Phase equilibria in the Tl<Subscript>2</Subscript>Se-PbSe system and growth and properties of Tl<Subscript>4</Subscript>PbSe<Subscript>3</Subscript> single crystals

The T-x phase diagram of the Tl₂Se-PbSe system has been mapped out. The system contains a congruently melting compound (803 K) of composition Tl₄PbSe₃, which forms a continuous series of solid solutions with Tl₂Se. Tl₄PbSe₃ single crystals have been grown by directional solidification, and their physicochemical and thermoelectric properties have been studied. Tl₄PbSe₃ crystals have high thermopower (α _T ) and thermoelectric figure of merit (Z _T ).     

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureT _C has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.     

Thermoelectric power of single-phase samples of Tl2CaBa2Cu2O y and Ba2CaSr2Cu2O y

Single-phase 2122 samples of thallium and bismuth superconductors were made by the precursor matrix method. The thermopower of these samples was measured in the temperature range 250 K-T _c . The thermopower was positive and decreased linearly with increasing temperature aboveT _c (onset). The exponential enhancement of thermopower seen in the undoped and doped YBCO was not observed in these samples. The linear variation of thermopower can be explained on the basis of either a two-band model or a narrow band model.     

Thermopower and resistance of superconducting-normal interfaces II. Experiment

This paper presents experimental data on the thermopower and resistance of superconducting-normal interfaces in SNS junctions made from pure copper foils to which rods of pure lead or PbBi alloys are attached, measured using a SQUID voltmeter. The data are analysed using the theory presented in the preceding paper, which is generally successful in its region of validity for the purer alloys near to T_c, and more successful than earlier theories, particularly as concerns the thermopower. The bulk thermopower for the superconducting excitations is found to be continuous with its normal value at T_c. The junctions made using dirty alloys in which the heat flow is phonon dominated show an unexplained large rise in boundary thermopower at low temperatures.     

Occurrence of Superconductivity and Magnetism in Nominally Undoped LaOFeAs

Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO_1−x F _x FeAs) even under ambient conditions and oxygen deficiency (LaO_1−x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools–resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T _con∼17 K and a tail-like behavior when R goes to zero at T _c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼−5.5 and −1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At T<T _c0 the change in MMA with B shows a low field (B∼1 mT) peak, which vanishes at T>T _c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T _min∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T _con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature.     

Thermopower of non-superconducting and superconducing Nd1·85Ce0·15CuO4−y samples

The thermopower of single-phase samples of Nd_1·85Ce_0·15CuO_4−y was measured from 250 K down to 10 K. The as-prepared sample was not superconducting. It had a negative thermopower at 250 K, whose magnitude increased as temperature was decreased to 95 K. A further reduction in temperature caused a decrease in magnitude of thermopower. The sign of the thermopower changes to positive at 12 K. The superconducting sample also showed the same behaviour but the change of sign now occurred at 40 K. Below 40 K, the thermopower showed a positive peak and reduced to zero at the superconducting transition. These results were compared with previous studies.     

Theoretical Study of Specific Heat, Density of States and Free Energy of Itinerant Ferromagnetic Superconductor URhGe

Following the equation of motion method and Green’s function technique, the coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is investigated in a single band homogeneous system. Self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (Δ_FM) are derived. It is shown that there generally exists a coexistent (Δ≠0 and Δ_FM≠0) solutions to the coupled equations of the order parameter in the temperature range 0<T<min (T _C,T _FM) where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for the specific heat, density of states and free energy are derived. The specific heat has a linear temperature dependence at low temperatures as opposed to the exponential decrease in the BCS theory. The density of states for a finite Δ_FM increases as opposed to that of a standard ferromagnetic metal. The free energy shows that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and therefore is realized at low enough temperature. The theory is applied to explain the observations of URhGe. The agreement between theory and experimental results is quite satisfactory.      

Nonequilibrium Casimir force on a particle in cold vacuum background near a heated plate

General expressions for a nonequilibrium Casimir force in a stationary situation for the “small particle-plate” system with arbitrary local dielectric properties of materials are obtained for the given particle (T ₁) and plate (T ₂) temperatures and a “cold” (T ₃ = 0) vacuum background over the plate.     

Thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9+δ</Subscript> with Lu substitution

A series of Ca_3−xLu_xCo₄O_9+δ (x = 0, 0.1, 0.2, and 0.4) has been prepared by sol–gel method. The effects of Lutetium substitution on thermoelectric properties of Ca₃Co₄O_9+δ have been systematically investigated from 4 to 335 K. With the partial substitution of Lu³⁺ for Ca²⁺, the resistivity and thermopower for Lu-doped samples increase, while their thermal conductivity decreases. The dimensionless figure of merit for Ca_2.8Lu_0.2Co₄O_9+δ material (ZT = 0.032) is about five times better than that for Ca₃Co₄O_9+δ (ZT = 0.007) at 335 K. Strikingly, for Lu-doped samples, the thermopower exhibits a steeper upturn at low temperatures.     

Influence of fluorine doping on the transport properties of GdBa2Cu3O7-X

Undoped and fluorine-doped materials of GdBa₂Cu₃O_7-XF_x (with x = O, 0.05, 0.1 and 0.2) were prepared at high temperature (950° C) by solid state reaction followed by oxygen treatment. X-ray diffractograms of the undoped sample showed single orthorhombic phase. Resistivity measurements showed thatT _c(0) decreases with fluorine concentration. Thermopower measurements indicated a peak just before the onset of the superconducting transition. The reasons for the decrease ofT _c and the anomalous behaviour of thermopower have been explained.     

Formation and properties of SnO–MgO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

A new glass system SnO–MgO–P₂O₅ with low viscosity has been developed by a melt-quenching method. Formation, thermal properties, and chemical durability of these glasses have been investigated. For a constant P₂O₅ concentration, the glass formation ability is enhanced with the increasing Sn/(Sn + Mg) ratio. The glasses exhibit low glass transition temperature (T _g = 270–400 °C), low dilatometric softening temperature (T _DS = 290–420 °C), and high thermal expansion coefficient (CTE = 110–160 × 10⁻⁷ K⁻¹). With the increasing Sn/(Sn + Mg) ratio, T _g and T _DS decrease, and CTE increases. When Sn/(Sn + Mg) ratio is varied, the relationship between chemical durability and thermal properties of the present glasses is not consistent with what expected in general cases. It is noted that the glasses with 32–32.5 mol% P₂O₅ exhibit excellent chemical durability and tunable T _g, T _DS, and CTE (by varying Sn/(Sn + Mg) ratio).     

Structural and thermoelectric properties of Zr<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NiSn solid solutions

We have studied the electronic and crystal structures and temperature-dependent resistivity and thermopower of Zr_{1 − x} Er _x NiSn (x = 0–0.20) substitutional semiconductor solid solutions (so-called half-Heusler alloys) in the temperature range 80–380 K. Heavily erbium doped semiconductors with the MgAgAs structure are described in terms of an amorphous semiconductor model. The erbium atoms in Zr_{1 − x} Er _x NiSn are shown to act as acceptors. Density of states calculation results for the Zr_{1 − x} Er _x NiSn alloys are consistent with experimental data.     

(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)     

Magnetic, electrical, and optical properties of (HgS)1.5(Al2S3)0.5?Mn? and (HgS)1.5(In2S3)0.5?Mn? crystals

We have studied the magnetic, electrical, and optical properties of (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 and (HgS)_1.5(In₂S₃)_0.5〈Mn〉 crystals and determined their magnetic and band structure parameters. The magnetic (Faraday method) and transport (four-probe method) properties of the (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 and (HgS)_1.5(In₂S₃)_0.5〈Mn〉 crystals were investigated in the ranges T = 77−300 K and H = 20−320 kA/m. The magnetic susceptibility data suggest that the magnetic properties of the crystals can be understood in terms of Mn-S-Mn-S clusters. The Hall coefficient of the crystals is temperature-independent. The electrical conductivity of (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 varies little with temperature and has a maximum, and that of (HgS)_1.5(In₂S₃)0.5〈Mn〉 shows metallic behavior and is a nearly linear function of temperature. The thermopower of the crystals increases with temperature. Optical data attest to direct allowed interband optical transitions. The magnetic and band structure parameters of the crystals are determined.     

Thermopower of chromel–AuFe0.07% thermocouples in magnetic fields

Chromel–AuFe_0.07% thermocouples are widely used to measure small temperature differences down to liquid-He temperatures. In this work we present the thermopower S of two chromel–AuFe_0.07% thermocouples over an extended temperature range from 1.5 K to 200 K in magnetic fields up to 7 T. For low temperatures T < 4 K we find a sizeable field dependence of S even in small fields. Furthermore, a field of 7 T affects the thermopower considerably up to 200 K. Thus, our data demonstrate a significant field dependence of the thermopower of chromel–AuFe_0.07% thermocouples in an extended temperature and field range compared to previous works.     

Theoretical Study of Specific Heat,Density of States,Free Energy,and Critical Field of High Temperature Cuprate Superconductors Based on Intra and Interlayer Interactions

The role of attractive interlayer and intralayer interactions in layered high T _c cuprate superconductors have been investigated using a one-band two layer tight binding Hamiltonian. Self-consistent equations for the superconducting order parameter (Δ) and critical temperature (T _c ) are derived using double time Green’s functions and equation of motion method. The expression for excitonic type correlation (γ _c ), specific heat, density of states, free energy, and critical field are obtained. The interlayer interactions play an important role in the enhancement of T _c in layered high T _c cuprates. The oxygen isotope effect is also analyzed. The agreement between theoretical and experimental results for the system YBa_2−x La _x Cu₃O₇ (0≤x≤0.5) is quite satisfactory.      

Anisotropy of Superconducting Single Crystal SmFeAsO0.8F0.2 Studied by Torque Magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_0.8F_0.2 with T _c≃45(1) K were investigated by torque magnetometry. The crystals of mass ≤0.1 μg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ≃8 at T≲T _c to γ≃23 at T≃0.4T _c. This unusual behavior of γ signals unconventional superconductivity.      

Study of copper ion superionic solids in the system (1 − x)Cul:xCdl2

Electrical transport studies of five compounds (x = 0, 0,25, 0.33, 0.50 and 0.75) in the system (1 −x) Cul:xCdl₂have been reported. It has been observed that all compounds of this system undergo phase transition from lower temperature low conductingβ-phase to higher temperature high conductivityα-phase. The phase transition temperature (T _p) decreases systematically withx being 642 forx = 0 and 500 K forx = 0.50. AboveT _p all compounds are typical superionic solids with Cu⁺ ion as the entity of charge carrier. From the stability, temperature span of superionic phase,T _p andσ values point of view Cu₃Cdl₅ (x = 0.25) appears to be the best superionic solid of this system. The extended lattice gas model has been shown to suit better for explaining the electrical transport of solids with O⩽x ⩽ 0.5 of this system in superionic solid phase. BelowT _p all compound of this sytem behave as normal ionic solid with relatively high conductivity. They yield usual “Knee temperature” and below this temperature conductivity becomes extrinsic. The activation enthalpy, heat of transport and enthalpy for the formation of defects have also been evaluated from experimental electrical conductivity and thermoelectric power data in each case.     

Point-Contact Spectroscopy of the Borocarbide Superconductor YNi2B2C in the Normal and Superconducting State

Point-contact (PC) spectroscopy measurements of YNi₂B₂C single crystals in the normal and superconducting (SC) state (T _c ≃ 15.4 K) for the main crystallographic directions are reported. The PC study reveals the electron–phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No “soft” modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in Δ_{[100] ≈ 1.5 meV for the a direction and Δ_{[001] ≈ 2.3 meV along the c axis; however the critical temperature T _c in PC in all cases is near to that in the bulk sample. The value 2Δ_[001]/k _B T _c ≈ 3.6 is close to the BCS value of 3.52, and the temperature dependence Δ_[001](T) is BCS-like, while the for small gap Δ_[100](T) is below BCS behavior at T > T _c /2 similarly as in the two-gap superconductor MgB₂. It is supposed that the directional variation Δ can be attributed to a multiband nature of the SC state in YNi₂B₂C.