly sensitive probe for the quantum-dot dynamics. Our work points out a powerful alternative to conventional spectral-domain measurements for a deeper understanding of the vacuum Rabi dynamics in quantum-dot-based–cavity quantum electrodynamics systems.

We examine the influence of the superconducting proximity effect on the transport properties of a T-shaped double quantum dot strongly coupled to two normal, nonmagnetic or ferromagnetic leads. We show that the two-stage Kondo screening may be suppressed or enhanced by the presence of pairing correlations, depending on the specific geometric arrangement of the device. We explain our results by invoking an effective decrease of Coulomb interactions by the proximity effect and find a qualitatively correct description in many cases, although the spin-filtering effect stemming from spin-dependent Fano-Kondo interference is surprisingly vulnerable to the presence of induced superconducting pairing correlations. The results are obtained within the numerical renormalization group framework in the limit of large superconducting gap, which allows for a reliable examination of the low-temperature subgap properties of the considered system. Nevertheless, finite-temperature effects are also taken into account.

Interactions between the surface and the bulk in a strong topological insulator (TI) cause a finite lifetime of the topological surface states (TSSs) as shown by the recent experiments. On the other hand, interactions also induce unitary processes, which, in the presence of anisotropy and the spin-orbit coupling (SOC), can induce nontrivial effects on the spin texture. Recently, such effects were observed experimentally in the ${\text{Bi}}_2{\text{X}}_3$ family, raising the question that the hexagonal warping (HW) may be linked with new spin-related anomalies. The most remarkable is the sixfold periodic canting of the in-plane spin vector. Here we show that this anomaly is the result of a triple cooperation between the interactions, the SOC, and the HW. To demonstrate it, we formulate the spin-off-diagonal self-energy. A unitary phase with an even symmetry develops in the latter and modulates the spin-1/2 vortex when the Fermi surface is anisotropic. When the anisotropy is provided by the HW, a sixfold in-plane spin canting is observed. Our theory suggests that the spin-canting anomaly in ${\text{Bi}}_2{\text{X}}_3$ is a strong evidence of the interactions.

Making use of infinite projected entangled pair states, we investigate the ground-state phase diagram of the nearest-neighbor spin-1 bilinear-biquadratic Heisenberg model on the triangular lattice. In agreement with previous studies, we find the ferromagnetic, ${120}^{\circ }$ magnetically ordered, ferroquadrupolar and antiferroquadrupolar phases, and confirm that all corresponding phase transitions are first order. Moreover, we provide an accurate estimate of the location of the ferroquadrupolar to ${120}^{\circ }$ magnetically ordered phase transition, thereby fully establishing the phase diagram. Also, we do not encounter any signs of the existence of a quantum paramagnetic phase. In particular, contrary to the equivalent square lattice model, we demonstrate that on the triangular lattice the one-dimensional Haldane phase does not reach all the way up to the two-dimensional limit. Finally, we investigate the possibility of an intermediate partially magnetic partially quadrupolar phase close to $\theta =\pi /2$, and we show that, also contrary to the square lattice case, this phase is not present on the triangular lattice.

The physical properties of several compositions of ${\mathrm{Ge}}_x{\mathrm{Ga}}_x{\mathrm{Te}}_{100-2x}$ liquids ($x=6$, 10, 14%) are studied using a combination of density functional based molecular dynamics simulations and neutron experiments. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions and compare the results with diffraction experiments. We find that Ga is essentially in tetrahedral configuration and also affects the tetrahedral character of germanium when the results are compared to similar telluride liquids, while Te atoms display a coordination number that is larger than 2 and depend substantially on composition. The vibrational density of states is also measured from inelastic neutron scattering and compared to the simulated counterpart which exhibits a very good agreement at low frequency. We finally determine the dynamics of the melts by evaluating the diffusivity and relaxation properties. These reveal that Ge-Ga-Te melts are extremely fragile ($\mathcal{M}\simeq 139$), similarly to Ge-Sb-Te liquids.

Coupling between optical phonons and light leads to the formation of phonon polaritons, resulting in important changes to the material dispersion. Commonly, the phonon-polariton dispersion curve has been determined by transient-grating impulsive stimulated Raman-scattering measurements, but reported results for the prototypical dispersion curve of $\mathrm{LiNb}{\mathrm{O}}_3$ have led to some controversy. We present a procedure to measure the phonon-polariton dispersion curve by varying the pump wavelength with fixed probe wavelength that allows measurement over a broad range of wave vectors. We determine the lowest-frequency $E$-mode phonon-polariton dispersion curve for $\mathrm{LiNb}{\mathrm{O}}_3$ with measured wave vectors ranging from $1\times {10}^3\mathrm{c}{\mathrm{m}}^{-1}$ to $19\times {10}^3\mathrm{c}{\mathrm{m}}^{-1}$, which is the broadest known optically measured phonon-polariton dispersion curve for $\mathrm{LiNb}{\mathrm{O}}_3$. The broad range offers improved understanding of the extracted material parameters. We also note some challenges that occur in interpreting the data at short pump wavelengths; these effects help explain controversial differences between previous transient-grating measurements of $\mathrm{LiNb}{\mathrm{O}}_3$ dispersion curves.

We study the effect of coupling a spin-bath environment to a system which, at low energies, can be modeled as a quantum Ising system. A field theoretic formalism incorporating both thermal and quantum fluctuations is developed to derive results for the thermodynamic properties and response functions, both for a toy model and for the ${\text{LiHoF}}_4$ system, in which spin-8 electronic spins couple to a spin-$7/2$ nuclear spin bath: The phase transition then occurs in a system of electronuclear degrees of freedom, coupled by long-range dipolar interactions. The quantum Ising phase transition still exists, and one hybridized mode of the Ising and bath spins always goes soft at the transition.

We present our calculation results for organic magnetic electrides. In order to identify the “cavity” electrons, we use maximally localized Wannier functions and the “empty atom” technique. The estimation of magnetic coupling is then performed based on magnetic force linear response theory. Both short- and long-range magnetic interactions are calculated with a single self-consistent calculation of a primitive cell. With this scheme we investigate four different organic electrides whose magnetic properties have been partly unknown or under debate. Our calculation results unveil the nature of magnetic moment and their interactions, and justify or defy the validity of preassumed spin models. Our work not only provides useful insight to understand magnetic electrides but also suggests a paradigm to study the related materials.

Weakly disordered superconducting films can be driven into an anomalous low-temperature resistive state upon applying a magnetic field. Recent experiments on weakly disordered amorphous ${\mathrm{InO}}_x$ have established that both the Hall resistivity and the frequency of a cyclotronlike resonance in the anomalous metal are highly suppressed relative to the values expected for a conventional metal. We show that both of these observations can be understood from the flux flow dynamics of vortices in a superconductor with significant vortex pinning. Results for flux flow transport are obtained using a systematic hydrodynamic expansion, controlled by the diluteness of mobile vortices at low temperatures. Hydrodynamic transport coefficients are related to microscopics through Kubo formulas for the longitudinal and Hall vortex conductivities, as well as a “vortoelectric” conductivity.

GeTe is a well-known ferroelectric and thermoelectric material that undergoes a structural phase transition from a rhombohedral to the rocksalt structure at $\sim 600–700$ K. We present a first-principles approach to calculate the thermal expansion of GeTe in the rhombohedral phase up to the Curie temperature. We find the minimum of the Helmholtz free energy with respect to the structural parameters, including the internal atomic displacement, in a manner similar to the traditional Grüneisen theory, explicitly accounting for the variation of the static elastic energy with respect to all structural parameters. We obtain the temperature dependence of the structural parameters of rhombohedral GeTe in very good agreement with experiments. In particular, we correctly reproduce a negative volumetric thermal expansion of GeTe near the phase transition. We show that the negative thermal expansion is induced by the coupling between acoustic and soft transverse optical phonons, which is also responsible for the low lattice thermal conductivity of GeTe.

Temperature-dependent synchrotron x-ray diffraction and extended x-ray absorption fine structure (EXAFS) studies were performed to understand the role of structural characteristics in driving the magnetoelectric multiferroic properties of ${\mathrm{GdCrO}}_3$. The results suggest that the distortion in the structure appears to be associated with the off-center displacement of Gd atoms together with octahedral rotations via displacement of the oxygen ions in ${\mathrm{GdCrO}}_3$. Further, a comparative EXAFS study of ${\mathrm{GdCrO}}_3$ with a similar system, ${\mathrm{YCrO}}_3$, suggests that the oxygen environment of Gd in ${\mathrm{GdCrO}}_3$ is different from that of Y in ${\mathrm{YCrO}}_3$, which results in an orthorhombic $Pna{2}_1$ structure in ${\mathrm{GdCrO}}_3$ in contrast to the monoclinic $P{2}_1$ structure in ${\mathrm{YCrO}}_3$.

The exact solution of a diffusion-reaction model for the trapping and annihilation of positrons in small extended spherical defects (clusters, voids, small precipitates) with competitive rate-limited trapping in vacancy-type point defects is presented. Closed-form expressions are obtained for the mean positron lifetime and for the intensities of the two positron lifetime components associated with trapping at defects. The exact solutions can be conveniently applied for the analysis of experimental data and allow an assessment in how far the usual approach, which takes diffusion limitation into account by means of effective diffusion trapping rates, is appropriate. The model is further extended for application to larger precipitates where diffusion- and reaction-limited trapping is not only considered for the trapping from the matrix into the precipitate-matrix interface, but also for the trapping from inside the precipitates into the interfaces. This makes the model applicable to all types of composite structures where spherical objects are embedded in a matrix, irrespective of their size and their number density.

Because of the presence of robust massless modes with no excitation gap, the heat current carried by phonons tends to survive at low temperatures even when scattering mechanisms are incorporated into the system. This becomes a fundamental obstacle to thermoelectric applications at low temperatures. In this study, we investigate the effect of energy broadening on phonon transport in mesoscopic systems coupled to leads or probes in various geometries using a nonequilibrium Green's function formalism. An analytic theory derived from a minimal model consisting of a harmonic chain shows that geometrically induced broadening sizably suppresses low-temperature phonon transport. It is also demonstrated from numerical calculations that this scheme for phonon blocking is viable for realistic systems in higher dimensions.

We theoretically analyze the spectrum of a magnetic molecule when its charge and spin can couple to the molecular vibrations. More specifically, we show that the interplay between charge-vibron and spin-vibron coupling leads to a renormalization of the magnetic anisotropy parameters of the molecule. This effect is discussed for a model device consisting of an individual magnetic molecule embedded in a junction. We study the transport properties of the device and illustrate how the differential conductance is affected by the vibrationally induced renormalization of the magnetic anisotropy. Depending on the total molecular spin and the bare (intrinsic) magnetic anisotropy, the induced modulation can lead to visible shifts and crossings in the spectrum, and it can even be the cause of a transport blockade. It is therefore of particular interest to use mechanically controllable break junctions, since in such a case, the relevant coupling between the molecular spin and vibrations can be controlled via deformations of the molecule when stretching or compressing the junction.

Using a combination of scanning tunneling spectroscopy and atomic lateral manipulation, we obtained a systematic variation of the Kondo temperature $T_{\mathrm{K}}$ of Co atoms on Ag(111) as a function of the surface-state contribution to the total density of states at the atom adsorption site ${\rho }_s$. By sampling the $T_{\mathrm{K}}$ of a Co atom on positions where ${\rho }_s$ was spatially resolved beforehand, we obtain a nearly linear relationship between the magnitudes. We interpret the data on the basis of an Anderson model including orbital and spin degrees of freedom [SU(4)] in good agreement with the experimental findings. The fact that the onset of the surface band is near the Fermi level is crucial to finding the observed linear behavior. In light of this model, the quantitative analysis of the experimental data evidences that at least a quarter of the coupling of Co impurities with extended states takes place through the hybridization to surface states. This result is of fundamental relevance in the understanding of Kondo screening of magnetic impurities on noble-metal surfaces, where bulk and surface electronic states coexist.

The use of energetic ion beams to induce nanopattern formation at surfaces has been well studied both experimentally and theoretically. However, the influence on morphological evolution of the implanted species themselves remains little understood, particularly in the case when the incident ion species does not interact chemically with the target material. In this work, MD simulation results are presented for cumulative ion bombardment of Si to a fluence of $3\times 10^{15}\mathrm{c}{\mathrm{m}}^{-2}$ or more for a range of incident ion energies (20–1000 eV), angles ($0-{85}^{\circ }$), and species (Ne, Ar, Kr, Xe). For most cases, the implanted ions are observed to form gas clusters or bubbles beneath the surface as the fluence increases. The implantation and cluster formation decrease in magnitude with increasing ion incidence angle, and remain fairly similar for the heavier-than-Si species (Ar, Kr, and Xe). However, the implantation and cluster formation are much more prominent for Ne irradiation. As the fluence continues to increase beyond $\sim 10^{15}\mathrm{c}{\mathrm{m}}^{-2}$, the gas clusters begin to become exposed to the vacuum as the Si layers trapping the gas atoms are eroded by the incident ions. The exposed gas clusters then degas very rapidly, leading to disruption at the surface and to viscous material flow of Si into the void left behind. Comparison to dynamic binary collision approximation (BCA) simulations indicates that cluster formation and degassing contributes to a wide distribution of single-impact emission yields of implanted ions, contrary to intuitive expectations based on BCA simulations. Notably, the increased size and frequency of many-atom implanted ion emission events contributes to a much lower concentration of the implanted species than is otherwise expected from BCA simulations. Additionally, this cluster degassing phenomenon is conjectured to provide a potential “antipatterning” mechanism by disrupting or destroying nanopattern “seeds” at the surface. This could provide an additional mechanism to improve model predictions of critical angles for patterning transitions, and may also provide at least a partial explanation for the difficulty of obtaining patterns on Ne-bombarded Si surfaces.

Controlled growth of metal monatomic wires would enable on-demand tuning of electronic and magnetic properties in this new class of materials. Prior to this work, it was believed that the binding energy can be determined from the distribution of the lengths of the wires. Another misconception was that the antiripening mechanism or quantum effects are responsible for the growth of wires during postdeposition annealing. Combining kinetic Monte Carlo and first-principles density-functional theory calculations, we study the growth of one-dimensional atomic wires on the steps of vicinal surfaces. We show that for a large value of the bond energy, the antiripening mechanism and quantum effects do not affect the length of metal monatomic wires. The conditions under which wires with magic length appear are determined. The observed mechanism of wire growth will be useful both for explaining the experimental data and for creating atomic wires with a given length.