H. Adrian

Superconductivity mediated by spin fluctuations in the heavy-fermion compound UPd2Al3

It is well known that any weak attractive electron–electron interaction in metals can in principle cause the formation of Cooper pairs, which then condense into a superconducting ground state. In conventional superconductors, this attractive interaction is mediated by lattice vibrations (phonons). But for the heavy-fermion and high-temperature superconductors, alternative pairing interactions are considered to be possible. For example, the low-temperature properties of heavy-fermion systems are dominated by antiferromagnetic spin fluctuations, which have been considered theoretically as a possible cause for Cooper-pair formation. This picture recently received some experimental support: the resistivity behaviour under pressure of two cerium-based heavy-fermion compounds was shown to be consistent with a magnetically mediated pairing mechanism. Here we use tunnelling spectroscopy to investigate the superconducting order parameter of a uranium-based heavy-fermion superconductor—epitaxial thin films of UPd2 Al3. Our observation of a strong-coupling feature in the tunnelling conductivity, combined with recent inelastic neutron scattering data strongly suggest a pairing interaction mediated by spin fluctuations.

Superconductivity in rare earth molybdenum selenides

Abstract The complete series of rare earth molybdenum selenides RExMo6Se8 (RE = rare earth) has been synthesized and with few exceptions these compounds are superconducting above 5 K. These results are discussed with respect to other superconducting compounds in this particular structure.

Tunneling conductance in layered oxide superconductors

The copper oxide superconductors are considered to have stacking structures of strongly and weakly super-conducting layers. Using this model, the tunneling conductance in a junction of the superconductor and a normal metal is calculated as a function of bias voltage. The calculation predicts that a characteristic fine structure appears inside the superconducting gap in the tuneling spectrum at low temperatures.

Epitaxial film growth and magnetic properties of Co2FeSi

We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron sputtering. On (100)-oriented MgO substrates we find fully epitaxial (100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film growth is (110)-oriented, and several in-plane epitaxial domains are observed. The temperature dependence of the electrical resistivity shows a power law with an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0 K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the epitaxial films are magnetically isotropic in the plane. Measurements of the X-ray magnetic circular dichroism of the films allowed us to determine element specific magnetic moments. Finally we have measured the spin polarization at the surface region by spin-resolved near-threshold photoemission and found it strongly reduced in contrast to the expected bulk value of 100%. Possible reasons for the reduced magnetization are discussed.

Extremely slow Drude relaxation of correlated electrons

The electrical conduction of metals is governed by how freely mobile electrons can move throughout the material. This movement is hampered by scattering with other electrons, as well as with impurities or thermal excitations (phonons). Experimentally, the scattering processes of single electrons are not observed, but rather the overall response of all mobile charge carriers within a sample. The ensemble dynamics can be described by the relaxation rates, which express how fast the system approaches equilibrium after an external perturbation. Here we measure the frequency-dependent microwave conductivity of the heavy-fermion metal UPd2Al3 (ref. 4), finding that it is accurately described by the prediction for a single relaxation rate (the so-called Drude response). This is notable, as UPd2Al3 has strong interactions among the electrons that might be expected to lead to more complex behaviour. Furthermore, the relaxation rate of just a few gigahertz is extremely low—this is several orders of magnitude below those of conventional metals (which are typically around 10 THz), and at least one order of magnitude lower than previous estimates for comparable metals. These observations are directly related to the high effective mass of the charge carriers in this material and reveal the dynamics of interacting electrons.

High-superconducting critical current densities in YBa2Cu3O7/PrBa2Cu3O7 superlattices

Well‐ordered [(YBa2Cu3O7)nY/(PrBa2Cu3O7)nPr)]m superlattices with high superconducting critical temperatures were prepared by sequential reactive dc sputtering. The superconducting critical current density at 4.5 K reaches values as high as 1.1×107 A/cm2 for a sample with nY = 2 and nPr = 1 and 3.8×106 A/cm2 for nY = 1 and nPr = 3, indicating that the coupling of the CuO2 layers is not necessary for high‐jc values. The angular dependence jc(φ) is determined by the component of the magnetic field perpendicular to the a‐b planes and shows that these superlattices are two‐dimensional systems.

CeO2: An alternative insulator material for superconducting field effect devices

The properties of high‐Tc superconducting field‐effect devices using CeO2 as an insulating layer have been studied. The dielectric constant and hence the achieved charge transfer for constant gate voltage and equivalent geometry is smaller as compared to the established SrTiO3 dielectric. This is mostly compensated by better insulating properties. The observed field effects for a fixed amount of charge transfer are very similar to those obtained with the SrTiO3 dielectric, indicating that the observed field effects are truly due to changes in the charge carrier density and not due to field‐induced stresses in the multilayer structure.

Preparation and structural characterization of thin epitaxial Bi2Sr2CaCu2O8+δ films with Tc in the 90 K range

Abstract Thin films of Bi 2 Sr 2 CaCu 2 O 8+δ with T c up to 92 K were prepared by an in situ sputtering method on SrTiO 3 (100) and LaAlO 3 (100) substrates. A strong c -axis orientation of the film growth with low mosaic spread and full epitaxy within the ab -plane were confirmed by X-ray diffraction in Bragg-Brentano and four-circle geometry. Rutherford backscattering and channeling confirm the correct film composition and highly textured growth with a minimum yield of 23%. The surface morphology of the films was examined by STM and SEM studies, revealing a low density of precipitates and the absence of screw dislocations. The depth profile of the composition was investigated by secondary neutrals mass spectroscopy (SNMS).

Superconducting transport properties of Bi2Sr2CaCu2O8+x bicrystal grain boundary junctions

We have performed a detailed study of the superconducting transport properties of Bi2Sr2CaCu2O8+x bicrystal grain boundary junctions (GBJs). The GBJs were fabricated by the deposition of c‐axis oriented epitaxial Bi2Sr2CaCu2O8+x films on SrTiO3 bicrystal substrates. The measured resistive transition R(T), the current‐voltage characteristics (IVCs), the temperature dependence of the critical current density Jc(T), and the magnetic field dependence of the critical current Ic(B) of the Bi2Sr2CaCu2O8+x GBJs are similar to those observed for YBa2Cu3O7−δ GBJs. However, for the same misorientation angle the magnitude of Jc is almost an order of magnitude smaller for the BSCCO‐GBJs. The R(T) dependencies and the IVCs of the BSCCO‐GBJs are close to the prediction of the resistively shunted junction (RSJ) model. The Ic(B) dependencies are close to those of large Josephson junctions with overlap geometry. Our results show that BSCCO‐GBJs may be useful for cryoelectronic applications.

Low temperature irradiation of Nb3Sn and V3Si with high energy sulphur ions

Abstract The critical temperature Tc, the transition width To and the residual resistance R0 of Nb3Sn and V3Si have been measured as functions of the fluence of 20 MeV sulphur ions below 20 K and isochronal annealing up to 290 K. The temperature dependence of the resistivity is reported for samples with different irradiation-induced states of damage. Tφ and the thermal part of the resistivity are unambiguously correlated to the Tcversus fluence behavior. At high fluences the Tc of Nb3Sn again shows a minimum as a function of fluence, as reported earlier for oxygen irradiation. The temperature coefficient of the resistivity changes from positive to negative when the Tc of Nb3Sn passes through the minimum. A qualitatively different annealing behavior of Tc and R0 is observed for samples irradiated to Tc. values before and after the minimum. Highly damaged Nb3Sn shows an increase of R0 with increasing annealing temperature. The results are compared with A15 irradiations with different projectiles and with heavy ion irradiations of superconducting elements.