Complex perovskite oxides are promising materials for cathode layers in solid oxide fuel cells. Such materials have intricate electronic, magnetic, and crystalline structures that prove challenging to model accurately. We analyze a wide range of standard density functional theory approaches to modeling a highly promising system, the perovskite ${\text{LaCoO}}_3$, focusing on optimizing the Hubbard $U$ parameter to treat the self-interaction of the B-site cation's $d$ states, in order to determine the most appropriate method to study defect formation and the effect of spin on local structure. By calculating structural and electronic properties for different magnetic states we determine that $U=4$ eV for Co in ${\text{LaCoO}}_3$ agrees best with available experiments. We demonstrate that the generalized gradient approximation ($\mathrm{PBEsol}+U$) is most appropriate for studying structure versus spin state, while the local density approximation ($\mathrm{LDA}+U$) is most appropriate for determining accurate energetics for defect properties.

Axion electrodynamics, first proposed in the context of particle physics, manifests itself in condensed matter physics in the topological field theory description of $3d$ topological insulators and gives rise to magnetoelectric effect, where applying magnetic (electric) field $\mathbf{B}\left(\mathbf{E}\right)$ induces polarization (magnetization) $\mathbf{p}\left(\mathbf{m}\right)$. We use linear response theory to study the associated topological current using the Fu-Kane-Mele model of $3d$ topological insulators in the presence of time-dependent uniform weak magnetic field. By computing the dynamical current susceptibility ${\chi }_{ij}^{{\mathbf{j}}_p{\mathbf{j}}_p}\left(\omega \right)$, we discover from its static limit an ‘order parameter’ of the topological phase transition between weak topological (or ordinary) insulator and strong topological insulator, found to be continuous. The ${\chi }_{ij}^{{\mathbf{j}}_p{\mathbf{j}}_p}\left(\omega \right)$ shows a sign-changing singularity at a critical frequency with suppressed strength in the topological insulating state. Our results can be verified in current noise experiment on $3d$ TI candidate materials for the detection of such topological phase transition.

We study the thermoelectric properties of a double-Weyl fermion system, possibly realized in ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$ and ${\mathrm{SrSi}}_2$, by a semiclassical Boltzmann transport theory. We investigate different relaxation processes including short-range disorder and electron-electron interaction on the thermoelectric transport coefficients. It is found that the anisotropy of the band dispersion for in-plane and out-of-plane momentum directions affects the relaxation time for transport in different directions. The transport also exhibits an interesting directional dependence on the chemical potential and model parameters, differing from a simple isotropic quadratic or linearly dispersing electron gas. By applying a static magnetic field along the linearly dispersing direction, the longitudinal and transverse electrical and thermal magnetoconductivity show a similar dependence on the in-plane cyclotron frequency to the linear dispersing Weyl nodes. By including internode scattering, we find that the chiral anomaly contribution to the thermoelectric coefficients doubles that of a linearly dispersing Weyl node in both the semiclassical and quantum regimes. A magnetic field applied along the quadratically dispersing direction will split the double-Weyl point into two single-Weyl points with the same chirality.

We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent quantum superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.

We experimentally demonstrate how coherent population trapping (CPT) for donor-bound electron spins in GaAs results in autonomous feedback that prepares stabilized states for the spin polarization of nuclei around the electrons. CPT was realized by excitation with two lasers to a bound-exciton state. Transmission studies of the spectral CPT feature on an ensemble of electrons directly reveal the statistical distribution of prepared nuclear-spin states. Tuning the laser driving from blue to red detuned drives a transition from one to two stable states. Our results have importance for ongoing research on schemes for dynamic nuclear-spin polarization, the central spin problem, and control of spin coherence.

We demonstrate the optical addressability of the spin of an individual chromium atom (Cr) embedded in a semiconductor quantum dot. The emission of Cr-doped quantum dots and their evolution in magnetic field reveal a large magnetic anisotropy of the Cr spin induced by local strain. This results in the zero field splitting of the 0, $\pm 1$, and $\pm 2$ Cr spin states and in a thermalization on the magnetic ground states 0 and $\pm 1$. The observed strong spin to strain coupling of Cr is of particular interest for the development of hybrid spin-mechanical devices where coherent mechanical driving of an individual spin in an oscillator is needed. The magneto-optical properties of Cr-doped quantum dots are modeled by a spin Hamiltonian including the sensitivity of the Cr spin to the strain and the influence of the quantum dot symmetry on the carrier-Cr spin coupling.

Light-matter momentum transfer in plasmonic materials is theoretically discussed in the framework of a plasmonic pressure mechanism taking into account nonequilibrium electron dynamics and the thermalization process. We show that our approach explains the experimentally observed relationship between the plasmon-related electromotive force and the absorption and allows one to correctly predict the magnitude of the plasmon drag electromotive force in flat metal films. We extend our theory to metal films with modulated profiles and show that the simple relationship between plasmonic energy and momentum transfer holds at relatively small amplitudes of height modulation and an approximation of laminar electron drift. The theoretical groundwork is laid for further investigations of shape-controlled plasmon drag in nanostructured metal.

We show that an interplay of double exchange and impurity randomness can explain the competition between metal-ferromagnetic and insulating charge ordered states in doped manganites. The double exchange is simplified in the Ising type, whereas the randomness is modeled by the Falicov-Kimball binary distribution. The combined model is considered in a framework of dynamical mean-field theory. Using the Kubo-Greenwood formalism, the transport coefficients are explicitly expressed in terms of single-particle spectral functions. Dividing the system into two sublattices we have pointed out a direct calculation to the checkerboard charge order parameter and the magnetizations. Numerical results show us that the checkerboard charge order can settle inside the ferromagnetic state at low temperature. An insulator-metal transition is also found at the point of the checkerboard charge order-ferromagnetic transition.

We report on a theoretical investigation of the interplay between vacuum fluctuations, Majorana quasiparticles (MQPs), and bound states in the continuum (BICs) by proposing a new venue for qubit storage. BICs emerge due to quantum interference processes as the Fano effect and, since such a mechanism is unbalanced, these states decay as regular into the continuum. Such fingerprints identify BICs in graphene as we have discussed in detail in Phys. Rev. B 92, 245107 (2015) and Phys. Rev. B, 92, 045409 (2015). Here, by considering two semi-infinite Kitaev chains within the topological phase, coupled to a quantum dot (QD) hybridized with leads, we show the emergence of a novel type of BICs, in which MQPs are trapped. As the MQPs of these chains far apart build a delocalized fermion and qubit, we identify that the decay of these BICs is not connected to Fano and it occurs when finite fluctuations are observed in the vacuum composed by electron pairs for this qubit. From the experimental point of view, we also show that vacuum fluctuations can be induced just by changing the chain-dot couplings from symmetric to asymmetric. Hence, we show how to perform the qubit storage within two delocalized BICs of MQPs and to access it when the vacuum fluctuates by means of a complete controllable way in quantum transport experiments.

The Bogoliubov-Born-Green-Kirkwood-Yvon or time-dependent density matrix (TDDM) hierarchy of equations for higher density matrices is truncated at the three-body level in approximating the three-body correlation function by a quadratic form of two-body ones, closing the equations in this way. The procedure is discussed in detail and it is shown in nontrivial model cases that the approximate inclusion of three-body correlation functions is very important to obtain precise results. A small amplitude approximation of this time-dependent nonlinear equation for the two-body correlation function is performed (STDDM*-b) and it is shown that the one-body sector of this generalized nonlinear second random phase approximation (RPA) equation is equivalent to the self-consistent RPA (SCRPA) approach which had been derived previously by different techniques. It is discussed in which way SCRPA also contains the three-body correlations. TDDM and SCRPA are tested versus exactly solvable model cases.

Angle-resolved photoemission spectroscopy has been performed to investigate the Fermi surface (FS) and the electronic structure of misfit cobaltites, by comparing ${\left[{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{O}}_4\right]}_p{\mathrm{CoO}}_2$ with $\left[{\mathrm{Bi}}_2{\mathrm{Ba}}_2{\mathrm{O}}_4\right]{}_{0.50}{\mathrm{CoO}}_2$. In $\left[{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{O}}_4\right]{}_{0.51}{\mathrm{CoO}}_2$, we observed a suppression of spectral weight near the Fermi energy. Nevertheless, the FS can be mapped out by measuring the photoelectron intensity distribution. The FS size changes with temperature. Additionally, results show that the electronic band structure at a higher binding energy was changed by changing the sample temperature. Our results provide insight into the characteristics of the pseudogap state in the misfit-layered cobaltites. The unusual properties are probably related to the local deformations of the incommensurate structure.

A theoretical approach using ab initio calculations was applied to study the interaction of an ultrashort laser pulse with the metal alloy ${\mathrm{Fe}}_{0.72}{\mathrm{Cr}}_{0.18}{\mathrm{Ni}}_{0.1}$ (AISI 304). The electronic structure was simulated by taking into account the chemical and magnetic disorder of the alloy by the coherent potential approximation implemented in a fully relativistic Korringa-Kohn-Rostoker formalism in the framework of spin density functional theory. A self-consistent calculation of the electronic structure using the Matsubara technique in the paramagnetic state of ${\mathrm{Fe}}_{0.72}{\mathrm{Cr}}_{0.18}{\mathrm{Ni}}_{0.1}$ for finite temperatures was applied. Utilizing these predictions we determined the electron heat capacity and the electron-phonon coupling factor of ${\mathrm{Fe}}_{0.72}{\mathrm{Cr}}_{0.18}{\mathrm{Ni}}_{0.1}$ in dependence on the electron temperature for two-temperature model applications. Compared with pure Fe a maximum deviation of 5% for the electron heat capacity and 25% for the electron-phonon coupling factor was found.

Electronic self-interaction poses a fundamental challenge in density-functional theory. It greatly limits, e.g., the physical interpretation of eigenvalues as electron removal energies. We here investigate whether local hybrid functionals that are designed to be free from one-electron self-interaction lead to occupied Kohn-Sham eigenvalues and orbitals that approximate photoemission observables well. We compare the local hybrid results to the ones from global hybrid functionals that only partially counteract the self-interaction, and to the results that are obtained with a Perdew-Zunger-type self-interaction correction. Furthermore, we check whether being nominally free from one-electron self-interaction translates into a reduced many-electron self-interaction error. Our findings show that this is not the case for the local hybrid functionals that we studied: In practice they are similar to global hybrids in many respects, despite being formally superior. This finding indicates that there is a conceptual difference between the Perdew-Zunger way and the local hybrid way of translating the one-electron condition to a many-electron system. We also point out and solve some difficulties that occur when using local hybrid functionals in combination with pseudopotentials.

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole in-filling offer a combination of strong charge confinement, optical efficiency, and high spatial symmetry advantageous for polarization entanglement and spin-photon interface. Here, we study experimentally electron and nuclear spin properties of such dots. We find nearly vanishing electron $g$ factors $\left(g_e<0.05\right)$, providing a potential route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to $65\%$ achieved. Nuclear magnetic resonance spectroscopy reveals two distinct types of quantum dots: with tensile and with compressive strain along the growth axis. In both types of dots, the magnitude of strain ${\epsilon }_b<0.02\%$ is nearly three orders of magnitude smaller than in self-assembled dots: On the one hand, this provides a route for eliminating a major source of electron spin decoherence arising from nuclear quadrupolar interactions, and on the other hand such strain is sufficient to suppress nuclear spin diffusion leading to a stable nuclear spin bath with nuclear spin lifetimes exceeding 500 s. The spin properties revealed in this work make this new type of quantum dot an attractive alternative to self-assembled dots for the applications in quantum information technologies.

We demonstrate that ultrafast carrier excitation can drastically affect electronic structures in nonplasmonic metals and determine a transient, brief surface plasmonic state, potentially creating the conditions for a plasmonic switch. The initial state can be related to $d$-band partial filling and splitting, with a pseudo-band-gap accommodating the chemical potential. This determines a quasi-resonant-like spectral behavior of the optical constants for pumping carriers across the $d$-band pseudogap, i.e., visible frequencies. The relation between real and imaginary parts of the refractive index does not fulfill surface plasmonic conditions in the visible photon range. Using first-principles molecular dynamics and Kubo-Greenwood formalism for laser-excited tungsten we show that carrier heating mobilizes $d$ electrons into collective inter- and intraband transitions leading to a sign flip in the imaginary optical conductivity, activating plasmonic properties for the initial nonplasmonic phase. The drive for the laser-induced optical evolution in this case does not rely on a variation of the free electron number but can be visualized as an increasingly damped character of the quasiresonance at visible frequencies. Here laser heating determines an energy-dependent degree of occupation with broadening profiles. The subsequent evolution of optical indices for the excited material is confirmed by time-resolved ultrafast ellipsometry. The large optical tunability extends the existence spectral domain of surface plasmons in ranges typically claimed in laser self-organized nanostructuring. Nonequilibrium heating is thus a strong factor for engineering optical control of evanescent excitation waves, particularly important in laser nanostructuring strategies.

We investigate the zero-temperature phase diagram of two-orbital $\mathrm{SU}(N$) fermionic models at incommensurate filling which are directly relevant to strontium and ytterbium ultracold atoms loading into a one-dimensional optical lattice. Using a low-energy approach that takes into account explicitly the $\mathrm{SU}(N$) symmetry, we find that a spectral gap for the nuclear-spin degrees of freedom is formed for generic interactions. Several phases with one or two gapless modes are then stabilized which describe the competition between different density instabilities. In stark contrast to the $N=2$ case, no dominant pairing instabilities emerge and the leading superfluid one is rather formed from bound states of $2N$ fermions.

Graphite is a prototypical solid lubricant demanding a thorough understanding of its low-friction behavior. The $E_{2g}$(1) Raman active vibrational mode of graphite is associated with the rigid-layer relative movement of its graphene sheets. Thus, this mode can provide a good means of exploring the low resistance of graphene layers to slip with respect to each other. To take advantage of this fact, the anharmonicity of the $E_{2g}$(1) mode has to be carefully characterized and evaluated since the atomic arrangement of carbon atoms in the ambient condition ABA stacking of graphite evidences potential asymmetry. The calculated one-dimensional energetic profile of the $E_{2g}$(1) mode reveals this local anisotropy around the energy minima and can be microscopically interpreted in terms of electron density interactions. Morse-type potentials accurately fit the energetic profiles at different interlayer separations, and provide simple analytical expressions for evaluating harmonic and anharmonic contributions to the $\mathrm{\Gamma }$-point $E_{2g}$(1) frequency ${\omega }_{E_{2g}\left(1\right)}$ under a perturbative algebraic treatment. We quantify how the anharmonic contribution increases with the available energy ($E$) at zero pressure, and how this contribution decreases as hydrostatic pressure ($p$) or uniaxial stress is applied for a given available energy. The calculated ${\omega }_{E_{2g}\left(1\right)}-p$ and ${\omega }_{E_{2g}\left(1\right)}-E$ trends indicate an increasing (decreasing) of frictional forces in graphite with pressure (temperature). Our conclusions are supported by the good agreement of the calculated frequencies with existing Raman experiments under hydrostatic pressure conditions.

Multiferroic materials have attracted great interest but are rare in nature. In many transition-metal oxides, charge ordering and magnetic ordering coexist, so that a method of engineering charge-ordered materials into ferroelectric materials would lead to a large class of multiferroic materials. We propose a strategy for designing new ferroelectric or even multiferroic materials by inserting a spacing layer into each two layers of charge-ordered materials and artificially making a superlattice. One example of the model demonstrated here is the perovskite ${\left({\mathrm{LaFeO}}_3\right)}_2/{\mathrm{LaTiO}}_3$ (111) superlattice, in which the ${\mathrm{LaTiO}}_3$ layer acts as the donor and the spacing layer, and the ${\mathrm{LaFeO}}_3$ layer is half doped and performs charge ordering. The collaboration of the charge ordering and the spacing layer breaks the space inversion symmetry, resulting in a large ferroelectric polarization. As the charge ordering also leads to a ferrimagnetic structure, ${\left({\mathrm{LaFeO}}_3\right)}_2/{\mathrm{LaTiO}}_3$ is multiferroic. It is expected that this work can encourage the designing and experimental implementation of a large class of multiferroic structures with novel properties.

Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary by up to a factor of 3 for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front-atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single-atom tips, while the intensity drops sharply for multiatom tips. Adsorption of the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that the molecule is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multiatom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111).