J. P. Franck

Evidence for Two Superconducting Gaps in MgB2

We have measured the Raman spectra of polycrystalline MgB2 from 25 to 1200 cm(-1). A superconductivity-induced redistribution in the electronic Raman continuum was observed. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. The measured spectra were analyzed using a quasi-two-dimensional model in which two s-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained Delta(1) = 22 cm(-1) ( 2.7 meV) and Delta(2) = 50 cm(-1) ( 6.2 meV). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron sigma bands and is responsible for the superconductivity of MgB2.

Oxygen isotope effect in superconducting (Y1-xPrx)Ba2Cu3O7-δ

Abstract Oxygen isotope effect studies were performed on the (Y 1−x Pr x ) Ba 2 Cu 3 O 7−δ system for 0⩽x⩽0.5. With increasing x, and falling transition temperature, we find a large isotope effect, the coefficient α 0 is approaching 0.5. The results show the importance of the electron-phonon interaction for the superconductivity of these compounds.

Boron isotope effect in Ni and Pd based borocarbide superconductors

Abstract The boron partial isotope effect (10B-11B) was investigated in YNi2B2C and in YPd5B3C0.3. Samples with pure 10B and 11B, as well as natural boron, and a 1 : 1 mixture of 10B and 11B were investigated. We find a shift of the magnetic transition temperature of ΔT c = T c ( 10 B )−T c ( 11 B ) = 690 mK for the Pd based samples, and of 380 mK for the Ni based samples. This corresponds to partial isotope exponents αB = 0.25 ± 0.04 (Ni based samples) and αB = 0.32 ± 0.04 (Pd based samples). This supports the classification of these materials as electron-phonon mediated superconductors. Preliminary investigations of the carbon isotope effect (12C-13C) did not show a change in Tc.

The copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

The copper isotope effect (63Cu —65Cu) was investigated for oxygen-deficient YBa2Cu3O7−δ with transition temperatures between 40 and 91 K. The isotope exponent αCu=−ΔℓnTc/ΔℓnmCu is negative for all transition temperatures. It is small, αCu≈−0.14, in the 60 K plateau, and unobservable for Tc=91 K. In regions away from these special conditions, αCu reaches large values of up to αCu=−0.4.

Whisker-growth of superconducting BiSrCaCu oxide

Abstract The growth of superconducting whiskers of Bi 2 Sr 2 Ca 1 Cu 2 O x is described. The whiskers are single-crystal, and show elastic behaviour. The superconducting transition temperature of the whiskers is near 75 K.

Experimental studies of the isotope effect in high Tc superconductors

Abstract A brief review of isotope effect studies in high T c cuprates is given. It is shown that in all systems a close connection exists between the isotope effect and the pressure dependence of T c . Both charge transfer and noncharge transfer mechanisms contribute to these effects.

Measure of the magnitude of Young's modulus in whisker-like samples of the Bi-based high-Tc materials Bi2Sr2CaCu2Ox and Bi2Sr2Ca2Cu3Ox

Abstract We have measured Youngs modulus in ribbon-like whisker samples of the high- T c materials: Bi 2 Sr 2 CaCu 2 O x and Bi 2 Sr 2 Ca 2 Cu 3 O x . This is the first direct measurement of Youngs modulus for whiskers of any of the high T c materials. We used a stress-strain probe that applies an axial force (uniaxial stress) to the whisker-like sample and simultaneously measures the corresponding sample elongation. The slope of the resulting stress-strain curve yields the magnitude of Youngs modulus ( Y ). We found these Bi based high- T c materials to be soft with Y ≈20 GPa for Bi 2 Sr 2 CaCu 2 O x and Y ≈25 GPa for Bi 2 Sr 2 Ca 2 Cu 3 O x .

The oxygen isotope effect in Pr, Ca, and Zn substituted YBa2Cu3O7−δ and EuBa2Cu3O7−δ

Abstract The oxygen isotope effect α was studied for Pr and Ca substituted YBCO, and for Zn substituted EuBCO and YBCO, α increases to values near 0.5 for the highest Pr concentrations, and lowest transition temperatures. In the Zn substituted system α remains small to the lowest transition temperatures. It is shown that the isotope exponent is a function of mobile hole concentration, it decreases with increasing hole concentration and remains low for over-doping.

Oxygen isotope effect in the superconductor YBa2Cu3O7-x

Abstract Shifts in the superconducting transition in 18 O substituted YBa 2 Cu 3 O 7-x are reported. Shifts as large as 1.31 K (resistively) and 1.70 K (magnetically) are found.

Doping dependence of the critical field Hc2 and the transition temperature in Zn doped YBa2(Cu1−xZnx)3O7−δ

We present data on the doping dependence of Tc in Zn doped YBa2(Cu1−xZnx)3O6.94, for x up to 10.6%. We find re-entrant behaviour in the range between 9.0 and 10.6% Zn. Data on the magneto-resistance of a sample with extremely low Tc (9% Zn, zero field resistive onset near 3.5 K) show linear normal resistance in all fields down to 1.8 K in 17.5 T. A resistively defined critical field H*, related to Hc2, shows divergent behaviour. The temperature dependence of H* is well represented by expressions derived for the Bose condensation of pre-formed pairs.