Liesbet Weckhuysen

Critical currents, pinning forces and irreversibility fields in (YxTm1−x)Ba2Cu3O7 single crystals with columnar defects in fields up to 50 T

Abstract We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6×1011 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1−x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2–90 K revealed that: (i) in fields up to μ0H≈20 T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T∼40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field Bφ· was defined. Moreover, introducing columnar defects also changes the pinning force fp qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: J c,6×10 11 Kr-ions/cm 2 (77 K, 5 T)≥10Jc,ref (77 K, 5 T).

Simulation and calibration of an open inductive sensor for pulsed field magnetization measurements

The relation between the voltage induced in an open sensor and the magnetic moment of a sample during pulsed field magnetization measurements has been calculated. The calculation takes into account the size of a sample, its internal flux distribution, and its position with respect to the center of the sensor. It has been found that the behavior of the response signal with respect to the sample position is the same for homogeneously magnetized Ni samples and high temperature superconductors. Because of this universality, it is possible to calibrate the open sensor even in the case of an unfavorable sample geometry. The calculations have been experimentally verified by using Ni samples with different geometries. Our results can easily be extended to samples with an arbitrary local field distribution.

Magnetism of C60-based molecular complexes: High-field-magnetization and magnetooptical study

Magnetization study of the C60 · TMTSF · 2CS2 molecular complex in magnetic fields up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)2C60 at T=1.8 K for the frequency range 60–90 GHz in magnetic fields up to 32 T provide experimental evidence that paramagnetic centers with reduced g-factor values g<1 control the magnetic properties of these solids. A model is suggested in which the renormalization of the g factor is due to the dynamic Jahn-Teller effect involving negative C60 ions that appear as defects in the crystalline structure with a weak charge transfer.

Magnetic properties of the fullerene organic compounds in strong magnetic fields

Abstract Magnetisation study of the C60·TMTSF·2CS2 molecular complex in magnetic field up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)2C60 at T=1.8 K for the frequency range 60–90 GHz in magnetic field up to 32 T provide the experimental evidence that a paramagnetic centers with the reduced g-factor values g

EVIDENCE FOR THE PAIR FORMATION FAR ABOVE Tc IN EPITAXIAL La2-xSrxCuO4 THIN FILMS

A large positive magnetoresistivity (up to ten percent or more) is observed for both under-doped and over-doped superconducting La2-xSrxCuO4 epitaxial thin films at temperatures far above the superconducting critical temperature Tc. This magnetoresistivity can be related to the pairs eventually forming the superconducting phase at low temperatures. Our observations support the idea of a close relation between the pseudogap and the superconducting gap and provide new indications for the presence of pre-pairs above Tc.

HIGH-FIELD MAGNETOTRANSPORT OF EPITAXIAL LA2-XSRXCUO4 THIN FILMS IN THE REGIME OF LOW SR-DOPING

High temperature superconductors have an unusual temperature (T) — doping (x) phase diagram, in which both antiferromagnetism and superconductivity occur. The spin gap and the superconducting gap are the two essential features of the T(x) plane which evolve with doping. A material allowing investigating the T-x plane in detail, from the underdoped (x 0.15) regime, is the La2-xSrxCuO4 superconductor. The temperature dependence of the resistivity strongly depends on the substitution ratio x. Overdoped films show metallic behavior, meaning that ρ(T) is linear above 100 K, while it saturates for lower temperatures. This saturation persists below Tc, when superconductivity is suppressed by sufficiently high pulsed magnetic fields. From the slightly overdoped (0.2<x) down to the strongly underdoped regime (x~0.03) we observe, instead of saturation, a ρ(T) dependence with logarithmic divergence in the field-induced normal state at T→0. For underdoped films (x<0.1) a superlinear ρ(T) behavior develops at intermediate temperatures (50 K<T<200 K), indicating the opening of the spin gap.