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Register a new userGating of Au-$Bi_2Se_3$-NbN ramp-type junctions with superconducting NbN gate-electrode and $SrTiO_3$ film as the gate-insulator
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Authors
Gad Koren
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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.
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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.
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.
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.
We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product q_Tl(q_T is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14-30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/tau_{e-ph} sim T^n with the exponents n = 3.2-3.8. We found that in this temperature range tau_{e-ph} and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9-17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.
We present a physically consistent interpretation of the dc electrical properties of niobiumnitride (NbN)-based superconducting hot-electron bolometer (HEB-) mixers, using concepts of nonequilibrium superconductivity. Through this we clarify what physical information can be extracted from the resistive transition and the dc current-voltage characteristics, measured at suitably chosen temperatures, and relevant for device characterization and optimization. We point out that the intrinsic spatial variation of the electronic properties of disordered superconductors, such as NbN, leads to a variation from device to device.
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