Showing papers in "European Physical Journal B in 1995"

Calculated electronic structure of the sandwichd 1 metals LaI2 and CeI2: Application of new LMTO techniques

Abstract: The electronic structures of the cubic layeredd1 metals LaI2 and CeI2 were calculated using local density-functional theory and the linear muffin-tin orbital method. Special care was taken in the sphere packing used for the atomic spheres approximation. the band structure and the bonding were analysed in terms of projections of the bands onto orthogonal orbitals. The conduction-band structure could be calculated with a down-folded two-orbital basis which then served for the construction of an analytical 2×2 orthogonal, two-center tight-binding Hamiltonian. The conduction band has almost pure Ln-Ln 5d egcharacter. Thex2−y2contribution dominates and is two-dimensional and short ranged. Strong hybridization with the 3z2−1 orbital occurs near the saddle point, which is thereby lowered in energy and bifurcated due to thekz-dispersion provided by the 3z2−1 orbital. This strengthens the metal-metal bonds and prevents the nesting instability of the Fermi surface of the half filledx2−y2band. Within the limited accuracy of the LDA, the band structure of CeI2 was found to be identical to that of LaI2. The conduction-band 4f hybridizationVdf2(0) was analysed and found to be several times smaller than in fcc γ-Ce, in qualitative agreement with recent photoemission results [1]. Of importance for this reduction seems to be that the conduction band is formed by essentially only one orbital,\(Ce 5d_{x^2 - y^2 } \),that the number of Ce nearest-neighbors is small, and that the Ce−Ce distance is relatively large.

Neutron scattering study of the two-dimensional spin S=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2

Abstract: We have carried out a neutron scattering investigation of the static structure factorS(q 2D ) (q 2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2. For the spin correlation length ξ we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr2CuO2Cl2-Monte Carlo data, which cover the length scale from ≈1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0)∼ξ 2 for the reduced temperature range 0.16

The concept of effective tension for fluctuating vesicles

Abstract: Vesicles are closed surfaces of bilayer membranes. Their mean shapes and fluctuations are governed by the competition of curvature energy and geometrical constraints on the enclosed volume and total surface area. A scheme to calculate these fluctuations to lowest order in the ratio of temperature to bending rigidity is developed. It is shown that for fluctuations that break a symmetry of the mean shape the area constraint indeed acts like a tension whose value is given by the Lagrange multiplier used to enforce the area constraint in the first place. As a consequence, these fluctuations are also insensitive to the specific variants of the curvature model. For fluctuations that preserve the symmetry of the mean shape the role of the area constraint is more subtle. The low temperature expansion breaks down in the spherical limit where with the excess area another small parameter enters. By incorporating the area constraint in this limit exactly, the validity of the conventional approach using an effective tension for fluctuations of quasi-spherical vesicles can be assessed.

The binding of alkali atoms to the surfaces of liquid helium and hydrogen

Abstract: Alkali atoms have been shown previously to have only unstable binding states inside liquid4He. We calculate the equilibrium configurations and binding energies of single alkali atoms near the liquid-vapor interface of4He and3He. A simple interface model is used to predict the surface deformation due to the presence of the atoms. A more realistic density functional model yields somewhat higher energies in the case of4He. For all alkali atoms, we find the surface binding energies to be around 10 to 20 K. A similar analysis with atom-H2 interactions finds that alkali atoms tend to submerge into liquid H2, with the exception of Li.

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

Abstract: We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.

Universal amplitudes and profiles for critical adsorption

Abstract: The analysis of published experimental data for seven different binary liquid mixtures allows us to test the concept of universality for critical phenomena at interfaces and to compare the values of three independent universal surface amplitudes with theoretical predictions. In spite of uncomfortable large experimental error bars in the case of one surface amplitude we find agreement for five different systems and disagreement for two. For the other two surface amplitudes we obtain a marginal agreement between theory and experiment which is impeded by the lack of experimental error bars. We also test the scaling functions for critical adsorption. In view of the complexity of the data analysis the agreement between theory and experiment is satisfactory and encouraging. Experimental improvements are suggested in order to obtain more stringent tests.

Adatom mobility for the solid-on-solid model

Abstract: The relaxation of a crystal surface through surface diffusion is studied within the solid-on-solid model. Two types of (conserved) dynamics are considered. ForArrhenius dynamics we show that the relevant transport coefficient, the adatom mobility, has a simple analytic form: It is independent of orientation, and depends exponentially on the inverse temperature, for any surface dimensionalityd. Together with the expression for the orientation-dependent stiffness this completely determines the macroscopic evolution equation for the surface. The predictions of the macroscopic theory are checked against simulations of profile evolution and roughening ind=1. For one-dimensionalMetropolis dynamics we provide an upper bound on the adatom mobility and obtain numerical estimates of its actual value, which indicate a nontrivial orientation dependence in this case. An alternative derivation of the macroscopic dynamics directly from the master equation is presented and discussed in relation to previous approximate work.

Bose-Einstein condensation in a cavity

Abstract: The effect of the physically correct boundary conditions and the nonvanishing ground state energy on Bose-Einstein condensation of quantum particles confined to a cubic volumeV=L 3 is evaluated. The transition point is shifted towards higher temperatures by the confinement, the specific heat below the onset of condensation is no longer proportional toT 3/2, and the pressure does depend on the volume. Precise expressions for the modification of the ground state population and for the shift of the condensation temperature are derived, together with an expansion of the internal energy and of the specific heat. Numerical computations confirm the accuracy of our analytical approximations.

Two-parameter expansion in the renormalization-group analysis of turbulence

Abstract: The renormalization of the solution of the Navier-Stokes equation for randomly stirred fluid with long-range correlations of the driving force is analysed near two dimensions. It is shown that a local term must be added to the correlation function of the random force for the correct renormalization of the model at two dimensions. The interplay of the short-range and long-range terms in the large-scale behaviour of the model is analysed near two dimensions by the field-theoretic renormalization group. A regular expansion in 2e=d-2 and δ=2-λ is constructed, whered is the space dimension and λ the exponent of the powerlike correlation function of the driving force. It is shown that in spite of the additional divergences, the asymptotic behaviour of the model near two dimensions is the same as in higher dimensions, contrary to recent conjectures based on an incorrect renormalization procedure.

Electron and hole doping in NiO

Abstract: We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.

Excited p-states of alkali atoms in liquid helium

Abstract: The configuration of liquid helium around an alkali atom in an excitedp state is investigated using a simple continuous and local description of the liquid. It is found that it is qualitatively different from the spherical cavity found around an impurity in as-state. Nodal planes or lines of the electronic wave function allow some helium atoms to come very close to the atomic core. These atoms are expected to play a key role in the non radiative decay of excited states. Large fine structures tend to reduce their number. The first excited state of Na and that of Cs are considered as examples.

Universality of "level curvature" distribution for large random matrices: systematic analytical approaches

Abstract: We perform an extensive analytical study of distributions of “level curvatures” (the second derivatives of eigenvalues with respect to a perturbation parameter) for different classers of random matrice. First, we consider the case of three Gaussian ensembles: GUE, GOE and GSE. This part of our calculation is complementary to that done recently by von Oppen [22, 23], but evaluation goes along different lines and allows to treat all the three cases uniformly. In the second part of the paper we exploit completely another method allowing to treat the problem analytically for the broad class of disordered systems subject to time-reversal symmetry breaking perturbation. That gives us a possibility to prove the conjecture by Zakrzewski and Delande [17] for the ensemble of symmetric sparse random matrices.

Spectroscopy of alkali atoms and molecules attached to liquid He clusters

Abstract: Large helium clusters, ranging in size from a few hundred to several thousand atoms, are produced in a nozzle expansion. Combining this source with a pick-up scattering cell in which the clusters can be seeded with chromophores allows us to probe the influence of the helium environment on the atoms and molecules attached to the clusters. Using an alkali as chromophore we recorded laser induced fluorescence spectra of Na atoms and molecules attached to helium clusters. Apart from the spectrum of the Na monomer, we have found spectroscopic bands which can unambiguous be assigned to two bound Na atoms. The first of this bands is due to 11 ∑ μ + (A) ← 11 ∑ + (X) excitations of the covalently bound singlet Na2 molecule while the second is due to 13 ∑ + ← 13 ∑ + excitations for the van der Waals bound triplet Na2 dimer. Both bands have been vibrationally resolved. Furthermore we found very large fluorescence intensities in the region 605–635 nm which are likely due to the excitation of a species containing three Na atoms attached to a helium cluster.

Flow equations for the spin-boson problem

Abstract: Using continuous unitary transformations recently introduced by Wegner [1], we obtain flow equations for the parameters of the spin-boson Hamiltonian. Interactions not contained in the original Hamiltonian are generated by this unitary transformation. Within an approximation that neglects additional interactions quadratic in the bath operators, we can close the flow equations. Applying this formalism to the case of Ohmic dissipation at zero temperature, we calculate the renormalized tunneling frequency. We find a transition from an untrapped to trapped state at the critical coupling constant αc=1. We also obtain the static susceptibility via the equilibrium spin correlation function. Our results are both consistent with results known from the Kondo problem and those obtained from mode-coupling theories. Using this formalism at finite temperature, we find a transition from coherent to incoherent tunneling atT2*≈2T1*, whereT1* is the crossover temperature of the dynamics known from the NIBA.

Atoms and ions in superfluid helium. I: optical spectra of atomic and ionic impurities

Abstract: Optical spectra of atomic impurities in liquid helium have been investigated. Comparison is made between the wavelength of the free atomic and ionic lines and those in the liquid heluim matrix. Simultaneously, the line width and a possible asymmetry is recorded. Presence and absence of radiative transitions depend on the species of the atom implanted in the quantum fluid. The absence of any optical transitions from states lying as low as 1, 8 eV velow the ionization limit will be explained in this paper

Structural relaxation and viscous flow in amorphous ZrAlCu

Abstract: The ternary metallic glass Zr65Al7.5Cu27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

Abstract: The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr0.7Ca0.34−xAxMnO3−δ (A=Sr, Ba), Pr0.7−xLaxCa0.3 MnO3−δ and Pr0.66Ca0.34−xSrxMnO3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr0.7Ca0.25Sr0.025MnO3−δ and Pr0.66Ca0.26Sr0.08MnO3−δ that exhibit a variation of resistance of 2.5. 107% and 109% at μ0 H=5 T for T=88 K and 50 K respectively, Pr0.58La0.12Ca0.3 MnO3−δ that exhibits a variation of 6.106% for μ0 H=5 T at T=80 K and Pr0.7Ba0.025Ca0.275MnO3−δ for which a resistance variation of 5.109%, at T=50 K, for μ0 H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature Tmax, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr0.7Ca0.3−xSrxMnO3−δ and Pr0.66Ca0.34−xSrxMnO3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation Tmax is decreased of about 50 K by introducing 0.034 hole per Mn mole.

STM characterisation of organic molecules on H-terminated Si(111)

Abstract: We present the first high resolution STM images of organic molecules on the technological important hydrogen terminated silicon surface. Ordered layers of PTCDA and PTCDI were prepared on this surface by organic molecular beam epitaxy. The submolecular contrast of these molecules on Si(111)/H obtained in the high resolution images agrees with the corresponding images on HOPG and MoS2 substrates.

Crystal electric field next to a hydrogen-like interstitial— µ + in PrNi 5

Abstract: Muon spin rotation measurements of the temperature dependence and the anisotropy of the µ+ Knight shift in single crystals of the crystal electric field singlet ground state system PrNi5 reveal pronounced deviations from a linear scaling of the Knight shift with the bulk magnetic susceptibility atT≤50 K. They are explained by a µ+ induced modification of the atomic susceptibility of neighboring Pr3+ ions. From the Knight shift anisotropy atT> 50 K it is determined that the implanted μ+ occupy a single intersitial site, namely the 6i site (0.5, 0, 0.21±0.02). Using this site information, good model fits to the measured data are obtained assuming a μ+ induced perturbation of the crystal electric field at the Pr3+ ions next to the µ+. Apparently, the presence of the µ+ leads to a lowering of the local symmetry, causing a lifting of the degeneracy and a pronounced rearrangement of the low lying crystal electric field levels for these ions.

From bunches of membranes to bundles of strings

Abstract: Bunches of membranes and bundles of strings exhibit unbinding transitions from a bound state at low temperatures to an unbound state at high temperatures.N freely suspended manifolds unbind continuously at the unique unbinding temperatureT u f which is independent ofN. The amplitudes of the critical singularities have a strongN-dependence, however, which implies that the critical region for the continuous transition becomes very small and the transition becomes very abrupt in the limit of largeN. IfN membranes or strings are bound to a rigid surface, they undergo a sequence of either two or ofN successive transitions. In general, the rigid surface affects the contact probabilities of the fluctuating manifolds. For effectively repulsive interactions, the contact exponent ς2 which governs the probability for local pair contacts satisfies the scaling relation ς2=d‖+ς whered‖ and ζ denote the dimensionality and the roughness exponent of these manifolds.

On the thermodynamic stability of odd-frequency superconductors

Abstract: The thermodynamic stability of odd-frequency pairing states is investigated within an Eliashberg-type framework. We find the rigorous result that in the weak coupling limit a continuous transition from the normal state to a spatially homogeneous odd-in-ω superconducting state is forbidden, irrespective of details of the pairing interaction and of the spin symmetry of the gap function. For isotropic systems, it is shown that the inclusion of strong coupling corrections does not invalidate this result. We discuss a few scenarios that might escape these thermodynamic constraints and permit stable odd-frequency pairing states.

Ions at helium interfaces

Abstract: The boundaries between different phases of condensed helium provide an interesting testing ground for studying ions in a quantum matter matrix. Here we consider the simplest positive and negative ions in helium — snowballs and electron bubbles, respectively — being trapped at the liquid-liquid interface of phaseseparated3He−4He mixtures and at the liquid-solid interface of4He. A comparison of experimental results for the trapping with predictions of the snowball and the bubble models shows that the models are in qualitative accord with the observations, but disagree in detail. It is suggested to use such studies for refinements of the ion models. In addition multielectron bubbles (=mesoscopic ions) and electrons on helium films are briefly discussed.

Pressure shift of atomic resonance lines in liquid and solid helium

Abstract: We show that for atoms trapped in a liquid He matrix the excitation and emission spectra can be quantitatively described by the bubble model which treats the matrix as an incompressible continuous medium. Comparison with experimental results shows that the model also holds for a solid matrix. The model is only valid in the case where the attractive He−He interaction dominates the attractive part of the foreign atom-helium interaction. Its failure in the case of implanted Na atoms is discussed.

Spectroscopy and dynamics of neutral atoms in superfluid helium

Abstract: Optical spectra and radiative lifetimes of neutral atoms in superfluid helium have been studied. The absorption and emission spectra of Ag, Mg, Yb, Al, Ga, and In were found to exhibit shifts and broadening typical of atoms residing in microscopic He bubbles, showing that this type of trapping is fairly general. The radiative lifetimes measured for these atoms are close to the free space values, indicating that the surrounding bubble hardly perturbs the electronic orbitals during photo emission. One exception observed in Sr is discussed, where competing autoionization substantially decreases the lifetime of a high excited triplet state. A transient non-bubble state with sharp, free atom like spectra is seen during the first few μs after dispersion for many atomic species. The dynamics of this state are unusual, with for example very short radiative lifetimes measured for light alkali atoms.

Quantum oscillations and the Sermi surface of LuB12

Abstract: To examine the Fermi surface of LuB12, measurements of the de Haas-van Alphen (dHvA) effect were made at temperatures between 0.35 and 2 K in magnetic fields up to 12 Tesla. Oscillations in the susceptibility occurred above 5 Tesla in any field direction relative to the single crystal sample. From the Fourier transform of the data obtained, we conclude the Fermi surface of both conduction bands to have multiple extremal cross sections. For some of these orbits, the temperature dependence of the dHvA signal was investigated to determine the corresponding cyclotron mass. For a better understanding, a Full Potential Linearized Augmented Plane Wave-(FLAPW-) band structure calculation was carried out and the shapes of the Fermi surfaces were determined. In addition, we investigated the transverse magnetoresistance as a function of the field and the field direction. Its anisotropy, as well as the Shubnikov-de Haas (SdH) oscillations occurring in certain geometries, are in agreement with the results of the dHvA measurements.

Crystal-field induced dipoles in heteropolar crystals II: Physical significance

Abstract: The significance of dipole moments induced by crystal fields in heteropolar crystals is discussed with respect to some aspects of solid state physics. Experimental results from structural analyses that provide data on induced dipoles are summarized. The concept of ionic radii is reconsidered, and a new tabulation scheme is proposed in terms of deformed charge distributions. It is shown that spontaneous polarization as well as the pyro- and piezoelectric coefficients are not independent sets of crystallographic constants, but are accounted for by the structural parameters, the ionic polarizabilities and the elastic constants. The dipole concept is extended to statistically induced or random dipoles. They can account for an important part of the binding energy of substitutionally disordered and non-stoichiometric compounds and, therefore, are concluded to stabilize disorder in solids.

Implantation and spectroscopy of metal atoms in solid helium

Abstract: The first implantation of neutral Ba and Cs atoms into solid4He is reported. We discuss details of the experimental setup and techniques used to load the helium with atoms at concentrations of 108 cm−3. If photodissociation of Cs molecules and clusters is performed twice per hour this atomic concentration can be kept without a second implantation for almost a day. From the optical spectra of Ba in solid helium we infer no significant difference in the trapping site with respect to that in liquid helium.

de Haas-van Alphen effect in the superconducting state of YNi2B2C

Abstract: To examine the Fermi surfaces of the recently discovered quaternery compounds RENi2B2C measurements of the de Haas-van Alphen (dHvA) effect were made on YNi2B2C single crystals in magnetic fields up to 12 T. ForB‖c we observe two dHvA frequenciesF1=0.499 kT andF2=6.933 kT which corresponds to approximately 1,5% and 21% of the Brillouin zone cross section area. Both frequencies could be observed deep in the vortex state of the type-II superconductor, the lower dHvA frequency down to 2 T corresponding to roughly 1/5 of the upper critical fieldBc2 which was found to be 10.6 T in resistivity measurements. The field dependent quasiparticle damping in the superconducting state is very weak, since the amplitude of the dHvA oscillations seems to be unaffected by the phase transition atBc2.

Order parameters and dielectric relaxation in betaine proton glasses

Abstract: Mixed crystals of betaine, phosphite and betaine phosphate have been investigated using broadband di-electric spectroscopy at frequencies 10−1Hz≤ν≤109Hz and temperatures 1.5 K≤T≤300 K for several betaine phosphate concentrationsx. For 0.2≤x≤0.65 an orientational glass state is found at low temperatures. The broad susceptibility spectra were analyzed using the concepts of distributions of relaxation times and of distributions of energy barriers. A critical comparison of the different approaches is given. In the mixed crystals that show antiferroelectric order at low temperatures, charge transport phenomena are studied. The static permittivity of the proton glass-forming crystals is analyzed, in terms of effectively one- and three-dimensional Ising models that incorporate random fields and random bonds.

Aging without disorder on long time scales

Abstract: We study the Metropolis dynamics of a simple spin system without disorder, which exhibits glassy dynamics at low temperatures. We use an implementation of the algorithm of Bortz, Kalos and Lebowitz [1]. This method turns out to be very efficient for the study of glassy systems, which get trapped in local minima on many different time scales. We find strong evidence of aging effects at low temperatures. We relate these effects to the distribution function of the trapping times of single configurations.