Electron localization effects on the low-temperature high-field magnetoresistivity of three-dimensional amorphous superconductors

he electrical resistivity ρ of three-dimensional amorphous superconducting films a-Mo3Si and a-Nb3Ge is measured in magnetic fields μ0H up to 30 T. At low temperatures and at magnetic fields above the upper critical field Hc2, ρ is temperature independent and decreases as a function of magnetic field. This field dependence is consistent with localization theory in the high-field limit [μ0H≫ħ/(4eLφ2), where Lφ is the phase-coherence length]. Above the superconducting transition temperature Tc, the temperature dependence of the conductivity is consistent with inelastic scattering processes which are destructive to the phase coherence for electron localization, thereby allowing estimates for Lφ(T). The Hall effect data on a-Mo3Si, in conjunction with the resistivity data, allow the determination of the carrier concentration and mean free path. The upper critical field is comparable to (in a-Mo3Si) and significantly larger than (in a-Nb3Ge) the Clogston-Chandrasekhar paramagnetic limit. This phenomenon is discussed in the context of electron localization

Evidence for Thermally Activated Spontaneous Fluxoid Formation in Superconducting Thin-Film Rings

We have observed spontaneous fluxoid generation in thin-film rings of the amorphous superconductor Mo$_3$Si, cooled through the normal-superconducting transition, as a function of quench rate and externally applied magnetic field, using a variable sample temperature scanning SQUID microscope. Our results can be explained using a model of freezout of thermally activated fluxoids, mediated by the transport of bulk vortices across the ring walls. This mechanism is complementary to a mechanism proposed by Kibble and Zurek, which only relies on causality to produce a freezout of order parameter fluctuations.Comment: 4 pages, 3 figure

c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions

Within a microscopic two band model of planes and chains with a pairing potential in the planes and off diagonal pairing between planes and chains we find that the chains make the largest contribution to the Josephson tunnelling current and that through them the d-wave part of the gap contributes to the current. This is contrary to the usual assumption that for a d-wave tetragonal superconductor the c-axis Josephson current for incoherent tunnelling into an s-wave superconductor is zero while that of a d-wave orthorhombic superconductor with a small s-wave component to its gap it is small but non-zero. Nevertheless it has been argued that the effect of twins in YBCO would lead to cancellation between pairs of twins and so the observation of a current in c-axis YBCO-Pb experiments is evidence against a d-wave type order parameter. We argue that both theory and experiment give evidence that the two twin orientations are not necessarily equally abundant and that the ratio of tunnelling currents in twinned and untwinned materials should be related to the relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur

Angular dependence of Josephson currents in unconventional superconducting junctions

Josephson effect in junctions between unconventional superconductors is studied theoretically within the model describing the effects of interface roughness. The particularly important issue of applicability of the frequently used Sigrist-Rice formula for Josephson current in d-wave superconductor / insulator / d-wave superconductor junctions is addressed. We show that although the SR formula is not applicable in the ballistic case, it works well for rough interfaces when the diffusive normal metal regions exist between the d-wave superconductor and the insulator. It is shown that the SR approach only takes into account the component of the d-wave pair potential symmetric with respect to an inversion around the plane perpendicular to the interface. Similar formula can be derived for general unconventional superconductors with arbitrary angular momentum l.Comment: 4 pages, 4 figure

Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron doped cuprate superconductor LaCeCuO. An applied magnetic field reduces the spectral weight around zero energy and shifts it non-linearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.Comment: 9 pages, 7 figures; December 2004, accepted for publication in Phys. Rev.

Simple theory of extremely overdoped HTS

We demonstrate the existence of a simple physical picture of superconductivity for extremely overdoped CuO2 planes. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^2 - y^2}$ symmetry of order parameter, and the coexistence of a single electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels. An orbital paramagnetism of preformed pairs is predicted.Comment: 7 pages, 1 figur

A quantitative model for IcR product in d-wave Josephson junctions

We study theoretically the Josephson effect in d-wave superconductor / diffusive normal metal /insulator/ diffusive normal metal/ d-wave superconductor (D/DN/I/DN/D) junctions. This model is aimed to describe practical junctions in high-$T_C$ cuprate superconductors, in which the product of the critical Josephson current ($I_C$) and the normal state resistance ($R$) (the so-called $I_{\rm C}R$ product) is very small compared to the prediction of the standard theory. We show that the $I_{\rm C}R$ product in D/DN/I/DN/D junctions can be much smaller than that in d-wave superconductor / insulator / d-wave superconductor junctions and formulate the conditions necessary to achieve large $I_{\rm C}R$ product in D/DN/I/DN/D junctions. The proposed theory describes the behavior of $I_{\rm C}R$ products quantitatively in high-$T_{\rm C}$ cuprate junctions.Comment: 4 pages, 6 figure

Ground state and bias current induced rearrangement of semifluxons in 0-pi long Josephson junctions

We investigate numerically a long Josephson junction with several phase pi-discontinuity points. Such junctions are usually fabricated as a ramp between an anisotropic cuprate superconductor like YBCO and an isotropic metal superconductor like Nb. From the top, they look like zigzags with pi-jumps of the Josephson phase at the corners. These pi-jumps, at certain conditions, lead to the formation of half-integer flux quanta, which we call semifluxons (SF), pinned at the corners. We show (a) that the spontaneous formation of SFs depends on the junction length, (b) that the ground state without SFs can be converted to a state with SFs by applying a bias current, (c) that the SF configuration can be rearranged by the bias current. All these effects can be observed using a SQUID microscope.Comment: ~8 pages, 6 figures, submitted to PR