This is a Mössbauer study of wüstite at pressures above 200 GPa using synchrotron Mössbauer spectroscopy. Synthetic $\mathrm{F}{\mathrm{e}}_{0.96}\mathrm{O}$-wüstite was investigated at 91(2), 95(4), 109(2), 114.5(3), 131.1(7), 133.2(2), 155(2), 167(2), 193(2) and 203(1) GPa at 300 K at the SPring-8 BL11XU beamline. The Mössbauer spectrum at 91 GPa consists of both magnetic and nonmagnetic components. The magnetic high-spin component decreases gradually with increasing pressure from 91 to 203 GPa, while the nonmagnetic low-spin component increases with pressure in the same pressure range. The result suggests that the spin state of Fe in the outer core at pressures above 203 GPa is the low-spin state. If oxygen exists in the core, the low-spin Fe-O bonding is shorter than high-spin Fe-O bonding, suggesting dense Fe-O liquid in the Earth's outer core. The gradual increase of the density of the metallic liquid with depth by the spin transition of Fe-O bonding in the shallow outer core region will stabilize the outer core against thermal convection.

The carrier dynamics of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ is studied using the femtosecond pump-probe technique. Three distinct processes, including free carrier absorption, band filling, and electron-hole recombination, are found to contribute to the reflectivity changes. The two-temperature model is used to describe the intraband energy relaxation process of carriers, and the Drude contribution well explains the intensity dependence of the peak values of the nonoscillatory component in the reflectivity signal. The combined effects of free carrier absorption and band filling result in a reflection minimum at about 2 ps after laser excitation. The nonzero background signal increases linearly with the pump fluence, which is attributed to the electron-hole recombination. Finally, our results provide an illustration of investigating the carrier dynamics in semiconductors from the ultrafast reflectivity spectra.

We derive a detailed analytical tight-binding (TB) model for a double helix emulating DNA with one type of nucleotide pair and a single oriented $\pi$ orbital per base. The TB model incorporates both kinetic and intrinsic spin-orbit (ISO) contributions as well as Rashba-type interactions coupled to an external electric field along the axis of the double helix. The helical structure of the molecule renders the ISO first order in the interaction strength (in the meV range) as in carbon nanotubes. The coupling between the ISO and the chirality of the molecule is manifest in the effective coupling parameters while the Rashba coupling is only weakly dependent on structural chirality. A continuum model at half filling is derived where the dispersion is linear around the Fermi level. Spin transport can be completely solved in the case of ISO and the dominant Rashba type term. Spin selectivity is shown to exist for this minimal model (with features similar to recent experimental findings) when the double helix is biased and thus time reversal symmetry is broken. The model also display robustness toward scattering because of the chiral nature of the eigenstates.

Owing to the existence of open channels, light could theoretically be transmitted through disordered media with 100% efficiency. However, because of the large number of channels to be controlled or measured, this has yet to be observed experimentally. To avoid this difficulty we consider observables that are easier to handle experimentally. By measuring the correlations of the field transmitted through a disordered medium, we were able to count the number of channels involved in transmission and to compare this number with theory. For a sample of transmission 1/25, we get a number of modes about 15 times smaller than for free space propagation. This figure is in good agreement with theoretical predictions.

Perovskite $\left(\mathrm{La}X{\mathrm{O}}_3\right){}_2/\left({\mathrm{LaAlO}}_3\right){}_4$(111) superlattices with $X$ spanning the entire $3d$ transition-metal series combine the strongly correlated, multiorbital nature of electrons in transition-metal oxides with a honeycomb lattice as a key feature. Based on density functional theory calculations including strong interaction effects, we establish trends in the evolution of electronic states as a function of several control parameters: band filling, interaction strength, spin-orbit coupling (SOC), and lattice instabilities. Competition between local pseudocubic and global trigonal symmetry as well as the additional flexibility provided by the magnetic and spin degrees of freedom of $3d$ ions lead to a broad array of distinctive broken-symmetry ground states not accessible for the (001)-growth direction, offering a platform to design two-dimensional electronic functionalities. Constraining the symmetry between the two triangular sublattices causes $X=\mathrm{Mn}$, Co, and Ti to emerge as Chern insulators driven by SOC. For $X=\mathrm{Mn}$ we illustrate how interaction strength and lattice distortions can tune these systems between a Dirac semimetal, a Chern and a trivial Mott insulator.

The interaction-driven insulator-to-metal transition has been reported in the ionic Hubbard model (IHM) for moderate interaction $U$, while its metallic phase only occupies a narrow region in the phase diagram. To explore the enlargement of the metallic regime, we extend the ionic Hubbard model to two coupled layers and study the interplay of interlayer hybridization $V$ and two types of intralayer staggered potentials $\mathrm{\Delta }$: one with the same (in-phase) and the other with a $\pi$-phase shift (antiphase) potential between layers. Our determinant quantum Monte Carlo (DQMC) simulations at lowest accessible temperatures demonstrate that the interaction-driven metallic phase between Mott and band insulators expands in the $\mathrm{\Delta }\text{−}V$ phase diagram of bilayer IHM only for in-phase ionic potentials; while antiphase potential always induces an insulator with charge density order. This implies possible further extension of the ionic Hubbard model from the bilayer case here to a realistic three-dimensional model.

We developed the theory of spin dependent transport through a spin-modulator device (so-called Datta-and-Das spin transistor) in the presence of a high-frequency electromagnetic field (dressing field). Solving the Schrödinger problem for dressed electrons, we demonstrated that the field drastically modifies the spin transport. In particular, the dressing field leads to renormalization of spin-orbit coupling constants that varies conductivity of the spin transistor. The present effect paves the way for controlling the spin-polarized electron transport with light in prospective spin-optronic devices.

We report on high-order magnetoplasmon resonances detected in photoresistance in high-mobility GaAs quantum wells. These resonances manifest themselves as a series of photoresistance extrema in the regime of Shubnikov–de Haas oscillations. Extending to orders as high as 25, the extrema exhibit alternating strength, being less (more) pronounced at even- (odd)-order magnetoplasmon modes. This experimental technique provides a sensitive and elegant means to detect and investigate multiple magnetoplasmon modes and could be applied to other systems.

We propose a self-consistent theoretical approach capable of describing the features of the anisotropic nanodomain formation induced by a strongly inhomogeneous electric field of a charged scanning probe microscopy tip on nonpolar cuts of ferroelectrics. We obtained that a threshold field, previously regarded as an isotropic parameter, is an anisotropic function that is specified from the polar properties and lattice pinning anisotropy of a given ferroelectric in a self-consistent way. The proposed method for the calculation of the anisotropic threshold field is not material specific, thus the field should be anisotropic in all ferroelectrics with the spontaneous polarization anisotropy along the main crystallographic directions. The most evident examples are uniaxial ferroelectrics, layered ferroelectric perovskites, and low-symmetry incommensurate ferroelectrics. Obtained results quantitatively describe the differences at several times in the nanodomain length experimentally observed on $X$ and $Y$ cuts of $\mathrm{LiNb}{\mathrm{O}}_3$ and can give insight into the anisotropic dynamics of nanoscale polarization reversal in strongly inhomogeneous electric fields.

We introduce an algorithm for the reconstruction of the complex transmission function of a specimen using segmented detectors in scanning transmission electron microscopy geometry. The phase of the transmission function can be related to magnetic and electric fields within the specimen and is sensitive to lighter elements. The technique is demonstrated for simulated data and also using experimental datasets taken from a ${\mathrm{MoS}}_2$ monolayer and a ${\mathrm{SrTiO}}_3$ crystal. We present an extension to the algorithm to account for uncertainties in the illuminating probe. The algorithm can be implemented using fast Fourier transforms, and this provides the possibility of reconstructing specimen transmission functions in real time.

A variety of heavy fermion superconductors, such as UCoGe, ${\mathrm{UGe}}_2$, and URhGe exhibit a striking coexistence of bulk ferromagnetism and superconductivity. In the first two materials, the magnetic moment decreases with pressure, and vanishes at a ferromagnetic quantum critical point (qcp). Remarkably, the superconductivity in UCoGe varies smoothly with pressure across the qcp and exists in both the ferromagnetic and paramagnetic regimes. We argue that in UCoGe, spin-orbit interactions stabilize a time-reversal invariant odd-parity superconductor in the high pressure paramagnetic regime. Based on a simple phenomenological model, we predict that the transition from the paramagnetic normal state to the phase where superconductivity and ferromagnetism coexist is a first-order transition.

An intrinsic property of antiferromagnetic materials is the compensation of the magnetic moments from the individual atoms that prohibits the direct interaction of the spin lattice with an external magnetic field. To overcome this limitation we have created artificial spin structures by heteroepitaxy between two bulk antiferromagnets ${\mathrm{SrMnO}}_3$ and ${\mathrm{NdMnO}}_3$. Here, we demonstrate that charge transfer at the interface results in the creation of thin ferromagnetic layers adjacent to $A$-type antiferromagnetism in thick ${\mathrm{NdMnO}}_3$ layers. A novel interference based neutron diffraction technique and polarized neutron reflectometry are used to confirm the presence of ferromagnetism in the ${\mathrm{SrMnO}}_3$ layers and to probe the relative alignment of antiferromagnetic spins induced by the coupling at the ferro- to antiferromagnet interface. A density functional theory analysis of the driving forces for the exchange reveals strong ferromagnetic interfacial coupling through quantifiable short range charge transfer. These results confirm a layer-by-layer control of magnetic arrangements that constitutes a promising step on a path towards isothermal magnetic control of antiferromagnetic arrangements as would be necessary in spin-based heterostructures like multiferroic devices.

Ultrasmall magnetic clusters and adatoms are of strong current interest because of their possible use in future technological applications. Here, we demonstrate that the magnetic coupling between the adsorbates and the substrate can be significantly changed through oxidation. The magnetic properties of Co adatoms and CoPd dimers deposited on a remanently magnetized Ni/Cu(100) substrate have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy at the Co ${\mathrm{L}}_{2,3}$ edges. Using spectral differences, pure and oxidized components are distinguished, and their respective magnetic moments are determined. The Co adatoms and the CoPd dimers are coupled ferromagnetically to the substrate, while their oxides, Co-O and CoPd-O, are coupled antiferromagnetically to the substrate. Along with the spin reorientation from the pure to the oxidized state, the magnetic moment of the adatom is highly reduced from Co to Co-O. In contrast, the magnetic moment of the dimer is of similar order for CoPd and CoPd-O.

We show that the S/F/S Josephson ${\phi }_0$ junction permits detection of macroscopic quantum tunneling and quantum oscillation of the magnetic moment by measuring the ac voltage across the junction. Exact expression for the tunnel splitting renormalized by the interaction with the superconducting order parameter is obtained. It is demonstrated that magnetic tunneling may become frozen at a sufficiently large ${\phi }_0$. The quality factor of quantum oscillations of the magnetic moment due to finite ohmic resistance of the junction is computed. It is shown that magnetic tunneling rate in the ${\phi }_0$ junction can be controlled by the bias current, with no need for the magnetic field.

To date, research on metasurfaces has predominantly focused on those with polarizabilities that are tangent to the metasurface. Only a few theoretical works have characterized metasurfaces with normal polarizabilities. Here, we provide full analytical, numerical, and experimental characterizations of a metasurface that can be described with polarizabilities in all three spatial directions. First, a set of equations is derived that allows a surface distribution of scatterers to be replaced by a sheet boundary condition. It is shown that the extraction of unknown polarizabilities in the normal direction necessarily requires scattering parameters obtained from oblique incidence. Closed-form expressions that relate scattering parameters to surface susceptibilities in all three spatial directions are given. It is shown that the reflection and transmission properties of the metasurface can be predicted, for an arbitrary angle of incidence, from the sheet parameters. In addition, we report the experimental characterization of a metasurface with polarizabilities in the normal direction. The free-space measurements were performed on a recently proposed DB metasurface at 10 GHz. Experiments confirmed that the sheet parameters do not change with angle of incidence. Consequently, it was possible to extract surface susceptibilities in the normal direction from measured transmission parameters alone.

The scaling of the transition temperature into an ordered phase close to a quantum critical point as well as the order parameter fluctuations inside the quantum critical region provide valuable information about universal properties of the underlying quantum critical point. Here, we employ quantum Monte Carlo simulations to examine these relations in detail for two-dimensional quantum systems that exhibit a finite-temperature Ising-transition line in the vicinity of a quantum critical point that belongs to the universality class of either (i) the three-dimensional Ising model for the case of the quantum Ising model in a transverse magnetic field on the square lattice or (ii) the chiral Ising transition for the case of a half-filled system of spinless fermions on the honeycomb lattice with nearest-neighbor repulsion. While the first case allows large-scale simulations to assess the scaling predictions to a high precision in terms of the known values for the critical exponents at the quantum critical point, for the later case, we extract values of the critical exponents $\nu$ and $\eta$, related to the order parameter fluctuations, which we discuss in relation to other recent estimates from ground-state quantum Monte Carlo calculations as well as analytical approaches.

A proper theoretical description of the electronic structure of the $3d$ orbitals in the metal centers of functional metalorganics is a challenging problem. We apply density functional theory and an exact diagonalization method in a many-body approach to study the ground-state electronic configuration of an iron porphyrin (FeP) molecule. Our study reveals that the consideration of multiple Slater determinants is important, and FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal-field effects, on-site Coulomb repulsion, and hybridization between the Fe-$d$ orbitals and ligand N-$p$ states. The mechanism of switching between two close-lying electronic configurations of Fe-$d$ orbitals is shown. We discuss the generality of the suggested approach and the possibility to properly describe the electronic structure and related low-energy physics of the whole class of correlated metal-centered organometallic molecules.

Recent angle-resolved photoemission spectroscopy measurements of Co-doped LiFeAs report a large and robust superconducting gap on the $\mathrm{\Gamma }$-centered hole band that lies 8 meV below the Fermi level. We show that, unlike a conventional superconductor described by BCS theory, a multiband system with strong interband Coulomb interactions can explain these observations. We model LiFeAs with a five-band model in which the shallow hole band is coupled with the other bands by only Coulomb interactions. Using Eliashberg theory, we find reasonable interaction parameters that reproduce the $T_c$ and all five gaps of LiFeAs. The energy independence of the Coulomb interactions then ensures the robustness of the gap induced on the shallow band. Furthermore, due to the repulsive nature of the Coulomb interactions, the gap changes sign between the shallow band and the other hole pockets, corresponding to an unconventional $s_{\pm }$ gap symmetry. Unlike other families of iron-based superconductors, the gap symmetry of LiFeAs has not been ascertained experimentally. The experimental implications of this sign-changing state are discussed.

Emergence of the topological invariant and the magnetic moment in topological insulators doped with magnetic impurities is studied based on a mutual cooperation between the spin-orbit coupling of electrons and the spin exchange of these electrons with magnetic impurity moments. The mutual cooperation is realized based on the Kane-Mele model in the presence of magnetic impurities. The topological invariants and the spontaneous magnetization are self-consistently determined within the dynamical mean-field theory. We find different magnetic topological phase transitions, depending on the electron filling. At half filling an antiferromagnetic topological insulator, which exhibits the quantum spin Hall effect, exists in the phase region between the paramagnetic topological insulator and the trivially topological antiferromagnetic insulator. At quarter and three-quarter fillings, a ferromagnetic topological insulator, which exhibits the quantum anomalous Hall effect, occurs in the strong spin-exchange regime.