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Register a new userMagnetoresistance and gating effects in ultrathin NbN-$rm Bi_2Se_3$ bilayers
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Gad Koren
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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.
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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.
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$10 MV/cm fields.
Transport measurements in thin film junctions of NbN-$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 properties of topological insulator - ferromagnet - superconductor trilayers comprised of thin films of 20 nm thick $rm Bi_2Se_3$ on 10 nm $rm SrRuO_3$ on 30 nm $rm YBa_2Cu_3O_x$. As deposited trilayers are underdoped and have a superconductive transition with $rm T_c$ onset at 75 K, zero resistance at 65 K, $rm T_{Cueri}$ at 150 K and $rm T^*$ of about 200 K. Further reannealing under vacuum yields the 60 K phase of $rm YBa_2Cu_3O_x$ which still has zero resistance below about 40 K. Only when $10times 100$ micro-bridges were patterned in the trilayer, some of the bridges showed resistive behavior all the way down to low temperatures. Magnetoresistance versus temperature of the superconductive ones showed the typical peak due to flux flow against pinning below $rm T_c$, while the resistive ones showed only the broad leading edge of such a peak. All this indicates clearly weak-link superconductivity in the resistive bridges between superconductive $rm YBa_2Cu_3O_x$ grains via the topological and ferromagnetic cap layers. Comparing our results to those of a reference trilayer with the topological $rm Bi_2Se_3$ layer substituted by a non-superconducting highly overdoped $rm La_{1.65}Sr_{0.35}CuO_4$, indicates that the superconductive proximity effect as well as ferromagnetism in the topological trilayer are actually strongly suppressed compared to the non-topological reference trilayer. This strong suppression is likely to originate in strong proximity induced edge currents in the SRO/YBCO layer that can lead to Majorana bound states, a possible signature of which is observed in the present study as zero bias conductance peaks.
Twisted bilayers of high-$T_c$ cuprate superconductors have been argued to form topological phases with spontaneously broken time reversal symmetry ${cal T}$ for certain twist angles. With the goal of helping to identify unambiguous signatures of these topological phases in transport experiments, we theoretically investigate a suite of Josephson phenomena between twisted layers. We find an unusual non-monotonic temperature dependence of the critical current at intermediate twist angles which we attribute to the unconventional sign structure of the $d$-wave order parameter. The onset of the ${cal T}$-broken phase near $45^circ$ twist is marked by a crossover from the conventional $2pi$-periodic Josephson relation $J(varphi)simeq J_csin{varphi}$ to a $pi$-periodic function as the single-pair tunneling becomes dominated by a second order process that involves two Cooper pairs. Despite this fundamental change, the critical current remains a smooth function of the twist angle $theta$ and temperature $T$ implying that a measurement of $J_c$ alone will not be a litmus test for the ${cal T}$-broken phase. To obtain clear signatures of the ${cal T}$-broken phase one must measure $J_c$ in the presence of an applied magnetic field or radio-frequency drive, where the resulting Fraunhofer patterns and Shapiro steps are altered in a characteristic manner. We discuss these results in light of recent experiments on twisted bilayers of the high-$T_c$ cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$.
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