S. Gygax

Anisotropy of the Ginzburg-Landau parameter κ in NbSe2

On small single crystals of 2H NbSe2 we have measured the temperature and angular dependence ofHc2 as well as the temperature dependence ofHc1 close toTc. Using an effective mass approach to the Ginzburg-Landau equations we have analyzed our data in terms of an anisotropic κ and obtained values for the coherence distances and penetration depths parallel and perpendicular to the layers. The connection between these parameters and normal-state properties is discussed and shown to be more complex than in the isotropic case.

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.

Raman scattering from YBaCuO high Tc-superconductors

Abstract Raman scattering experiments have been carried out on polycrystalline samples of the high temperature superconductors Y 1.2 Ba 0.8 CuO 4− x and YBa 2 Cu 3 O 7− y throughout the temperature range 15 K ≤ T ≤ 300 K. The vibrational frequencies in each compound have been measured and compared to values obtained from Raman and infrared experiments carried out on similar compounds. The appearance of two new modes in Y 1.2 Ba 0.8 CuO 4− x at 170 K and 90 K and a careful investigation of the temperature dependence of a 644 cm −1 mode in YBa 2 Cu 3 O 7− y has indicated that a structural modification occurs at approximately 170 K. This result is discussed in relation to the results of previous EXAFS experiments.

The oxygen isotope effect in superconducting YBa2Cu3O7−δ

Abstract We report on measurements of the effect of 18O-16O substitution on the superconducting properties of Y1Ba2Cu3O7−δ. Substitutions were made by gas-exchange alone, and by gas exchange in conjunction with substituted BaCO3 and CuO. From Raman scattering we find that gas-exchange alone leads to lower substitution levels, particularly in the bridging oxygen O(4) position. Gas-exchanged samples give an isotope effect of 0.3 to 0.4 K. Use of substituted materials leads to samples with an isotope effect of up to 0.9 K in the resistance transition and 1.7 K in in the magnetically determinded transition.

CONSTRUCTION AND PROPERTIES OF WEAK‐LINK DETECTORS USING SUPERCONDUCTING LAYER STRUCTURES

A new technique for the construction of weak‐link loop devices (SQUID) is presented, which uses single crystals of NbSe2, a superconducting layer structure. The devices display excellent stability against mechanical shock and thermal cycling to cryogenic temperatures. A comparison is made with existing point‐contact devices.

Zero-wavevector vibrational modes in EuBa2Cu3O7−x

Abstract The Raman spectrum of the high-T C superconductor EuBa 2 Cu 3 O 7−x has been investigated. Phonon features arising from both first and second order processes have been observed. The spectra were very sensitive to the oxygen concentration in the samples. A simple lattice dynamics model has been used to obtain the frequencies and eigenvectors of the normal modes of vibration at the centre of the Brillouin zone. Using these results an attempt has been made to identify the symmetry character of the observed spectral features.

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.

Growth and characterization of BiSrCaCuO whiskers

Abstract Superconducting whiskers grown from BiSrCaCuO powders during a sintering process have been studied as a function of the starting mixture and sintering cycles. Experiments using X-ray diffraction, electron diffraction, EDX, HREM, magnetization and resistance were used to investigate the composition, structure and superconducting properties of these whiskers. They are found to be very homogeneous, free of stacking faults and exfoliations. They also show a full Meissner effect. Under proper conditions layers of the 2223 phase appear to be intercalated in the predominant 2212 phase.

Superconducting Maser: A Critical Re‐examination

The condition that the threshold current has to be less than the critical current in a superconducting injection maser is shown to be insufficient. Irrespective of the material, the number of states above the gap edge is too small to accommodate the overpopulation required to exceed all the losses in a superconductor.