Transport studies in a Corbino disk geometry suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows a sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe 2 crystals.These features are found to be absent in the Corbino geometry, in which the circulating vortices do not cross the sample edges.
A new procedure for analysis of random telegraph signals in time domain has been developed and applied to the analysis of voltage fluctuations in the current induced dissipative state in superconducting thin films. The procedure, based entirely on the difference in the statistical properties of discrete Marcovian telegraph fluctuations and Gaussian background noise, ascribes each point of the experimental time record to one of the telegraph states. The average statistical lifetimes and amplitudes of the telegraph signal are then determined in an iterative way by fitting the amplitude histogram of thus obtained record of the redistributed data to the two-Gaussian histogram of the original experimental signal. The procedure allows for analyzing “noisy” random telegraph signals with low ratio between the signal amplitude and the intensity of the background noise that cannot be analyzed by the classical approach. Separation of the time record into two subrecords relative to two telegraph states also enables in-depth analysis of the spectral properties of the background noise observed together with the telegraph fluctuations.
Magnetic properties of compacted La 0.8 Ca 0.2 MnO 3 manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures T C Ϸ 231 K and T C Ϸ 261 K, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances T C of La 0.8 Ca 0.2 MnO 3 nanoparticles at a rate dT C / dP Ϸ 1.8-1.9 K / kbar, independently on the average particle size. Pronounced irreversibility of magnetization below T irr Ϸ 208 K and strong frequency dependent ac susceptibility below T C for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled ͑ZFC͒ and field-cooled magnetization. These features indicate the appearance of spin-glasslike state, partially reminiscent the behavior of La 1−x Ca x MnO 3 crystals, doped below the percolation thresholdx Ͻ x C = 0.225. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of La 1−x Ca x MnO 3 crystals with x Ͼ x C . The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent 1.89Ϯ 0.56. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.
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