Frustrated percolation,spin glasses and glasses

Summary The static and dynamic properties of the frustrated percolation model are investigated. This model, which contains frustration as an essential ingredient, exhibits two transitions: a percolation transition at a temperatureT _p with critical exponents of the ferromagnetic (s=1/2)-state Potts model, and a second transition at a lower temperatureT _g in the same universality class of the Ising spin glass model. AboveT _p the time-dependent autocorrelation function is characterized by a single exponential, while forT _p>T>T _g preliminary numerical results show a broad shoulder or plateau typical of a structural glass transition. BelowT _g the system is in glassy state with an infinitely long relaxation time. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO₃ shows a sharp anomaly at the spin-Peierls transition temperatureT _sp . The experimental decrease of the magnetic specific heat belowT _sp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesT _sp only slightly but reduces the spin gap by a factor of two.     

On the behaviour of the new Kondo lattice CeCuAl3

The temperature dependence of the electrical resistivity, thermopower, specific heat, susceptibility and magnetization of CeCuAl₃ are presented. CeCuAl₃ behaves as a Kondo lattice system with antiferromagnetic ground state properties (T _N 2.8 K). The valency of Ce in this tetragonal compound is close to 3 and the overall crystal field splitting found from our results is about 150 K. The Kondo temperatureT _K in the crystal field ground state, estimated from the magnetic susceptibility and the specific heat, is of the order of 8 K.     

Dynamic properties of the anderson model at finiteU

TheN-fold-degenerate single impurity Anderson model is studied at finiteU, using the perturbation expansion in the hybridizationV. The 1/N-expansion and the energy-dependence of the self-energies allow to set up a system of self-consistent integral equations for the self-energies, and thef-spectral function. Thef-spectral density and transport coefficients (the electrical resistivity , the Lorenz ratioL/L ₀ , the thermoelectric powerS, and the Hall-coefficientR _H ) are calculated numerically via the self-consistent integral equations. We are particularly interested in the effect of the Coulomb interaction on the dynamic properties. Thef-spectral function is calculated at small and largeU. It shows a sharp peak at the Fermilevel (the Kondo-Abrikosov-Suhl resonance) and two broad peaks around _f and _f +U. The structure at the Fermi-level is very sensitive to the Coulomb interactionU. In order to get more information about the model, we have studied the transport coefficients at differentU, which are universal functions of the temperatureT/T _K , where the Kondo-temperature sets the scale. We find that the transport coefficients show Kondo behavior aroundTT _K , and are also sensitive toU. The Hall-coefficient is negative forTT _K already within the impurity model. This competes with the usual interpretation as a coherence effect caused by the lattice.     

Exact quantum partition function of the BCS model

The exact calculation of the reduced BCS model quantum partition function (QPF) in the thermodynamic limit is carried out by the path integration method. The expression for the QPF and the phase transition temperatureT _c in the regular phase coincide with the results of Bogolyubov. In the nonregular phase a temperature singularity appears in the expression for the QPF: the QPF diverges in the region of temperaturesT _c which are smaller than some critical temperatureT _c ^* , and it turns out that in all casesT _c ^* > T _c and the differenceT _c ^* –T _c is not small. The interpretation of the temperatureT _c ^* is given.     

Superconductivity and normal electrical conductivity of amorphous lead films nucleated with molybdenum

Lead films vapor quenched onto nucleating monolayers of Mo or W exhibit strong lattice disorder and can be considered to be amorphous. The amorphous-to-crystalline transformation temperatureT _tr is indicated by a sharp drop of the electrical resistivity in the course of annealing.T _tr is found to be proportional tod ^–2 for Pb thicknessd smaller than 30 nm. The superconducting transition temperatureT _c is by 0.6 to 1 K smaller in the amorphous state than after crystallization. In both states,T _c is proportional tod ^–1. Prenucleation with about half a monolayer of Mo leads to quite the sameT _c depression as observed earlier by Strongin et al. on Pb films vapor quenched onto predeposited films of SiO, Ge or Al₂O₃. For comparison, experiments have been carried out with 2.5 nm Ge predeposits. As with Mo prenucleation, a well defined transformation temperatureT _tr of about the same value has been observed.T _c of bulk amorphous Pb can be extrapolated to be about 6.6 K.     

Nuclear spin-electron coupling of copper at microkelvin temperatures

The nuclear spin-electron coupling of copper has been determined by establishing stationary temperature differences between nuclei and electrons at electronic temperatures of 15 K to 5 mK in magnetic fields of 2 mT to 440 mT using a well-defined heat flow from the electrons to the nuclear spin system. The measured Korringa constantk increases proportional toB/T _e ² with decreasing electronic temperatureT _e and with increasing magnetic fieldB. At the lowest temperaturesk is more than an order of magnitude larger than its high temperature value.     

Recoilless fraction studies of iron near the Curie temperature

The recoilless fractionf of⁵⁷Fe in iron is determined as a function of temperatureT in the vicinity of the Curie temperatureTc using a source of⁵⁷Co diffused in a foil of 100% enriched⁵⁶Fe. Our results show thatf does not undergo an abrupt change atTc, but that in the ferromagnetic phase |df/dT| is larger than in the non-ferromagnetic phase. This indicates that the ferromagnetic interaction increases the bond between the iron atoms, and thus contributes to the stability of the iron lattice belowTc.     

Mechanoluminescence excitation in alkali halide crystals and colouration decay in microcrystalline powders

The mechanoluminescence (ML) of NaCl, NaBr, NaF, LiCl and LiF crystals ceases at 105, 58, 170, 151 and 175°C respectively. Both the temperatureT _c at whichML disappears and the temperatureT _s required to induce a particular percentage of colouration decay in a given time, decreases with increasing nearest neighbour distance in alkali halide crystals. This perhaps suggests that similar processes cause the disappearance ofml in alkali halide crystals and the colouration decay in their microcrystalline powders. It is shown that mobile dislocations may cause the leakage of surface charge and the decay of colouration in microcrystalline powders.     

Nonlinearity in cooperative systems-dynamical Bethe-Ising model

The dynamics of the short-range order as well as the long-range order in the nonlinear cooperative system is investigated specifically for a kinetic Ising model in the Bethe approximation. The phenomena of critical slowing down near the transition temperatureT _c and anomalous fluctuation belowT _c are directly related to the instability of the long-range order. The dynamics of the short-range order is essentially a fast mode and is noncritical. However, through the nonlinear coupling the short-range order is also influenced by the critical behavior of the long-range order.     

Dielectric properties of superionic lead fluoride

Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition temperatureT _c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation mechanism,ε is found to diverge at the threshold temperatureT _th where ionic conduction starts.     

A mode coupling analysis of the central peak at order disorder phase transitions

The influence of local and translation invariant memory effects on the critical dynamics of a model undergoing a continous structural phase transition from a disordered (T>T _c ) to an ordered distorted phase (T>T _c ) is studied by mode coupling theory above the critical temperatureT _c . It is shown that besides the existence of critical slowing-down modes there also exists a central peak as a consequence of correlations of the critical modes, increasing with the critical exponent when approachingT _c . The dependence of the central peak on the wavevector , temperatureT and on the spatial dimensiond is investigated. Ford=3 a scenario withlocal long time memory correlations is compared with respect to its temperature and momentum dependence with a model using translation invariant long time memory correlations.     

Transport properties of the Anderson lattice

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 temperatureT ₀, with high temperature logarithmic decrease and low temperatureT ² 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.     

Superconductivity and magnetic properties in Pr0.2Yb0.8−xLaxBa2Cu3O7−δ

The crystal structure, superconductivity and magnetic properties of the title compounds have been investigated. The results revealed that with the concentration of Pr being constant, the transition temperatureT _c of this system decreases with the increasing of La, or rather with the increasing of effective ionic radii of rare earth elements and the superconductivity disappears at aboutx=0.6. The coefficient of electron specific heat and the density statesN (0) which were infered from the magnetic measurements also decrease with the increasing of the effective radii. The relation between the radii of rare earth elements and hybridization is briefly discussed.     

CePd2Ga3: A new ferromagnetic Kondo lattice

Various temperature-, pressure- and field dependent investigations on CePd₂Ga₃ indicate this ternary compound as belonging to the group of ferromagnetically ordered Kondo lattices, with the Curie temperatureT _C =6K and the Kondo temperatureT _K =4K. The first excited crystal field level of this hexagonal compound is about 40 K above the crystal field ground state, while the overall splitting is much larger.     

CO2-laser gas discharges in narrow gaps

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO₂ lasers. For CO₂ laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 m required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO₂ lasers we have measured the electron temperatureT _e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT _e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV.We have estimated the electron densityn _e and the current densityj _e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj _e is in the range of abnormal glow discharges.In the gas discharges between narrow gaps the electron oscillation amplitudex _e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT _e is necessary to sustain the gas discharge. Because of this high electron temperatureT _e an efficient pumping of the upper laser level is not possible.     

On the relaxation behaviour of a cluster glass aboveT f

The cluster glass Y(Fe _x Al_1−x )₂,x=0.75, 0.65 and 0.40 was investigated in external fields between 4.5 and 13.5 T at temperatures above the freezing temperatureT _f. To explain the relaxation effects, which dominate the magnetic behaviour up toT∼7·T _f, a model is proposed, where magnetic clusters are formed stochastically and decay after a mean life time.     

Two-valley model for studying the low temperature phases inn-type inversion layers of semiconductors

The low temperature phases of a two-valley model forn-type inversion layers are studied. If the coupling constants are in a certain range charge-density waves exist below a transition temperatureT _c. For reasonable values of the coupling constantsT _c is around 30 °K.     

Analysis of the temperature dependence of the heat capacity of solid tin in the vicinity of its melting temperature

Summary The experimental temperature dependence of the heat capacityC _p(T) of solid tin (Sn) in its premelting region 402.07–502.22 K was investigated and two regions with different temperature dependences ofC _p(T) were found. In the far (from the melting temperatureT _m) region 402.07–485.88 K the experimentalC _p(T) of Sn is described by the standard vacancy model. In the close region 485.88–502.22 K it is described by the formation of complicated volumetric defects in the crystalline lattice of solid Sn near itsT _m.