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Register a new userTunneling and bound states in near-edge NbN-$rm Bi_2Se_3$ junctions fabricated by deposition through a wire shadow mask
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Authors
G. Koren
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Transport measurements in thin film junctions of NbN-$rm Bi_2Se_3$ exhibit tunneling as well as bound state resonances. The junctions are prepared by pulsed laser deposition of a NbN layer through a 25 $mu$m wide gold wire shadow mask bisecting the wafer into two halves, on a $rm Bi_2Se_3$ blanket film without further patterning. This results in two independent near-edge junctions connected in series via the 25 $mu m$ long and 10 mm wide area of the uncapped $rm Bi_2Se_3$ layer. Conductance spectra measured across the wire masked trench at different locations on the wafer show that some junctions have tunneling behavior with pronounced coherence peaks at $pm 2Delta$ where $Delta simeq$ 1 meV, while others have zero bias conductance peaks and series of bound states at higher bias. The later can be attributed to zero energy Majorana bound states or to the more conventional Andreev bound states. Based on the present results, we can not distinguish between these two scenarios.
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Ultrathin $rm Bi_2Se_3$-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick $rm Bi_2Se_3$ film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR=[R(H)-R(0)]/{[R(H)+R(0)]/2}), and under three electric gate-fields of 0 and $pm2$ MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the $rm Bi_2Se_3$ cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime.
Ramp-type junctions of $rm Au-Bi_2Se_3-NbN$ were prepared on top of a bottom gate comprised of a $rm SrTiO_3$ gate-insulator film on $rm NbN$ gate-electrode layer on (100) $rm SrTiO_3$ wafer. Two wafers with gate-insulator thickness of 120 and 240 nm were studied, with the former showing higher gate leakage currents Ig at high gate voltages Vg, leading to heating effects and shifting of the junctions conductance spectra versus the voltage bias. At Vg=0 V, the conductance spectra of the low resistance junctions showed zero bias conductance peaks inside a tunneling gap with typical conductance drops when the critical current Ic was reached, while the high resistance ones exhibited tunneling conductance only. For Vg$>$-0.2 V ($rm Esimeq$ -2 MV/cm) of the wafer with 120 nm thick gate-insulator linear Ig vs Vg was found, while for Vg$<$-0.2 V, Ig saturation was observed, leading to quadratic and linear heating effects at positive and negative high Vg values, respectively. This led to asymmetric conductance spectra shifts versus Vg which followed almost exactly the Ig vs Vg behavior. In the wafer with twice the gate-insulator thickness (240 nm), heating effects were strongly suppressed, and symmetric small peak shifts appeared only under the highest Vg values of Vg=$pm$2 V ($rm Esimeq pm$ 10 MV/cm). Under Vg=2 V, a 5% lower conductance was observed as compared to Vg=-2 V, indicating a small Fermi energy shift in our junctions under $pm$10 MV/cm fields.
Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s$^{pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.
In a search for a simple proximity system of a topological insulator and a superconductor for studying the role of surface versus bulk effects by gating, we report here on a first step toward this goal, namely the choice of such a system and its characterization. We chose to work with thin film bilayers of grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick topological layer of $rm Bi_2Se_3$ and compare the transport results to those obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with ex-situ and in-situ prepared $rm NbN-Bi_2Se_3$ interfaces were studied and two kinds of proximity effects were found. At high temperatures just below the superconducting transition, all bilayers showed a conventional proximity effect where the topological $rm Bi_2Se_3$ suppresses the onset or mid-transition $T_c$ of the superconducting NbN films by about 1 K. At low temperatures, a cross-over of the resistance versus temperature curves of the bilayer and reference NbN film occurs, where the bilayers show enhancement of $T_c(R=0)$, $I_c$ (the supercurrent) and the Andreev conductance, as compared to the bare NbN films. This indicates that superconductivity is induced in the $rm Bi_2Se_3$ layer at the interface region in between the NbN grains. Thus an inverse proximity effect in the topological material is demonstrated.
The tunneling characteristics of planar junctions between a normal metal and a non-centrosymmetric superconductor like CePt3Si are examined. It is shown that the superconducting phase with mixed parity can give rise to characteristic zero-bias anomalies in certain junction directions. The origin of these zero-bias anomalies are Andreev bound states at the interface. The tunneling characteristics for different directions allow to test the structure of the parity-mixed pairing state.
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