Mössbauer effect of tin-119 doped in highT c Y−Ba−Cu−O superconductor

Mössbauer measurements at temperatures ranging from 12 K to 300 K have been carried out on highT _c superconductor YBa₂Cu₃O_6+δ samples doped with Sn (5,2 and 0.5 wt%). The temperature dependent recoilless fractionf(T) has a narrow and small drop nearby transition temperatureT _c which is dependent on the Sn-concentration and has a drop between 200–220 K. This indicates the occurrence of phonon softening at these temperatures. The Debye temperature θ_D of the samples containing 5, 2 wt% Sn atoms was determined as (290±25) K and (320±13) K respectively. These results show that phonons may play an important role for electron pairing in novel oxide superconductor.     

Metal-nonmetal transition and superconductivity in frozen aluminium-hydrogen films

A study of the influence of the hydrogen concentration c_H on the superconducting transition temperature T_c and the d.c.-conductivity σ of Al-films has been performed. The samples were prepared by co-deposition of Al and H (H₂) onto a sapphire substrate held at a temperature of about 5 K. Within the experimental errors, there is no difference between the behaviour of Al-H and Al-H₂ respectively. σ shows a decrease with increasing c_H and a metal-insulator transition at c_H?0.68. The T_c values exhibit a maximum T_c ? 5.3 K for 0.22 ? c_H ? 0.5. Annealing experiments result in a decrease of T_c and an increase of σ. An analysis of the annealing experiments leads us to the conclusion that the as-prepared Al-H (Al-H₂) films have an amorphous structure, which transforms during annealing in a granular system of Al grains embedded in insulating AlH₃. No H-specific effect has been detected.     

Effect of γ irradiation on the structural and thermal properties of [N(C2H5)4]2ZnBr4 in the vicinity of a first-order phase transition

The unit cell parameters a and c of nonirradiated [N(C₂H₅)₄]₂ZnBr₄ crystals in the temperature region 90–300 K and of samples irradiated with γ rays to doses of 10⁶ and 5 × 10⁶ R in the 270-to 300-K interval were measured using x-ray diffraction. The data obtained were used to derive the thermal expansion coefficients α_a and α_c. It is shown that the parameter a increases and the parameter c decreases with increasing temperature. In the vicinity of the phase transition (PT) at T = 285 K, the temperature dependences of a(T) and c(T) reveal anomalies in the form of jumps and the α_a(T) and α_c(T) curves have a maximum and a minimum, respectively. The heat capacity of nonirradiated and irradiated [N(C₂H₅)₄]₂ZnBr₄ samples was measured by adiabatic calorimetry. A maximum was found in the C _p(T) curve at T = 285 K. Both x-ray diffraction and heat capacity measurements showed that the PT temperature decreased after γ irradiation.     

Manifestation of pseudogap features in structurally inhomogeneous optimally doped HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.92</Subscript>

Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < T _c (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state T _c < T < 400 K for optimally oxygen-doped samples of YBa₂Cu₃O_6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties.     

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature T_c and critical magnetic field H_c) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature T_c and transition width ΔT_c) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of T_c of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.     

Experimentelle Bestimmung der Supraleitungsparameter einiger Verbindungen vom β-Wolframtyp aus Magnetisierungsmessungen

A complete analysis of magnetization curves is given for bulk sintered samples of Nb₃Sn with low hysteresis and for finely powdered sintered and melted samples of Nb₃Sn, Nb₃Al, V₃Si and V₃Ga. The Ginzburg-Landau parameterκ and the penetration depthλ of weak magnetic fields is calculated from the temperature dependence of the thermodynamic and upper critical fieldsH _c andH _c2. The slope of theH _c ?T curve is about 360 Oe/°K and approximately the same for all measured samples. There is an extremely high increase of the lower critical fieldH _c1 if the grain size approachesλ. The occurence of field expulsion and partially reversible magnetization curves of sintered Nb₃Sn samples with high sintering temperatures and high pore volumes is explained by means of connections between single grains which have lower transition temperatures.     

Hall coefficients for A15 compounds: Nb-Sn and V-Si

The Hall coefficients, R_H, for A15 structure single crystal V₃Si and polycrystal V25.25 at.% Si and Nb 26 at.% Sn have been measured as functions of temperature. The data between the superconducting transition temperature, T_c, and 80 K for the Nb-Sn show a small dip centred arounf 30 K which accompanies a cubic-to-tetragonal lattice distortion commencing at about 45 K. R_H for the VSi samples, which are believed to be non-transforming, is temperature independent in the range 17–40 K and equal to 2.0 ± 0.2×10^?10m³C^?1. These result are discussed in relation to the effect of the tetragonal distortion on the band structures of these compounds.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

Synthesis of superconducting compounds by thermolysis of volatile hydrides and organometallic compounds on glowing wires

The synthesis of high temperature superconducting phases in the NbGe, NbSn, VSi, VGe, VSn, NbC and MoC systems is described by method consisting in the thermolysis of volatile hydrides or organometallic compounds on resistively heated wires. For face-centred cubic NbC a higher transition temperature than previously reported was obtained. The A15 phase boundary of NbGe is extended towards the stoichiometric 3:1 composition, affording samples of Nb₃ Ge with a T_c onset of 15.8°K.     

Influence of irradiation induced atomic disorder on Hc2 of Nb3Sn

A significant monotonous decrease of H_c2(0) almost proportional to T_c has been observed after heavy ion irradiation of the A15 superconductor Nb₃Sn at low temperature < 30 K and isochronal annealing up to 300 K has been measured. The effects are discussed in connection with the Goodman—Gorkov-relation.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Superhard superconducting materials based on diamond and cubic boron nitride

Superhard superconducting samples with a critical temperature of T_c = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with T_c = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with T_c = 36.1–37.5 K have been obtained in the systems diamond-MgB₂ and cubic boron nitride-MgB₂.     

Metal-insulator transition and superconductivity in SnAr films

We studied the conductivity and superconducting transition temperature T_c of SnAr films. The films were prepared by condensing the SnAr mixture on a sapphire substrate held at 5 K. A plot of the conductivity as a function of Sn concentration shows a metal-insulator transition which agrees with a percolation model consisting of Sn clusters embedded in solid Ar. A drop of T_c is observed below the percolation threshold.     

Tunneling in single-layer Bi2Sr2CuO6+δ single crystals in high magnetic field

In tunneling experiments with high-quality single crystals of a single-layer cuprate superconductor Bi₂Sr₂CuO_6+δ using the break junction and point-contact techniques at T<T _c, the coexistence of the superconducting-state gap and the normal-state gap was observed. The values of the superconducting energy gap 2Δ_p?p are in the range from 13.4 to 15 meV (Δ_p?p=6.7–7.5 meV). The values of 2Δ_p?p are similar for two samples with T _c=4 K and for two samples with T _c=9–10 K and are independent of the carrier concentration. The normal-state gap, with the magnitude approximately equal to 50 meV, persists at T<T _c and in the magnetic field H?H _c2 up to 28 T. After the transition of the sample to the normal state, the intensity of the tunneling conductance rapidly decreases with increasing magnetic field strength and temperature. The observed large broadening of the tunneling spectra and large zero-bias conductances can be caused by a strong angular dependence of the superconducting gap. The tunneling results are in full agreement with the data of the angle-resolved photoemission spectroscopy measurements.     

Electron transport,penetration depth,and the upper critical magnetic field in ZrB<Subscript>12</Subscript> and MgB<Subscript>2</Subscript>

We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field H_c2, for polycrystalline samples of dodecaboride ZrB₁₂ and diboride MgB₂. We conclude that ZrB₁₂ behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature T_R = 280 K (to be compared to T_R = 900 K for MgB₂). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB₁₂ at T_c = 6.0 K. Although a clear exponential λ(T) dependence in MgB₂ thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB₁₂ below T_c/2. These features indicate an s-wave pairing state in MgB₂, whereas a d-wave pairing state is possible in ZrB₁₂. In disagreement with conventional theories, we found a linear temperature dependence, of H_c2(T) for ZrB₁₂ (H_c2(0) = 0.15 T).     

Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×10¹⁹cm^?2) on the superconducting transition temperature T _c and the upper critical field H _c2 of polycrystalline MgB₂ samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T _c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH _c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T _c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.     

Increase in Tc and spin glass transition in La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2)

We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La_0.9Ca_0.1Mn_1−yCr_yO₃ (y=0, 0.1 and 0.2) prepared by sol-gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at T_c=136 K. Both for y=0.1 and 0.2, T_c increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at T_f<T_c which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to T_c and T_f. The value of MR decreases for the Cr substituted samples.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Evidence for a three Tc behavior of the Kondo superconductor (La,Y)Ce

The dependence of the superconducting transition temperature T_c on Ce impurity concentration is reported for the (La_0.80Y_0.20)Ce system. Susceptibility and resistivity measurements on samples with about 0.85 at. % Ce show unique behavior which must be interpreted as a superposition of three transition curves: a first transition at 0.55 K from the normal to the superconducting state, a second one at 0.27 K back to the normal state, and finally a third one at about 0.05 K again to the superconducting state. The experimental evidence of this third transition (T_c3) is of special importance for the decision about the relevant physical mechanism of pair-breaking at low temperature (T ? T_K).     

Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

The electrical resistance of a linear chain metal Nb₃Te₄ were measured from 1.3 to 320 K. The residual resistance ratio $\text{R(300}\text{K}\text{)}\text{R(4.2}\text{K}\text{)}$ is about 3. Nb₃Te₄ shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature T_c, the critical magnetic field H_c2 is proportional to δT=T_c?T. The angular dependence of H_c2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.