Magnetoresistance, Gating and Proximity Effects in Ultrathin NbN-Bi2Se3 Bilayers

Abstract

Ultrathin Bi 2 Se 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 Bi 2 Se 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 ±2 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 Bi 2 Se 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.

Strongly suppressed superconducting proximity effect and ferromagnetism in trilayers of $\rm Bi_2Se_3$ / $\rm SrRuO_3$ / underdoped $\rm YBa_2Cu_3O_x$: A possible new platform for Majorana nano-electronics

“…2 (d) depicts R vs magnetic field H of two resistive TTL microbridges of (c) at 1.95 K. The result is very similar to the magnetoresistance of a stand-alone BS topological layer. It shows weak anti-localization (WAL) at low fields up to about 1 T with tendency to saturation above it [22,23]. Unlike previous magnetoresistance results in bilayers of 15 nm BST/15 nm SRO where the WAL signature of the TI was observed together with that of the coercive field of the FM layer [9], here no effect of either the FM or SC layers is observed.…”

Magnetic proximity effect of a topological insulator and a ferromagnet in thin-film bilayers of Bi0.5Sb1.5Te3 and SrRuO3

“…Next we turn to resistance hysteresis loops measurements of the BL, TI and SRO samples versus magnetic field at low temperature of 1.85±0.05 K. Fig. 3 depicts such measurement results of the 15 nm BST on 15 nm SRO bilayer, together with the non-hysteretic R versus H curve of a second reference film of the TI (15 nm thick BST on (100) STO wafer) [27]. By increasing the field after ZFC from 0 to 4 T, the resistance shows a peak at 0.26 T which results from the combined effect of the weak anti-localization (WAL) in the 15 nm BST layer where the magnetoresistance increases with field [27], and the resistance decrease due to realignment of the magnetic domains in the SRO layer of the bilayer.…”

“…3 depicts such measurement results of the 15 nm BST on 15 nm SRO bilayer, together with the non-hysteretic R versus H curve of a second reference film of the TI (15 nm thick BST on (100) STO wafer) [27]. By increasing the field after ZFC from 0 to 4 T, the resistance shows a peak at 0.26 T which results from the combined effect of the weak anti-localization (WAL) in the 15 nm BST layer where the magnetoresistance increases with field [27], and the resistance decrease due to realignment of the magnetic domains in the SRO layer of the bilayer. The latter originates in reduction of the overall domain walls resistance [24] due to merging of domains with increasing field into a single domain.…”

Gating of Au-$Bi_2Se_3$-NbN ramp-type junctions with superconducting NbN gate-electrode and $SrTiO_3$ film as the gate-insulator

“…Under these conditions, quantum operations by braiding of MBS are expected to be topologically protected, thus enabling basic quantum computations [2,5]. The present study is an extension of our previous reports on similar proximity systems comprised of Au − Bi 2 Se 3 − NbN junctions and Bi 2 Se 3 − NbN bilayers [6,7,9,10,14], but here we focus on gating effects of ramp-type junctions. To keep compatibility with standard fabrication processes, the junctions and gates are prepared from thin films only.…”