We investigate and compare experimental and numerical excitonic spectra of the yellow series in cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) in the Voigt configuration and thus partially extend the results of F. Schweiner et al. [Phys. Rev. B 95, 035202 (2017)], who considered only the Faraday configuration. The main difference between the configurations is given by an additional effective electric field in the Voigt configuration, caused by the motion of the exciton through the magnetic field. This magneto-Stark effect was already postulated in 1961 by E. F. Gross et al. [Sov. Phys. Solid State 3, 221 (1961)] and D. G. Thomas and J. J. Hopfield [Phys. Rev. 124, 657 (1961)]. Group-theoretical considerations show that, most of all, the field significantly increases the number of allowed lines by decreasing the symmetry of the system. This conclusion is supported by both the experimental and numerical data.

The out-of-plane magnetoresistance of a $\mathrm{LaAl}{\mathrm{O}}_3$/$\mathrm{SrTi}{\mathrm{O}}_3$ interface is commonly positive due to significant orbital effect. In this paper, a large negative magnetoresistance is observed at a $\mathrm{LaAl}{\mathrm{O}}_3$ and Fe-doped $\mathrm{SrTi}{\mathrm{O}}_3$ interface upon the application of an out-of-plane magnetic field. Such large negative magnetoresistance is attributed to enhanced two-dimensional weak localization and Kondo effect. We think that these two effects are mainly the consequences of intentional Fe dopants in the substrate, which introduce holes, disorders, and spin scattering centers into the electronically conducting interface, directly modifying the spin-orbit coupling, orbital effect, scale of electrons motion, and local magnetic moments at the $\mathrm{LaAl}{\mathrm{O}}_3$/$\mathrm{SrTi}{\mathrm{O}}_3$ interface. The ${\mathrm{LaAlO}}_3\text{/Fe-doped}{\mathrm{SrTiO}}_3$ interfacial system provides a prospective platform for studying quantum interference between strongly correlated electrons. The substrate doping method may facilitate the application of $\mathrm{LaAl}{\mathrm{O}}_3$/$\mathrm{SrTi}{\mathrm{O}}_3$ interfaces in versatile electronic devices.

We study the dynamical behavior of a non-Hermitian moire superlattice system, which consists of two-coupled Su-Schrieffer-Heeger (SSH) chains with staggered imaginary on-site potentials. The slight difference, which indicates that the in-lattice spacing of nearest-neighbor lattice sites at the top is different from that in the bottom chain, can result in periodical occurrence of the two main spatial regions. We show that two such distinct regions are dimerized and tetramerized, which determine the distinct dynamical behaviors. The Dirac probability can oscillate periodically, increase quadratically, and increase exponentially, which correspond to the unbroken phase, exceptional points, and the broken phase of the tetramerized region. In comparison, the Dirac probability can exhibit high-frequency oscillation in the dimerized region. These phenomena demonstrate the dynamical signature and provide insightful information of the moire pattern in the non-Hermitian regime.

Dielectric layers composed of metal oxides are routinely subjected to external electric fields during the course of normal operation of electronic devices. Many phenomenological theories suggest that electric fields strongly affect the properties and mobilities of defects in oxide films and can even facilitate the creation of new defects. Although defects in metal oxides have been studied extensively both experimentally and theoretically, the effect of applied electric fields on their structure and migration barriers is not well understood and still remains subject to speculations. Here, we investigate how static, homogeneous electric fields affect migration barriers of canonical defects—oxygen vacancies and interstitial ions—in a prototypical oxide, MgO. Using the modern theory of polarization within density functional theory (DFT), we apply electric fields to defect migration pathways in three different charge states. The effect of the field is characterized by the change of the dipole moment of the system along the migration pathway. The largest changes in the calculated barriers are observed for charged defects, while those for the neutral defects are barely significant. We show that by multiplying the dipole moment difference between the initial and the transition states, which we define as the effective dipole moment, by the field strength, one can obtain an estimate of the barrier change in excellent agreement with the DFT calculated values. These results will help to assess the applicability of phenomenological models and elucidate linear and nonlinear effects of field application in degradation of microelectronic devices, electrocatalysis, batteries, and other applications.

We present the implementation of dynamical mean-field theory (DMFT) in the CASTEP ab initio code. We explain in detail the theoretical framework for DFT+DMFT and we demonstrate our implementation for three strongly-correlated systems with $f$-shell electrons: $\gamma$-cerium, cerium sesquioxide ${\mathrm{Ce}}_2{\mathrm{O}}_3$, and samarium telluride SmTe by using a Hubbard I solver. We find very good agreement with previous benchmark DFT+DMFT calculations of cerium compounds, while for SmTe we show the improved agreement with the experimental structural parameters as compared with LDA. Our implementation works equally well for both norm-conserving and ultrasoft pseudopotentials, and we apply it to the calculation of total energy, bulk modulus, equilibrium volumes, and internal forces in the two cerium compounds. In ${\mathrm{Ce}}_2{\mathrm{O}}_3$ we report a dramatic reduction of the internal forces acting on coordinates not constrained by unit cell symmetries. This reduction is induced by the many-body effects, which can only be captured at the DMFT level. In addition, we derive an alternative form for treating the high-frequency tails of the Green function in Matsubara frequency summations. Our treatment allows a reduction in the bias when calculating the correlation energies and occupation matrices to high precision.

Epitaxial strain offers an effective route to tune the physical parameters in transition metal oxides. So far, most studies have focused on the effects of strain on the bandwidths and crystal field splitting, but recent experimental and theoretical works have shown that also the effective Coulomb interaction changes upon structural modifications. This effect is expected to be of paramount importance in current material engineering studies based on epitaxy-based material synthesization. Here, we perform constrained random phase approximation calculations for prototypical oxides with a different occupation of the $d$ shell, ${\mathrm{LaTiO}}_3$ ($d^1$), ${\mathrm{LaVO}}_3$ ($d^2$), and ${\mathrm{LaCrO}}_3$ ($d^3$), and systematically study the evolution of the effective Coulomb interactions (Hubbard $U$ and Hund's $J$) when applying epitaxial strain. Surprisingly, we find that the response upon strain is strongly dependent on the material. For ${\mathrm{LaTiO}}_3$, the interaction parameters are determined by the degree of localization of the orbitals, and grow with increasing tensile strain. In contrast, ${\mathrm{LaCrO}}_3$ shows the opposite trend: the interaction parameters shrink upon tensile strain. This is caused by the enhanced screening due to the larger electron filling. ${\mathrm{LaVO}}_3$ shows an intermediate behavior.

We report the room-temperature dielectric function (DF) of GaPN grown lattice-matched on Si(100). Data were derived from spectroscopic ellipsometry measurements on a series of films prepared by metalorganic vapor phase epitaxy. The Kramers-Kronig analysis reveals good self-consistency between the real and the imaginary parts of the determined DF. The onset of a strong absorption in GaPN is clearly redshifted with respect to the direct band gap of GaP. In contrast, direct optical transitions at the $E_0$ and $E_1$ critical points (CPs) in GaPN are remarkably blueshifted. The experimental data near the absorption edge are analyzed using the k · p model of the valence-band structure and the experimentally motivated band anticrossing model to describe the conduction band. We found that the resulting Kane's matrix element for the lower conduction subband is quite small ($E_{\mathrm{P},-}=0.64\mathrm{eV}$), which indicates a relatively low probability of the respective direct optical transitions compared to typical direct band-gap semiconductors, such as GaAs, InP, or GaN. In addition, a considerable contribution of phonon-assisted optical absorption processes occurs. These observations are interpreted in terms of quasilocalization of nitrogen-induced electronic states responsible for optical transitions below the $E_0$ CP and their significant vibrational coupling to the lattice.

One of the most fascinating ground states of an interacting electron system is the so-called Wigner crystal where the electrons, in order to minimize their repulsive Coulomb energy, form an ordered array. Here, we report measurements of the critical filling factor (${\nu }_C$) below which a magnetic-field-induced, quantum Wigner crystal forms in a dilute, two-dimensional electron layer when a second, high-density electron layer is present in close proximity. The data reveal that the Wigner crystal forms at a significantly smaller ${\nu }_C$ compared to the ${\nu }_C$ ($\simeq 0.20$) in single-layer two-dimensional electron systems. The measured ${\nu }_C$ exhibits a strong dependence on the interlayer distance, reflecting the interaction and screening from the adjacent, high-density layer.

We present a transparent and computationally efficient approach for the first-principles calculation of Hubbard parameters from linear-response theory. This approach is based on density-functional perturbation theory and the use of monochromatic perturbations. In addition to delivering much improved efficiency, the present approach makes it straightforward to calculate automatically these Hubbard parameters for any given system, with tight numerical control on convergence and precision. The effectiveness of the method is showcased in three case studies—${\mathrm{Cu}}_2\mathrm{O}$, NiO, and ${\mathrm{LiCoO}}_2$—and by the direct comparison with finite differences in supercell calculations.

Multimode waveguiding in the visible and near-ultraviolet spectral regime is observed and characterized in thermally grown ${\mathrm{SiO}}_2$ layers on silicon using photoemission electron microscopy (PEEM). Comparison with finite-element-method simulations allows identifying order and character of the attenuated modes. Real-time investigations on mode propagation support these findings and give additional evidence for the existence of radiative modes. Finally, the presented experimental results illustrate how a defined deposition of gold nanoparticles can substantially enhance the sensitivity of the PEEM technique to electromagnetic field modes supported by thin dielectric and insulating layers.

We consider a system that is effectively a quantum particle on a one-dimensional ring for which the points $\varphi$ and $\varphi +2\pi$ are indistinguishable. We show that interactions with another particle on a neighboring ring can modify the configuration space and make the points $\varphi$ and $\varphi +2\pi$ distinguishable. As a consequence, the orbital motion acquires a periodicity of $2\pi n$ with $n>1$ which leads to changes in the energy spectrum and in all observable properties. In particular, the fundamental magnetic flux period ${\mathrm{\Phi }}_0=h/q$ of the Aharonov-Bohm effect is reduced to ${\mathrm{\Phi }}_0/n$.

We present a study of the static magnetic properties and spin dynamics in cobalt valence tautomers (VT), molecules where a low spin (LS) to high spin (HS) crossover driven by an intramolecular electron transfer can be controlled by the temperature, by the external pressure, or by light irradiation. In the investigated complex, a LS-Co(III) ion bound to a dinegative organic ligand can be reversibly converted into the HS-Co(II) bound to a mononegative one. By combining magnetization measurements with nuclear magnetic resonance (NMR) and muon spin relaxation ($\mu \mathrm{SR}$), we have investigated the static magnetic properties and the spin dynamics as a function of the temperature. Moreover, the effect of the external pressure as well as of the infrared light irradiation has been explored through magnetometry and NMR measurements to determine the spin dynamics of the HS state. The photoinduced HS state, which can have a lifetime of several hours below 30 K, is characterized by spin dynamics in the MHz range, which persist at least down to 10 K. The application of an external pressure causes a progressive increase of the LS-HS crossover, which reaches room temperature for pressures around 10 kbar.

One of the salient signatures of Majorana zero modes and topological superconductivity is a $4\pi$-periodic Josephson effect due to the combination of fermion parity conservation and the presence of a topologically protected odd number of zero-energy crossings in the Andreev spectrum. In this paper, we study this effect in Josephson junctions composed of two semiconducting nanowires with Rashba spin-orbit coupling and induced superconductivity from the proximity effect. For certain orientations of the external magnetic field, such junctions possess a chiral symmetry, and we show how this symmetry allows the Andreev spectrum and the protected crossings to be shifted by introducing a relative angle between the two wires. The junction then displays a geometrically induced anomalous Josephson effect, the flow of a supercurrent in the absence of external phase bias. Furthermore, we derive a proportionality relation between the local current density and the local curvature for a single curved wire. This result can be viewed as a one-dimensional analog of the recently proposed geo-Josephson effect [T. Kvorning et al., Phys. Rev. Lett. 120, 217002 (2018)]. Our two proposed effects can, in principle, be used as signatures of topological superconductivity in one dimension.

Numerous two-dimensional superconductors exhibit a characteristic dome-shaped dependence of the critical temperature $T_{\mathrm{c}}$ on the carrier density. The reduction of the critical temperature with increasing charge carrier density on the overdoped side of this dome is frequently related to scattering arising from conduction in multiple bands. Multiband superconductivity can be resolved by tunneling spectroscopy, but the interpretation of spectroscopic data can be ambiguous. Superconductivity in multiple bands is also expected to affect the temperature dependence of the upper critical field $H_{\mathrm{c}2}$ of the superconductor. We have therefore measured the temperature dependence of $H_{\mathrm{c}2}$ across the dome of the ${\mathrm{LaAlO}}_3\text{−}{\mathrm{SrTiO}}_3$ interface superconductor and found $H_{\mathrm{c}2}(T$) to be linear across the entire dome. This result places constraints on scenarios of multiband pairing in this interface superconductor.

Antiferromagnetic materials present us with rich and exciting physics, which we can exploit to open new avenues in spintronic device applications. We explore perpendicularly magnetized exchange biased systems of Pt/Co/IrMn and Pt/Co/FeMn, where the crossover from paramagnetic to antiferromagnetic behavior in the IrMn and FeMn layers is accessed by varying the thickness. We demonstrate, through magneto-optical imaging, that the magnetic domain morphology of the ferromagnetic Co layer is influenced by the Néel order of the antiferromagnet (AFM) layers. We relate these variations to the anisotropy energy of the AFM layer and the ferromagnet-antiferromagnet (FM-AFM) interlayer exchange coupling. We also quantify the interfacial Dzyaloshinskii-Moriya interaction (DMI) in these systems by Brillouin light scattering spectroscopy. The DMI remains unchanged, within experimental uncertainty, for different phases of the AFM layers, which allows us to conclude that the DMI is largely insensitive to both AFM layer spin order and exchange bias. Understanding such fundamental mechanisms is crucial for the development of future devices employing chiral spin textures, such as Néel domain walls and skyrmions, in FM-AFM heterostructures.

We report on the electronic structure and magnetic properties of hybrid heterostructures of the ferromagnetic double perovskite ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ (SCRO) and ferroelectric ${\mathrm{BaTiO}}_3$ (BTO) calculated in the GGA approach using the fully relativistic spin-polarized Dirac LMTO method. The electronic band structure and optical properties are studied in the BTO and SCRO oxides as well as in SCRO/BTO heterostructures with different supercells: SCRO/BTO monolayered $(1\times 1\times 1)$, $(2\times 2\times 1),$ and $(1\times 1\times 2)$ heterostructures. We investigated theoretically the magnetization, spin and orbital magnetic moments as well as magnetocrystalline anisotropy energy (MAE) in the SCRO on the BTO substrate as a function of the temperature assuming that the finite temperature can be mimicked by the experimental lattice constants corresponding to this temperature. We found that the spin and orbital magnetic moments are monotonic, almost linear functions of temperature but change abruptly at the points of the BTO structural phase transitions. The magnetization $M$ also possesses “jumps” at the structural phase transitions in agreement with the experiment. However, the theoretically predicted jumps in the total magnetic moment are much smaller than the experimentally measured ones. We found that the Cr/Re disorder increases the jump in the magnetic moment by an order of magnitude compared to the calculated data without disorder. The microscopic origin of such huge effect is discussed. The theoretically calculated temperature dependence of the total magnetic moment with 25% of the Cr/Re disorder is in excellent agreement with the critical amplitude, obtained from the fitting of the experimentally measured magnetization $M\left(T\right)$. From the theoretically calculated MAE we found that the easy axis of magnetization for the tetragonal and orthorhombic phases is along the $\langle 001\rangle$ direction in agreement with the experiment. However, it changes to the $\langle 11\overline{1}\rangle$ direction in the rhombohedral phase. The intersite Cr/Re disorder reduces the MAE for the tetragonal and orthorhombic phases and increases it for the rhombohedral phase. We found that the major contribution to the MAE is due to the orbital magnetic anisotropy at the Re site. We also found that the MAE in the monolayered SCRO/BTO heterostructure is significantly increased in comparison with the MAE in SCRO on BTO substrate. The element-specific x-ray absorption spectra as well as the x-ray magnetic circular dichroism at the Ti, Ba, and Re $L_{2,3}$ edges in SCRO/BTO heterostructures are investigated theoretically from first principles and compared with available experimental spectra.

Problems of finite-temperature quantum statistical mechanics can be formulated in terms of imaginary (Euclidean) -time Green's functions and self-energies. In the context of realistic Hamiltonians, the large energy scale of the Hamiltonian (as compared to temperature) necessitates a very precise representation of these functions. In this paper, we explore the representation of Green's functions and self-energies in terms of a series of Chebyshev polynomials. We show that many operations, including convolutions, Fourier transforms, and the solution of the Dyson equation, can straightforwardly be expressed in terms of the series expansion coefficients. We then compare the accuracy of the Chebyshev representation for realistic systems with the uniform-power grid representation, which is most commonly used in this context.

Building on a previously introduced block Lanczos method, we demonstrate how to approximate any operator function of the form $\mathsf{Tr}f\left(A\right)$ when the argument $A$ is given as a Hermitian matrix product operator (MPO). This gives access to quantities that, depending on the full spectrum, are difficult to access for standard tensor network techniques, such as the von Neumann entropy and the trace norm of an MPO. We present a modified, more efficient strategy for computing thermal properties of short- or long-range Hamiltonians, and we illustrate the performance of the method with numerical results for the thermal equilibrium states of the Lipkin-Meshkov-Glick and Ising Hamiltonians.

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the buildup of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes.

We study locally gated silicene nanoribbons as spin active devices and we solve the quantum scattering problem in the atomistic tight-binding formalism. Particular attention is paid to the low energy range for which only four subbands appear at the Fermi level. We find that the gated segments of zigzag nanoribbons can be used for spin inversion. The strong intrinsic spin-orbit coupling in the presence of an external vertical electric field provides a fast spin precession around the axis perpendicular to the silicene plane. The spin inversion length can be as small as 10 nm. On the other hand, in the armchair nanoribbons the spin inversion occurs via the Rashba effect which is weak and the spin inversion lengths are of the order of microns.