Remanent fields and critical currents in Tl2 Ba2Ca2Cu3Ox ceramic

The temperature and magnetic-field dependences of the remanent magnetization have been determined from far-field measurements for thallium ceramic samples. The current flow that gives rise to the observed magnetization is intragranular. The results are consistent with flux-creep-controlled transport with a distance between pinning centers of - 1/2/m. Using near-field measurements, the superconducting properties are found to be heterogeneous on a scale of 1 mm. Magnetic domains have been induced by pplying spatial variation in the applied field.     

Homogeneity and critical current density of Sn-doped PbMo6S8 superconductors

With the overall critical current density,J _c , remaining low for PbMo₆S₈, the addition of 0.3 at. % Sn noticcably improved the performance. The enhancement occurs despite the presence of excess Pb and a slightly suppressedT _c . The increase inJ _c can be attributed to a better homogeneity with respect to the chemical composition of the samples, which may be as a consequence of Sn substitution in the lattice. Detailed calorimetric investigation of undoped PbMo₆S₈ showed aT _c distribution down to 9 K evidenced by a smearing ofT _c . This may be due to a progressive degradation of the sample quality leading to a plausible off-stoichiometry. High-resolution Auger electron spectroscopic experiments suggest a variation in composition indicating excellent correspondence with the specific heat data. However, no such deviation was detected for the Sn-doped samples and therefore the observed raise inJ _c . The results are discussed in conjunction with oxygen, lead and tin incorporated PbMo₆S₈.     

Some parameters of superconductors with an extreme singularity in the density of state: (s+d) model

We study some parameters of superconductors with δ-function type singularities in the electronic density of states (DOS), exhibiting (s+d)-wave symmetry of the order parameter. Starting with a pure s-wave pairing potential V_s, the critical temperature T_c at first slightly increases with increasing the d-wave interaction potential V_d, being determined by this interaction only for stronger V_d values. The ratio R=2|Δ(0)|/k_BT_c of the mean value of the zero temperature energy gap |Δ(0)| to T_c increases with increasing V_d, reaching a maximum which depends on the mixing interaction term. The maximum values of R are comparable with very high values obtained in some gap measurements. The jump in the specific heat at critical temperature is by a factor 2.4 higher for the extreme singularity of pure s-wave symmetry, as compared with the BCS theory with constant DOS. Such higher values of the jump are in agreement with the experimentally observed values, as well as with the calculations determined by extended saddle points in the electronic bands. By switching the d-wave channel, the value of the jump decreases. The results show the usefulness of calculations with δ-type singularities as a limiting case of very strong singularities in the DOS.     

Effect of polarizable atmospheres on superconductivity of the Y-Ba-Cu-O compounds

This letter reports that there may exist an intense interaction between the Y-Ba-Cu-O superconductors and polarizable molecules, which results in a change in structure and affects the superconductivity. This phenomenon can be explained in terms of the polarization and recombination of electron orbits of copper-oxygen plane.     

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.     

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.     

A new understanding of the resistive state and the V-I characteristic of high-Tc superconductors

Bardeen-Stephen dissipation in the regime of thermally activated flux flow (TAFF) and flux creep has been considered with respect to its contribution to the activation energy for thermally initiated flux movement. The inclusion of this flux flow dissipation term leads to a new expression for the voltage-current (V-I, or E-J) characteristics. The extended model describes the flux dynamics in the three regimes of flux movement: TAFF, creep, and flux flow, in a satisfactory way and explains the main experimental observations.     

Vortex glass phase and flux creep in YBa2Cu3O y epitaxial film

Nonlinear I–V characteristics of YBa₂Cu₃O _y epitaxial film at different temperatures in a magnetic field of up to 5T and angles between the direction of the field and the a–b plane of 0°, 30°, 45°, 60°, and 90° were measured. We find that there are the vortex glass phase and vortex liquid phase in the mixed state. The flux creep exists in the vortex glass phase, and it obviously appears in the area around the boundary between the vortex glass phase and the vortex liquid phase.     

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.     

The Tl-Ba-Ca-Cu-O superconducting ceramics containing Al2O3

The Tl-Ba-Ca-Cu-O superconducting ceramics containing Al₂O₃ have been prepared utilizing the reaction, Tl₂O₂+BaCuO₂+Ca₂CuO₃+A1₂O₃ or Tl₂O₃+BaCuO₂ +Ca₂CuO₃+AlCuO_2.5. It is shown that the resulting ceramics show the superconductivity at temperatures over 100 K when the Al/Cu molar ratio is kept below 0.5. EPMA analyses revealed that there is always an Al₂BaO₄ layer between the superconducting Tl-Ba-Ca-Cu-O and Al₂O₃ phases, leading to the separation between the superconducting and A1₂O₃ phases.     

Crossover from high-temperature vortex-liquid to low-temperature vortex-glass behavior in single phase HgBa2Ca2Cu3O8+δ superconductor

The ρ-T curves in our single phase HgBa₂Ca₂Cu₃O_8+δ superconductor were measured as a function of temperature and magnetic field, ρ=ρ₀exp(−U_eff/κ_BT). It can be transformed to another form d(lnρ)/d(1/T)=−U_eff+TdU_eff/dT, then this becomes a plot of the activation energy U_eff as a function of temperature. Our data plotted in these ways show a clear crossover from high-temperature two-dimensional vortex-liquid to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents v(z−1)=3.9±1.9 in this system are little different with previous measurements in BSCCO and YBCO systems.     

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.     

Upper critical fields of chromium-rhenium alloys

Alloys Cr_1–x Re _x in theirbcc phase withx up to 0.31 have been studied through measurements of the upper critical magnetic fields, electrical resistivity and thermoelectric power. For this alloy system itinerant electron antiferromagnetism has recently been postulated to coexist with superconductivity. The critical field curvesB _c2(T) measured down to 0.06 K may be adequately described by the theroetical results obtained by Werthamer et al. and by Maki for normal dirty superconductors. Hence there is no indication of magnetic order in these data. The influence of spin orbit scattering is found to increase with the Re concentration. The sign and temperature variation of the thermoelectric power measured up to 300 K gives evidence of a drastic change of the electronic structure of the alloys with increasing Re concentration.     

Upper critical fields of cubic and tetragonal single crystal and polycrystalline Nb3Sn in DC fields to 30 tesla

Measurements of the upper critical field, H_c2(T), in single crystal Nb₃Sn were extended to 30 tesla (300 kG) with dc fields produced by a Hybrid magnet. Observations of H_c2(T) were made for materials which remain in the cubic (c) phase and those which show a martensitic transformation at the tetragonal (t) phase. H_c2(T) measurements of Nb₃Sn for a pure crystal for which the de Haasvan Alphen (DHVA) effect was observed and for polycrystalline (t) phase and (c) phase materials are also reported. Measured values of H_c2(4.2 K) and calculated values of H_c2(0) are: 1) along the [100] direction for our earlier Nb₃Sn, H_c2(4.2 K) = 26 T for the (c) phase and 21.5 T for the (t) phase; H_c2(0) = 29T for the (c) phase and 24 T for the (t) phase; 2) along the [100] direction for the DHVA material H_c2(4.2 K) = 18 T and H_c2(0) = 20 T; 3) for polycrystalline Nb₃Sn (t) phase material H_c2(4.2 K) = 23 T and H_c2(0) = 25 T and for (c) phase material H_c2(4.2 K) = 26 T and H_c2(0) = 29 T. The values of (dH_c2/dT)_T=Tc vary from 2.4T/K for the highest H_c2(T) material to 1.6T/K for the DHVA material. The anisotropy for various Nb₃Sn single crystal materials is small and independent of temperature from T_c to 0.1 T_c. δT_c between the (c) and (t) phase is <0.3 K. Within experimental error excellent fits of H_c2(T) with theory are obtained assuming a dirty or clean Type II superconductor with no Pauli paramagnetic limiting. Experimental details and strong-coupling effects are discussed. When strong-coupling is included, the effects of any paramagnetic limiting would be small and not detectable within our present experimental error. Brief comments also are made concerning H_c2 of V₃Si.     

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.     

The collective nature of a flux creep in high-T c superconductor YBa2Cu3O x as observed in microwave absorption experiments

Studies of the flux lattice dynamics via microwave absorption measurements with the help of an ESR spectrometer are presented. The experiments give evidence that for the temperatures below 35 K the absorbed power depends on the input microwave power in a non-linear way. For the case of absorption due to the motion of vortexes via thermally activated flux creep this corresponds to the dependence of the activation barrier on the current density. The measurements lead to the conclusion, that the activation barrier should grow with decreasing current. Fitting of the expression for the dissipated power obtained in terms of the collective creep theory to the obtained experimental data enable us to determine the character of pinning and creep at small amplitudes of the external exciting current. The obtained numerical values give evidence that the flux lattice is frozen in a vortex-glass state and in the limit of zero current the resistance at finite temperatures vanishes.     

Influence of heat treatment on the superconducting properties of Bi1.6Pb0.3Sb0.1Sr2Ca2Cu3O y

Differently heat-treated samples with the nominal composition Bi_1.6Pb_0.3Sb_0.1Sr₂Ca₂Cu₃O _y were studied by resistance and ac susceptibility measurements in the temperature range 77 K<T<200 K. The X-ray diffraction patterns at room temperature were also investigated. The experimental results show that repeated quenchings can depress the formation of the 80 K phase, and enhance the formation of the 110 K phase.     

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.     

Effect of compositional variation on the properties of Y-Ba-Cu-O compounds

Single phase YBa₂Cu₃O_7– as well as multi-phase compositions of the Y-Ba-Cu-O system have been prepared by ceramic techniques. The effects of the additional phases on the superconducting transition and normal state properties have been investigated. Measurements show that while the transition temperature is not effected, grain growth, resistivity and unit cell volume of the 123 phase are dependent on the presence of multi phases. The observed changes in cell volume and resistivity can both be explained by a change in the oxygen concentration in the range 0 to 0.15.     

Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe

Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ∼5 to ∼30. Magnetization M(T) and resistivity ρ(T) measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 μ_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s=0.65 K.