C.J. van der Beek

Thermally assisted flux flow at small driving forces

The theory for thermally assisted flux flow (TAFF) in the limit of small driving forces is used to derive exact expressions for the time-dependent behaviour of the magnetisation and permeability. The problem is especially relevant for the high-temperature superconductors where large variations of the transition temperature in small DC fields are observed as a function of the frequency of the probing AC field. The parameters of the theory are extensively discussed in relation to the present understanding of flux pinning.

'Inverse' melting of a vortex lattice

Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to ‘dither’ the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.

Magnetization and relaxation curves of fast relaxing high-Tc superconductors

Abstract The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.

Strong pinning in high-temperature superconducting films

Detailed measurements of the critical current density

High-temperature superconductivity in LaBaCaCu3O6.85

{j}_{c}

High-field state of the flux-line lattice in the unconventional superconductor CeCoIn5

of

Flux pinning and creep in the vortex-glass phase in Bi2Sr2CaCu2O8+δ single crystals

{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}

Field-induced suppression of the phase transition in Bi2Sr2CaCu2O8.

films grown by pulsed laser deposition reveal the increase of

Nonlinear current diffusion in type-II superconductors

{j}_{c}

Superconductivity and 2-dimensional magnetism in orthorhombic and tetragonal GdBa2Cu3O7−δ

as a function of film thickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024 (1991)]. From the model, we deduce values of the defect density