Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Anomalously strong small-q electron-phonon coupling in cuprates due to dielectricity

We consider the role of the enormous ionic dielectric constant ε₀≈ 30–50 in the cuprates. The ionically-screened Thomas-Fermi screening parameter ~q_TF = [4θe²N(E_F)/εin0 ^1/2 is extremely small, namely 0.2-0.3 A^-1. The electron-phonon coupling constant I(q,w) is found to be anomalously large forq ≈ ~q_TF and smallco, accounting for the CDW observed in overdoped BSCCO with wavevector 0.24 A^-1. Using the Eliashberg theory we derive from this I(q,w) a maximumT _c of about 200 K. The small electron-phonon scattering angle △θ = ~q _{F ≈} 0.3 rad acounts for the observedd-wave pairing, as originating from a phonon-mediated mechananism.     

The'thin-film' state of 4He on Cs and Rb

At temperatures less than the wetting temperature, T _w, ⁴ He exists in equilibrium, either as a thin film on the surface of Cs or Rb, or as a macroscopic drop with a finite contact angle. The thin-film state is at most around a few monolayers thick and can exist as a two-dimensional (2D) gas or a 2D liquid. We present evidence of a gas /liquid transition at T _gl 1.5K on Cs. For T < T _gl the gas density falls rapidly with decreasing temperature to 10 ^–6 monolayers at T = 0.1K. On Rb there can be a superfluid thin liquid film for T T _w but no 2D gas state has been seen. We discuss the free energy of the solid/vapour interface and its temperature dependence.     

Samarium diffusion in polycrystalline samarium sulfide

The diffusion of samarium in polycrystalline samarium sulfide with a superstoichiometric composition of Sm_1.13S has been studied at T = 1000 and 1100°C. It is concluded that Sm atoms predominantly migrate over the boundaries of monocrystalline domains of the polycrystalline sample. The diffusion coefficient varies within D ≈ 10⁻²−10⁻³ cm²/s. In the temperature dependence of the diffusion coefficient, the diffusion activation energy is evaluated at E ∼ 4.6 eV and the preexponential term at D ₀ ≈ 1.8 × 10¹⁵ cm²/s.     

Thermochemistry of Alkali Metal Uranoborates and Their Crystal Hydrates

The standard enthalpies of formation at T = 298.15 K were determined for A^IBUO₅·nH₂O (A^I = Li, Na, K, Rb, Cs). The heat capacity of crystalline NaBUO₅·H₂O was determined by adiabatic vacuum calorimetry, and its thermodynamic functions were calculated. The thermodynamic characteristics of dehydration of alkali metal uranoborates were evaluated.     

Generalized Bose–Einstein Condensation in Superconductivity

Unification of the BCS and the Bose–Einstein condensation (BEC) theories is surveyed via a generalized BEC (GBEC) finite-temperature statistical formalism. Its major difference with BCS theory is that it can be diagonalized exactly. Under specified conditions it yields the precise BCS gap equation for all temperatures as well as the precise BCS zero-temperature condensation energy for all couplings, thereby suggesting that a BCS condensate is a BE condensate in a ternary mixture of kinematically independent unpaired electrons coexisting with equally proportioned weakly-bound two-electron and two-hole Cooper pairs. Without abandoning the electron–phonon mechanism in moderately weak coupling it suffices, in principle, to reproduce the unusually high values of T _c (in units of the Fermi temperature T _F ) of 0.01–0.05 empirically reported in the so-called “exotic” superconductors of the Uemura plot, including cuprates, in contrast to the low values of T _c /T _F ≤10⁻³ roughly reproduced by BCS theory for conventional (mostly elemental) superconductors.     

Thermodynamics of a Trapped Bose Condensate with Negative Scattering Length

We study the Bose–Einstein condensation (BEC) for a system of ⁷Li atoms, which have negative scattering length (attractive interaction), confined in a harmonic potential. Within the Bogoliubov and Popov approximations, we numerically calculate the density profile for both condensate and non-condensate fractions and the spectrum of elementary excitations. In particular, we analyze the temperature and number-of-boson dependence of these quantities and evaluate the BEC transition temperature T _BEC. We calculate the loss rate for inelastic two- and three-body collisions. We find that the total loss rate is strongly dependent on the density profile of the condensate, but this density profile does not appreciably change by increasing the thermal fraction. Moreover, we study, using the quasi-classical Popov approximation, the temperature dependence of the critical number N _c of condensed bosons, for which there is the collapse of the condensate. There are different regimes as a function of the total number N of atoms. For N<N _c the condensate is always metastable but for N>N _c the condensate is metastable only for temperatures that exceed a critical value T _c.     

MAEAM Investigation of Phonons for Alkali Metals

Within the harmonic approximation, the atomic force constants have been derived and the phonon dispersion curves along three major symmetry directions [00ζ], [0ζ ζ] and [ζ ζ ζ] (or in group-theory notation Δ, Σ, and Λ) have been calculated for five alkali metals Li, Na, K, Rb and Cs by combining the modified analytic embedded atom method (MAEAM) with the theory of lattice dynamics. The results are in agreement with available experimental results for Li, Na and K, which may imply that the predicted phonon dispersion curves for Rb and Cs are correct.      

Re−Ni2B2C superconducting borocarbides: “In situ” growth of thin films, transport and point contact spectroscopy

High quality c-axis oriented films of the intriguing intermetallic superconducting compound YNi₂B₂C have been obtained “in situ” by magnetron sputtering on MgO substrates held at about 800°C. The films showed maximum T_c=15.3 K, †T_c≈0.1 K, room temperature resistivity ρ≈50μΩ·cm, critical current J_c≈10⁵ A/cm² and B_c2≈6 T. Superconducting films were also obtained on Al₂O₃ and LaAlO₃ single-crystal substrates. From the ρ(T) dependence a value of the Debye temperature Θ _D =330±20 K can be deduced. At low temperatures the resistivity follows a quadratic power law possibly indicative of a high value of the electron-phonon interaction parameter λ. In order to clarify the role of λ in these compounds, point contact spectroscopy measurements have been performed on YNi₂B₂C and HoNi₂B₂C bulk samples prepared by inductive melting using a Low Temperature Scanning Tunneling Microscope (LTSTM). In the point contact regime clear evidence of a superconducting gap have been found in both compounds, corresponding to a moderate strong coupling behaviour (2†/KT_c≈3.8).     

(111)⇒(001) orientational phase transitions and nature of the insulating state in BiSrCaCuO (2212 and 2223) films

The influence of the growth temperature T _s on the structure, optical absorption, and electrical conductivity of BiSrCaCuO films has been studied. It was observed that nonmonotonic changes in the parameters of the films with T _s are caused by (111)⇒(001) phase transitions at T _s ≈550 °C for the 2212 phase and T _s ≈600 °C for the 2223 phase. These phase transitions stimulate the formation of metallic conductivity and are caused by a change in the system of preferential ordering planes of the atoms. Pis’ma Zh. Tekh. Fiz. 24, 13–20 (January 12, 1998)     

Microwave Response of Perfect YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films Deposited on CeO<Subscript>2</Subscript>-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z _s(T), of epitaxial YBCO thin films deposited on CeO₂-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R _s(T) at low temperatures obeys the exponential law: R _s(T) = R _res+R ₀⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T _c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R _s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R _s(T) behavior.     

In situ observation of the relaxation of conductivity in γ-irradiated <Emphasis Type="Italic">n</Emphasis>-type silicon under the action of ultrasound pulses

Reversible change of the electric conductivity σ_US in a temperature range of T = 110–180 K has been observed for the first time in gamma-irradiated and partly annealed (280°C) floating-zone grown silicon (n-Si-Fz) under the action of pulsed ultrasound (longitudinal wave) at a frequency of 6–10 MHz, intensity up to 4 × 10³ W/m², and pulse duration within 10⁻⁵–10⁻³ s. It is established that the temperature dependences of the parameters of acoustic-wave-induced change of σ_US (increase time, τ _i ; decay time, τ _d ) obey the Arrhenius law. Experimental τ _{i, d} (T) curves have been used to determine the corresponding activation energies (E _i ≈ 0.09 eV, E _d ≈ 0.13 eV) and preexponential factors (τ _i ⁰ ≈ 4 × 10⁻⁸ s, τ _d ⁰ ≈ 10⁻⁹ s). The observed phenomenon is interpreted as an acoustic-wave-induced transition between the states of a metastable structural defect.     

Very Low Temperature Electronic Transport in Al-Pd-Re Quasicrystalline Alloys

Electrical resistivities of two icosahedral (I) Al-Pd-Re alloys have been measured between room temperature and mK temperatures. One quasicrystalline (QC) polygrain Al-Pd-Re sample exhibited insulating behavior in its resistivities, increasing by a factor of r=R(4 K)/R(300 K)=7.76; its room temperature resistivity was 9,890 μΩ cm. A “phenomenological” expression fitted the conductivity data well between 300 K to 0.5 K. Below 0.4 K a crossover to an activated variable-range hopping law was observed. Low temperature magnetoresistance ratio data and fits using the wave function shrinkage theory are presented. A second QC Al-Pd-Re sample had a small resistance temperature ratio r=2.12. The room temperature resistivity was extremely large, ρ(300 K)≈40,980 μΩ cm. Its conductivity could be described well using a simple temperature power law between 300 K to 20 K. Below 20 K there was a crossover to a new behavior. Below 1 K, the conductivity could be fitted using a very weakly insulating power law where σ(T)≈11.37T ^0.032 in (Ω cm)⁻¹, suggesting that this sample is located just below the metal-insulator transition. The magnetoconductivity data could not be fitted successfully using the 3D weak localization (WL) theory and inserting into it physical and realistic fitting magnitudes for the inelastic magnetic field B _in.      

Reaction behaviour of Ni1?x M x O’s (M?=?Li, Na) formation and its thermoelectric properties

Reaction behaviour of the mixed reactant powder of NiO and M₂CO₃ (M = Li, Na) for the formation of Ni_1−x M _x O (M = Li, Na) was investigated. Ni_1−x M _x O (M = Li, Na) compacts were fabricated by sintering green compacts of the mixed reactant powder at 1,173 K for 5 h in air. The results indicated that Ni_1−x M _x O (M = Li, Na) was successfully synthesized when the sintering temperature was approximately above 800 K. In addition, the resultant Ni_1−x M _x O (M = Li, Na) kept the same crystal structure with that of NiO (except for x = 0.4), but the diffraction peaks were found to shift to larger diffraction degrees compared with that of NiO. Interestingly, Li ions distribute in the lattice of NiO uniformly. Meanwhile, the electrical resistivity of the sintered compacts decreased and their semiconductor behaviour was always maintained with the increase of doping Li or Na. The power factor was increased significantly and exceeded 1.8 × 10⁻⁴ Wm⁻¹ K⁻² at 823 K.     

Magnetotransport Properties and Mechanism of the A-Site Ordering in the Nd–Ba Optimal-Doped Manganites

The crystal structure and magnetotransport properties of the Nd_0.70Ba_0.30-MnO_3.00−δ manganites have been investigated. The stoichiometric A-site ionic disordered Nd_0.70Ba_0.30MnO_3.00 sample is orthorhombic (SG=Imma) ferromagnet below T _C ≈151 K. It has metal-insulator transition at T _MI ≈140 K and peak of magnetoresistance ∼50% in field of 9 kOe. The anion-deficient partly A-site ionic ordered Nd_0.70Ba_0.30MnO_2.60 sample is ferrimagnet with T _N ≈130 K and has nanometric ∼440 nm grains. The oxygen annealed stoichiometric partly A-site ordered Nd_0.70Ba_0.30MnO_3.00 sample consists of two perovskite phases: (i) A-site ionic completely ordered tetragonal (SG=P4/mmm) NdBaMn_2.00O_6.00 with a Curie point of ∼310 K and (ii) superstoichiometric orthorhombic (SG=Pnma) NdMnO_3.00+γ with a Curie point of ∼120 K. The magnetoresistance of this sample at the room temperature is about 7% in a field of 9 kOe. Considerable changes of the magnetic properties are interpreted as due to the ordering of Nd³⁺ and Ba²⁺ cations as well as the formation of nanometric grains.      

Solubility and Thermodynamic Properties of Alkali Metal Unanoborates

The solubility of uranoborates M^IBUO₅·nH₂O (M^I = Li, Na, K, Rb, Cs) in aqueous HCl solutions was measured at 25°C, and the standard thermodynamic functions of the compounds were calculated.     

Heat transport in dilute mixtures of3He in superfluid4He

We report new measurements of the effective thermal conductivity K_eff and relaxation time τ in dilute mixtures of³He in superfluid⁴He, with molar concentrationsX≤10⁻³. The temperature range extended fromT≈1.4 K toT _λ. Both K_cff and τ are found to agree with theoretical predictions, in contrast to previous experiments where significant differences were observed. A new thermal conductivity cell design was used which almost completely eliminates extraneous volumes and surfaces, and the earlier results are explained in relation to these design changes.     

The three isentropic exponents of wet steam

Real gas isentropic changes may be described using the three well known ideal gas relations,pv ^k =const, p^(1−k)T^k=const andTv ^k-1=const, where exponent k has for each equation a different value k_{p, ν}, k_{p, T} and k_{T, ν} respectively. In this paper the three isentropic exponents theory for real gases is extended to the two phase region. As an application the numerical values of the three exponents are calculated for wet steam.     

Understanding Heavy Fermion from Generalized Statistics

Heavy electrons in superconducting materials are widely studied with the Kondo lattice t–J model. Numerical results have shown that the Fermi surface of these correlated particles undergoes a flattening effect according to the coupling degree J. This behaviour is not easy to understand from the theoretical point of view within standard Fermi–Dirac statistics and non-standard theories such as fractional exclusion statistics for anyons and Tsallis nonextensive statistics. The present work is an attempt to account for the heavy electron distribution within incomplete statistics (IS) which is developed for complex systems with interactions which make the statistics incomplete such that ∑ _i=1 ^w p _i ^q =1. The parameter q, when different from unity, characterizes the incompleteness of the statistics. It is shown that the correlated electrons can be described with the help of IS with q related to the coupling constant J in the context of Kondo model.