Showing papers in "European Physical Journal B in 1987"

Multiple Light Scattering from Disordered Media. The Effect of Brownian Motion of Scatterers

Abstract: We have measured the time autocorrelation function of the light intensity multiply scattered from turbid aqueous suspensions of submicron size polystyrene spheres in directions near backscattering. It is found strongly non-exponential at short times revealing the very fast decay of coherence in extended scattering loops due to the thermal motion of the many spheres involved; the longest living decay time is found remarkably close to the single particle backscattering relaxation time even under conditions of interparticle interactions. These features are only weakly affected by the particular interference effect between time-reversed pairs of loops giving rise to the coherent backscattering enhancement. A simple argument is presented which accounts for these observations.

Electrical transport in open and closed systems

Abstract: Electrical conductance is typically calculated by approaches which view the electrical field as a causative source and the motion of carriers as a response. An alternative viewpoint, which starts from the flux of carriers maintained at the edges of a sample, and then calculates how charges build up and fields develop, has gained acceptance in the treatment of disordered systems, the solid state Aharanov-Bohm effect, and universal fluctuations. We analyze some of the less appreciated concomitants of this viewpoint, emphasizing both the generality and limitations of the viewpoint. Particular emphasis is given to the Residual Resistivity Dipole; localized scatterers in metallic conductivity are accompanied by highly localized transport fields. Closed Hamiltonian systems, e.g. a metallic ring with elastic scattering and driven by a time-dependent magnetic flux, are conservative. They cannot exhibit dissipation, under our conventionally accepted forms of physics. It is suggested that the limited precision available,in principle, in calculating the behavior of physical systems limits our ability to retrieve energy from supposedly conservative systems. This can be regarded as the ultimate source of dissipative processes.

Classical and quantum chaos for a kicked top

Abstract: We discuss a top undergoing constant precession around a magnetic field and suffering a periodic sequence of impulsive nonlinear kicks. The squared angular momentum being a constant of the motion the quantum dynamics takes place in a finite dimensional Hilbert space. We find a distinction between regular and irregular behavior for times exceeding the quantum mechanical quasiperiod at which classical behavior, whether chaotic or regular, has died out in quantum means. The degree of level repulsion depends on whether or not the top is endowed with a generalized time reversal invariance.

Transport as a consequence of state-dependent diffusion

Abstract: Overdamped particles subject to a drift in a force field with sinusoidal space dependence and also a sinusoidally modulated space-dependent diffusion, with the same period as the drift, experience a net driving force. The resulting current depends on the amplitude of the modulation of the diffusion and is a periodic function of the phase difference between the sinusoidal drift and the sinusoidal modulation of the diffusion. For small modulation amplitudes a particle subject to state-dependent noise behaves the same way as a particle subject to thermal noise but with a drift which, in addition to the sinusoidal term, contains a net force term.

Experimental electronic structure studies of La2−xSrxCuO4

Abstract: X-ray induced photoelectron spectroscopy, bremsstrahlung-isochromat spectroscopy, and electron energy-loss spectroscopy were used to investigate the electronic structure of the high-temperature superconductor La2−xSrxCuO4 In general, good agreement was obtained with band structure calculations of the tetragonal phase of La2CuO4 Near the Fermi energyEF the systems shows forx=0 a smeared out gap or a region of very low density of states ∼2eV wide Upon replacement of La by Sr, the gap is filled with defect states, the intensity of which is proportional tox The experimental data are not consistent with the closing of a Peierls gap alone

The Cu valence in the highT c superconductors and in monovalent, divalent and trivalent copper oxides determined from XPS core level spectroscopy

Abstract: A number of theoretical models have been developed to explain the unexpected high temperature superconductivity in La-Me-Cu-oxides (Me=Ba, Sr) and Y-Ba-Cu-oxides. Some of these models invoke charge fluctuations on the copper ions in the superconductors between a 2+ and a 3+ state. In order to test these possibilities we have measured the Cu−2p 3/2-core level spectra of NaCuO2 in which the copper ion is in a 3+ state and compared it with the core line position in pure CuO and to superconducting oxides. The data strongly suggest, that there is, if any very little Cu3+ in the superconducting compounds present. However, we notice, that in comparison to trivalent and monovalent copper oxides the Cu−2p 3/2 line in CuO and the superconducting oxides is unexpectedly broad. The cause of this large linewidths remains so far unexplained.

A theorist's view of high-T c superconductivity

Abstract: A brief review is given of the current proposals to explain the observation of superconductivity with high transition temperatures in the copper oxides.

Superconductivity in the Bi — Sr — Cu — O System

Abstract: During the investigation of the system Bi—Sr—Cu—O a novel family of superconducting oxides, close to the composition Sr2Bi2Cu207+δhas been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The X-ray diffraction and high-resolution electron microscopy studies have shown its relationships with perovskite and demonstrated its bidimensional character. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.

Numerical simulations of a high-Tc-superconductive glass model

Abstract: Numerical simulations for anXY-spin-glass model are presented. The connection to recent experiments on high-T c superconductors is outlined. In particular, the glass behaviour in experiments is reproduced by a 2-D model. A phase diagram important for future experiments is also provided. A substantial gain inT c is predicted for enhanced couplings between planes.

Photoemission on the highT c superconductors Y?Ba?Cu?O

Abstract: XPS and UPS photoemission experiments on the highTc superconductors (Tc≈90 K) with nominal composition YBa2Cu3O9-y (y≈2) show the following: a) The density of electronic states at the Fermi energy is very small, much smaller than in pure Cu. b) The Cu 2p spectra show only a Cu2+ contribution. c) The Ba core levels show a structure with two components of nearly equal magnitude, which leads to the suggestion that these compounds have large O2− vacancies coordinated to Ba2+ sites. d) Annealing at 400°C under UHV conditions leads possibly to a partial reduction of Cu2+ to lower Cu valence states and to a small increase of the O2− vacancy component of the Ba2+ line.

Localization, quantum interference, and the metal-insulator transition

Abstract: The density of states, and the phase diagram of localization in a three-dimensional disordered tight binding model with box, Gaussian, and Lorentzian distribution of the site energies is numerically determined. The influence of the behaviour of the spectral density on the position of the mobility edge is discussed. Extended states are found outside the unperturbed band for the box and the Gaussian distribution. This implies the existence of two metal-insulator transitions at certain energies which may be attributed to the breakdown of potential localization, and the breakdown of quantum interference localization, respectively. The general validity of the scaling hypothesis in the localization theory is discussed in view of these results.

A stable superconducting state at 8 K and ambient pressure in $$\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1 $$

Abstract: We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.

Superconducting instability in the large- U limit of the two-dimensional Hubbard model

Abstract: We have investigated numerically the pairing instabilities of Gutzwiller wavefunctions. These are equivalent to a certain form of the resonant valence bond wavefunction. The case considered is a nearly half-filled two dimensional band with interactions given by a Hubbard model with large on-site Coulomb interactions. We find that the paramagnetic normal state is unstable tod-wave pairing but stable againsts-wave pairing. The antiferromagnetic state is marginally stable against both types of pairing. These results can be explained as an interference effect resulting in enhanced antiferromagnetic spin correlation in the paired state.

The superconductive properties and crystal-structure of ba2(y1-xprx)cu3o9-y solid-solutions

Abstract: A series of Ba2(Y1−x Pr x )Cu3O9−y solid solutions were prepared with the orthorhombic distorted perovskite structure. The compound Ba2PrCu3O9−y belonging to tetragonal system is a good semiconductor. The superconductivity is observed for samples withx<0.6. With the increase of Y content, the distortion of crystal structure is greater and the superconductive transition temperature of zero resistanceTc(0) raises.

A high resolution neutron spectrometer for quasielastic scattering on the basis of spin-echo and magnetic resonance

Abstract: We propose a high resolution neutron spectrometer, which combines the spin-echo principle with the separated coil magnetic resonance technique. The introduction of magnetic resonance instead of static spin-flippers in the spin-echo spectrometer allows the replacement of its high magnetic fields by low guide fields. The new technique represents a generalisation of the conventional spin-echo spectrometer.

Fast neutron irradiation of YBa2Cu3O7

Abstract: The influence of fast neutron irradiation on the superconducting transition temperature,T c , the transition width, upper critical field,B c2 , and critical current density,j c , has been investigated in YBa2Cu3O7 up to a fluence of 1019 cm−2 (E>1 MeV). TheT c degradation with fluence is slightly less than in PbMo6S8, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj c and the superconducting volume fraction.

Phonon density-of-states for the high-T c superconductor La1.85Sr0.15CuO4 and its non-superconducting reference La2CuO4

Abstract: From inelastic neutron-scattering experiments, we determined the generalized phonon density-of-states for the high-Tc superconductor La1.85Sr0.15CuO4 and its semiconducting reference La2CuO4 atT=300K andT=6K. The spectra differ only little from each other. In particular, we do not observe any pronounced soft-mode behaviour either on cooling or when comparing the superconducting sample to the non-superconducting reference. We find, however, modest but distinct indications of coupling between electrons and oxygen breathing-modes at high frequencies.

Mean first passage time and the lifetime of a metastable state

Abstract: The escape rate from quite general metastable states is calculated by means of the mean first passage time. Our result generalizes the known expression for transition rates in equilibrium systems in a very natural way. Possible limitations of its validity are discussed.

Core-electron binding energies in free and supported metal clusters

Abstract: Core-electron binding energy shifts in free and supported clusters are discussed using the Born-Haber formalism. For grounded clusters this approach shows that the shift reflects the decrease in the average atomic cohesive energy of the cluster relative to that of the bulk metal. This shift is closely related to the surface-atom core level shift. For free clusters there is a second term reflecting the unit charge left on the cluster by the emission of the photoelectron. In small clusters this term results in the suppression of conduction electron screening. Clusters supported on amorphous carbon remain charged in the final-state, and are similar to free clusters, but have smaller shifts because the substrate reduces the energy of the final state by forming an image charge. The shift in monolayer islands on metallic substrates is determined largely by the adsorption enthalpy of the adatoms.

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

Abstract: A sintered bulk sample of YBa2Cu3O7 has been investigated resistively and inductively with respect to its resistivity ϱ(B, T), upper critical fieldBc2(T), and current carrying capabilityjc(B, T). The inductive measurements were repeated after powdering the sample to a maximum particle size of 60 μm. Results show that the intrinsic-intragrain properties as approximately revealed by the powder measurements, are severely masked in bulk measurements by the intergrain weak-link structure. As an example, the current densityjc(≃0 T, 4.2 K) in the powdered YBa2Cu3O7 has a value of 4·106 A/cm2, which is about three orders of magnitude higher than in the bulk material. The high values of the intragrainjc are probably due to pinning at twin boundaries. — Besidesjc the normal state resistivity just aboveTc, ρn, is influenced by the interconnecting structure, both properties being directly related: a smaller ρn leads to a higherjc in bulk samples. — The upper critical field shows a pronounced anisotropyBc2|/Bc2⊥ of about 20 to 30, whereBc2|=330 T is the extrapolated upper critical field at zero temperature for the field orientation parallel to the (Cu−O)-planes. — For the time being, theBc2 anisotropy and the weak-link structure of sintered bulk samples are the main obstacles for high field applications.

Superconductivity at 40 K in La1.8Sr0.2CuO4

Abstract: High-Tc superconductivity withTc onsets up to 42 K (midpoint: 37 K, zero resistance: 34 K) is observed in X-rays homogeneous single phase La1.8Sr0.2CuO4. The quarternary compounds La2−xBaxCuO4 and La2−xSrxCuO4 (0≤x≤0.3 for Ba and 0≤x<1 for Sr, depending on the heating conditions) are mixed-valence oxygen defect oxides, characterized by the presence of Cu2+ and Cu3+ simultaneously. These oxides have a tetragonal symmetry (space group:I 4/mmm) similar to that of K2NiF4. We report the synthesis, characterization, and superconducting properties of various high-temperature superconducting La−Ba−Cu−O and La−Sr−Cu−O compounds. Through the substitution of Sr for Ba in these oxygen defect compounds an increasing superconducting transition temperature onset from 28 K to 35 K for La1.8Sr0.1Ba0.1CuO4 was observed. A positive initial pressure coefficient ofdTc/dp=290 (mK/kbar) has been found for La1.8Sr0.2CuO4 with a magnetic susceptibility change consistent with the 100% diamagnetic expectation value.

Symmetry classification of states in high temperature superconductors

Abstract: The symmetry classification of possible singlet and also triplet states in the case of the 2-dim square lattice, and 3-dim tetragonal and orthorhombic lattices is examined. If particle-hole symmetry is present then an additional symmetry classification is possible. However in the lower symmetry crystal structures that actually occur, no distinction on symmetry grounds can be drawn between usuals-wave, extendeds-wave and some of thed-wave states.

Phase diagram and phase transitions of Krypton on graphite in the extended monolayer regime

Abstract: We report high resolution x-ray diffraction studies of the structures and phase transitions of monolayer krypton, adsorbed on both powder and single crystal graphite substrates. A comprehensive series of powder diffraction profiles is used to construct the two dimensional phase diagram. The melting of the $$\sqrt 3 x\sqrt 3$$ commensurate solid is shown to be strongly first order throughout the region where tricritical behavior was previously thought to occur; fluid solid coexistence extends up to the termination of the commensurate phase at 130 K. A disordered weakly incommensurate phase is shown to be a reentrant fluid, a system which may be described as a disordered network of domain walls and which evolves continuously into a more conventional 2D fluid. This evolution is marked by the disappearance of satellite peaks which are caused by the modulation of the overlayer by the substrate. The freezing of the reentrant fluid into the commensurate phase is shown to be consistent with a chiral Potts transition, its freezing into the incommensurate solid consistent with a dislocation binding transition. Single crystal experiments reveal the orientation of the weakly incommensurate phase. The reentrant fluid is found to have no visible orientational fluctuations, manifesting isotropic diffraction peaks. This is attributed to the strong epitaxy of domain walls. The incommensurate solid is shown to undergo an aligned-rotated transition which is well described by zerotemperature calculations.

Phase transformation and microtwinning in crystals of the high-T c superconductor YBa2Cu3O8−x,x≅1.0

Abstract: We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T c superconductor YBa2Cu3O8−x (x≅1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa2Cu3O8−x . The domain size ranges from typically 100 A to 1000 A. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.

Static structure function of turbulent flow from the Navier-Stokes equations

Abstract: The velocity field of a fully developed isotropic turbulent flow is decomposed into a smoothed fieldu and a strongly fluctuating partũ depending on a lengthr which is varied through all scales. The eddies described byu loose their energy either by direct dissipation or by energy transfer to theũ -eddies. Both contributions can be traced back to the second order static structure functionD(r) and the Lagrangian time correlation function. The latter can also be evaluated in terms ofD(r). The energy balance then gives an integro-differential equation forD(r) which determinesD(r) uniquely. The solution is not only in agreement with the scaling behaviour in the viscous and inertial subranges as predicted by dimensional arguments, but it also gives the correct transition between the two regimes. A comparison with experimental data is offered.

Critical wetting and surface-induced continuous phase transition on Cu3Au(001)

Abstract: In the bulk Cu3Au is known to undergo adiscontinuous order-disorder phase transition atT 0=663 K. Spin polarized low-energy-electron diffraction (SPLEED) reveals acontinuous surface-induced order-disorder phase transition-equivalent to a critical wetting transition—on the surface of Cu3Au. The results indicate that nearT 0 the observed singularity is related to the surface order parameter which can be described by a functional dependence of the formt β1 with β1=0.77±0.06.

X-ray absorption spectroscopy of the 90K superconductor YBa2Cu3O7−δ

Abstract: X-ray absorption spectroscopy has been used to probe the Cu environment of the new highT c YBa2Cu3O7−δ superconductor. Evidences of other valencies than two are found for copper. Besides Cu1+ which is localized on Cu1 sites when the unit cell is oxygendeficient, one invokes a ground state which merges bivalent 3d 9 to 3d 9 L configurations as proposed in a recent paper (L stands for a ligand hole in the CuO bond) very similar to that observed in many light rare earth oxides.

The glass transition singularity

Abstract: Formulae for the β-relaxation process as obtained within the mode coupling theory for the dynamics of supercooled liquids are derived for states near anA4 glass transition singularity. A discussion of the expected anomalies of susceptibility spectra is presented. In particular the parameter regions for (1/f)-noise, for spectra exhibiting two minima and regions of linear variations in ln(1/f) are identified. The results are used to interprete quantitatively dielectric data for the following polymeric systems: polychlorotrifluoroethylene (PCTFE), polyhexamethylene sebacamide (nylon 610), polyoxymethylene (delrin) and polyparaethylene oxybenzoate (PEOB).

On the structure of non-superconducting YBa2Cu3O6+ɛ

Abstract: The crystal structure of non-superconducting, tetragonal YBa2Cu3O6+ɛ (ɛ close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa2Cu3O7 samples under vacuum. For ɛ≈0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T c superconductor YBa2Cu3O7. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu−O−Cu−O-chains alongb in YBa2Cu3O7, is vacant in YBa2Cu3O6. Some crystals show a\(2\sqrt {2a} \times 2\sqrt {2a} \) superstructure with superstructure reflections which are sharp in thea*−a*-plane but form diffuse streaks alongc*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to ɛ>0 in the chemical formula YBa2Cu3O6+ɛ).

Influence of magnetic fields on taylor vortex formation in magnetic fluids

Abstract: The flow of a magnetic fluid placed inside a small gap between concentric rotating cylinders is investigated for axial, radial and azimuthal magnetic fields. An equation of motion is derived phenomenologically to describe the hydrodynamics of magnetic fluids. Studied are the changes in the critical Taylor numberTcand wave numberTcwhich characterize the instability of primary circular Couette flow towards Taylor vortices. It is found that all above magnetic fields have a stabilizing effect on circular Couette flow and thatTcincreases or decreases, depending on the direction of the magnetic field. Besides this, the influence of the magnetic fields on the correlation length ξ0, the wave number of maximal growthkmand the linear growth rate amplitude σ0 is determined.