M. J. Higgins

Instabilities and disorder-driven first-order transition of the vortex lattice

Transport studies in a Corbino disk suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows 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 NbSe2 crystals. These features are found to be absent in the Corbino geometry in which the circulating vortices do not cross the sample edges.

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.

Varieties of dynamics in a disordered flux-line lattice

Abstract Dynamics of a flux-line lattice (FLL) has been studied in the anisotropic superconductor 2H-NbSe 2 , where pinning is extremely weak (critical current/depairing current ∼ 10 −6 ), the lattice is well-formed and a robust “peak effect” occurs slightly below H c2 . The strong H dependence of the rigidity of the FLL is used to explore the crossover between interaction-dominated and disorder-dominated dynamics. Three distinct types of dynamics, describing elastic flow, plastic flow and fluid flow, respectively, are observed as the lattice softens with the approach to the superconducting-to-normal phase boundary. A power-law behavior, V ∼ ( I − I c ) β , describes the I – V curves for both elastic and fluid flow (with different β) reasonably well, but not for plastic flow. The latter dominates the intermediate regime, where the peak effect occurs, and is accompanied by qualitative changes in the I – V curves related to tearing of the FLL. In this regime, a “fingerprint phenomenon” is observed in the current-dependent differential resistance describing a specific sequence of depinning of “chunks” or filaments in a given sample. At large driving forces, the disorder due to pinning is less significant, the dynamically generated defects heal and a nearly defect-free coherent motion is recovered. In some samples, the I – V curves at the onset of motion in this regime become discontinuous and hysteretic as in first-order transitions. A “non-equilibrium phase diagram” describing various regimes of dynamics is constructed. A time-averaged velocity correlation length characterizes the spatial inhomogeneity of a moving FLL and thus distinguishes between the various dynamical regimes. These dynamical phenomena are compared and contrasted with phenomena attributed to thermodynamic phase transitions in the cuprate superconductors.

Metastability and Glassy Behavior of a Driven Flux-Line Lattice

Strong metastability and history dependence are observed in dc and pulsed transport studies of fluxline lattices in 2H-NbSe 2, leading to the identification of two distinct states of the lattice with different spatial ordering. The metastability is most pronounced upon crossing a transition line marked by a large jump in the critical current (the peak effect). Current-induced annealing of the metastable state towards the stable state is observed with a strongly current dependent annealing time, which diverges as a threshold current is approached from above. [S0031-9007(96)01054-X] In the absence of disorder the physics of a magnetic flux-line lattice (FLL) is governed by the interplay between thermal fluctuations, which favor melting, and interactions, that lead to ordering. The resulting phase diagram consists of a liquid and a solid phase with relatively simple dynamics. Quenched disorder causes the system to develop additional phases and complex dynamic effects such as pinning and irreversibility in the magnetic and electric responses. The role played by disorder and pinning in the physics of FLL in equilibrium has recently become an area of intense study [1,2]. A related but distinct topic of current interest concerns the role of motion on the spatial ordering of the FLL, the resulting dynamical transitions or crossovers that may occur, and the relation they bear to the disorder free situation [3‐7]. In this Letter we report on the existence of two distinct states of the FLL, one disordered, the other much less disordered (hereafter referred to as the ordered state), with strikingly large differences between their transport properties. As a result, the system displays a wide range of phenomena such as history dependence, metastability, current-induced annealing, and glassy relaxation. Each of these states is stable in its own sharply defined region of the (H, T ) plane and metastable elsewhere, and each can be accessed with a simple reproducible procedure. Our experiments show directly that the metastable state can be annealed into the “equilibrium” state by applying a current that depins the FLL. The annealing kinetics is found to be strongly current dependent, with the annealing time diverging as the depinning current is approached from above. The variation of these phenomena with field, temperature, and driving current provides direct access to the interplay between static and dynamic transitions and can elucidate the role of disorder in different parts of the FLL phase diagram. Our results can also be used to interpret the rich and complex history dependence studied earlier in low Tc superconducting films [8,9].

Anomalous peak effect in CeRu2 and 2H-NbSe2: Fracturing of a flux line lattice

CeRu 2 and 2H-NbSe 2 display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for CeRu 2 .

Two coexisting vortex phases in the peak effect regime in a superconductor

The critical current in the vortex phase of a type-II superconductor such as NbSe2 displays a striking anomaly in the vicinity of the superconductor-to-normal-metal transition. Instead of going to zero smoothly, it rebounds to a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect, has remained an unsolved problem for 40 years. Here we use a scanning a.c. Hall microscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter phases with intrinsically different pinning strengths coexist on a macroscopic scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects and anomalous voltage response commonly seen when external parameters such as temperature, magnetic field or transport current are varied. We argue that the observed phase coexistence is, in fact, the hallmark of a disorder-driven non-thermal phase transition.

Supercooling of the disordered vortex phase via minor hysteresis loops in 2 H − NbSe 2

We report on the observation of novel features in the minor hysteresis loops in a clean crystal of

Metastability and switching in the vortex state of 2H-NbSe2

2\mathrm{H}\ensuremath{-}{\mathrm{NbSe}}_{2},

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

which displays a peak effect. The observed behavior can be explained in terms of a supercooling of the disordered vortex phase while cooling the superconductor in a field. Also, the extent of spatial order in a flux-line lattice formed in ascending fields is different from (and larger than) that in the descending fields below the peak position of the peak effect; this is attributed to a different degree of reorganization of the vortex state induced by changes in the field in the two cases.

Nature of phase transitions of superconducting wire networks in a magnetic field.

History-dependent metastable states with different bulk properties are formed in the vortex state of the type-II superconductor 2H-NbSe2. Magnetic measurements demonstrate the difference between the shielding responses of a field- and a zero-field-cooled state, and provide a procedure for switching the system from one state to the other.