Y. Paltiel

Dynamic instabilities and memory effects in vortex matter

The magnetic flux line lattice in type II superconductors serves as a useful system in which to study condensed matter flow, as its dynamic properties are tunable. Recent studies have shown a number of puzzling phenomena associated with vortex motion, including: low-frequency noise and slow voltage oscillations; a history-dependent dynamic response, and memory of the direction, amplitude duration and frequency of the previously applied current; high vortex mobility for alternating current, but no apparent vortex motion for direct currents; and strong suppression of an a.c. response by small d.c. bias. Taken together, these phenomena are incompatible with current understanding of vortex dynamics. Here we report a generic mechanism that accounts for these observations. Our model, which is derived from investigations of the current distribution across single crystals of NbSe2, is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current. For an alternating current, only narrow regions near the edges are in the disordered phase, while for d.c. bias, most of the sample is in the disordered phase—preventing vortex motion because of more efficient pinning. The resulting spatial dependence of the disordered vortex system serves as an active memory of the previous history.

Dynamic creation and annihilation of metastable vortex phase as a source of excess noise

The large increase in voltage noise, commonly observed in the vicinity of the peak-effect in superconductors, is ascribed to a novel noise mechanism. A strongly pinned metastable disordered vortex phase, which is randomly generated at the edges and annealed into ordered phase in the bulk, causes large fluctuations in the integrated critical current of the sample. The excess noise due to this dynamic admixture of two distinct phases is found to display pronounced re-entrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears.

V-I characteristics in the vicinity of the order-disorder transition in vortex matter

The shape of the V-I characteristics leading to a peak in the differential resistance r{sub d}=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r{sub d} is large when measured by dc current. However, for a small Iac on a dc bias r{sub d} decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r{sub d} are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.

Magnetization decay due to vortex phase boundary motion in BSCCO

Abstract We identify a new regime of decay of the irreversible magnetization in clean Bi 2 Sr 2 CaCu 2 O 8 crystals, at induction values close to the “second peak field” at which the bulk critical current density steeply increases. A time window is identified during which the decay of the induction is controlled by the slow propagation of the phase transformation front across the sample.

Abrupt crossover between thermally activated relaxation and quantum tunneling in a molecular magnet

We report Hall sensor measurements of the magnetic relaxation of Mn12 acetate as a function of magnetic field applied along the easy axis of magnetization. Detailed data taken at a series of closely spaced temperatures between 0.24 K and 1.4 K indicate an abrupt shift between thermally activated and ground-state tunneling over a narrow range of temperature.

Measurement of the magnetic induction vector in superconductors using a double-layer Hall sensor array

We describe an experimental technique for simultaneous measurement of both the normal (Bz) and the in-plane (Bx) components of the magnetic induction field near the surface of a superconducting sample. This technique utilizes a novel design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. The effectiveness of this technique is demonstrated in measurements of Bx and Bz and the current distribution at the surface of a thin YBa2Cu3O7 crystal.

Magnetic noise measurements using cross-correlated Hall sensor arrays

An experimental technique for measuring magnetic fluctuations by means of a double-layer Hall sensor array is described. The technique relies on cross-correlating Hall signals from two independent sensors positioned one above the other in two separate two-dimensional-electron-gas layers of a GaAs/AlGaAs heterostructure. The effectiveness of the technique is demonstrated by a reduction of the magnitude of the background noise floor of the correlated sensors with respect to the noise level of the best single sensor.

Flux pinning, surface and geometrical barriers in YNi2B2C

Abstract In the majority of the (RE)Ni 2 B 2 C family of superconductors (RE=Dy, Ho, Er, Tm, Y, Lu), a magnetic ordering of the RE moments (RE=Dy, Ho, Er, Tm) appears to directly influence the formation and structure of the vortex lattice as well as the weak residual flux pinning properties. To study the residual pinning in these materials, it is, therefore, necessary to investigate the non-magnetic members such as YNi 2 B 2 C or LuNi 2 B 2 C, without the influence of magnetic order. Here, we present data from local Hall probe and global magnetization measurements used to examine flux pinning and superconducting hysteresis in YNi 2 B 2 C ( T c ≈15.8 K). At high fields, a pronounced peak effect in the magnetization indicates that bulk pinning becomes significant as the vortex lattice softens for fields approaching B c2 . On the other hand, for small applied fields close to H c1 , direct measurements of the local induction using linear micro-Hall probe arrays show dome-like field profiles, as expected when surface and geometrical barrier effects dominate the vortex behaviour over bulk pinning. We discuss the competing roles of weak residual bulk pinning and surface and geometrical barrier effects in YNi 2 B 2 C.

Hall-array gradiometer for measurement of the magnetic induction vector in superconductors

An experimental technique for measuring the distribution of the normal and planar components of the magnetic induction near the surface of a superconducting sample is described. This technique utilizes a design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. Applications of this technique are demonstrated in measuring the current density distribution and in characterizing the flux creep process in a thin YBa2Cu3O7 crystal.

Edge Contamination Effects in the Dynamics of Vortex Matter in Superconductors: Memory Effects and Excess Flux-flow Noise

The magnetic flux line lattice in type II superconductors serves as a useful system in which to study condensed matter flow, as its dynamic properties are tunable. Recent studies have shown a number of puzzling phenomena associated with vortex motion, including: low-frequency noise and slow voltage oscillations; a history-dependent dynamic response, and memory of the direction, amplitude duration and frequency of the previously applied current; high vortex mobility for alternating current, but no apparent vortex motion for direct currents; negative resistance and strong suppression of an a.c. response by small d.c. bias. A generic edge contamination mechanism that comprehensively accounts for these observations is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current. For an alternating current, only narrow regions near the edges are in the disordered phase, while for d.c. bias, most of the sample is in the disordered phase-preventing vortex motion because of more efficient pinning. The resulting spatial dependence of the disordered vortex system serves as an active memory of the previous history. Random injection of the strongly pinned metastable disordered vortex phase through the sample edges and its subsequent random annealing into the weakly pinned ordered phase in the bulk results in large critical current fluctuations causing strong vortex velocity fluctuations. The resulting excess low frequency flux-flow voltage noise displays pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise is absent.