We have investigated vortex motion below the quasistatic melting transition in a detwinned YBaqCu307 q single crystal by measuring the differential resistance and the voltage noise. With the magnetic field parallel to the copper-oxygen planes and the large, flat surface of the sample, the noise power spectrum permits us to deduce that the surfaces are the dominant source of the noise and that the vortices appear to be fiowing in channels. In certain regimes, the noise power spectrum exhibits unusually sharp and distinct peaks which are periodic in frequency.PACS numbers: 74.60.GeIn a type-II superconductor, in the mixed state, the sample is penetrated by an array of magnetic vortices each of which contains a quantized amount of magnetic Aux. In an applied current, there will be a Lorentz force density f = J X B on the vortices, where 1 is the current density and B is the flux density [1]. If the vortices are at rest, there will be no electric field and the resistance

The resistance of small Cuo. 9lMno. p9 samples in the spin-glass phase showed random changes on thermal cycling, quantitatively showing that random spin configurations affect universal conductance fluctuations.Low-frequency resistance noise showed large non-Gaussian effects, which were inconsistent with the simplest droplet models for spin glasses. A typical spin-rearrangement event involved about 10 spins. Several detailed statistical parameters were measured for comparison with droplet and hierarchical models.

The dynamics of atomic motions in the amorphous conductors Si-Au and C-Cu were probed using conductance fluctuations in mesoscopic samples in the temperature range 4-300 K. Near 4 K the number of mobile sites was close to that expected from the anomalous heat capacity of amorphous materials. Although some sites showed two-state-switching kinetics, a variety of statistical indicators all indicated that intersite interactions were important. Amorphous materials show a surprisingly universal term in their low-temperature heat capacity, which is ordinarily attributed to two-level systems (TLS), largely on the basis of nonlinear acoustic properties. ' However, the extent to which it is appropriate to view such TLS as independent sites, or even whether the TLS picture might break down due to intersite interactions, remains uncertain. 3 In this paper we describe the use of conductance fluctuations in mesoscopic samples of amorphous materials to examine whether the mobile sites are generally independent two-state systems.The spectral densities of conductance noise in macroscopic samples of amorphous conductors has been shown to be consistent with a conventional picture of double-well systems (TLS at low temperatures) coupled to the conductance (below about 50 K) via universal conductance fluctuations (UCF), assuming a TLS density comparable to that expected from extrapolated heat-capacity measurements. According to standard UCF theory, 5 the conductance noise should be very nonselective, picking up all types of atomic motions, and thus is expected a priori to see the same mobile sites that appear in time-dependent heat capacity. Since the temperature range of our measurements is above 4 K, and particularly in view of the low conductivities of the materials, the relaxation of the TLS or other structural fluctuations should be dominated by the same phonon processes as in insulating amorphous materials. ' With mesoscopic samples we could directly measure the density offluctuating sites using the size of spectral features, and use statistical properties to test if the f1uctuators were actually independent TLS.The materials studied were Cl -"Cu"and Sii -"Au" (with x typically 0.15 to 0.20) prepared by rf sputtering of a composite target. Transmission-electron-microscopy analysis showed that the samples were almost entirely amorphous, although about 2% of crystalline inclusions occurred. Both samples of Si-Au and four of the five samples of C-Cu were in the metallic conduction regime (with resistivity in the 10 Acm range). One C-Cu sample was near the metal-insulator transition.A new double-step-edge technique, described in detail elsewhere, 6 allowed four-probe mesoscopic samples (necking down to small wires of 10 ' to 10 ' cm ) of continuous films to be prepared by ordinary photolithography. The approximate dimensions of the short, narrow necks in the samples (about 30 nm laterally by 150-nm length) were confirmed by scanning electron microscopy and by resistance measurements.Noise measurements were made with standard dc te...