J. V. Kunzler

Electrical resistivity of heusler alloys

Abstract The electrical resistivity of eight different Heusler alloys is discussed in terms of electron phonon and spin-disorder scattering. The systematics common to the alloys can be explained in the intermediate to high temperature range while the low temperature range still presents interpretation difficulties.

Splitting of the bulk resistive transition in high-Tc superconductors : evidence for unconventional pairing

Abstract We report on measurements of the resistive transition in several samples of polycrystalline YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ x , and single crystal Bi 2 Sr 2 CaCu 2 O 8+ x . The results are analyzed in terms of the temperature derivative of the resistivity, d ϱ /d T . In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d ϱ /d T , which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.

Spin-disorder resistivity in the Cu2Mn(Al1−xSnx) heusler alloys

Abstract Electrical resistivity measurements have been performed on the Cu 2 Mn(Al 1− x Sn x ) Heusler alloys, where x = 0; 0.05; 0.10; 0.15, in the temperature range of 4–800 K. The experimental curves clearly show the importance of the ferromagnetic character on the resistivities of the alloys. The results obtained are in agreement with an interpretation in terms of a spin-disorder model.

Critical and Gaussian conductivity fluctuations in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8☆

Abstract Systematic conductivity fluctuation measurements are reported for polycrystalline YBa 2 Cu 3 O 7−° and single-crystal Bi 2 Sr 2 CaCu 2 O 8 . Gaussian and critical regimes are observed. For both systems, the critical exponents are consistent with s-wave pairing.

Electrical resistivity of amorphous Fe0.92Zr0.08

Abstract We report on precise resistivity measurements between 2.5 K and 300 K of a-Fe0.92Zr0.08. The resistivity varies as − T below 9 K. There is a singularity in the temperature derivative of the resistivity at the Curie point from which the critical exponent α = −1.1 ± 0.2 is deduced. This value for α supports the image of a disordered ferromagnetic-like state dominated by short-range correlations.

Breaking of ergodicity in high-Tc superconductor YBa2Cu3O7

Abstract We report on magnetic measurements on a high-Tc oxide superconductor YBa2Cu3O7. Below a characteristic temperature, glassy features are revealed by the diamagnetic susceptibility which splits in two separate branches corresponding to zero field cooling (ZFC) and field cooling (FC) the sample. The metastable ZFC susceptibility is considerably larger than the completely reversible FC susceptibility. The temperature which marks the onset of the irreversibility effects depends on the applied field and the line separating ergodic and non-ergodic regimes in the H-T plane closely resembles those found in spin glasses.

Spin-disorder resistivity in Ni2Mn(Sn1−xInx) heusler alloys

Abstract Electrical resistivity measurements have been performed on Ni 2 Mn(Sn 1− x In x ) Heusler alloys, where x = 0, 0.02, 0.05, 0.10 and 0.15, in the temperature range of 4–300 K. The experimental data clearly show the existence of two distinct kinds of resistivity behaviour, both described by an aT n law, for 7 ⩽ T ⩽ 20 K. Between 170 and 300 K the data are well described by an AT + BT 2 phenomenological fit, and the results obtained are in good agreement with an interpretation based on electron-phonon and spin-disorder scattering.