P. L. Gammel

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.

Real-time magneto-optical imaging of vortices in superconducting NbSe2

We present here a new experimental tool for the direct observation of magnetic vortices in type-II superconductors. The magneto-optical imaging technique has been improved to enable single vortex observation at low flux densities. The main advantage of the new method is its high temporal resolution combined with the applicability to any superconducting sample with a flat surface. We give a short description of the experimental set-up and show examples of results obtained for a NbSe2 single crystal at 4.0 K.

Observation of smectic and moving-Bragg-glass phases in flowing vortex lattices

The defining characteristic of the superconducting state is its ability to carry electrical currents without loss. The process by which it does this has been extensively studied for decades but there are still many unresolved issues. In particular, the critical current, which is the maximum electrical current that a superconductor can carry without loss, remains a poorly understood concept at the microscopic level. In a type II superconductor, a flux-line lattice (FLL) forms if a magnetic field between Hc1 and Hc2, the lower and upper critical fields, is applied: flowing electrical currents will exert a force on this FLL. If the FLL remains pinned, the current flows without loss of energy and the effective resistance remains zero. However, if the lattice moves in response to the current, energy is dissipated and the zero-resistance state is lost. Because of its relevance to the critical current, the types of structures that these moving lattices can form have attracted much recent theoretical attention. Here we report magnetic decoration studies of flowing vortex lattices which show evidence for a transition, as a function of increasing flux density, from a layered (or smectic) FLL to a more well-ordered moving Bragg glass.

Observation of mesoscopic vortex physics using micromechanical oscillators

It has long been known that magnetic fields penetrate type II superconductors in the form of quantized superconducting vortices. Most recent research in this area has, however, focused on the collective properties of large numbers of strongly interacting vortices,: the study of vortex physics on the mesoscopic scale (a regime in which a small number of vortices are confined in a small volume) has in general been hampered by the lack of suitable experimental probes. Here we use a silicon micromachined mechanical resonator to resolve the dynamics of single vortices in micrometre-sized samples of the superconductor 2H-NbSe2. Measurements at and slightly above the lower critical field, H c1 (the field at which magnetic flux first penetrates the superconductor), where only a few vortices are present, reveal a rich spectrum of sharp, irreversible vortex rearrangements. At higher fields, where tens of vortices are present, the sharp features become reversible, suggesting that we are resolving a new regime of vortex dynamics in which the detailed configuration of pinning sites, sample geometry and vortex interactions produce significant changes in the measurable vortex resonse. This behaviour can be described within the framework of interacting vortex linesin a ‘1 + 1’-dimensional random potential—an important (but largely untested) theoretical model for disorder-dominated systems,.

Intertwined symmetry of the magnetic modulation and the flux-line lattice in the superconducting state of TmNi 2 B 2 C

Materials that can in principle exhibit both superconductivity and ferromagnetism are caught in a dilemma: both states represent long-range order, but are in general mutually exclusive. When the material favours a ground state with a large magnetic moment, as is the case for Er4Rh4B (ref. 1), superconductivity is destroyed. For superconductivity to persist, the magnetic structure would need to adopt an antiferromagnetic modulation of short enough wavelength to ensure a small net moment on the length scale of the superconducting coherence length. The intermetallic borocarbide superconductors RNi2B2C (where R is a rare-earth element) have shed new light on this balance between magnetism and superconductivity. The response of these materials in the superconducting state to a magnetic field is dominated by the formation of a flux-line lattice—a regular array of quantized magnetic vortices whose symmetry and degree of order are easily modified and thus can be expected to interact with an underlying magnetic modulation. In TmNi2B2C, superconductivity and antiferromagnetic modulated ordering coexist below 1.5 K (refs 5–7). Here we present the results of a small-angle neutron-scattering study of this compound which show that the structure of the magnetic modulation and the symmetry of the flux-line lattice are intimately coupled, resulting in a complex phase diagram.

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

Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNi2B2C

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

Monolithic MEMS optical switch with amplified out-of-plane angular motion

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.

Determination of ZnO temperature coefficients using thin film bulk acoustic wave resonators

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C ( T(c) = 16.6 K). At temperatures below 10 K the transition onset field, H2(T), is only weakly temperature dependent. Above 10 K, H2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T) merging with the upper critical field and suggests that just below the H(c2)(T) curve the flux line lattice might be hexagonal.

Temperature characteristics of ZnO-based thin film bulk acoustic wave resonators

We describe an array of electrostatically actuated surface micromachined mirrors for fiber-optic switching. Using a pure flexure angle amplification mechanism the mirrors can be tilted continuously from 0 to 14 degrees with less than 60 V.