a-b plane anisotropy of single-domain crystals of Bi2Sr2CaCu2O8

The polarized reflectance of thea-b plane of single-domain Bi₂Sr₂CaCu₂O₈ crystals is anisotropic above and belowT _c . The normal-state infrared conductivity is higher forEa whereas the high-frequency conductivity is higher alongb, particularly for transitions associated with the Bi–O layers. BelowT _c there is a definite anisotropy to the far-infrared absorption, with a finite absorption forEb down to 20 meV. This anisotropy of thea-b plane could be due either to anisotropy of the superconducting gap or to anisotropy of the midinfrared component to the conductivity.     

Electronic phase separation in La-Cuprates

The mechanisms driving the phase separation in high-T _c La cuprates is studied by magnetic susceptibility and electrical resistivity measurements. The experiments are performed under different thermal treatment. Three temperature ranges can be resolved characteristic for different physical processes: At 150T170K a space limited diffusion of superparamagnetic clusters (single hole clusters) occurs leading to the formation of a small but rather tight percolative subphase which at highT _c values becomes superconducting. Above 180 K the cluster motion becomes unrestricted in space and a more extended subphase is built up, however, with lowerT _c values. Above 230 K oxygen diffusion starts leading to an unexpected destruction of the conducting/superconducting phase.     

Anisotropic singlet superconductivity for systems with strongly correlated electrons

The periodic Anderson model extended by pairing interactions amongf- and conduction electrons is investigated in the context of the singlet-like superconductivity. We evaluate the transition temperatureT _c and the ratios of superconducting order parameters atT _c as functions of an effective parameter which describes the correlation off-electrons. The relative stability of thes- andd -superconductivity depends on the values of the Fermi wave vector and on the magnitude of . Due to the absence of on-site pairing forf-electrons, the increase of Coulomb correlations makes thes-like superconductivity more favourable than thed -superconductivity. This behaviour is accompanied by crossover effects: with the increase of Coulomb correlations amongf-electrons the superconductivity is taken over by conduction electrons.Supported by PAN, CPBP 01.12     

Specific heat of the two dimensional antiferroelectric squaric acid

The heat capacity of a single crystal of the two dimensional antiferroelectric squaric acid was measured particularly near the phase transition. In a wide temperature range belowT _c a strong temperature dependence of the anomaleous specific heat is found which can be described by a critical exponent=0.5. The behaviour close toT _c is determined by a latent heat thus indicating the phase transition to be of the first order. Near the phase transition the thermal behaviour can be described by a Landau type theory. The smallness of the transition entropy detected may be ascribed possibly to proton tunneling.     

Pressure-volume relationship,quasiparticle density of states and hybridization gap of mixed valence EuO

Thes-f model, extended by a hybridization term, is used to investigate the pressure-dependence of the quasiparticle density of states of paramagnetic EuO (T=300K). We find a semiconductor-metal transition of first order atp _c =301 kbar, which manifests itself by a striking discontinuity in the pressure-volume relationship, in almost exact agreement with the experiment [5]. The transition is due to the valence-change Eu²⁺Eu³⁺ of the Europium ion.-Special attention is devoted to the competing influence of thes-f exchange interaction and thes-f hybridization between extendeds-band states and localizedf-levels on characteristic details of the phase transition. Exchange and hybridization are therefore taken into account exactly, while the electron hopping is treated in an approximate many body theory.-As a typical feature we find a small hybridization gap in the quasiparticle density of states. Forp<p _c the chemical potential lies always within this gap, so that the material is a narrow gap semiconductor. Forp<p _c the gap does survive, but then is below the gap allowing metallic conductivity.     

On the absence of intermediate phases in the two-dimensional Coulomb gas

For a simple, continuum two-dimensional Coulomb gas (with soft cutoff), Gallavotti and Nicoló [J. Stat. Phys. 38:133–156 (1985)] have proved the existence of finite coefficients in the Mayer activity expansion up to order 2n below a series of temperature thresholdsT _n =T [1+(2n–1)^–1] (n=1, 2,...). With this in mind they conjectured that an infinite sequence of intermediate, multipole phases appears between the exponentially screened plasma phase aboveT ₁ and the full, unscreened Kosterilitz-Thouless phase belowT T _KT. We demonstrate that Debye-Hückel-Bjerrum theory, as recently investigated ford=2 dimensions, provides a natural and quite probably correct explanation of the pattern of finite Mayer coefficients while indicating the totalabsence of any intermediate phases at nonzero density ; only the KT phase extends to >0.     

Quantum BRST charge for quadratically nonlinear Lie algebras

We consider the construction of a nilpotent BRST charge for extensions of the Virasoro algebra of the form {T _a ,T _b }=f _ab ^c T _c +V _ab ^cd T _c T _d , (classical algebras in terms of Poisson brackets) and [T _a ,T _b ]=h _ab I+f _ab ^c T _c +V _ab ^cd (T _c T _d )(quantum algebras in terms of commutator brackets; normal ordering of the product (T _c T _d ) is understood). In both cases we assume that the set of generators {T _a } splits into a set {H _i } generating an ordinary Lie algebra and remaining generators {S }, such that only theV _ij are nonvanishing. In the classical case a nilpotent BRST charge can always be constructed; for the quantum case we derive a condition which is necessary and sufficient for the existence of a nilpotent BRST charge. Non-trivial examples are the spin-3 algebra with central chargec=100 and theso(N)-extended superconformal algebras with levelS=–2(N–3).     

High-resolution measurements of thermal expansion along the uniqueb-axis of squaric acid

The thermal expansion of squaric acid along the uniqueb-axis was measured between 323 K and 418 K with a resolution of 2·10^–7 in strain and 5·10^–4 K in temperature using a capacitance dilatometer. The anomaly associated with the antiferro-electric phase transition occuring atT _c=373.5 K was observed over the rangeT _c–50 K<T<T _c+7 K. The regular lattice expansion follows a generalized Grüneisen's law with a Debye-temperature _D =660 K and a uniaxial pressure-dependence of _D /p _b . The critical behaviour close toT _c was studied with a sweep rate of 36 mK/h. A hyteresis loop of width 23 mK was observed in the strain curve, indicating a 1st order transition. However, no discontinuity in dilatation was observed, and therefore the transition is close to a multicritical point. The anomalous increase in the expansion coefficient can be described with critical exponents ==0.56 in the interval 0.4 K<|T–T _c|<12 K. Closer toT _c the divergence is stronger. A generalized Pippard relation between the expansion coefficient and specific heat holds to within 1 K from the transition point. The inconsistencies apparent closer toT _c are interpreted as a consequence of a weak 1st order transition.     

The induced action ofW 3 gravity

We obtain the induced action [h, b] for chiralW ₃ gravity in thec± limit from the induced action of a gaugedSl(3,R) Wess-Zumino-Witten model by imposing constraints on the currents of the latter. In the process we find a closed gauge algebra for the gauge sector ofW ₃ gravity in which the currentsT andW become auxiliary fields. An explicit realization ofT andW in terms of the gauge fields is given. In terms of new fieldsr ands, which are a generalization of Polyakov'sf variable for ordinary gravity, the complete induced action [h, b; c±] becomes local.Work supported in part by NSF grant No. PHYS 89-08495Address after September 1, 1991: Physics Department, U.C. Berkeley, Berkeley, CA 94720, USA     

Absence of the partially ordered phase in theq-state models

We investigate theq-state models called (N ,N ) model using an infinitesimal Migdal-Kadanoff renormalization-group method. We distinguish two cases namely the isotropic model and the anisotropic model. The first one presents a critical value ofq,q _c such that forq _c we obtain an Ashkin-Teller phase diagrams while forq>q _c the partially ordered phase disappears then the model exhibits only phase transition between ferromagnetic phase and disordered one. The phase diagrams in the second case are qualitatively similar to one obtained forZ(6) model for all values ofq.     

Transition temperature and isotope effect near an extended van Hove singularity

Using Eliashberg theory and a model density for ² F the transition temperatureT _c and the isotope effect are calculated near an extended van Hove singularity. We show that, at least in the one-particle and the Migdal approximation, even the considered strong van Hove singularity cannot yield large enhancements ofT _c and strong reductions of of the kind observed in experiment around optimal doping.     

The wetting transition in a random surface model

We continue our analysis of the phase diagram of a discrete random surface, with no downward fingers, lying above a flat two-dimensional substrate. The surface is closely related to the 2D Ising model and its free energy is exactly solvable in much (but not all) of the phase diagram. There is a transition at temperatureT _w from a high-T infinite height or wet phase to a low-T finite height or partially wet phase. Previously it was shown that when a parameterb, related to the contact interaction, is positive,T _w is independent ofb and there is a logarithmic specific heat divergence asT _w is approached fromeither side. Here we show that forb<0,T _w does depend onb and there isno thermodynamic singularity from the wet phase. The partially wet phases forb0 andb>0 differ in the absence or presence of a monolayer covering the entire substrate; this results in a first-order transition across the lineb=0,T<T _w.This paper is dedicated to Jerry Percus.     

Growth of clusters in a first-order phase transition

The results of computer simulations of phase separation kinetics in a binary alloy quenched from a high temperature are analyzed in detail, using the ideas of Lifshitz and Slyozov. The alloy was modeled by a three-dimensional Ising model with Kawasaki dynamics. The temperature after quenching was 0.59T _c, whereT _c is the critical temperature, and the concentration of minority atoms was=0.075, which is about five times their largest possible single-phase equilibrium concentration at that temperature. The time interval covered by our analysis goes from about 1000 to 6000 attempted interchanges per site. The size distribution of small clusters of minority atoms is fitted approximately byc ₁(1-)³ w(t),c ₁ (1–)⁴ Q _l w(t)^l(2l10); wherec _l is the concentration of clusters of sizel;Q ₂,...,Q ₁₀ are known constants, the cluster partition functions;t is the time; andw(t)=0.015(1+7.17t ^–1/3). The distribution of large clusters (l20) is fitted approximately by the type of distribution proposed by Lifshitz and Slyozov,c _l ,(t)=–(d/dl) [lnt+p (l/t)], where is a function given by those authors and is defined by(x)=C _o e–x-C ₁ e ^–4x/3-C ₂ e ^–5x/3;C ₀,C ₁,C ₂ are constants determined by considering how the total number of particles in large clusters changes with time.Supported by the U.S. Air Force Office of Scientific Research under Grant No. 78-3522 and by the U.S. Department of Energy under Contract No. EY-76-C-02-3077*000.     

Structural changes in α → γ phase transformations of polycaprolactam

The transition from the to the modification of polycaprolactam caused by treatment with iodine was studied by means of X-ray diffraction. The measurements were carried out on foils with a plane texture. During treatment with aqueous solutions of LiI₃, NaI₃, KI₃ and RbI₃ stable iodine complexes of polycaprolactam were produced. These complexes have a three-dimensional, not quite ordered structure and are similar to one another. Their unit cell has the dimensionsa=15,5 å,b=4,4 å,c=24,7 å; the edges of the unit cell are perpendicular to one another. The iodine forms in the complexes columns that are parallel to the polycaprolactam chains which are arranged into planes parallel to the plane of the foil. The action of water causes the complexes to disintegrate and when the iodine is completely eliminated from the foil the modification of polycaprolactam, which has an equally perfect texture as the initial modification, is formed.In conclusion, the author would like to thank Dr. K. Toman, Dr.Sc., for guiding the work and Ing. B. Sedláek, Dr.Sc., for his interest.     

Thermal fluctuations in high-temperature superconductors

The effect of fluctuations and anisotropy on the transition from the normal to the superconducting state are studied. Neglecting magnetic fluctuations, which is justified as long as the Ginzburg-Landau parameter 1, the critical behavior belongs to thexy-universality class including superfluid helium. Since (t)=₀ t ^2/3, wheret=1-T/T _c , upon approachingT _c further, the intrinsic fluctuating magnetic field might change the nature of the transition. Concentrating on thexy-regime, we derive with the aid of the helicity modulus a universal relation betweenT _c and the amplitudes of the phase correlation length and penetration depth. We also extend the universal critical point amplitude relations to the case of superconductors with uniaxial mass anisotropy. Our analysis of recent specific heat and excess dc conductivity measurement suggest that for both static and dynamic properties three-dimensional critical behavior has been observed. The -like specific heat singularity points to criticalxy-behavior. Further evidence is provided in terms of the universal amplitude relations, providing estimates for the amplitudes of the correlation lengths for the magnitude and phase of the order parameter and the London penetration depth. We find remarkable agreement with experiment and the correlation volume is comparable to that in superfluid helium.     

Analysis of anisotropic electrical resistivity and fluctuation conductivity of YBa2Cu3O7−δ single crystals

For a large number of YBa₂Cu₃O_7– single crystals which were grown by a flux method in different crucibles the anisotropic resistivity has been measured. An analysis of the metallic in-plane resistivity _ab gives evidence for different carrier concentrations in the individual samples caused by different amounts of oxygen disorder effects in theab-planes. The temperature dependence of the out-of-plane resistivity c which is also metallic for the best samples is determined by the applied crucible material. From the luctuation conductivity in theab-plane aboveT _c the coherence length _c (0)2.5 Å can be determined in good agreement with values obtained fromH _c2 measurements.Dedicated to Prof. Dr. G. von Minnigerode on the occasion of his 60th birthday     

Deviation of Upper Bounds of Critical Temperature of Two-Dimensional O(N) Spin Models

Critical temperature of the classical O(N) spin model in two dimensions is investigated. We show that no phase transition exists in the system if the inverse temperature is less than _c=_c(N), where _c(N) is a constant such that _c(N) > const. N log N.     

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt₃. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingT _c. Our results provide evidence that the lower superconducting phase in theH-T phase diagram of UPt₃ is characterised by broken time-reversal symmetry. Measurements of URu₂Si₂ and CeCu_2.2Si₂ indicate that the magnetically ordered volume fraction is temperature dependent in both systems.     

Calculation of quasi-stationary state parameters within the algebraic version of the resonating-group method

The dynamical equations of the algebraic version of the resonatinggroup method are used to determine the positionsE _res,l and the widths _l of quasi-stationary states or, what is equivalent, to find the polesE _l =E _resl –i _l /2 of theS-matrix in the fourth quadrant of thek-plane. Concrete calculations are performed for the -decay of⁸Be. A comparison with the results of other authors and with experiment is made.     

Far-infrared reflectivity of sintered YBa2Cu3O7 in the normal and superconducting state

We report on a study of the far-infrared reflection for a sintered YBa₂Cu₃O₇ sample that contained a large portion of preferentially oriented crystallites with thea–b plane parallel to the surface and that showed extraordinary high far-infrared reflectivity. From experimental reflection data we determined, by Kramers-Kronig analysis, the dynamical conductivity and extracted the contributions due to free charge carriers and phonons, respectively. We find evidence for an anomalous behavior of the dynamical conductivity at temperatures aboveT _c ; the dynamical conductivity increases strongly for temperatures approachingT _c and is strongly frequency dependent. By use of the Mattis-Bardeen theory we obtain an estimate for the superconducting energy gap of 2/kT _c 4.6 (forTT _c ). We find that the lowest frequency infrared-active phonon mode is less damped in the superconducting state than in the normal state.