On the lower critical field and phase diagram of a thin cylindrical type-II superconductor

The lower critical field H _c1 ^cyl (T) of a superconducting cylinder with radius r ₀∼ξ(T)≪λ(T) is found on the basis of the Ginzburg-Landau theory with various boundary conditions. These results together with the well-known results for the upper critical field are used to construct phase diagrams in terms of the field versus the reduced radius r ₀∼ξ(T) variables. The jump in the average magnetization at H _c1 ^cyl (T) is calculated as a function of the reduced radius. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 537–542 (25 April 1999)     

Dynamical Correlation Functions for an Impenetrable Bose Gas with Neumann or Dirichlet Boundary Conditions

Abstract

We study the time and temperature dependent correlation functions for an impenetrable Bose gas with Neumann or Dirichlet boundary conditions ψ(x ₁, 0)ψ ^?(x ₂ , t) _±,T . We derive the Fredholm determinant formulae for the correlation functions, by means of the Bethe Ansatz. For the special case x ₁ = 0, we express correlation functions with Neumann boundary conditions ψ(0, 0)ψ ^?(x ₂ , t) _+,T , in terms of solutions of nonlinear partial differential equations which were introduced in [1] as a generalization of the nonlinear Schrödinger equations. We generalize the Fredholm minor determinant formulae of ground state correlation functions ψ(x ₁)ψ ^?(x ₂) _±,0 in [2], to the Fredholm determinant formulae for the time and temperature dependent correlation functions ψ(x ₁, 0)ψ ^?(x ₂ , t) _±,T , t ∈ R, T ≥ 0.     

Spin-wave theory for finite classical magnets and superparamagnetic relation

Analytical calculations based on finite-size spin-wave theory and Monte Carlo (MC) simulations are performed to investigate the validity of the well-known relation m(H, T) = M(H, T)B _D[M(H, T) H/T] between the induced magnetization m of the magnetic particle and its intrinsic magnetization M for the Ising and isotropic classical models (B _D(x) is the Langevin function, D is the number of spin components, is the number of atoms in the particle). It follows from general arguments and from our analytical results for the Heisenberg model at T≪T _c that this relation is not exact for any finite D and nonzero temperature. Nevertheless, corrections to this formula remain very small practically in the whole range T < T _c if ≫ 1, as confirmed by our Monte Carlo calculations. At T T _c/4 there is a good agreement between the MC and finite-size spin-wave calculations for the field dependence of m and M for the Heisenberg model with free boundary conditions. Received 1st December 2000     

Analysis of inelastic scattering processes of electrons by localized electrons in quantum dots

The inelastic Coulomb scattering rate 1/τ_in of conduction electrons has been theoretically evaluated in the presence of localized states such as quantum dots. By a diagrammatical method, we have formulated 1/τ_in and its relation to the conductivity σ_loc(ω) through localized states. The dependence of τ_in on temperature T is examined in the case that σ_loc(ω) follows the Mott's model. It is found that 1/τ_in varies as T²(ln Δ/T)^d+1 where d is the dimensionality and Δ is tunneling energy between the localized states in the asymptonic T = 0 limit, in agreement with Imry's calculation. It is also found that calculated 1/τ_in deviates from T²(ln Δ/T)^d+1 as T increases, suggesting the importance of correction term at high temperature.     

NMR and quantum chemistry study of mesoscopic structuring in 3-methylpyridine/water/NaBr mixture in the vicinity of the phase separation boundary

²³Na and ⁸¹Br NMR spin-lattice relaxation times and signal half widths (Δ_1/2) have been measured in 3-methylpyridine (3MP)/H₂O/NaBr mixture along T?=?294 and 301 K isotherms gradually increasing the mass fractions of salt (X) up to the phase separation boundary. The extreme narrowing condition and thus 1/T ₁?=?1/T ₂?=?πΔ _1/2 was found to be valid in all cases. Discontinuous changes in slope of 1/T _1,2?=?f(X) were detected, and then corresponding points on the phase diagram (X, T) were attributed to the borderline between the molecular-ionic solution and the area of enhanced mesoscopic structuring. A very strong relaxation effect was observed for ⁸¹Br nuclei reaching relaxation rates of 14,000 s^–1. ²³Na and ⁸¹Br NMR relaxation data together with calculations of quantum chemistry model of electrical field gradient tensor evidence the migration of 3-methylpyridinium at increasing X from the anions hydration shells towards cations. An interchange from migration to steady distribution regimes is observed for anions and vice versa for cations at the borderline of the structured phase.     

Subextensive Singularity in the 2D ± <Emphasis Type="Italic">J</Emphasis> Ising Spin Glass

The statistics of low energy states of the 2D Ising spin glass with +1 and −1 bonds are studied for L × L square lattices with L≤ 48, and p = 0.5, where p is the fraction of negative bonds, using periodic and/or antiperiodic boundary conditions. The behavior of the density of states near the ground state energy is analyzed as a function of L, in order to obtain the low temperature behavior of the model. For large finite L there is a range of T in which the heat capacity is proportional to T ^{5.33 ± 0.12}. The range of T in which this behavior occurs scales slowly to T = 0 as L increases. Similar results are found for p = 0.25. Our results indicate that this model probably obeys the ordinary hyperscaling relation d ν = 2 − α, even though T _c = 0. The existence of the subextensive behavior is attributed to long-range correlations between zero-energy domain walls, and evidence of such correlations is presented. PACS numbers: 75.10.Nr, 75.40.Mg, 75.60.Ch, 05.50.+q     

La0.67Sr0.08Na0.25MnO3的奇特输运性质及CMR效应

用固相反应法制备了La_0.67Sr_0.08Na_0.25MnO₃样品.通过磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线以及ρ-T拟合曲线研究了样品的输运性质及庞磁电阻(colossal magnetoresistance，CMR)效应.结果表明，ρ-T曲线和磁电阻-温度(MR-T)曲线均出现双峰现象；高温峰是伴随顺磁-铁磁(PM-FM)相变出现绝缘体-金属(I-M)相变，低温峰是颗粒界面效应；两个绝缘相输运机理不同：较低温度下(248K<T<274K)，ρ(T)符合极化子的可变程跃迁模型，而在更高温区(330K<T<374K)，ρ(T)符合极化子近邻跃迁模型；两个类金属相输运机理也不同：在低温区(67K<T<186K)，满足ρ-T^2.5关系，输运机理是自旋波散射和电-磁子散射作用，而在高温区(292K<T<304K)，满足ρ-T²关系，输运机理是单磁子散射作用. 关键词： 庞磁电阻 金属-绝缘体转变 晶界效应 输运行为     

Properties of (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 analysed within dielectric permittivity,conductivity, electric modulus and derivative techniques approach

The (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a?=?3.920?±?0.001?Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10^?1–3?·?10⁷?Hz was applied within the range of 100–650?K. The real, ?′(f,?T), and imaginary, ?″(f,?T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f,?T)?∝?exp(?E _a/kT) but the variable range hopping (VRH) dependence σ?∝?exp[?(T ₀/T)^1/4] is manifested at low temperatures. The derivatives technique in the frequency (??log??/??log?ω) and temperature (??log??/?T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f,?T)?=?1/?*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6?eV range and characteristic time of the electric conductivity relaxation of the order of 10^?12?s. The relaxation times can be fitted at better accuracy with the VRH dependence where T ₀ are of the order of 10⁸?K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0)?～?ω_m?～?τ_m ^?1 typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σ_dc?∝?(T/T ₀)^q (e ²/kT) ω_c, where ω_c?=?ν_ph exp[?(T ₀/T)^1/4] is valid for the locally disordered (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ compound.     

Scaling properties of inhomogeneity kinetic energy in some diatomic molecules,in relation to dissociation energies

The ground-state density ρ(r) at equilibrium internuclear distances is utilized to calculate the inhomogeneity kinetic energy T ₂ = (?²/72 m) ε (?ρ)²/ρ dr for a number of diatomics. The scaling properties of T ₂ are exhibited, together with some brief comments on the relation of T ₂ to molecular dissociation.     

Insulator-metal transition in the single-crystal high-T c superconductor Bi-2201

Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T _c superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T ^−1/3, changes to ln ρ ∝ T ^−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator state. The behavior of the width of the superconducting transition and of the temperature of its onset, T _con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity of the IM transition the superconducting transition becomes broader, and the onset of the transition T _con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in the crystal of superconducting droplets with different values of T _c , which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions, superconductivity has a percolation character. Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998)     

Modified relaxor–PbTiO3 piezoelectrics for high temperature applications

In this study, Pb_{1? x} (Yb_1/2Nb_1/2)_0.515Ti_0.485O₃?x{(2SrO?+?BaO)/3} [PYNT–SBx] compositions with x?=?0.00–0.05 at the rhombohedral side near the morphotropic phase boundary were investigated. The SB substitution for Pb²⁺ in PYNT was found to increase the tetragonality, but greatly reduce the Curie temperature (T_c ) and broaden the dielectric permittivity maximum. The temperature dependence of the relative permittivity follows a Curie–Weiss law above the deviation temperature (T_D ) at high temperatures for the SB-doped PYNT. The optimized properties with a piezoelectric coefficient of 610?pm?V^?1, maximum dielectric constant (ε _max) of 25,000 (at the T_c ?=?315°C and 1?kHz), and a remnant polarization (P_r ) of 33?µC?cm^?2 were obtained in PYNT–SB0.04 ceramics.     

Annealing effect on the superconductivity of In2O3–ZnO thin films

We have investigated the relation among ρ–T characteristics, superconductivity, annealing conditions and the crystallinity of polycrystalline (In₂O₃)_1−x–(ZnO)_x films. We annealed as-grown amorphous films in air by changing annealing temperature and time. It is found that the films annealed at 200 °C or 300 °C for a time over 0.5 h shows the superconductivity. Transition temperature T_c and the carrier density n are T_c < 3.3 K and n ≈ 10²⁵–10²⁶ m⁻³, respectively. Investigations for films with x = 0.01 annealed at 200 °C have revealed that the T_c, n and crystallinity depend systematically on annealing time. Further, we consider that there is a suitable annealing time for sharp resistive transition because the transition width becomes wider with longer annealing times. We studied the upper critical magnetic field H_c2(T) for the film with different annealing time. From the slope of dH_c2/dT for all films, we have obtained the resistivity ρ dependence of the coherence length ξ(0) at T = 0 K.     

NMR in highly correlated superconductors

Results of our systematic NMR study in highT _c cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La_1.85Sr_0.15CuO₄. YBa₂Cu₃O₇ and Tl₂Ba₂CuO_6+y . 1/T ₁ of⁶³Cu in the CuO₂ plane in the normal state follows essentially a Curie-Weiss law at high temperature andT ₁ T = const. law at low temperature. The temperature dependence of 1/T ₁ and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation betweenT _c andv _Q, and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum inT _c.     

Thermal conductivity of fluids with steeply repulsive potentials

We consider the thermal conductivity of steeply repulsive inverse power fluids (SRP) in which the particles interact with a pair potential, φ(r) = ε(σ/r)ⁿ. The time correlation function for the heat flux, C_λ(t), and the time average, C_λ(0) are calculated numerically by molecular dynamics simulations, and accurate expressions for these are also derived for the SRP fluid. We show, by molecular dynamics simulations, that close to the hard-sphere limit this time correlation function has the same analytic form as for the shear and pressure correlation functions for the shear and bulk viscosity, i.e. C_λ(t)/C_λ(0) = 1 ?T* (nt*)² + 0((nt*)⁴), where T* = k _B T/ε, is the reduced temperature, k _B is Boltzmann's constant and t* = (ε/σ²)^1/2 t is the reduced time. The thermal conductivity for the limiting case of hard spheres is numerically very close to that given by the traditional Enskog relation. At low densities the normalized relaxation times are typically largest for the thermal conductivity, followed by shear and then bulk viscosity. Close to the maximum fluid density, the latter two increase rapidly with density (especially for the shear) but continue a monotonic decline for the thermal conductivity. This reflects the relative insensitivity of the thermal conductivity to the approach to the fluid-solid phase boundary.     

QCD sum rule determination of the axial-vector coupling of the nucleon at finite temperature

A thermal QCD Finite Energy Sum Rule (FESR) is used to obtain the temperature dependence of the axial-vector coupling of the nucleon, g_A(T). We find that g_A(T) is essentially independent of T, in the very wide range 0≤T≤0.9 T_c, where T_c is the critical temperature. While g_A at T=0 is q²-independent, it develops a q² dependence at finite temperature. We then obtain the mean square radius associated with g_A and find that it diverges at T=T_c, thus signalling quark deconfinement. As a byproduct, we study the temperature dependence of the Goldberger-Treiman relation.     

Positron annihilation response and broadband dielectric spectroscopy: Salol

A phenomenological analysis of the ortho-positronium (o-Ps) annihilation from positron annihilation lifetime spectroscopy (PALS) and the dynamics from broadband dielectric spectroscopy (BDS) are reported on a small molecular glass former of intermediate H-bonding and fragility: salol. The dielectric spectra extend over a very broad frequency range of about 2 × 10⁻²−3.5 × 10¹¹ Hz, providing information on the α-relaxation, the secondary relaxation giving rise to the excess wing, and the shallow high-frequency minimum in the micro- to milli-meter wave range. A number of empirical correlations between the o-Ps lifetime, τ ₃(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the τ ₃(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: T _g ^PALS, T _b1 ^L = 1.15T _g ^PALS and T _b2 ^L = 1.25T _g ^PALS, which are discussed in relation to similar findings for some typical small molecular vdW- and H-bonded glass formers. A slighter change of the slope at T _b1 ^L appears to be related to the transition from excess wing to the primary α-process-dominated behavior, with the secondary process dominating in the deeply supercooled liquid state below T _b1 ^L . The high-temperature plateau effect in the τ ₃(T) plot occurs at T _b2 ^L and agrees with the characteristic Stickel temperature, T _B ^ST, marking a qualitative change of the primary α process, but it does not follow the relation T _b2 ^L < T _α [τ ₃(T _b2) < τ _α ]. Both effects at T _b1 ^L and T _b2 ^L correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, β _KWW. Finally, the application of the two-order parameter (TOP) model to the structural relaxation as represented by the primary α relaxation times from BDS leads to the characteristic TOP temperature, T _m ^c , close to T _b1 from PALS. Within this model the phenomenological interpretation is offered based on changes in the probability of occurrence of solid-like and liquid-like domains to explain the dynamic as well as PALS responses. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in small molecule glass formers.     

Mechanical and dielectric dissipation related to phase transitions

Abstract

(1) First Order Phase Transition There always appear two dielectric loss and/or two mechanical dissipation peaks (P₂ and P₃ ) related to domain walls in many martensitic and ferroic materials. The P₂ near T_c (or M_s ) originates from first-order phase transition or phase-like transition and shows static hysteresis character, but in a certain frequency range (kHz) manifests relaxation behavior, with t—t ₀exp[? B/(T_c—T)]. The transient componant of P₂ is also reviewed.

(2) Glass Transition The peak P ₃ at kHz often appears near a freezing temperature Tf as in KDP, TGS and occasionally in Mn-Cu at Hz, with T = T ₀exp[U/(T- Tf)], a Vogel-Fulcher relation, showing a “Transition” type of glass-transition of domain wall. Whereas the glass-transition in C₆₀ crystal obeys the Arrhenius relation, exibiting “Kinetic” type of glass-transition.

(3) Flux Melting The peak just below T_c was measured for melt-textured YBa₂Cu₃O_{7 +}δ superconductors in an applied magnetic field and various frequencies. The peak hight satisfies the power law Q ^?1 ∝ ω ^?n (1 < n < 2), being explained by the melting and viscous motion of flux.     

Transport properties of the Lorentz gas: Fourier's law

We investigate the stationary nonequilibrium (heat transporting) states of the Lorentz gas. This is a gas of classical point particles moving in a region gL containing also fixed (hard sphere) scatterers of radiusR. The stationary state considered is obtained by imposing stochastic boundary conditions at the top and bottom of , i.e., a particle hitting one of these walls comes off with a velocity distribution corresponding to temperaturesT ₁ andT ₂ respectively,T ₁ <T ₂. Letting be the average density of the randomly distributed scatterers we show that in the Boltzmann-Grad limit,,R 0 with the mean free path fixed, the stationary distribution of the Lorentz gas converges in theL ¹-norm to the stationary distribution of the corresponding linear Boltzmann equation with the same boundary conditions. In particular, the steady state heat flow in the Lorentz gas converges to that of the linear Boltzmann equation, which is known to behave as (T ₂-T ₁)/L for largeL, whereL is the distance from the bottom to the top wall: i.e., Fourier's law of heat conduction is valid in the limit. The heat flow converges even in probability. Generalizations of our result for scatterers with a smooth potential as well as the related diffusion problem are discussed.Research supported in part by NSF Grant no. Phy 77-22302.On leave of absence from the Fachbereich Physik der Universität, München. Work supported by a DFG fellowship.     

Influence of magnetic impurities on proton spin relaxation of water in clay

The¹H nuclear magnetic spin relaxation of water in slurry of kaolin clay was investigated in the presence of magnetite (black iron oxide, Fe₃O₄) at 0.2 T and room temperature. The water spectra at high magnetite contents showed two different resonances, presumably from surface-associated water and free interstitial water. The difference in observed resonance frequencies increased as much as 200 ppm with increasing magnetite content. The apparent nuclear magnetic resonance intensity decreased biexponentially as a function of magnetite added. The observedT ₂^* values at low magnetite contents were in accordance with the predicted values from the resonance intensities and the estimated magnetic susceptibilities. TheT ₁ relaxation was multiexponential in character, so a uniform penalty program was used for the analysis of distribution. At 0.2 T for¹H, kaolin slurry containing less than 5.5 ppm magnetite did not differ significantly from magnetite-free clay in the longitudinal relaxation rates of water. However, higher concentrations of magnetite produced features in theT ₁ distribution significantly different from those of magnetite-free clay. TheT ₂ could be approximated by monoexponential relaxation, probably because the fast-decaying components relaxed before they could be recorded. The apparent transverse relaxation ratesR ₂ increased linearly as a function of magnetite content. On the basis of the comparison of spin-echo and Carr-Purcell-Meiboom-Gill data, an empirical relation was derived to describe the signal loss due to diffusion. It can be expressed by a power function of magnetite amount, which is multiplied by the sum of volume-dependent and volume-independent terms.     

Interface, Surface Tension and Reentrant Pinning Transition in the 2D Ising Model

We develop a new way to look at the high-temperature representation of the Ising model up to the critical temperature and obtain a number of interesting consequences. In the two-dimensional case, it is possible to use these tools to prove results on phase-separation lines in the whole phase-coexistence regime, by way of a duality transformation. We illustrate the power of these techniques by studying an Ising model with a boundary magnetic field, in which a reentrant pinning transition takes place; more precisely we show that the typical configurations of the model can be described, at the macroscopic level, by interfaces which are solutions of the corresponding thermodynamic variational problem; this variational problem is solved explicitly. There exist values of the boundary magnetic field and temperatures 0<T ₁<T ₂<T _c such that the interface is not pinned for T<T ₁ or T>T ₂, but is pinned for T ₁<T<T ₂; we can also find values of the boundary magnetic field and temperatures 0<T ₁<T ₂<T ₃<T _c such that for T<T ₁ or T ₂<T<T ₃ the interface is pinned, while for T ₁<T<T ₂ or T>T ₃ it is not pinned. An important property of the surface tension which is used in this paper is the sharp triangle inequality about which we report some new results. The techniques used in this work are robust and can be used in a variety of different situations. Received: 28 January 1998 / Accepted: 22 December 1998