Physical Review B

Dynamics of the charge-transfer (CT) pairs has been investigated for the cyano-bridged Co²⁺-Fe³⁺ particles grown in a hydrophilic cavities of Nafion 117 film. We decomposed the differential absorption spectra into the fast and slow components. We ascribed the fast and slow components to the Frank-...

By gradually changing the degree of the anisotropy in an XXZ chain, we study the defect formation in a quantum system that crosses an extended critical region. We discuss two qualitatively different cases of quenches, from the antiferromagnetic to the ferromagnetic phase and from the critical to t...

We have performed a resonant x-ray scattering study at the Co pre- K edge on a single crystal of Ca₃Co₂O₆ . The measurements reveal an abrupt transition to a magnetically ordered state immediately below T_N=25   K , with a magnetic correlation length in excess of 5500   Å along the c...

The scaling behavior of the entanglement entropy in the two-dimensional random transverse field Ising model is numerically studied through the strong disordered renormalization group method. We find that the leading term of the entanglement entropy always linearly scales with the block size. However...

Quantum phase modulation is achieved in a metallic nanoring with a FeNi∕Cu∕FeNi (ferromagnet/normal-metal/ferromagnet) layer. Both frequency and amplitude of Aharonov-Bohm oscillation are found to be modified with respect to the magnetization configurations, indicating that the phase modulatio...

We describe the thermal (κ) and electrical (σ) conductivities of quasi-one-dimensional wires, across a quantum phase transition from a superconductor to a metal induced by pair-breaking perturbations. Fluctuation corrections to BCS theory motivate a field theory for quantum criticality. We des...

We investigate the effect of substituting Si for Ge in the ferromagnetic superconductor UCoGe. dc-magnetization, ac-susceptibility, and electrical resistivity measurements on polycrystalline UCoGe_1−xSi_x samples show that ferromagnetic order and superconductivity are progressively depressed with ...

We study a hybrid system consisting of a spin-incoherent Luttinger liquid adjoined at one or both ends to a superconductor. We find that the tunneling density of states diverges at low energies and exhibits a universal frequency dependence independent of the strength of the interactions in the syste...

The temperature dependent evolution of the renormalization effect in optimally doped Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+δ along the nodal direction has been studied via angle-resolved photoemission spectroscopy. Fine structure is observed in the real part of the self-energy (Re   Σ) , including a subk...

Inelastic x-ray scattering and ab initio calculation are applied to investigate the lattice dynamics and electron-phonon coupling of the ternary silicide superconductor CaAlSi (P6̅ m2) . A soft c -axis polarized mode is clearly observed along the Γ-A-L symmetry directions. The soft mod...

It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases of superfluid ³He contained within a highly porous silica aerogel. For example, global anisotropy introduced via uniaxial compression of aerogel might stabilize the axial state, which is called the A...

The effective attenuation length (EAL) of low-energy electrons in CoO is investigated by photoemission spectroscopy experiments (5≤hν≤19   eV) by measuring the Ag Fermi-edge signal through a CoO overlayer of increasing thickness. The EAL is found to increase when lowering the electron en...

Bosonization predicts that the specific heat C(T) of a one-dimensional interacting Fermi system is a sum of the specific heats of free collective charge and spin excitations, plus the term with the running backscattering amplitude which flows to zero logarithmically with decreasing T . We verify ...

For finite systems, the real part of the conductivity is usually decomposed as the sum of a zero frequency delta peak and a finite frequency regular part. In studies with periodic boundary conditions, the Drude weight, i.e., the weight of the zero frequency delta peak, is found to be nonzero for int...

We present the Kelvin probe force microscopy measurements of the Einstein relation, i.e., the relation between the diffusion coefficient of charge carriers and their mobility, in undoped and doped disordered organic thin films. The theoretical prediction of a large deviation of the Einstein relation...

The origin of ferromagnetism is investigated in epitaxial Co:ZnO thin films which become weakly ferromagnetic after annealing in Zn vapor. Conventional characterization techniques indicate no change after treatment. However, x-ray photoelectron spectroscopy depth profiling clearly indicates the pres...

We investigate phase-coherent transport in InN nanowires of various diameters and lengths. The nanowires were grown by means of plasma-assisted molecular beam epitaxy. Information on the phase-coherent transport is gained by analyzing the characteristic fluctuation pattern in the magnetoconductance....

The puzzling behavior of the transition phase through a quantum dot can be understood in a natural way via formation of the electron molecule in the quantum dot. In this case, the resonance tunneling takes place through the quasistationary (doorway) state, which emerges when the number of electrons ...

We report the first-principles GW -Bethe–Salpeter equation and quantum Monte Carlo calculations of the optical and electronic properties of molecular and crystalline rubrene (C₄₂H₂₈) . Many-body effects dominate the optical spectrum and quasiparticle gap of molecular crystals. We interpret the o...

We investigate exciton spin memory in individual InAs∕GaAs self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. ...

Amorphous Si was completely transformed to a nanocrystalline phase in nanometer thick layers of Si-SiO₂ multiple quantum wells deposited on quartz substrates employing an illumination with a continuous-wave laser. The process was controlled by micro-Raman spectroscopy. Preferential heating of amor...

We study the optical emission from single semiconductor quantum dots coupled to the optical modes of photonic crystal nanocavities. For dots that are both spectrally and spatially coupled, autocorrelation measurements reveal efficient single photon generation, with a drastically reduced lifetime due...

We report interlayer tunneling measurements between very dilute two-dimensional GaAs hole layers. Surprisingly, the shape and temperature dependence of the tunneling spectrum can be explained with a Fermi liquid-based tunneling model, but the peak amplitude is much larger than expected from the avai...

A Fabry-Perot-type interferometer is experimentally realized for electrons in a semiconductor device. A special experimental geometry creates interference conditions for copropagating electrons in quantum Hall edge states, which results in oscillations of the current through the device. The visibili...

We have studied electron transport in clean single-walled carbon nanotube quantum dots. Because of the large number of the Coulomb blockade diamonds simultaneously showing both shell structure and the Kondo effect, we are able to perform a detailed analysis of tunneling renormalization effects. Thus...

The optical response of a thin metallic film with shallow corrugations on both surfaces is explored and the structure is found to support a strongly coupled surface plasmon polariton when transverse magnetic radiation is incident in a plane parallel to the grating grooves. Modeling confirms that thi...

The dielectric function of Ag and Au for wavelengths ranging from the infrared to the vacuum ultraviolet regime was measured with reflection electron energy-loss spectroscopy (REELS). The spectra are compared to density functional theory (DFT) calculations and to experimental optical data available ...

Using a transmission electron microscope combined with a scanning tunneling microscope, we find that a gold (111) or (001) atomic sheet is formed between two gold electrodes. Simultaneous conductance measurements indicate a value in the vicinity of G₀ ( =2e²∕h : conductance quantum), 2G₀ , 3G₀...

We studied the interaction between an Ar⁷⁺ ion and a graphene sheet by combining real-time propagation of electron wave functions with molecular dynamics simulations. Our calculation supports the fluorescence from Ar⁷⁺ ions penetrating the carbon foil [S. Bashkin , Phys. Rev. A 25, 417 (1982)], ...

The photoelectron spectrum of the (0001) surface of single-crystalline yttrium bulk samples is rich in structure from the surface state near the Fermi energy to a series of spectral features between 2 and 10   eV binding energies. Many of them appear on other rare-earth surfaces, e.g., Gd, Ho...

The effective attenuation length (EAL) of low-energy electrons in CoO is investigated by photoemission spectroscopy experiments (5≤hν≤19   eV) by measuring the Ag Fermi-edge signal through a CoO overlayer of increasing thickness. The EAL is found to increase when lowering the electron en...

As an advanced quantum system, a three terminal triple quantum dot with one lead attached to each dot allows us to simultaneously measure a transport along two different paths. Quadruple points with all three dots in resonance are prepared and investigated for different electron numbers in the indiv...

We present an exact analytic result for the time-dependent and steady-state current and the distribution function in a nonlinear electric field for an electron gas in a one-dimensional superlattice miniband by employing a relaxation-time approximation for inelastic scattering. Our transparent result...

The energetics of adsorbed hydrogen and surface germanium on vicinal silicon-germanium surfaces is important in understanding the growth of a silicon-germanium film at the atomic scale. By using plane wave pseudopotential density functional theory methods, we have calculated the molecular hydrogen a...

Nonequilibrium transport through a quantum dot with one spin-split single-particle level is studied in the cotunneling regime at low temperatures. The Coulomb diamond can be subdivided into parts differing in at least one of two respects: what kind of tunneling processes (i) determine the single-par...

The origin of ferromagnetism is investigated in epitaxial Co:ZnO thin films which become weakly ferromagnetic after annealing in Zn vapor. Conventional characterization techniques indicate no change after treatment. However, x-ray photoelectron spectroscopy depth profiling clearly indicates the pres...

Temperature-driven structural transformations in Pb-based perovskite-type relaxors are studied by using polarized Raman spectroscopy, high-resolution powder, and synchrotron single-crystal x-ray diffraction applied to PbSc_0.5Ta_0.5O₃ (PST) and Pb_0.78Ba_0.22Sc_0.5Ta_0.5O₃ (PBST). The two compounds we...

Despite extensive research work on the α-ε phase transition occurring in shock-loaded iron, the kinetics of this transformation remain largely unknown. Here, we present time-resolved free surface velocity measurements in iron foils of thicknesses ranging from 150 to 520 μm subjected to lase...

Nonpolar amorphous and polar quasiamorphous phases of substrate-supported BaTiO₃ and SrTiO₃ were studied with x-ray photoelectron spectroscopy (XPS) to characterize the structural and chemical changes accompanying the transformation of the former into the latter. It was found that there are two ...

The first onset of spatial dispersion, i.e., the variation of velocity of an elastic wave when its wavelength approaches the natural scale of length of a medium, can be accommodated within continuum elasticity theory by the incorporation of third and fourth order spatial derivatives of the displacem...

The suitability of the Green–Kubo method for predicting the thermal conductivity of nanocomposites is assessed by studying model Lennard-Jones superlattices. Good agreement is found when comparing the predicted cross-plane thermal conductivities to independent predictions from the direct method. T...

The spin dynamics in individual Permalloy nanodisks has been investigated by using time-resolved magneto-optical Kerr effect microscopy. Transitions between the vortex and quasisingle domain states have been observed by sweeping the applied bias field, and the critical bias fields for triggering vor...

The thermodynamic properties (magnetization, magnetic susceptibility, transverse and longitudinal correlation lengths, and specific heat) of one- and two-dimensional ferromagnets with arbitrary spin S in a magnetic field are investigated by a second-order Green-function theory. In addition, quantu...

We investigate the two-magnon Raman scattering from the S=1 / 2 Heisenberg antiferromagnet on the triangular lattice by considering both the effect of the renormalization of the one-magnon spectrum by 1/S corrections and the final-state magnon-magnon interactions. The bare Raman intensit...

We report on the influence of strain on the magnetic properties of thin Cr(001) layers in the Fe/Cr/Fe(001) trilayers. The epitaxial strain in the Cr layer is measured via channeling Rutherford backscattering spectroscopy and x-ray diffraction spectroscopy. The magnetic properties of the Cr spacer l...

The antiferromagnetic Heisenberg model on an anisotropic kagome lattice may be a good minimal model for real magnetic systems as well as a limit from which the isotropic case can be better understood. We therefore study the nearest-neighbor Heisenberg antiferromagnet on an anisotropic kagome lattice...

We consider a two-species hard-core boson Hubbard model for a supersolid, where two types of bosons represent vacancies and interstitials doped into a commensurate crystal. The on-site interspecies interaction can create bound states of vacancies and interstitials facilitating vacancy condensation a...

The electronic structure of the oxygen vacancy in ZnO has been found to be sensitive to the corrections applied to the local (spin) density approximation (LSDA) band gap underestimate. Here, the “ LSDA+U ” approach, in which Hubbard- U corrections are added to the local density approximation, i...

In this paper, the average density of states (ADOS) in graphene with binary alloy disorders is calculated by the recursion method. We observed an obvious resonant peak and a dip in ADOS curves near the Dirac point, which result from interactions with surrounding impurities. We also found that the re...

Exciton effects in metallic carbon nanotubes with and without magnetic flux are studied in an effective-mass approximation. For parallel polarization, an exciton associated with the first excited bands has an appreciable binding energy even in the presence of strong screening of linear bands. The Ah...

The formation of nanoscale self-assembled compositional patterns in monolayer bulk-immiscible alloy films is studied from first principles within the framework of a previously proposed hybrid atomistic-continuum model [V. Ozoliņš , Phys. Rev. Lett. 88, 096101 (2002)]. The details surrounding the p...

To understand contrast formation in atomic resolution noncontact atomic force microscopy (NC-AFM), we investigate whether or not repulsive tip-sample interaction contributes to contrast formation. We relate attractive and repulsive interactions to contrast features depending on both oscillating ampl...

We studied the step dynamics during sublimation and growth when the adatoms on the crystal surface have a drift velocity D_sF/kT , where D_s is the surface diffusion coefficient and F is a force acting on the adatoms ( F is related to the electric current heating the crystal). In the limit of fa...

The radial diffraction lattice behavior of CaF2 was analyzed in its low pressure (fluorite) and high pressure phase up to 11.5 GPa using radial X-ray diffraction techniques in the diamond anvil cell. Between 3.5 and 7.1 GPa, fluorite develops a radial diffraction strength of  0.8 GPa. The high pressure lattice anisotropy was measured to be equal to 0.65, in good agreement with previous Brillouin spectroscopy measurements. By 8.8 GPa, CaF2 has undergone a phase transformation to its high pressure (orthorhombic) phase, with a corresponding volume decrease of 10.89the hydrostatic direction. The high pressure phase is found to withstand a significantly larger differential stress than the low pressure fluorite phase; however its bulk modulus is lower. In the maximum stress direction at 8.8 GPa, we observe a time-dependent evolution of the lattice parameters of CaF2, indicating that the kinetics of the structural transition are hindered at these pressures.

A good choice of model formulation and model parameters is one of the most important and difficult aspects in mesoscale modeling and requires a systematic and quantitative analysis. In this article, it is studied how the model parameters of a generalized phase field model affect the landscape of the free energy density functional, the phase field profiles at the grain boundaries and the corresponding trajectory along the free energy landscape. The analysis results in quantitative relations between the model parameters, on the one hand, and grain boundary energy and mobility, on the other hand. Based on these findings, a procedure is derived that generates a suitable set of model parameters that reproduces accurately a material's grain boundary energy and mobility for arbitrary misorientation and inclination dependence. The misorientation and inclination dependence are formulated so that the diffuse interface width is constant, resulting in uniform stability and accuracy conditions for the numerical solution. The proposed model formulation and parameter choice allow us to perform quantitative simulations with excellent controllability of the numerical accuracy and therefore of the material behavior.

First-principles calculations have been performed for V, Nb, Ta, Mo and W. The recently discovered bcc®rhombohedral transition for vanadium [Phys. Rev. Lett. 98, 085502 (2007)] was confirmed as the mechanical instability of c44 was found at P=80 GPa. Furthermore, the c11, c12 and c44 constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb and Ta, not seen for Mo and W. From electronic topological transition (ETT) contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.

Amorphous carbon multilayered films have been studied by molecular dynamic simulations. Samples were grown on diamond and silicon substrates alternating deposition energies of 1 and 40 eV; atomic interactions were described by the semiemperical Tersoff potential. Structural analysis of the obtained films shows that the film density and sp^3/sp^2 ratio oscillate with deposition energy. The alternation of layers with different local atomic stresses is also observed. High density sp^3 rich profiles and compressive stress are associated to high deposition energy regions whereas low density sp^3 poor profiles and tensile stress are associated to low deposition energy ones. The grown films are thermally stable in the bulk under simulated annealing at 2500 K; appreciable changes, similar to those observed in monolayer films, occur mainly on the last superficial layer.

The L3-edge x-ray absorption spectrum of Pa(V) fluoride in aqueous solution show clear evidence for the double photoexcitation involving 2p and 4f electrons. A comparison with the [2p4f] double-electron excitations observed in the L3-edge x-ray absorption spectra of other actinides (thorium, uranium, neptunium, plutonium and americium) indicates a monotonic increase of the excitation energy. The sharp edge-like structure of the multielectron excitation reveals the origin of a shake-up channel.

We demonstrate the existence of different density-density functionals designed to retain selected properties of the many-body ground state in a non-interacting solution starting from the standard density functional theory ground state. We focus on diffusion quantum Monte Carlo applications that require trial wave functions with optimal Fermion nodes. The theory is extensible and can be used to understand current practices in several electronic structure methods within a generalized density functional framework. The theory justifies and stimulates the search of optimal empirical density functionals and effective potentials for accurate calculations of the properties of real materials, but also cautions on the limits of their applicability. The concepts are tested and validated with a near-analytic model.

The structural and electronic properties of both inorganic and hybrid organic-inorganic perovskites based on tin halides are investigated from first-principles. In particular, we contrast the inorganic CsSnCl3 and CsSnI3 to their hybrid counterparts (CH3NH3)SnCl3, (CH3NH3)SnI3 and (NH2CH=NH2)SnI3, obtained by substituting the inorganic Cs cation with the methylammonium (MA) CH3NH3 and the formamidinium (FO) NH2CH=NH2 cations. The comparison between the hybrid perovskites and the inorganic counterparts sheds light on the effects of the filling molecule on the structural and electronic properties of the compound. We show that the stability against distortion of the perovskitic cage strongly depends on the embedded cation. The electronic properties (in particular the band gap) can be tuned by a suitable choice of the organic molecule, and, in particular, of its size.

The tunneling-percolation mechanism of conduction in disordered conductor-insulator composites is studied for a realistic continuum model where conducting and impenetrable spherical particles are dispersed in a three dimensional continuum insulating material. Conduction between particles is via tunneling processes and a maximum tunneling distance d is introduced. We determine the percolation critical concentration for several values of d. By so doing we relax the restrictions applied in the previous studies of the problem i.e. the considerations of the underlying lattice and the contribution of only the nearest neighbours. The tunneling percolation transport is then analyzed by studying the conductance of the composite at and near the percolation threshold using a decimation procedure and a conjugate gradient algorithm. We show that at the critical concentration, and independently of the tunneling parameters, the critical transport exponent t reduces to the universal value t0 @ 2, while moving away from the percolation threshold, the conductance exponent becomes larger than t0, acquiring a strong concentration dependence. We interpret this feature as arising from the peculiar form of the distribution function for the local tunneling conductances. Consequently, apparent nonuniversality of transport appears when the conductance of the composite is fitted by forcing the exponent to be independent of the concentration. This leads us to believe that our tunneling-percolation theory is sufficient to explain the nonuniversal transport exponents observed in real disordered conductor-insulator compounds.

The distributions of inter-atomic distances of the first four coordination shells of copper and their leading cumulants have been determined by a path-integral Monte Carlo calculation on a many-body potential model, in the temperature range 4 to 300 K. The asymmetry of the distance distribution, measured by the third cumulant, is much larger for the first shell than for the outer shells. The mean value of the distance between neighboring atoms is given for each shell by the first cumulant of the distribution. This allowed us to test a well-known method of estimating the thermal expansion of each shell from the second and third cumulants of its distribution. This method gave values smaller by 40% for the first shell, and much smaller for all outer shells.

Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.

Element specific x-ray magnetic dichroism measurements have been carried out at the Ga K- and Ga and Gd L3-edges of the dilute magnetic semiconductor (DMS) Gd:GaN grown by molecular beam epitaxy. This DMS material has previously been reported to exhibit room temperature magnetic order accompanied by colossal effective magnetic moments. We detect only a very small magnetic polarization of the order of at most 10-5mB/Ga atom in Gd:GaN which cannot account for the colossal effective magnetic moments. Further, the element specific magnetic properties at the Gd sites do not reveal any ferromagnetic-like signatures but rather behave paramagnetic. Thus, the ferromagnetism in Gd:GaN is caused by polarization of the N site or by extrinsic mechanisms such as magnetic polarization of defects or residual oxygen in these samples.

By considering GaAs(110) and Si(100)(2×1) surfaces, it is shown that the use of the pseudohydrogen atoms can lead to distorted surface core-level shifts within the complete screening picture. The effect is linked to the polarization of the slab and to the change in the bulk-like electronic structure close to pseudohydrogenated part of the slab. It is demonstrated that these problems can be avoided, if the pseudohydrogenated slab is large enough and the bulk reference layer is properly chosen. One easy way to control these errors is to monitor the planar-averaged potential energy curve of the slab.

Low temperature Hall measurements have been performed on two-dimensional electron gases (2DEGs) induced by deposition of Cs or Na on in situ cleaved surfaces of p-type InAs. The surface donor level, at which the Fermi energy of the 2DEG is pinned, is calculated from the observed saturation surface electron density using a surface potential determined self-consistently. The results are compared to those of previous photoelectron spectroscopy measurements.

A double-quantum-dot coupled to electrodes with spin-dependent splitting of chemical potentials (spin bias) is investigated theoretically by means of the nonequilibrium Kyldysh Green's functions formalism. By applying a large spin bias, the quantum spin in a quantum dot (the dot 1) can be manipulated in a fully electrical manner. To noninvasively monitor the manipulation of the quantum spin in the dot 1, it is proposed that the second quantum dot (the dot 2) is weakly coupled to the dot 1. In the presence of the exchange interaction between the two dots, the polarized spin in the dot 1 behaves like an effective magnetic field and weakly polarizes the spin in the nearby quantum dot 2. By applying a very small spin bias to the dot 2, the spin-dependent transport through the dot 2 can be probed, allowing the spin polarization in the dot 1 to be identified nondestructively. These two steps form a complete scheme to manipulate a trapped spin while permitting this manipulation to be monitored in the double-dot system using pure electric approaches.

We present a density functional theory (DFT) investigation of the electronic and magnetic properties of a linear chain model of the antiferromagnetic Cr8 molecular ring. The chain model system is characterized by a smaller size of the simulation cell needed to perform the calculations, with respect to the one necessary for Cr8. By the thorough comparison between the model complex and the Cr8 ring, we prove that the chain model is reliable and mimics with good approximation the electronic and magnetic properties of Cr8.

The edge-cubic spin model on square lattice is studied via Monte Carlo simulation with cluster algorithm. By cooling the system, we found two successive symmetry breakings, i.e., the breakdown of Oh into the group of C3h which then freezes into ground state configuration. To characterize the existing phase transitions, we consider the magnetization and the population number as order parameters. We observe that the magnetization is good at probing the high temperature transition but fails in the analysis of the low temperature transition. In contrast the population number performs well in probing the low- and the high-T transitions. We plot the temperature dependence of the moment and correlation ratios of the order parameters and obtain the high- and low-T transitions at Th = 0.602(1) and Tl=0.5422(2) respectively, with the corresponding exponents of correlation length nh=1.50(1) and nl=0.833(1). By using correlation ratio and size dependence of correlation function we estimate the decay exponent for the high-T transition as hh=0.260(1). For the low-T transition, hl = 0.267(1) is extracted from the finite size scaling of susceptibility. The universality class of the low-T critical point is the same as the 3-state Potts model.

We study the magnetization reversal in elliptical nanodots with the external field applied exactly along the minor (hard) axis. By varying the magnitude of the applied field, several first and second order transitions take place and the system proceeds through magnetic configurations characterized by different symmetry properties. The dynamical matrix method is used to calculate the spin excitations as function of the applied field. This model system allows us to investigate the relationship between the singularities of the magnetization, the presence of soft spin excitations, and the symmetry properties of the static and dynamic magnetization fields. Rules are formulated that govern the transitions.

By means of first-principles calculations using spin-polarized generalized gradient approximation method and ultrasoft pseudopotentials, the electronic and magnetic structure of the recently synthesized orthorhombic phase of bilayer manganite Ca2.5Sr0.5GaMn2O8 compound is obtained. Our calculations have shown that the specific antiferromagnetic ordering, in accordance with experimental findings, is the most stable one. The total energies calculated for the several possible magnetic states of the title compound in the orthorhombic phase enable us to estimate spin exchange interactions for nearest Jnn = -1.60 meV and next nearest neighbors Jnnn= -0.21 meV. Calculated Curie-Weiss temperature in the mean-field approximation is in excellent agreement with the measured magnetic phase transition temperature. PACS numbers: 75.10.Lp,71.27.1a,75.30.Vn

In this paper we study the lattice CP1 model in (3+1) dimensions coupled with a dynamical compact U(1) gauge field. This model is an effective field theory of the s=1/2 antiferromagnetic Heisenberg spin model in three spatial dimensions at zero temperature. By means of Monte Carlo simulations, we investigate its phase structure. There exist the Higgs, Coulomb and confinement phases, and the parameter regions of these phases are clarified. We also measure the magnetization of O(3) spins, the energy gap of spin excitations, and the mass of gauge boson. Then we discuss the relationship between these three phases and magnetic properties of the high-Tc cuprates, in particular the possibility of deconfined-spinon phase. Effect of dimer-like spin exchange coupling and ring-exchange coupling is also studied.

In order to clarify whether VO2 is a Mott insulator or a Peierls insulator, the metal-insulator transition (MIT) and the structural phase transition (SPT) are simultaneously monitored for VO2 films by current-voltage and diffraction measurements using a synchrotron micro x-ray beam. In the regime showing a metallic conductivity below the SPT temperature (approximately 67oC), only diffraction planes of the monoclinic structure are observed while the planes of the tetragonal structure are absent. This reveals the presence of a monoclinic and metal phase between the MIT and the SPT characteristic of a Mott insulator.

In this paper, a geometrical approach is presented for the study of the double-resonance process giving rise to the G¢ band in monolayer graphene, bilayer graphene, and bulk graphite. It is shown that there are four discrete peaks present in the G¢ band spectrum obtained from the stacking of two graphene layers, and these discrete peaks arise from the quantization of the first Brillouin zone caused by its finite size along the c axis. Our analysis includes the study of the selection rules imposed on the electron-radiation and electron-phonon Hamiltonian interactions involving p electrons near the K point. We show that the anisotropy in the optical absorption (emission) near the K point in the first Brillouin zone of graphite should be taken into account in order to gain an understanding of the selection rules for optical transitions in bilayer graphene. The validity of considering a linear dispersion for p electrons along the K-G direction is taken into consideration. We present four numerical equations giving the dependencies of the frequencies of the four peaks composing the G¢ feature in the Raman spectrum of bilayer graphene on the laser excitation energy in the visible range. We also show that the two-peak shape of the G¢ band in the Raman spectrum of bulk graphite is in fact caused by the convolution of an infinite number of peaks.

Interaction with a substrate can modify the graphene honeycomb lattice and thus alter its outstanding properties. This could be particularly true for epitaxial graphene where the carbon layers are grown from the SiC substrate. Extensive ab initio calculations supported by Scanning Tunneling Microscopy experiments demonstrate here that the substrate indeed induces a strong nanostructuration of the interface carbon layer. It generates an apparent 6x6 modulation different from the interface 6Ö3×6Ö3R30 symmetry used for the calculation. The top carbon layer roughly follows the interface layer morphology. This creates soft 6x6 ripples in the otherwise graphene-like honeycomb lattice. The wavelength and height of the ripples are much smaller than the one found in exfoliated graphene. Their formation mechanism also differs: They are due to the weak interaction with the interface layer and not to a roughening of the plane due to the instability of a strictly two-dimensional crystal.

We study the effect of magnetic Mn ions on the two-band superconductor MgB2, and compute both the total and spin resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion's d-shell is taken into account, multiple Shiba states appear in the spectrum. The presence of these multiplets could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.

The effect of Li substitution for Mg and of Li-C co-substitution on the superconducting properties and crystal structure of MgB2 single crystals has been investigated. It has been found that hole doping with Li decreases the superconducting transition temperature Tc, but at a slower rate than electron doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for Mg and C for B decreases more than in the case where C is substituted alone. This means that holes introduced by Li cannot counterbalance the effect of decrease of Tc caused by introduction of electrons coming from C. The possible reason of it can be that holes coming from Li occupy the ... band while electrons coming from C fill the ... band. The temperature dependences of the upper critical field Hc2 for Al and Li substituted crystals with the same Tc show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of much different substitution level. This indicates that the mechanism controlling Hc2 and Tc is similar in both hole and electron doped crystals. Electrical transport measurements show an increase of resistivity both in Li substituted crystals and in Li and C co-substituted crystals. This indicates enhanced scattering due to defects introduced by substitutions including distortion of the lattice. The observed behavior can be explained as a result of two effects, influencing both Tc and Hc2. The first one is doping related to the changes in the carrier concentration, which may lead to the decrease or to the increase of Tc. The second one is related to the introduction of new scattering centers leading to the modification of the interband and/or intraband scattering and therefore, to changes in the superconducting gaps and to the reduction of Tc.

We investigate the two-magnon Raman scattering from the S=1 / 2 Heisenberg antiferromagnet on the triangular lattice by considering both the effect of the renormalization of the one-magnon spectrum by 1/S corrections and the final-state magnon-magnon interactions. The bare Raman intensit...

The electronic contribution to friction at semiconductor surfaces was investigated by using a Pt-coated tip with 50 nm radius in an atomic force microscope sliding against an n -type GaAs(100) substrate. The GaAs surface was covered by an approximately 1 nm thick oxide layer. Charge accumulation or...

Scanning tunneling spectroscopy (STS) measurements find that the surface of Bi-2212 is characterized by nanoscale sized regions, “gap patches,” which have different magnitudes for the d -wave energy gap Δ₀(r) . Recent studies have shown that the tunneling conductance can be fitted using a BCS...

We present an analysis of recent [M. Yogi , J. Phys. Soc. Jpn. 75, 124702 (2006)] Sb-NQR measurements of the heavy-fermion superconductor PrOs₄Sb₁₂ and the related La-doped series of superconductors Pr_1−xLa_xOs₄Sb₁₂ . We find that this data, along with previous temperature-dependent superfluid d...

We study quasiparticle and electron-hole pair excitations in the presence of spin polarization within the framework of many-body perturbation theory. The influence of magnetic ordering on linear optical response is examined in detail for collinear spins. Whereas Hedin’s expression for the exchange...

Quantum electrodynamics in 2+1 dimensions is an effective gauge theory for the so-called algebraic quantum liquids. A new type of such a liquid, the algebraic charge liquid, has been proposed recently in the context of deconfined quantum critical points [R. K. Kaul , Nat. Phys. 4, 28 (2008)]. In t...

The crystal structure and numerous normal and superconducting state properties of layered tetragonal (P4∕nmm) LaFeAsO, with F doping of ≈11% , are reported. Resistivity measurements give an onset transition temperature T_c=28.2   K , and low field magnetic susceptibility data indicate bu...

We report on a magneto-optical Kerr effect (MOKE) investigation of microstructured rectangular islands, which have been arranged on kagome lattices. The magnetization reversal was studied by regular longitudinal vector MOKE in specular geometry as well as in Bragg MOKE geometry, using the diffractio...

We describe the thermal (κ) and electrical (σ) conductivities of quasi-one-dimensional wires, across a quantum phase transition from a superconductor to a metal induced by pair-breaking perturbations. Fluctuation corrections to BCS theory motivate a field theory for quantum criticality. We des...

We theoretically show how constant-energy maps of the angle-resolved photoemission intensity can be used to test wave function symmetry in graphene. For monolayer graphene, we demonstrate that the observed anisotropy of angle-resolved photoelectron spectroscopy spectra is a manifestation of what has...