No abstract

No abstract

No abstract

No abstract

No abstract

We report on fluctuation magnetoconductivity and magnetic irreversibility of Y 1−x Pr x Ba 2 Cu 3 O 7−␦ single crystals and a polycrystalline sample. Although our samples are all single phase ͑orthorhombic͒ and the single crystals show no sign of structural inhomogeneity, all the samples exhibit two close and sharp genuine superconducting transitions. On the other hand, while the resistive transition of the polycrystalline sample exhibits in addition a coherence transition characteristic of a granular superconductor and the magnetic irreversibility displays the signature of the intergrain flux dynamics, the single crystals show no sign of these features. In view of these facts, we conclude that the well characterized split superconducting transition must result from a peculiar phase separation related with oxygen doping.

No abstract

The extraordinary Hall effect coefficients Rs(T ) of the canonical spin glass alloys AuF e8at% and AuM n8at% and of an archetype reentrant system AuF e18at% were measured as functions of temperature. The data show a critical cusp-like term superimposed on a smooth background for the spin glasses, and change of sign with temperature for the reentrant. The results can be interpreted consistently by invoking a chiral Hall effect contribution as proposed by Kawamura.Spin glasses have been intensively studied for more than thirty years, as paradigms for the statistical mechanics of the whole vast family of complex systems. However the origin the spin freezing in the canonical spin glasses, dilute alloys such as AuF e or CuM n where the spins are Heisenberg, has long remained an enigma. Experiments and in particular critical exponent measurements [1] have shown definitive evidence for a non-zero ordering temperature, while numerical work on Heisenberg spin glasses in dimension three indicated that there was an Edwards-Anderson type of ordering only at zero temperature [2] though recent simulations contradict this [3]. For vector spin glasses there exists a chiral order parameter in addition to the Edwards-Anderson spin parameter and there were early suggestions that chirality might play a role in vector spin glass ordering [4]. Kawamura and coworkers [5,6,7] have made concrete large scale numerical investigations of the chiral driven ordering mechanism which they postulated to explain freezing in Heisenberg spin glasses. In real samples the spin and chiral order are linked through Dzaloshinsky-Moriya (DM) random anisotropy terms which are inevitably present. Magnetic torque experiments on a range of spin glasses have shown in-field transition lines up to high applied fields [8], in excellent agreement with extensive simulations on Heisenberg systems submitted simultaneously to anisotropy and field, where both chiral and spin ordering were monitored [6,9]. The robustness of order under applied fields is a clear indication that chirality is a primary ingredient of spin glass ordering in vector systems. The torque measurements were interpreted using the chiral ordering scenario.In this context it would be of considerable interest to have a complementary direct observation of chirality in the spin glasses. Chirality is a "hidden" parameter and no technique was known with which to monitor it experimentally, until Kawamura proposed that the Hall effect should present a direct signature of the chiral susceptibility [10]. On the chiral scenario the extraordinary Hall signal (linked to the sample magnetization rather than the magnetic field) should have a critical term due to chirality. The key point is whether there is critical behaviour of the Hall coefficient R s (T ) at the glass temperature T g , and accompanying non-linear effects. R s is defined as the ratio of the Hall signal to the magnetization, once corrections have been made for the ordinary Hall term. Careful Hall experiments on spin glasses have been reported [11...