Showing papers in "European Physical Journal B in 1988"

Photon emission with the scanning tunneling microscope

Abstract: By placing a photon detector near the tip-sample region of a scanning tunneling microscope, we have measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV. Such spectra contain electronic-structure information comparable to inverse photoemission spectroscopy, but with high lateral/spatial resolution. The implications of this new observation are discussed.

Electron tunneling into thin films of Y1Ba2Cu3O7

Abstract: We have fabricated tunnel junctions on thin films of Y1Ba2Cu3O7 which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO3 substrates. These junctions of the type Y1Ba2Cu3O7/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y1Ba2Cu3O7. The gap-like structure was found to disappear atTc mainly by weakening and not by a shift of the peaks to lower voltages.

Study of the glass transition order parameter in amorphous polybutadiene by incoherent neutron scattering

Abstract: The temperature dependent elastic incoherent scattering from a glass forming polybutadiene was studied using high resolution neutron spectroscopy. This elastic scattering measures directly the non-ergodicity order parameter of the glass transition. We observed an anomalous decrease of this scattering setting in around 30 K below the thermodynamic glass transition,T g , the temperature dependence of which is in agreement with the square root ofT prediction of the mode coupling approach. The critical temperature of 220 K lies about 30 K aboveT g . The missing elastic intensity reappears as inelastic scattering in the 1 meV range. Within the μeV resolution of the backscattering spectrometer no quasielastic scattering can be detected up to 20 K aboveT g . The observed inelastic scattering may be interpreted as resulting from a continous shift of the density of states towards low frequencies as a consequence of a general softening of the structure.

Spin-dependent photoabsorption at theL-edges of ferromagnetic Gd and Tb metal

Abstract: The spin-dependent absorption of circularly polarized x-rays is studied at theL-edges of ferromagnetic Gd and Tb metal. At theL1-edge a spin-dependent part of the absorption coefficient of 10−3–10−2 is observed. Strong resonance absorption known as “white line” occurs at theL2- andL3-absorption onset. Correlated with it one finds large spin-dependent absorption effects with amplitudes of a few percent. The spin-dependent absorption spectra reflect the profiles of the spin densities of the states populated in the absorption process. Thep-states show spin densities correlated with the first two flat bands above the Fermi level. The spin density of thed-like states is concentrated in the energy range of the white line. In Gd a splitting of ∼(0.5–0.6) eV of the unoccupied 5d spin up and spin down bands is indicated for both spin-orbit partners. In Tb a large dependence of the 5d spin density on the spin-orbit configuration is observed. The experimental results on the spin densities in Gd are compared with band structure calculations for the ferromagnetic ground-state. The theoretical and experimental spin density profiles agree well for thep-states but not for thed-states. The discrepancy concerning thed-states may be attributed to core-hole polarization effects in the absorption process.

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

Abstract: From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x is constructed.

How universal are the low temperature acoustic properties of glasses

Abstract: We evaluated already published data on ultrasound absorption and velocity changes below 1 K for a set of 18 different glasses. By uniformly analysing these data according to the tunnelling model we have calculated the standard spectral density $$\bar P$$ of two-level-systems (TLS) and their coupling energyγ 1 andγ 1 to longitudinal and transversal phonons. Contrary to literature, we have not observed any correlation between $$\bar P$$ and the glass transition temperatureT g for our set of 18 glasses. As far asT g is concerned we only found a relation τ min 0.1K withn≃2.5 and where τmin is the shortest TLS-phonon relaxation time at 0.1 K. Since the values for $$\bar P$$ andγ 1,t scatter within half an order of magnitude, we conclude that the TLS parameters of a certain glass are only weakly dependent on bulk properties. We find that the ratioγ t 2 /C 44 (whereC 44 is the shear elastic constant) is a more sensitive parameter for tests on the variation of the TLS spectral density of glasses.

Crystallographic study of tetragonal, superconducting YBa2(Cu0.93Fe0.05)3O7

Abstract: Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (T c ≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 A in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.

A Cud-d excitation model for the pairing in the high-T c cuprates

Abstract: A new model for the pairing mechanism in the ceramic superconductors is presented. Like the magnetic models, we assume the limit of large correlation energies for the Cud electrons. We postulate that the pairing of the Op conduction holes occurs viad−d orbital excitations within thee g manifold of thed hole of Cu++, which is split because of tetragonal or lower symmetry at the Cu sites. This valence conserving charge degree of freedom has been ignored in the magnetic pairing models. Thed−d excitation model may provide a simple qualitative understanding of many experimental results.

Transport properties of the Anderson lattice

Abstract: Transport properties including electrical resistivity, thermoelectric power, Lorenz number and a.c. conductivity are evaluated in an approximate fashion for the Anderson lattice model for six-fold degenerate Ce ions. Coherence (Bloch's theorem) is explicitly included while the effects of intersite interactions which may be responsible for magnetic and superconducting instabilities are neglected. The calculations utilize the AverageT-matrix Approximation (ATA) with the self-consistent Non-Crossing Approximation (NCA) perturbation theory employed to give the single siteT-matrix estimate. The resistivity peaks near the characteristic Kondo temperatureT0, with high temperature logarithmic decrease and low temperatureT2 behavior. The thermoelectric power is positive and similar to the impurity result except for low temperatures; sign changes in the thermopower are in principle possible with momentum dependent hybridization. Frequency and temperature dependent optical conductivity calculations are in qualitative agreement with experimental data, although a suitably defined optical effective mass and scattering rate do not agree at least for large orbital degeneracy. The behavior of these latter quantities is qualitatively different for twofold degeneracy. Unanswered questions arising from the experimental literature are summarized.

Spin-correlations and low lying excited states of the spin-1/2 Heisenberg antiferromagnet on a square lattice

Abstract: The ground state and the lowest excited states of the spin 1/2-Heisenberg model are investigated by exact diagonalization and variational Monte Carlo techniques. Our trial state represents a generalization of a wave function introduced by Hulthen, Kasteleijn and Marshall. The long range character of the spin-correlation function is in excellent agreement with exact diagonalization and also with recent neutron scattering results for La2CuO4. The asymptotic behavior of the spin-correlation function is found to differ from spin-wave theory. From the exact (N<=20 spins) and variational (N<=400) ground state energies we determine as asymptotic values 1.3025 and 1.288, respectively. We calculate the dispersion for the spin-wave excitations and identify an excited triplet which becomes degenerate with the ground state in the thermodynamic limit. This triplet state allows spontaneous symmetry breaking to occur atT=0 K. Quantum fluctuations reduce the sublattice magnetization to an effective value of 0.195 (3) as compared to the Neel-state value of 1/2.

Magnetic frustration model for superconductivity in planar CuO2 systems

Abstract: We present a model for the superconductivity in CuO2 planar systems based on the magnetic frustration mechanism introduced recently by Aharony et al.; our model incorporates explicitly the concentration dependence of the Cu++ spin-spin correlation function. We argue that the transport is via holes in the non-bonding in-plane oxygen orbitals. These two features lead tod-state pairing with, in the BCS approximation, a predictedT c vsx dependence which agrees well experiments on La2−xSrxCuO4.

Thermal expansion, sound velocities, specific heat and pressure derivative of Tc in YBa2Cu3O7

Abstract: We report measurements, between helium and room temperature, of the thermal expansion α(T), sound velocitiesv L (T) andv T (v) and specific heatC(T) of YBa2Cu3O7 samples, all cut from the same sintered pellet. A linear term in α(T) is resolved at low temperatures. It is compared with a corresponding term in the low temperature specific heat and, using the bulk modulus from our sound propagation measurements, a Gruneisen parameter of usual size (1.2±0.4) is derived-in conformity with the possible existence of non-superconducting carriers well belowT c . Longtime drift effects in the length of the sample between 30 K and 60 K point to the vicinity of a structural instability. The Debye temperature derived fromC(T) amounts to about 450 K atT=T c =91 K and forT→0 approaches 350 K, in agreement with the value deduced fromv L (T) andv T (T). The superconducting transition is indicated by anomalies with idealized discontinuities Δα and ΔC. A thermodynamic relation between Δα and ΔC yields the relative pressure derivative ofT c ,t p =T −1 (∂T c /∂p) p→0=+(0.7±0.2)·10−7 kbar−1.

Polarized light and X-ray precession study of the ferroelastic domains of YBa2Cu3O7-δ

Abstract: The ferroelastic domains of the orthorhombic phase of YBa2Cu3O7-σ have been observed in polarized light on ceramics and single crystals. By combining polarized light microscopy with the X-ray precession technique, the correlation of the orientation of the orthorhombica- andb-axes with that of thea-b-plane bireflectance, reflection dichroism, transmission dichroism (at a thickness of about 1μm), reflection tints generated with compensators and upon uncrossing of polars, as well as the orientation of etch pits has been realized on ferroelastic single domains, bi-domains and more complicated domain patterns. Four ferroelastic orthorhombic domain states have been identified, at variance with former group theoretical considerations, predicting only two states. Ensembles of lamellar domains beyond optical resolution generate strong bireflectance with principal axes rotated by 45° relative to the truea, b-directions.

Chaotic diffusion and 1/f-noise of particles in two-dimensional solids

Abstract: We investigate the classical nonlinear dynamics of a particle moving conservatively in a two-dimensional periodic potential. The particle exhibits diffusive motion in the absence of random forces. In a broad range of energies above the potential barrier, the diffusion process is anomalously accelerated and associated with 1/f-noise in the power spectrum of velocity fluctuations. The analysis of Poincare surfaces of section and the distribution of free paths indicate that the phenomenon is caused by a trapping of orbits in a self-similar hierarchy of nested cantori. We describe a statistical theory for this mechanism in terms of a renewal process and a random walk on a hierarchical lattice.

Strong changes in the phonon spectra of 123 superconductors by varying oxygen concentration

Abstract: By means of inelastic neutron scattering we have studied the phonon densities of states (PDOS) ofY based 123 superconductors with oxygen concentrations varying between O7 and O6. We find drastic changes in the PDOS above 40 meV which develop in a systematic manner if we switch from superconducting to semiconducting samples. Model calculations clearly show that this cannot be explained solely by structural changes but is likely to reflect strong differences in the electron-phonon coupling.

Imaging of self-generated multifilamentary current patterns in GaAs

Abstract: Two-dimensional imaging of current filament patterns generated in homogeneousn-type GaAs during avalanche breakdown at low temperatures is reported The self-generated formation and subsequent growth behavior of distinct single- and multifilament configurations could globally be visualized by means of a scanning electron microscope equipped with a liquid-helium stage From local conductivity measurements in the smallest possible filaments (typical diameter of about 10 μm) carrier mobilities as high as about 4·106 cm2/Vs at 42 K were estimated Such high-mobility filament channels may become interesting for applications in ultrafast electronic circuits

Interfacial properties of elastically strained materials

Abstract: A general thermodynamics of surfaces between elastic media is developed which extends an earlier treatment by Andreev and Kosevich to embody the Gibbsian description of fluid-fluid interfaces. Complementary to the well known surface stress the concept of surface strain is introduced. The thermodynamic variables of the surface energy and the corresponding Gibbs-Duhem- and Maxwell-relations are investigated. We study two examples in detail:a) a surface between solids that cannot transform into each other, andb) an interface between a solid and its melt. In the latter case we show that the surface tension only depends on the tangential strain.

A relativistic theory of X-ray absorption by spin-polarized targets

Abstract: We show that, as a consequence of relativistic quantum mechanics, photons with left handed and right handed circular polarizations are absorbed differently by spin-polarized targets. We illustrate this effect by explicit calculation of the K-absorption edge in ferromagnetic Fe.

From physical dielectric-breakdown to the stochastic fractal model

Abstract: We show how the stochastic growth model we have formulated for dielectric breakdown is related to microscopic mechanisms. Beginning with gas discharges we focus on the origin of the stochastic features and the dependence of growth probability on the local electric field. For dielectric breakdown in solid insulators we argue that quenched disorder does not appreciably modify the patterns which appear in the gaseous case.

Further phase transition scenarios described by the self consistent current relaxation theory

Abstract: Two scenarios for dynamical phase transitions from ergodic to non ergodic dynamics as described by the self consistent current relaxation theory are examined and exemplified by numerical results. The first deals with endpoints of type B transition lines and its signature is logarithmic decay in time or (1/f)-noise for the spectra. The second deals with crossings of phase transition lines and it is characterized by the known relaxation pattern of one transition accompanied by strong precursors following the pattern of another transition. The possible relevance of the results for a description of liquid glass transitions and spin glass transitions is indicated.

Photoelectric quantum yield of free silver particles near threshold

Abstract: The absolute photoelectric quantum yieldY(hv), 4.6 eV≦hv≦5.6 eV, of clean free silver particles with radii 2.7 nm, 3.8 nm and 5.4 nm is measured for the first time. The particles are produced and transported in ultrapure Helium. The photoemission constantc in the Fowler-Nordheim law is enhanced by a factor of ≈200 for particles compared to a flat surface.Y(hv) shows the same structure for different radii and flat surfaces. The dependence of the work function Φ on the radius follows the classical electrostatic model.

Investigation of the low temperature behaviour of the Anderson lattice

Abstract: A recently developed new perturbational approach to the nonlocal correlations in the Anderson lattice model is used to study low temperature properties. These include the one particle excitation spectra and possible instabilities towards magnetic or superconducting states. The method rests upon a systematic and in principle exact incorporation of the large interaction energy, the Coulomb repulsionU between local electrons on the same lattice site, into the low energy dynamics of quasiparticles and phonons. The resulting dressing of quasiparticles and their resulting effective interactions are treated in the frame of established approximations: The former in NCA, self-consistently adapted to the lattice (LNCA), and the interacting low energy problem with help of ladder summations for the two-particle scattering. Numerical results for the one particle properties are presented, which show an interesting collective effect in the formation of a Kondo lattice state. Stoner-like expressions for various susceptibilitites are derived. Criteria for the occurrence of magnetic and different superconducting phases are given which clearly exhibit the role of quasiparticle band structure, electron-electron interaction and electronphonon interaction.

Study of the ferroelectric phase transition of TlGaSe2 by dielectric, calorimetric, infrared and X-ray diffraction measurements

Abstract: The real part of the dielectric constant e′, the heat capacitycp, the infrared reflectivity, and the X-ray diffraction of TlGaSe2 have been measured in the temperature range from 12 K (30K) to 300 K. Both e′ andcp show two anomalies at about 110 K and 120 K. A study of the hysteresis loop as well as an investigation of the dielectric dispersion in the microwave region show that the phase below 110 K is ferroelectric. The crystal structure remains nearly unchanged in the course of the phase transition. The loss of the\(\bar 1\) symmetry (C2/c→Cc) results from small positional shifts of the T1 atoms in the ab plane accompanied by a discontinuity in the axial ratios. We suggest, that the ferroelectricity is caused by the stereochemically active electron lone pair configuration of the Tl+ ion. Thus TlGaSe2 may provide the first example for ferroelectricity caused by this mechanism.

Phonon density-of-states for high-T c (Y, RE) Ba2Cu3O7 superconductors and non-superconducting reference systems

Abstract: From inelastic neutron scattering experiments, we determined the generalized phonon density-of-states (PDOS) for the high-T c superconductors MEBA2Cu3O7 (ME=Y, Y0.5Pr0.5, Nd) and the non-superconducting references YBa2Cu3O6 and PrBa2Cu3O7. A detailed study of the Y-O7 compound at temperatures between 300 K and 6 K gave no evidence of any pronounced soft-mode behaviour. Moderate changes only occur in the PDOS between Nd and Pr samples and Y and Y0.5Pr0.5 samples, respectively, but drastic differences show up in the Y-O7 and Y-O6 spectra. Thus, we could not find correlations betweenT c -values and particular features in the PDOS although there is some evidence that electron-phonon coupling is strong in this class of materials.

Oxidation states of Cu, Ba and Y in superconducting YBa2Cu3O7−x

Abstract: A high temperature (91.2 K) Y−Ba−Cu−O superconductor has been studied by monochromatized XPS core level measurements. The position, shape and satellite emission of Cu 2p indicate a mixture of Cu valencies 2+ and 1+ with some indication for the existence of a third oxidation state of 3+. The two component structure of the Ba 3d line suggests two principal Ba−O configurations due to oxygen vacancies. The shape and position of the Y 3d doublet imply only weak Y−O interactions.

Measurement of the diffusion-length of carriers and excitons in CdS using laser-induced transient gratings

Abstract: Using a laser-induced grating arrangement we measure the probe beam diffraction intensity under stationary excitation conditions as function of the grating constant. We developed a mathematical model and extract by comparison with experimental data the diffusion lengths of the quasiparticles existing at various temperatures and excitation levels in the CdS sample from the experimental findings.

Eigenvalues of the eight-vertex model transfer matrix and the spectrum of theXYZ Hamiltonian

Abstract: We study the eigenvalue problem for the transfer matrix of the eight-vertex model. By using an inversion relation which was recently discovered we develop a new method to calculate all eigenvalues of the transfer matrix in the thermodynamic limit. This leads to a complete classification of the spectrum. The results are used to determine all energy excitations of theXYZ-model.

Dielectric relaxation of mixed crystals of Rb1−x(NH4)xH2PO4 at microwave frequencies

Abstract: Measurements of the clamped dielectric constant on mixed single crystals of Rb1-x(NH4)xH2PO4,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to ≊ 100 MHz, with a freezing temperature ofT o≅8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.

Fluctuation conductivity in inhomogeneous superconductors

Abstract: We analyze the superconducting transition of inhomogeneous and fractal materials. We show that the relevant dimensionality for the fluctuations spectrum is the spectral dimension of the self-similar regime. We model general inhomogeneous materials by percolation theory and calculate their superconducting transition as a superposition of the homogeneous and the self-similar regimes. Our calculations agrees well with measurements on Al−Ge thin films that exhibit percolation structure. We further analyze various possibilities of inhomogeneous materials where an homogenization procedure is possible.

Analysis of the dc-resistivity of YBa2Cu3Ox: Deduction of the intragrain resistivity

Abstract: The resistivity ρ(100 K) scatters very much for YBa2Cu3Ox prepared as single crystals, epitaxial films or bulk samples which consist of grains of 1 to 10 μm diameter. An analysis of ρ(T) for granular bulk samples is presented indicating the existence of a low intrinsic resistivity ρ i (T)=ρ 0 +α i T with ρ 0 ≈0 and α i ≃0.5µΩcm/K. The large grain boundary resistivity ρ b between the grains (>1 μm) yield a macroscopic percolative conduction path lengtheningL/L 0>1 with a reduced effective cross sectionC