Surface and bulk properties of hot-pressed PbMo6S8 superconductor studied by Auger electron spectroscopy and calorimetry

A direct observation of the grain-boundary phases for several well-characterized hot-pressed PbMo₆S₈ samples were made by Auger electron spectroscopy. The surface elemental concentrations are completely different when compared to its bulk composition. The thickness of this altered composition is in the range 100–200 Å gradually increasing with increase in hotpressing temperature. Also, evidence of segregation of impurities, such as carbon and SiO_x, to the grain-boundaries were noticed for some of the samples. Calorimetric experiments show a continuous broadening and a reduction in amplitude of the specific heat anomaly atT _c . This in terms of superconducting volume fraction indicates aT _c distribution in the range 9–15 K. Such an observation can be related to the local inhomogeneities with respect to the ternary composition, i.e., a deviation from ideal stoichiometry, PbMo₆S₈. The results are discussed in conjunction with X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis. By considering the grain-boundary phases and the calorimetric observation of inhomogeneities, a plausible explanation is given for the low critical current densities in these materials.     

Superconductivity of thin films of lead,indium, and tin prepared in the presence of oxygen

We have studied the influence of oxygen on the superconducting properties of thin films of lead, indium and tin deposited on glass or sapphire substrates. In addition, the morphological microstructure was investigated by scanning electron microscopy. The film thickness was 1.0 μm, and the partial pressure of O₂ during the film deposition was raised up to 1×10⁻⁴ Torr. In all three materials the development of a granular structure and a strong increase in the residual electric resistivity was observed due to the O₂-treatment. Whereas in the Pb films no change of the critical temperature was found, the In films deposited on glass substrates showed a slight increase ofT _c due to the oxygen. The strongest increase ofT _c (up to 8%) was observed in the O₂-treated Sn films. These results are discussed in terms of the McMillan theory. From our measurements of the critical current densityj _c we conclude that edge pinning is dominant in the undoped films. All three materials showed a strong increase ofj _c due to the O₂-treatment which must be interpreted in terms of bulk pinning.     

Low temperature normal state resistance of ternary molybdenum sulfides

The resistance varies as T² over the range of T from T_c to nearly 40 K, in sintered, sputtered and evaporated thin film Cu_xMo₆S₈, and in sputtered PbMo₆S₈ films. In sintered PbMo₆S₈ the resistance varies as T. These results can neither be explained by existing theory for PbMo₆S₈, nor by a Mo₂S₃ phase argument.     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

Irradiation effects on the YBa2Cu3O7-δ superconducting compound

The effects of neutron irradiation on the magnetic and superconducting properties of the YBa₂Cu₃O_7- (Y123) superconducting compound have been investigated. No significant change of the superconducting transition temperature (T _c) was found. The effect of irradiation on the magnetization critical current density (J _c) was studied. Bean's critical state model was used to estimate J _c. Around 40% increase in J _c was found. The temperature dependence of J _c was also studied.     

Magnetic field alignment of high-T c superconductorsRBa2Cu3O7–δ (R=rare earth)

Finely ground powders ofRBa₂Cu₃O_{7 –} (R=Y, Nd, Sm, Eu, Dy, Ho, Er, Tm, Yb) have been mixed at dilute 3%-by-volume concentrations into epoxy matrices which were then allowed to harden in applied magnetic fieldsH _A=18 kOe. X-ray diffractometry studies and 4.3 K measurements of supercurrent-induced magnetization hysteresisM are interpreted as indicating at least partial alignment of single-crystal-grain c-axes (1) parallel toH _A forR=Y, Nd, Sm, Dy, Ho (as earlier found by Farrell et al. forR=Y), and (2) perpendicular toH _A forR=Eu, Er, Tm, Yb. With a few exceptions (Y, Sm, Eu) the alignment direction correlates with the sign of the second-order Stevens factor _J of the crystalline electric field Hamiltonian in the manner suggested by Livingston et al. For the best aligned specimens (Ho, Dy) critical current densitiesJ _c (4.3 K, 5 kOe) for individual grains are estimated fromM and the Bean model to be of order 10⁷ A/cm² for the measuring fieldH parallel to the original alignment fieldH _A, and of order 10⁶ A/cm² forH perpendicular toH _A.     

Critical current density of a YBa2Cu3O7 film: Comparison between experiment and collective pinning theory

With the vibrating reed technique we have measured the critical current densityJ _c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO₂ planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ _c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ _c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB _c (1–T/T _c )² forT20 K which agrees with the anomalous temperature dependence ofB _c2 observed recently in highly anisotropic high temperature superconductors.     

反应扩散法制备的PbMo6S8超导带的超导特性

本文报道反应扩散法制备Chevrel相PbMo₆S₈超导带材的过程以及带材的超导特性。Mo带与H₂S气体反应生成单相的Mo-S化合物(MoS₂或Mo₂S₃,根据反应温度而定),再与Pb的蒸汽反应生成单相的PbMo₆S₈薄层。PbMo₆S₈超导层的临界温度T_c=13.0 关键词：     

Thermally induced oscillatory relaxation of dc resistivity and ac susceptibility in Y−Ba−Cu−O during N−S transition

Dc resistivity and ac susceptibility have been measured simultaneously on Y–Ba–Cu–O specimens under the effect of a temperature gradient. During cooling process, an intermediate (normal-superconducting) state of samples forms in the vicinity ofT _c and an artificial, local, step-like change of temperature induces an oscillatory relaxation of the measured parameter. This phenomenon has been interpreted in the framework of a vortexglass model as a collective relaxation of vortices inside a domain volume defined by vortex-glass correlation length. Close to theT _c the domain relaxation time and the correlation length have been determined as 51 s and 2 mm, respectively.     

A new 1223-type high Tc cuprate (Tl,V) Sr2Ca2Cu3O9

A new 1223-type (Tl, V)-based superconducting layered cuprate (Tl_1–x V _x ) Sr₂Ca₂Cu₃O₉ withx=0.25 and 0.50 has been successfully synthesized in the nearly-pure form and identified by powder X-ray diffration analyses. An excessive quantity of Tl is necessary for the preparation of the (Tl, V)-based 1223-type compound. Resistance and ac susceptibility measurements showed that the new (Tl, V)-based 1223 cuprate exhibits T_c above 110 K.     

Magnetotransport study of in situ magnetic field textured Dy1Ba2Cu3O7−δ superconductors

The electrical resistivity, thermoelectric power and the Nernst effect have been studied as a function of temperature and magnetic field for a typical superconductor DyBa₂Cu₃O_7– magnetically textured in situ at 1035°C. The three transport coefficients show hybrid microscopic features. In particular, we show the anisotropy with respect to the field direction (H//a,H//c) in all transport coefficients. We verify that Tinkham's law is obeyed for the broadening of the resistive transition in a magnetic field. Similarly we obtainanisotropic broadening exponents for each integrated excess property. From Arrhenius plots we obtain orders of magnitude for the activation energies characterizing each property. They are markedly different from each other.     

A pseudopotential approach to the superconducting state parameters of (Ni33Zr67)1? x M x (M=Ti, V, Co, Cu) ternary metallic glasses

Superconductivity in ternary metallic glasses has been investigated using the model pseudopotential approach, which has been found quite successful in explaining superconductivity in metals, binary alloys and binary glasses. It is observed that this simple methodology successfully explains superconducting behaviour of ternary glasses without requiring the solution of Dirac equation for a many body problem or estimation of various interactions as required in ab-initio pseudopotential theory. In the present work superconducting state parameters of fourteen metallic glasses of (Ni-Zr)-M system (M=Ti, V, Co, Cu) have been determined in the BCS-Eliashberg-McMillan framework. It is observed that addition of V, Co, and Cu as the third element (M) to a binary metallic glass (Ni₃₃ Zr₆₇) causes the parameters λ,T _c, α, andN ₀ V to decrease, and Coulomb pseudopotential (μ*) to increase with concentration of M, showing that the presence of third element (M) causes suppression of superconducting behaviour of the alloy. The decrease inT _c with increasing concentration of third element (M) may be attributed to the modifications in density of states at the Fermi levelN(E _F), and probable changes in the band structure of the alloy due to addition of the third element (M). Slight difference is noticed when Ti is added to the Ni₃₃ Zr₆₇ alloy. In this caseT _c rises initially and then decreases with concentration of M, showing a peak at aboutx=0.05. This indicates that on addition of Ti, 3d states grow near the Fermi level and hence contribute substantially toN(E _F), favouring superconducting behaviour in this case. The present results forT _c show an excellent agreement with the experimental data. QuadraticT _c equations have been proposed, which provide successfully theT _c values of ternary metallic glasses under consideration. Paper presented at National Conference on Current Trends in Condensed Matter Research, Warangal, India, September 20–22, 2004.     

Superconducting tunneling on Chevrel phase thin films

Electron tunneling spectroscopy has been applied for the investigation of Cu_1.8Mo₆S₈, PbMo_6.35S₈ and LaMo₆S₈ d.c. getter-sputtered thin films. Cryogenically deposited Al₂O₃ as the artificial barrier with Al counter-electrode formed the tunnel junctions. The energy gap Δ₀ and the ratio 2Δ₀/k_BT_c were determined from the differential conductance dJ/dV vs. voltage V. Preliminary measurements showed phonon structure in LaMo₆S₈ films.     

Low temperature structural distortion in the high Tc Chevrel-phase superconductors PbMo6S8 and SnMo6S8

High-resolution powder neutron diffraction data reveal that, at temperatures below 100 K, oxygen-free, high-T_c samples of PbMo₆S₈ and SnMo₆S₈ exhibit a small structural distortion from the R$\text{3}$ rhombohedral space group. The data cannot be refined in a simple P$\text{1}$ triclinic space group as previously reported for EuMo₆S₈ and BaMo₆S₈. Thus, a supercell ordering is suggested.     

Correlations between structure and the superconducting transition temperature in Chevrel phase molybdenum chalcogenides

The superconducting transition temperatures for Chevrel phase materials, M_xMo₆X₈(M=Pb, Sn, rare eart …, X = S, Se, Te) show wide variability even for a single phase, e.g. PbMo₆S₈ with reported T_c's of 9.8–14.7 K. The T_c is shown to be a linear function of the hexagonal crystallographic $\text{c}\text{a}$ ratio when materials are grouped according to the formal charge on the ternary element M. This correlation is discussed in terms of the electron-phonon coupling and electronic density of states.     

Preparation of ternary lead molybdenum sulfide with high superconducting transition temperature

The high field superconducting compound PbMo₆S₈ has been prepared by an isothermal chemical vapour transport reaction at 1273 K. The reaction product was single phase PbMo₆S₈ with lattice parameters of $\text{a}{}_0\text{= 6.545}\text{A}\text{?}$, and α = 89° 12′ and has been found to transform to the superconducting state at a midpoint T_c of 14.7 K.     

Fast neutron irradiation of YBa2Cu3O7

The influence of fast neutron irradiation on the superconducting transition temperature,T _c , the transition width, upper critical field,B _c2 , and critical current density,j _c , has been investigated in YBa₂Cu₃O₇ up to a fluence of 10¹⁹ cm^–2 (E>1 MeV). TheT _c degradation with fluence is slightly less than in PbMo₆S₈, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj _c and the superconducting volume fraction.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

Onset of superconductivity at 107K in YBa2Cu3O7 − δ at high pressure

Electrical resistivity measurements under pressure have been carried out on the high-temperature superconductor YBa₂Cu₃O_{7 –} as a function of temperature T between 1 and 300 K at various pressures between 8 kbar and 149 kbar. The superconducting transition temperature T_c increases almost linearly with pressure at the rate dT_c/dP - 0.13 K/kbar. The onset of T_c, defined as the temperature at which(T) drops to 90% of its extrapolated normal state value, increases from 95 K at 8 kbar to 107 K at 149 kbar. These results suggest that higher pressures will yield yet higher values of T_c.     

A new (Hg,V)-based 1212-type cuprate (Hg,V) Sr2(Y,Ca) Cu2Oz with Tc(onset) up to 110 K

We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg_1?x V _x )Sr₂(Y_1?y Ca _y )Cu₂O _z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT _c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed.