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We study the electronic structure of Ta2NiSe5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ta L3 edge. We also investigate the electronic properties of Ta2NiSe5 within the density-functional theory using the generalized gradient approximation in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. While ARPES, dc transport, and optical measurements indicate that Ta2NiSe5 is a small band-gap semiconductor, DFT gives a metallic nonmagnetic solution in Ta2NiSe5 . To obtain the semiconducting ground state in Ta2 NiSe5 we use a self-interaction correction (SIC) procedure by introducing an orbital-dependent potential Vl into the Hamiltonian. We investigate theoretically the x-ray absorption spectroscopy (XAS) and RIXS spectra at the Ni and Ta L3 edges and analyze the spectra in terms of interband transitions. We investigate the RIXS spectra as a function of momentum transfer vector Q and incident photon energy. Because Ta2 NiSe5 possesses only fully occupied (Ni 3d and Se 4p) and completely empty (Ta 5d) shells with the formal valencies Ta5+ (5d0), Ni0 (3d10 ), and Se2- (4p6 ), both the Ni and Ta L3 RIXS spectra belong to a charge transfer type with ligand-to-metal excitations., Comment: 11 pages, 15 figures

We have investigated the electronic structure of the transition metal oxide Ca3Ru2O7 within density functional theory using the generalized gradient approximation while considering strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. Ca3Ru2O7 can be classified as a Mott insulator since it was expected to be metallic from band structure calculations. We have investigated the resonant inelastic x-ray scattering spectra at the Ru and Ca K, L3, and M3 edges as well as at the O K edge. The experimentally measured resonance inelastic x-ray spectrum of Ca3Ru2O7 at the Ru L3 edge possesses a sharp feature below 2 electron-volts corresponding to transitions within the Ru t_2g levels. The excitation located from 2 to 4 electron-volts is due to t2g -> e_g transitions. The third wide structure situated at 4.5-11 electron-volts appears due to transitions between the Ru 4dO states derived from the tails of oxygen 2p states and t_2g and e_g states. The measured resonance inelastic x-ray spectra at the Ru L3 and M3 edges are very similar. However, the corresponding measured resonance inelastic x-ray spectra at the Ca site quite differ from each other due to the significant difference in the widths of core-levels., Comment: 11 pages, 14 figures. arXiv admin note: text overlap with arXiv:2310.08228

We have investigated the electronic structure of Sr2IrO4 within the density-functional theory using the generalized gradient approximation while taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear MT orbital band structure method. We have investigated the x-ray absorption spectra, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering spectra at the Ir L3 and O K edges. The calculated results are in good agreement with the experimental data. The RIXS spectrum of Sr2IrO4 at the Ir L3 edge in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 1.5 eV corresponding to transitions within the Ir t2g levels. The excitation located from 2 eV to 5 eV is due to t2g-eg transitions. The third wide structure situated at 5-12 eV appears due to transitions between the Ir 5d_O states derived from the tails of oxygen 2p states and eg and t2g states. The RIXS spectrum of Sr2IrO4 at the O K edge consists of three major inelastic excitations at 0.7 eV, 3.5 eV, and around 6.2 eV. We have found that the first low energy feature is due to interband transitions between occupied and empty O t2g transitions, which appear due to the strong hybridization between oxygen 2p and Ir t2g states in the close vicinity of the Fermi level. The next two peaks at around 3.5 and 6.2 eV reflect the interband transitions from the occupied O 2p states and the empty oxygen states which arise from the hybridization with Ir t2g and eg states, respectively. We have found that the theory reproduces well the shape and energy position of the low energy feature, but the second and the third peaks are shifted towards smaller energy in comparison with the experimental measurements.

We study the electronic and magnetic properties of T$_2$AlC (T=Ti and Cr) compounds in the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations (GGA+$U$) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band-structure method. The X-ray absorption spectra and X-ray magnetic circular dichroism (XMCD) at the Cr $L_{2,3}$ and Cr, Ti, and C $K$ edges were investigated theoretically. The calculated results are in good agreement with experimental data. The effect of the electric quadrupole $E_2$ and magnetic dipole $M_1$ transitions at the Cr $K$ edge has been investigated., Comment: 16 pages, 7 figures, 1 table. arXiv admin note: text overlap with arXiv:cond-mat/0210482 by other authors

The AC Josephson effect predicted in 1962 and observed experimentally in 1963 as quantised voltage steps (the Shapiro steps) from photon assisted tunnelling of Cooper pairs is among the most fundamental phenomena of quantum mechanics and is vital for metrological quantum voltage standards. The physically dual effect, the AC coherent quantum phase slip (CQPS), photon assisted tunnelling of magnetic fluxes through a superconducting nanowire, is envisaged to reveal itself as quantised current steps. The basic physical significance of the AC CQPS is also complemented by practical importance in future current standards; a missing element for closing the Quantum Metrology Triangle. In 2012, the CQPS was demonstrated as superposition of magnetic flux quanta in superconducting nanowires. However the direct sharp current steps in superconductors; the only unavailable basic effect of superconductivity to date, was unattainable due to lack of appropriate materials and challenges in circuit engineering. Here we report the direct observation of the dual Shapiro steps in a superconducting nanowire. The sharp steps are clear up to 26 GHz frequency with current values 8.3 nA and limited by the present setup bandwidth. The current steps have been theoretically predicted in small Josephson junctions (JJs) 30 years ago. However, broadening unavoidable in JJs prevents their direct experimental observation. We solve this problem by placing a thin NbN nanowire in an inductive environment., Comment: 5 pages 4 figures

Plasmonic metasurfaces form a convenient platform for light manipulation at the nanoscale due to their specific localized surface plasmons. Nevertheless, despite the high degree of light localization in metals, their intrinsic Joule losses are often considered prevention from applications in high-quality dielectric structures. Here, we experimentally demonstrate that in some cases, the capabilities of plasmonic particles for light manipulation prevail over the negative impact of absorption. We show the lattice of plasmonic nanoparticles onto a dielectric waveguide that efficiently couples the light of both circular polarizations to guided modes propagating in opposite directions. We demonstrate 80% degree of circular polarization for the out-coupled emission of GaAs-waveguide-embedded quantum dots. The results allow us to consider the lattice as a circular-polarization-controlled grating coupler operating at normal incidence and make this structure prospective for further implementation as an efficient coupling interface for various integrated devices.

We studied the electronic structure of $\beta$-Li$_2$IrO$_3$ insulator within the density-functional theory using the generalized gradient approximation with taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The $\beta$-Li$_2$IrO$_3$ undergoes a pressure-induced structural and magnetic phase transitions at $P_c$ $\sim$4 GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by the formation of Ir$_2$ dimers on the zigzag chains, with an Ir-Ir distance of $\sim$2.66~\AA, even shorter than that of metallic Ir. The strong dimerization stabilizes the bonding molecular-orbital state, leads to the collapse of the magnetism and opens the energy gap with a concomitant electronic phase transition from a Mott insulator to band insulator. The resonant inelastic x-ray scattering spectra (RIXS) at the Ir $L_3$ edge were investigated theoretically from first principles. The calculated results are in good agreement with the experimental data. We show that the the drastic reconstruction of the RIXS spectral peak at 0.7 eV associated with the structural $Fddd \rightarrow C2/c$ phase transition at $P_c$ can be related to disappearing of the Coulomb correlations in the high-pressure $C2/c$ phase

Using density functional theory (DFT) methods, we have calculated X-ray absorption spectroscopy (XAS) and X-ray circular dichroism (XMCD) spectra in bulk and thin films of Fe$_3$GeTe$_2$, CrI$_3$, and CrGeTe$_3$. DFT+$U$ methods are employed for better handling of correlation effects of 3$d$ electrons of transition metals. We discuss relations between the density of states, radial matrix elements, and the corresponding spectra. By comparing the calculated spectra with previously measured spectra, we discuss the reliability of DFT+$U$ methods to describe the electronic structures of these materials and determine the corresponding optimal $U$ and $J$ parameters.

Magnetic Tunnel Junctions whose basic element consists of two ferromagnetic electrodes separated by an insulating non-magnetic barrier have become intensely studied and used in non-volatile spintronic devices. Since ballistic tunnel of spin-polarized electrons sensitively depends on the chemical composition and the atomic geometry of the lead/barrier interfaces their proper design is a key issue for achieving the required functionality of the devices such as e.g. a high tunnel magneto resistance. An important leap in the development of novel spintronic devices is to replace the insulating barrier by a ferroelectric which adds new additional functionality induced by the polarization direction in the barrier giving rise to the tunnel electro resistance (TER). The multiferroic tunnel junction Co/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ (Co/PZT/LSMO) represents an archetype system for which - despite intense studies - no consensus exists for the interface geometry and their effect on transport properties. Here we provide the first analysis of the Co/PZT interface at the atomic scale using complementary techniques, namely x-ray diffraction and extended x-ray absorption fine structure in combination with x-ray magnetic circular dichroism and ab-initio calculations. The Co/PZT interface consists of one perovskite-type cobalt oxide unit cell [CoO$_{2}$/CoO/Ti(Zr)O$_{2}$] on which a locally ordered cobalt film grows. Magnetic moments (m) of cobalt lie in the range between m=2.3 and m=2.7$\mu_{B}$, while for the interfacial titanium atoms they are small (m=+0.005 $\mu_{B}$) and parallel to cobalt which is attributed to the presence of the cobalt-oxide interface layers. These insights into the atomistic relation between interface and magnetic properties is expected to pave the way for future high TER devices.

A two-level quantum system can absorb or emit not more than one photon at a time. Using this fundamental property, we demonstrate how a superconducting quantum system strongly coupled to a transmission line can be used as a sensor of the photon flux. We propose four methods of sensing the photon flux and analyse them for the absolute calibration of power by measuring spectra of scattered radiation from the two-level system. This type of sensor can be tuned to operate in a wide frequency range, and does not disturb the propagating waves when not in use. Using a two-level system as a power sensor enables a range of applications in quantum technologies, here in particular applied to calibrate the attenuation of transmission lines inside dilution refrigerators.

The observations the dc voltage on asymmetric superconducting ring testify that one of the ring segments is a dc power source. The persistent current flows against the total electric field in this segment. This paradoxical phenomenon is observed when the ring or its segments are switched between superconducting and normal state by non-equilibrium noises. We demonstrate that the dc voltage and the power increase with the number of the identical rings connected in series. Large voltage and power sufficient for practical application can be obtained in a system with a sufficiently large number of the rings. We point to the possibility of using such a system for the observation of the dc voltage above superconducting transition and in the asymmetric rings made of normal metal., Comment: 7 pages, 5 figures

We study four- and higher-order wave mixing of continuous coherent waves on a single superconducting artificial atom. Narrow side peaks of different orders of nonlinearity resulting from elastic multi-photon scattering on the atom are observed and investigated. We derive an analytical expression for the peak amplitudes and show that the ratio of any two adjacent peaks is a function of driving amplitudes and detuning. This is attributed to the photon distribution in the coherent states and provides a measure for characterisation of photon statistics in non-classical coherent waves. We also demonstrate an Autler-Townes-like splitting of side peaks, the magnitude of which scales with the scattering order., Comment: 6 pages, 4 figures, revtex

We study operation of a new device, the superconducting differential double contour interferometer (DDCI), in application for the ultra sensitive detection of magnetic flux and for digital read out of the state of the superconducting flux qubit. DDCI consists of two superconducting contours weakly coupled by Josephson Junctions. In such a device a change of the critical current and the voltage happens in a step-like manner when the angular momentum quantum number changes in one of the two contours. The DDCI may outperform traditional Superconducting Quantum Interference Devices when the change of the quantum number occurs in a narrow magnetic field region near the half of the flux quantum due to thermal fluctuations, quantum fluctuations, or the switching a loop segment in the normal state for a while by short pulse of an external current., Comment: 11 pages, 8 figures, A version of the article has been accepted for publication in Nano Letters

Superconducting quantum systems (artificial atoms) have been recently successfully used to demonstrate on-chip effects of quantum optics with single atoms in the microwave range. In particular, a well-known effect of four-wave mixing could reveal a series of features beyond classical physics, when a non-linear medium is scaled down to a single quantum scatterer. Here we demonstrate a phenomenon of the quantum wave mixing (QWM) on a single superconducting artificial atom. In the QWM, the spectrum of elastically scattered radiation is a direct map of the interacting superposed and coherent photonic states. Moreover, the artificial atom visualises photon-state statistics, distinguishing coherent, one- and two-photon superposed states with the finite (quantized) number of peaks in the quantum regime. Our results may give a new insight into nonlinear quantum effects in microwave optics with artificial atoms., Comment: 6 pages, 5 figures; accepted version

Analyzing temperature dependent photoemission (PE) data of the ferromagnetic Kondo-lattice (KL) system YbNiSn in the light of the Periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature T_K, defined empirically from resistivity and specificic heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f-derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of T_K ~ TC= 5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi liquid like T- linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF-states., Comment: Accepted to Phys. Rev. B

The electronic structure and magnetic ordering in La$_{1-x}$Pr$_x$Co$_2$P$_2$ ($x=$0, 0.25, and 1) phosphides have been studied theoretically using the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band-structure method. The X-ray absorption and X-ray magnetic circular dichroism spectra at the Co$L_{2,3}$ and Pr$M_{4,5}$ edges have been investigated theoretically within the framework of the LSDA+$U$ method. The core-hole effect in the final state as well as the effects of the electric quadrupole $E_2$ and magnetic dipole $M_1$ transitions have been investigated. Good agreement with experimental measurements has been found., Comment: 11 pages, 8 figures

Magnetic quantum periodicity in the dc voltage is observed when asymmetric rings are switched between superconducting and normal state by a noise or ac current. This quantum effect is used for detection of a non-equilibrium noise with the help of a system of 667 asymmetric aluminum rings of $1 \ \mu m$ in diameter connected in series. Any noise down to the equilibrium one can be detected with the help of such system with enough great number of asymmetric rings., Comment: 6 pages, 6 figures

We studied structural, electronic and magnetic properties of a cubic perovskite BaFeO$_{3-\delta}$ ($0 \le \delta \le 0.5$) within the density functional theory using a generalized gradient approximation and a GGA+U method. According to our calculations, BaFeO$_3$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with extremely high Curie temperature ($T_C$) of 700 - 900 K. However, a such estimate of $T_C$ disagrees with all available experiments, which report that $T_C$ of the BaFeO$_3$ and undoped BaFeO$_{3-\delta}$ films varies between 111 K and 235 K or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for BaFeO$_3$, we concluded that the presence of oxygen vacancies in our model enables a good agreement. Thus, the relatively low $T_C$ measured in BaFeO$_3$ can be explained by oxygen vacancies intrinsically presented in the material. Since iron species near the O vacancy change their oxidation state from $4+$ to $3+$, the interaction between Fe$^{4+}$ and Fe$^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $\delta$ in BaFeO$_{3-\delta}$, its $T_C$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO$_{3-\delta}$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above the room temperature.

The electronic structure and numerous magnetic properties of MnBi magnetic systems are investigated using local spin density approximation (LSDA) with on-cite Coulomb correlations (LSDA+$U$) included. We show that the inclusion of Coulomb correlations provides a much better description of equilibrium magnetic moments on Mn atom as well as the magnetic anisotropy energy (MAE) behavior with temperature and magneto-optical effects. We found that the inversion of the anisotropic pairwise exchange interaction between Bi atoms is responsible for the observed spin reorientation transition at 90 K. This interaction appears as a result of strong spin orbit coupling on Bi atoms, large magnetic moments on Mn atoms, significant $p-d$ hybridization between Mn and Bi atoms, and it depends strongly on lattice constants. A better agreement with the magneto-optical Kerr measurements at higher energies is obtained. We also present the detailed investigation of the Fermi surface, the de Haas-van Alphen (dHvA) effect and the X-ray magnetic circular dichroism in MnBi.

The electronic structure, optical and x-ray absorption spectra, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface, phonon spectra, electron-phonon Eliashberg and transport spectral functions, temperature dependence of electrical resistivity of the MB2 (M=Ti and Zr) diborides were investigated from first principles using the full potential linear muffin-tin orbital method. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. A good agreement with experimental data of optical and x-ray absorption spectra, phonon spectra, electron-phonon spectral functions, electrical resistivity, cyclotron masses and extremal cross sections of the Fermi surface was achieved., Comment: arXiv admin note: substantial text overlap with arXiv:1302.1960

The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface, phonon spectra, electron-phonon Eliashberg and transport spectral functions, temperature dependence of electrical resistivity of the HfB2 diboride were investigated from first principles using the fully relativistic and full potential linear muffin-tin orbital methods. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. A good agreement with experimental data of electron-phonon spectral functions, electrical resistivity, cyclotron masses and extremal cross sections of the Fermi surface was achieved.

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the 6H5/2 ground state of the Sm3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.

The states of two phase-coupled superconducting rings have been investigated. Multiple current states have been revealed in the dependence of the critical current on the magnetic field. The performed calculations of the critical currents and energy states in a magnetic field have made it possible to interpret the experiment as the measurement of energy states into which the system comes with different probabilities because of the equilibrium and non-equilibrium noises upon the transition from the resistive state to the superconducting state during the measurement of the critical current, Comment: 5 pages, 5 figures

The origin of x-ray magnetic circular dichroism (XMCD) at the Cu L2,3 edge in YBa2Cu3O7/La(1-x)Ca(x)MnO3 superlattices is revealed by performing first-principle electronic structure calculation using fully-relativistic spin-polarized linear muffin-tin orbital and projected augmented plane wave methods. We show that the XMCD spectra at the Cu L2,3 edges are proportional to the difference of the densities of majority- and minority-spin Cu 3z2-1 states. Although the Cu 3z2-1 states lie well below the Fermi level, a small number of majority-spin 3z2-1 holes is created by the Cu 3z2-1 - O p_z - Mn 3z2-1 hybridization across the interface. Even this tiny number of holes is sufficient to produce appreciable Cu L2,3 XMCD. The robustness of this conclusion is verified by studying the influence of doping, atomic relaxation, correlation effects, and antiferromagnetic order in a CuO2 plane on the XMCD spectra., Comment: 11 pages, 8 figures

The quantum oscillations in magnetic field of the critical current of asymmetric superconducting rings with different widths of the half-rings are shifted to opposite sides for measurement in the opposite direction. The value of this shift found before equal half of the flux quantum for single ring with radius 2 \mu m has smaller value at lower critical current and for system of the rings with smaller radius., Comment: 3 pages, 3 figures

The wave-vector q and doping (x,y) dependences of the magnetic energy, iron moment, and effective exchange interactions in LaFeAsO{1-x}F{x} and Ba{1-2y}K{2y}Fe2As2 are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds (x=0, y=0), the minimum of the calculated total energy, E(q), is for q corresponding to stripe antiferromagnetic order. Already at low levels of electron doping (x), this minimum becomes flat in LaFeAsO{1-x}F{x} and for x>=5, it shifts to an incommensurate q. In Ba{1-2y}K{2y}Fe2As2, stripe order remains stable for hole doping up to y=0.3. These results are explained in terms of the band structure. The magnetic interactions cannot be accurately described by a simple classical Heisenberg model and the effective exchange interactions fitted to E(q) depend strongly on doping. The doping dependence of the E(q) curves is compared with that of the noninteracting magnetic susceptibility for which similar trends are found., Comment: 17 pages, 5 figures

The outstanding discrepancy between the measured and calculated (local-density approximation) Fermi surfaces in the well-characterized, paramagnetic Fermi liquid Sr2RhO4 is resolved by including the spin-orbit coupling and Coulomb repulsion. This results in an effective spin-orbit coupling constant enhanced 2.15 times over the bare value. A simple formalism allows discussion of other systems. For Sr2RhO4, the experimental specific-heat and mass enhancements are found to be 2.2., Comment: 4 pages, 2 figures

The electronic structure of the low-temperature (LT) monoclinic magnetite, Fe3O4, is investigated using the local spin density approximation (LSDA) and the LSDA+U method. The self-consistent charge ordered LSDA+U solution has a pronounced [001] charge density wave character. In addition, a minor [00{1/2}] modulation in the phase of the charge order (CO) also occurs. While the existence of CO is evidenced by the large difference between the occupancies of the minority spin t_{2g} states of ``2+'' and ``3+'' Fe_B cations, the total 3d charge disproportion is small, in accord with the valence-bond-sum analysis of structural data. Weak Fe orbital moments of ~0.07 mB are obtained from relativistic calculations for the CO phase which is in good agreement with recent x-ray magnetic circular dichroism measurements. Optical, magneto-optical Kerr effect, and O K-edge x-ray absorption spectra calculated for the charge ordered LSDA+U solution are compared to corresponding LSDA spectra and to available experimental data. Reasonably good agreement between the theoretical and experimental spectra supports the relevance of the CO solution obtained for the monoclinic LT phase. The results of calculations of effective exchange coupling constants between Fe spin magnetic moments are also presented., Comment: 32 pages, 10 figures

Charge ordering in the low-temperature triclinic structure of titanium oxide (Ti4O7) is investigated using the local density approximation (LDA)+U method. Although the total 3d charge separation is rather small, an orbital order parameter defined as the difference between t2g occupancies of Ti$^{3+}$ and Ti$^{4+}$ cations is large and gives direct evidence for charge ordering. Ti 4s and 4p states make a large contribution to the static "screening" of the total 3d charge difference. This effective charge screening leads to complete loss of the disproportionation between the charges at 3+ and 4+ Ti sites. The occupied t2g states of Ti$^{3+}$ cations are predominantly of $d_{xy}$ character and form a spin-singlet molecular orbital via strong direct antiferromagnetic exchange coupling between neighboring Ti(1) and Ti(3) sites, whereas the role of superexchange is found to be negligible., Comment: 6 pages, 4 figures

Charge ordering in the low-temperature monoclinic structure of iron oxoborate (Fe2OBO3) is investigated using the local spin density approximation (LSDA)+U method. While the difference between t_{2g} minority occupancies of Fe^{2+} and Fe^{3+} cations is large and gives direct evidence for charge ordering, the static "screening" is so effective that the total 3d charge separation is rather small. The occupied Fe^{2+} and Fe^{3+} cations are ordered alternately within the chain which is infinite along the a-direction. The charge order obtained by LSDA+U is consistent with observed enlargement of the \beta angle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole L-type ferrimagnetic spin structure., Comment: 7 pages, 8 figures

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory., Comment: 4 pages, 2 figures

We have studied the electronic properties of the ladder compound $\alpha'$-NaV$_{2}$O$_{5}$, adopting a joint experimental and theoretical approach. The momentum-dependent loss function was measured using electron energy-loss spectroscopy in transmission. The optical conductivity derived from the loss function by a Kramers-Kronig analysis agrees well with our results from LSDA+U band-structure calculations upon application of an antiferromagnetic alignment of the V~3$d_{xy}$ spins along the legs and an on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the calculated optical conductivity into contributions from transitions between selected energy regions of the DOS reveals the origin of the observed anisotropy of the optical conductivity. In addition, we have investigated the plasmon excitations related to transitions between the vanadium states within an effective 16 site vanadium cluster model. Good agreement between the theoretical and experimental loss function was obtained using the hopping parameters derived from the tight binding fit to the band-structure and moderate Coulomb interactions between the electrons within the ab plane., Comment: 23 pages, 8 figures; submitted to PRB

We present a joint theoretical and experimental investigation of the electronic and optical properties of vanadium pentoxide. Electron energy-loss spectroscopy in transmission was employed to measure the momentum-dependent loss function. This in turn was used to derive the optical conductivity, which is compared to the results of band structure calculations. A good qualitative and quantitative agreement between the theoretical and the experimental optical conductivity was observed. The experimentally observed anisotropy of the optical properties of V_2O_5 could be understood in the light of an analysis of the theoretical data involving the decomposition of the calculated optical conductivity into contributions from transitions into selected energy regions of the conduction band. In addition, based upon a tight binding fit to the band structure, values are given for the effective V3d_xy-O2p hopping terms and are compared to the corresponding values for alpha'-NaV_2O_5., Comment: 6 pages (revtex),6 figures (jpg)

Interference of proximity induced superconducting correlations in mesoscopic metallic rings is sensitive to the magnetic flux $\Phi$ inside these rings. This is the reason for magnetoconductance oscillations in such systems. We detected experimentally and explained theoretically a novel effect: the phase of these oscillations can switch between 0 and $\pi$ depending on the resistance of intermetallic interfaces and temperature. The effect is due to a nontrivial interplay between the proximity induced enhancement of the local conductivity and the proximity induced suppression of the density of states at low energies., Comment: 4 Pages, 5 postscript figures. Submitted to Phys. Rev. Lett