A single parameter scaling theory in a disordered layered system

We show that in a disordered layered system all states are localized if k_Fl<4Πg_c, irrespective of the interlayer coupling t _⊥. The crossover of dimensionality depends on t_⊥. Near the critical value t_⊥^c at which the Anderson transition occurs, the correlation length ζ^m and the localization length ζ^loc are calculated.     

Quasi-two-dimensional transport properties of the layered superconductor Nd2−xCe x CuO4+δ

The temperature dependences of resistivities ρ_ab in the ab plane and ρ_c along the c axis have been studied for single-crystal Nd_{2 ? x} Ce_xCuO_{4 + δ} (x = 0.12, 0.15, 0.17, 0.20) films with (001) and (1 $\bar 1$ 0) orientations. The superconducting transition temperature and anisotropy coefficient are shown to be maximal in optimally annealed samples (the oxygen content is close to the stoichiometric content, δ → 0). A combination of the metallic behavior of the ρ_ab(T) dependence and the nonmetallic behavior of the ρ_c(T) dependence for the optimally annealed samples is an intrinsic property of the substance and an indication of the fact that the system is quasi-two-dimensional. This layered quasi-two-dimensional system is an Anderson dielectric with a strongly anisotropic localization radius (R _loc ^ab ? R _loc ^c ). An increase in the oxygen content and, hence, in the degree of disorder in the Nd_{2 ? x} Ce_xCuO_{4 + δ} system is found to decrease the resistivity anisotropy coefficient. Thus, a disorder-induced Anderson transition takes place in this quasi-two-dimensional system.     

Instability of polarons and bipolarons in conducting polymers at various 3-d ordering types

Interchain electron hopping t_⊥ in conducting polymers with the small confinement parameter γ leads to the antiferromagnetic (AB) 3-d ordering of the dimerization pattern only at 2t²_γ >γΔ² (2Δ being the Peierls gap), otherwise the ferromagnetic (AA) one occurs. Polarons (Ps, υ = 1) and bipolarons (BPs, υ = 2) stability areas: t_⊥/Δ<t_c (υ,γ) are found in the phase plane “t_⊥ vs γ” with critical t_c being larger in the AB phase and for BPs. At the AA-AB transition interface, “deconfinement” of kink-solitons is possible. Photogeneration of Ps and BPs may proceed via metastable free states separated by the self-trapping barriers (STBs) which are also different in AA and AB (in the latter STBs exist even in 2-d systems) and are lower for BPs.     

Localization and transmission coefficient for two coupled metal chains with disorder

The equation for distribution of probabilities for the transmission coefficient T has been obtained for an electron passing through the finite section of the length L, consisting of two coupled disordered chains. The behavior of the mean 〈lnT〉 = - L/l_loc suggests localization of the electron on the length l, which depends on the coupling energy t between the chains. The ratio l_loc(t =)/l_loc(t?v_F/l) is found to be equal to 1 ? 1/π.     

The behaviour of an isolated impurity near a phase transition in a spherical-like model

The behaviour of an isolated impurity in a crystal undergoing a displacive phase transition is investigated in an exactly soluble spherical-like model. We find, depending on the parameter of the impurity, either a global phase transition at the bulkT _c or a freezing-out of local order at a temperatureT _c ^loc >T _c driven by a soft local mode. We furthermore discuss the dynamic autocorrelation function of the impurity by introducing a phenomenological damping.Work Supported by The Swiss National Science Foundation     

Study of phase transitions and properties of tetragonal (Pb,La)(Zr,Ti)O3 ceramics—II: Diffuse phase transitions and thermodynamics

The diffuse phase transition in tetragonal PLZT materials has been investigated. Strongly broadened permitivity curves were observed for materials with a relative high La content. Curie-Weiss behaviour in the permittivity curves is observed only at temperatures far above the apparent Curie temperature.A latent heat effect is observed at the sharp FE_t → PE_c transition in PLZT x/30/70 materials with a low La content (x<16 at° La). This heat effect decreases linearly with increasing La content. The diffuse tetragonal → cubic transition is attended with a very small heat-effect and a low value (≈0.04 cal/mol°C) for the entropy difference between high and low temperature phase could be determined.Consequently, polarisation fluctuations are possible in a certain temperature interval around the transition.     

Transverse-momentum dependence of J/Ψ production in nuclear collisions

The suppression of J/Ψ production at transverse moment p_⊥ < 2 GeV/c in central ¹⁶O+²³⁸U→Ψ+X at 200 A GeV has been interpreted as a possible signature of quark-gluon plasma formation. We show, however, that the observed p_⊥ dependence is consistent with extrapolations from p+A→Ψ+X data, and that quasielastic initial-state parton scattering together with final-state inelastic hadronic reactions may explain the preliminary data.     

Spatial-temporal dispersion of the kinetic coefficients near the Anderson transition

A generalization of the Vollhardt-Wölfle localization theory is proposed to make it possible to study the spatial-temporal dispersion of the kinetic coefficients of a d-dimensional disordered system in the low-frequency, long-wavelength range (ω?_F and q?k _F ). It is shown that the critical behavior of the generalized diffusion coefficient D(q,ω) near the Anderson transition agrees with the general Berezinskii-Gor’kov localization criterion. More precisely, on the metallic side of the transition the static diffusion coefficient D(q,0) vanishes at a mobility threshold λ _c common for all q: D(q, 0)∝t=(λ _c ?λ)/λ _c →0, where λ=1/(2π?_F τ) is a dimensionless coupling constant. On the insulator side, q≠0 D(q,ω)∝?iω as ω→0 for all finite q. Within these limits, the scale of the spatial dispersion of D(q,ω) decreases in proportion to t in the metallic phase and in proportion to ωξ ², where ξ is the localization length, in the insulator phase until it reaches its lower limit ～λ _F. The suppression of the spatial dispersion of D(q,ω) near the Anderson transition up to the atomic scale confirms the asymptotic validity of the Vollhardt-Wölfle approximation: D(q,ω)?D(ω) as |t|→0 and ω→0. By contrast, the scale of the spatial dispersion of the electrical conductivity in the insulator phase is of order of the localization length and diverges in proportion to |t|^?v as |t|→0.     

Statics and dynamics of adhesion between two soap bubbles

An original set-up is used to study the adhesive properties of two hemispherical soap bubbles put into contact. The contact angle at the line connecting the three films is extracted by image analysis of the bubbles profiles. After the initial contact, the angle rapidly reaches a static value slightly larger than the standard 120^° angle expected from Plateau rule. This deviation is consistent with previous experimental and theoretical studies: it can be quantitatively predicted by taking into account the finite size of the Plateau border (the liquid volume trapped at the vertex) in the free energy minimization. The visco-elastic adhesion properties of the bubbles are further explored by measuring the deviation Δθ_d(t) of the contact angle from the static value as the distance between the two bubbles supports is sinusoidally modulated. It is found to linearly increase with Δr _c/r _c , where r_c is the radius of the central film and Δr _c the amplitude of modulation of this length induced by the displacement of the supports. The in-phase and out-of-phase components of Δθ_d(t) with the imposed modulation frequency are systematically probed, which reveals a transition from a viscous to an elastic response of the system with a crossover pulsation of the order 1rad · s^-1. Independent interfacial rheological measurements, obtained from an oscillating bubble experiment, allow us to develop a model of dynamic adhesion which is confronted to our experimental results. The relevance of such adhesive dynamic properties to the rheology of foams is briefly discussed using a perturbative approach to the Princen 2D model of foams.     

Random magnetic field and weak localization effects in a dimpled 2D electron gas

We have studied negative magnetoresistance due to the weak localization effects in a 2D electron gas (2DEG) grown on dimpled substrates. Since the 2DEG is sensitive only to the normal component of B, depending on the orientation of the external magnetic field, electrons will move in a spatially inhomogeneous (B perpendicular to the substrate-B_⊥) or sign alternating, random magnetic field (B parallel to the substrate B_∥). A difference in the magnetoresistance at B_∥ and B_⊥ is seen for the sample with a coherence length larger than the spatial periodicity of magnetic field. We believe that the difference in the magnetic flux through the closed electron trajectories at B_∥ and B_⊥, taken into account random character of B_∥, is responsible for this behaviour. Features connected with Aharonov Bohm flux through the different areas on the dimpled surface were observed.     

Thermoreflectance of single crystal Gd

Thermoreflectance spectra for E∥c and E⊥c were measured on a Gd single crystal. The thermoconductance spectra, Δσ_∥ and Δσ_t], are very anisotropic. The strong interband edge at about 0.5 eV is shown to arise from a transition to or from the Fermi level for E⊥c. For E∥c additional effects occur.     

Observation of πo mesons with large transverse momentum in high-energy proton-proton collisions

Invariant cross-sections are presented for the inclusive reaction p + p → π^o + anything, Measurements of large transverse momentum π^o's (2.5 GeV/c<p_⊥<9 GeV/c) were made near 90° at the CERN ISR at five centre-of-mass energies (√s = 23.5, 30.6, 44.8, 52.7 and 62.4 GeV. At large p_⊥, the invariant cross-sections are seem to vary with s and p_⊥, in good agreement with a fit of the form Ap_⊥^?nF(p_⊥/√s), with n≈8 and F(p_⊥/√s)≈exp(?26p_⊥/√s).     

Interacting one dimensional Ising spin chains as a model of the layered antiferroelectric squaric acid

Starting from the structure of the layered antiferroelectric squaric acid and assuming an ice rule type interaction mechanism of the protons it is shown that aboveT _c the molecular layers may be considered as a system of parallel and orthogonal nearly independent linear Ising spin chains with strong intrachain and comparatively weak interchain interactions. Whereas the former are treated exactly as nearest neighbour interactions the interchain interactions are taken into account either by the molecular field approximation or, in computer simulations, by nearest neighbour interactions. The parameters of the model are related to the unusually small transition entropy, to the high and nearly temperature independent dielectric constant aboveT _c ,and to neutron scattering data.     

Change in character of the paramagnetic to helimagnetic transition in CrAs upon MnAs substitution

The structural and magnetic characteristics of the para- (P) to helimagnetic (H_c) transition in Cr_1-tMn_tAs (0.00 ⩽ t ⩽ 0.67 ±0.01) have been studied by X-ray diffraction, differential scanning calorimetry and magnetic susceptibility measurements. The distinctly first order character of the P to H_c transition is retained at low substitution levels of Mn for Cr, as manifested by the observation of latent heat, temperature hysteresis and discontinuous jumps in the unit cell dimensions. All these effects decrease with increasing t, and in the vicinity of a pseudo tricritical point at t = 0.17 ± 0.02 the discontinuities in the a and c axes (Pnma setting of the MnP type unit cell) and the temperature hysteresis are no longer unambiguously detactable. However, the gradually diminishing discontinuity in the b axis is maintained, and, together with the behaviour of the heat capacity, this shows that the first order character of the P to H_c transition persists up to t = 0.32 ± 0.02. These unusual findings are discussed in terms of the concept of pseudo tricritical point, predicted by Bergman and Halperin for a compressible Ising system.     

无序层状系统的电导率

本文运用顶角的Post-CPA方法研究了无序层状系统的维数跨越效应,层间耦合t的强弱决定了系统的有效维数。对于耦合的临界值t_c,Anderson转变发生。我们求出了系统的玻耳兹曼电导率、对应于最大交叉顶角的动力学电导率和直流电导率以及临界点附近的局域长度。 关键词：     

Commensurate-incommensurate transition in two-coupled chains of nearly half-filled electrons

We investigate the physical properties of two coupled chains of electrons, with a nearly half-filled band, as a function of the interchain hopping t _⊥ and the doping. We show that upon doping, the system undergoes a metal-insulator transition well described by a commensurate-incommensurate transition. By using bosonization and renormalization we determine the full phase diagram of the system, and the physical quantities such as the charge gap. In the commensurate phase two different regions, for which the interchain hopping is relevant and irrelevant exist, leading to a confinement-deconfinement crossover in this phase. A minimum of the charge gap is observed for values of t _⊥ close to this crossover. At large t _⊥ the region of the commensurate phase is enhanced, compared to a single chain. At the metal-insulator transition the Luttinger parameter takes the universal value K _ρ ^* = 1, in agreement with previous results on special limits of this model. Received 31 July 2000     

Analyticity of the density of states and replica method for random schrödinger operators on a lattice

We analyze the density of states and some aspects of the replica method for Anderson's tight binding model on a lattice of arbitrary dimension, with diagonal disorder. We give heuristic arguments for the conjectures that the classical value of the exponent of the localization length is 1/2 and that the upper critical dimension,d _c ^loc , is bounded by 4d _c ^loc 6.Work supported in part by NSF Grant No. DMR 8100 417.     

液晶N相→SA相相变点附近的反常声衰减

本文考虑在N→S_A相变中,当T→T_c⁺(相交点)时,声在液晶中传播引起的压力效应。使用模-模耦合方法计算了声的衰减和色散,得到在高频区间,声的衰减有反常发散行为,在低频极限声衰减的临界指数为2.5.当T→T_c⁺时,本文的理论曲线大体上与实验结果相一致。     

Superconductivity in Au(10Å)/Ge(13Å) alternating ultra-thin layered films

We report the observation of a unique tricritical superconducting transition behaviour (a peak in the R-T curve below T_c) in Au(10Å)/ Ge(13Å) alternating ultra-thin layered sandwich films. A first transition from the normal to the superconducting state occurs at around 2K, a second (back to the normal state) at about 1.8K, and finally a third at about 1.7K (again to the superconducting state). The tricritical transition appearance is accompanied by a resistance anomaly and an electromotive force anomaly in the superconducting transition regime. The H_c2 behaviour is strongly anisotropic. From an analysis of the H_c2 anisotropy, the system seems to be a three dimensional coupled-layer superconductor.     

Critical dynamics and stability of elastically coupled systems at marginal dimensionalities

Static and dynamic behavior of an-component classical model atd=4 has been investigated assuming a coupling to a fluctuating lattice displacement field. Solutions of renormalization-group (RG) equations are given for elastically isotropic and anisotropic systems, and the temperature dependences of elastic constants and of the corresponding damping coefficients are calculated. For isotropic and weakly anisotropic systems it is found that forn<4 the critical regime can be split into a rigid regimet>t _s , and a compressible regimet<t _s , wheret=(T–T _c )/T _c andt _s is a crossover temperature. In the rigid regime, the logarithmic correction factors characterizing deviations from Landau theory have the same form as in systems without elastic coupling; in the compressible regime the exponents are renormalized by the coupling. Forn4 rigid behavior prevails at all temperatures; similarly only rigid behavior is found for strongly anisotropic systems for alln. The thermodynamic stability of the system is investigated by evaluating the contribution of ring diagrams for the casen=1. It is thus shown that under constant hydrostatic pressure a first-order transition occurs in both isotropic and anisotropic systems, and the corresponding equations for the transition temperature and the value of the order parameter atT _c are derived.