Method of producing apparatus comprising a low-reflection optical fiber connection

It has been discovered that tilted optical fiber endfaces useful in low-reflection optical fiber joints can be produced by cleaving, provided the fiber is twisted through an appropriate twist angle. By way of example, for a particular, commercially available single mode fiber, the normalized twist angle typically is in the range of 5-15 degrees/cm, resulting in a nominal tilt angle of the fiber endface in the approximate range 5-20 degrees.Published versio

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

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

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

Methods for making mirrors

A turning mirror in an optical waveguide structure is made by etching in the upper surface of the structure a cavity (18) that intercepts the path of light propagated by the waveguide (15, 16, 13). Preferably, the cavity is made to be asymmetric with the side (25) of the cavity remote from the waveguide sloping at typically a forty-five degree angle. The asymmetry can be introduced by using mask and etch techniques and treating the surface of the structure such that the etchant undercuts the mask on the side of the cavity remote from the waveguide to a greater extent than it undercuts the mask on the side of the cavity adjacent the waveguide.Published versio

Proximity Effect, Andreev Reflections, and Charge Transport in Mesoscopic Superconducting-Semiconducting Heterostructures

In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting Niobium layers the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting-normal-superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green's functions in the clean limit. For such a junction alternating Josephson currents and current-voltage characteristics (CVCs) are computed, using the non-equilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations. The CVCs exhibit features found experimentally by the Kroemer group: A steep rise of the current at small voltages ("foot") changes at a "corner current" to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective mode excitations.Comment: 8 pages, 4 figures (consisting of 5 .ps-files), added referenc

Angle dependence of Andreev scattering at semiconductor-superconductor interfaces

We study the angle dependence of the Andreev scattering at a semiconductor-superconductor interface, generalizing the one-dimensional theory of Blonder, Tinkham and Klapwijk. An increase of the momentum parallel to the interface leads to suppression of the probability of Andreev reflection and increase of the probability of normal reflection. We show that in the presence of a Fermi velocity mismatch between the semiconductor and the superconductor the angles of incidence and transmission are related according to the well-known Snell's law in optics. As a consequence there is a critical angle of incidence above which only normal reflection exists. For two and three-dimensional interfaces a lower excess current compared to ballistic transport with perpendicular incidence is found. Thus, the one-dimensional BTK model overestimates the barrier strength for two and three-dimensional interfaces.Comment: 8 pages including 3 figures (revised, 6 references added

Anomalous Behavior near T_c and Synchronization of Andreev Reflection in Two-Dimensional Arrays of SNS Junctions

We have investigated low-temperature transport properties of two-dimensional arrays of superconductor--normal-metal--superconductor (SNS) junctions. It has been found that in two-dimensional arrays of SNS junctions (i) a change in the energy spectrum within an interval of the order of the Thouless energy is observed even when the thermal broadening far exceeds the Thouless energy for a single SNS junction; (ii) the manifestation of the subharmonic energy gap structure (SGS) with high harmonic numbers is possible even if the energy relaxation length is smaller than that required for the realization of a multiple Andreev reflection in a single SNS junction. These results point to the synchronization of a great number of SNS junctions. A mechanism of the SGS origin in two-dimensional arrays of SNS junctions, involving the processes of conventional and crossed Andreev reflection, is proposed.Comment: 5 pages, 5 figure

Residual conductance of correlated one-dimensional nanosystems: A numerical approach

We study a method to determine the residual conductance of a correlated system by means of the ground-state properties of a large ring composed of the system itself and a long non-interacting lead. The transmission probability through the interacting region and thus its residual conductance is deduced from the persistent current induced by a flux threading the ring. Density Matrix Renormalization Group techniques are employed to obtain numerical results for one-dimensional systems of interacting spinless fermions. As the flux dependence of the persistent current for such a system demonstrates, the interacting system coupled to an infinite non-interacting lead behaves as a non-interacting scatterer, but with an interaction dependent elastic transmission coefficient. The scaling to large lead sizes is discussed in detail as it constitutes a crucial step in determining the conductance. Furthermore, the method, which so far had been used at half filling, is extended to arbitrary filling and also applied to disordered interacting systems, where it is found that repulsive interaction can favor transport.Comment: 14 pages, 10 EPS figure

Josephson Frequency Singularity in the Noise of Normal Metal-Superconductor Junctions

A singularity at the Josephson frequency in the noise spectral density of a disordered normal metal -- superconductor junction is predicted for bias voltages below the superconducting gap. The non-stationary Aharonov-Bohm effect, recently introduced for normal metals, is proposed as a tool for detecting this singularity. In the presence of a harmonic external field, the derivative of the noise with respect to the voltage bias reveals jumps when the applied frequency is commensurate with the Josephson frequency associated with this bias. The height of these jumps is non-monotonic in the amplitude of the periodic field. The superconducting flux quantum enters this dependence. Additional singularities in the frequency dependent noise are predicted above gap.Comment: 4 pages, 2 figures, revised versio