Non-local electron transport and cross-resistance peak in NSN heterostructures

We develop a microscopic theory describing the peak in the temperature dependence of the non-local resistance of three-terminal NSN devices. This peak emerges at sufficiently high temperatures as a result of a competition between quasiparticle/charge imbalance and subgap (Andreev) contributions to the conductance matrix. Both the height and the shape of this peak demonstrate the power law dependence on the superconductor thickness $L$ in contrast to the zero-temperature non-local resistance which decays (roughly) exponentially with increasing $L$. A similar behavior was observed in recent experiments.Comment: 4 pages, 3 figure

Directional tunnelling spectroscopy of a normal metal-s+gs+g-wave superconductor junction

We calculate the normal metal-$s+g$-wave superconductor tunnelling spectrum for various junction orientations and for two forms of the superconducting gap, one which allows for point nodes and the other which allows for line nodes. For a junction oriented with its normal parallel to the ab plane of the tetragonal superconductor, we find that the tunnelling spectrum is strongly dependent on orientation in the plane. The spectrum contains two peaks at energies equivalent to the magnitudes of the gap function in the direction parallel to the interface normal and in the direction making a $\pi/4$ angle with the normal. These two peaks appear in both superconductors with point nodes and line nodes, but are more prominent in the latter. For the tunnelling along the c axis, we find a sharp peak at the gap maximum in the conductance spectrum of the superconductor with line nodes, whereas with point nodes we find a peak occurring at the value of the gap function along the c axis. We discuss the relevance of our result to borocarbide systems.Comment: 16 pages, 10 figure

Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure

Nonlinear Transport through NS Junctions due to Imperfect Andreev Reflection

We investigate a normal metal -- superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various non-linear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis.Comment: 4 pages, RevTeX, three postscript figure

Josephson current through a long quantum wire

The dc Josephson current through a long SNS junction receives contributions from both Andreev bound states localized in the normal region as well as from scattering states incoming from the superconducting leads. We show that in the limit of a long junction, this current, at low temperatures, can be expressed entirely in terms of properties of the Andreev bound states at the Fermi energy: the normal and Andreev reflection amplitudes at the left-hand and at the right-hand S-N interface. This has important implications for treating interactions in such systems.Comment: 25 pages, 5 figure

Spin polarized tunneling in ferromagnet/unconventional superconductor junctions

We study tunneling in ferromagnet/unconventional superconductor (F/S) junctions. We include the effects of spin polarization, interfacial resistance, and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev reflection (AR) at the F/S interface, governing tunneling at low bias voltage, is strongly modified by these parameters. The conductance exhibits a very wide variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of minor error

The effects of a magnetic barrier and a nonmagnetic spacer in tunnel structures

The spin-polarized transport is investigated in a new type of magnetic tunnel junction which consists of two ferromagnetic electrodes separated by a magnetic barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method and the nearly-free-electron-approximation the dependence of the tunnel magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer thickness and the applied bias voltage are studied theoretically. The TMR and spin polarization show an oscillatory behavior as a function of the spacer thickness and the bias voltage. The oscillations originate from the quantum well states in the spacer, while the existence of the magnetic barrier gives rise to a strong spin polarization and high values of the TMR. Our results may be useful for the development of spin electronic devices based on coherent transport.Comment: 15 pages, 5 figure

Tunnelling spectroscopy of the interface between Sr2RuO4 and a single Ru micro-inclusion in eutectic crystals

The understanding of the zero bias conductance peak (ZBCP) in the tunnelling spectra of S/N junctions involving d-wave cuprate superconductors has been important in the determination of the phase structure of the superconducting order parameter. In this context, the involvement of a p-wave superconductor such as Sr2RuO4 in tunnelling studies is indeed of great importance. We have recently succeeded in fabricating devices that enable S/N junctions forming at interfaces between Sr2RuO4 and Ru micro-inclusions in eutectic crystals to be investigated.3 We have observed a ZBCP and have interpreted it as due to the Andreev bound state, commonly seen in unconventional superconductors. Also we have proposed that the onset of the ZBCP may be used to delineate the phase boundary for the onset of a time reversal symmetry broken (TRSB) state within the superconducting state, which does not always coincide with the onset of the superconducting state. However, these measurements always involved two interfaces between Sr2RuO4 and Ru. In the present study, we have extended the previous measurements to obtain a deeper insight into the properties of a single interface between Sr2RuO4 and Ru.Comment: To appear in J. Phys. Soc. Jpn. Vol. 75 No.12 issu

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of high-quality single-crystal-like MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In out-of-plane, high-pressure point contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Delta = 2.6 +/- 0.2 (clean limit). These results support the two-gap model of superconductivity, the presence of a modified layer at the surface of the crystals and an important and non-conventional role of the impurities in MgB2.Comment: 5 pages, 4 eps figures, SNS 2001 conferenc

DC current through a superconducting two-barrier system

We analyze the influence of the structure within a SNS junction on the multiple Andreev resonances in the subgap I-V characteristics. Coherent interference processes and incoherent propagation in the normal region are considered. The detailed geometry of the normal region where the voltage drops in superconducting contacts can lead to observable effects in the conductance at low voltages.Comment: 11 pages, including 7 postscript file