Visualization of geometric influences on proximity effects in heterogeneous superconductor thin films

Abstract

The proximity effect is a central feature of superconducting junctions that plays a key role in many devices and can be exploited in the design of new systems with quantum functionality [1][2][3][4][5][6][7][8][9][10][11][12] . Recently, exotic proximity effects have been observed in various systems, including superconductor-metallic nanowires 5-7 and graphenesuperconductor structures 4 . However, it is still not clear how superconducting order propagates spatially in a heterogeneous superconductor system. Here we report on intriguing junction geometry effects in a heterogeneous system consisting of electronically two-dimensional superconducting islands on a metallic substrate. Depending on the local geometry, the superconducting gap induced at the metallic surface sometimes decays within ∼20 nm of the superconductor, and sometimes survives at distances that are several coherence lengths from a superconductor. We show in particular that the curvature of the junction plays an essential role in the proximity effect.The sample system comprises superconducting two-dimensional (2D) Pb islands on top of a single-atomic-layer surface metal, the striped incommensurate (SIC) phase of the Pb overlayer on Si(111) (refs 13-16). The scanning tunnelling microscopy (STM) image shown in Fig. 1 captures a variety of junction configurations. Figure 1a shows an interesting 'π'-shaped Pb island five monolayers (ML) thick on top of the SIC surface. Previous scanning tunnelling spectroscopy (STS) studies have shown that the SIC phase is superconducting with T C_SIC = ∼1.8 K (ref. 17) and the 2D Pb islands have a T C around 6 K (ref. 18), although the actual T C value also depends on the lateral size as well as its thickness 19 . At 4.3 K, the SIC template is in the normal state. At locations far from the Pb islands, the tunnelling spectrum exhibits no gap (spectrum no. 2 in Fig. 1b), whereas the spectrum acquired at the 2D Pb island shows a clear superconducting gap (spectrum no. 3). In the SIC region near the 2D Pb island, a superconducting gap can also be observed (spectrum no. 1), indicative of a proximity effect. To address the spatial dependence, we performed spectroscopic mapping over the same area, whose differential conductance at zero bias (zero-bias conductance (ZBC)) is shown in Fig. 1c. As the ZBC directly correlates with the size of the tunnelling gap (the smaller the value of ZBC, the larger the tunnelling gap), the landscape of ZBC is a good representation of the landscape of the superconducting gap.The ZBC image reveals a rich landscape. In some regions (for example, region α), the induced superconducting gap decays very quickly within a very short distance from the SIC/superconductor (S) interface, whereas in region β where the SIC wetting layer is surrounded by Pb islands from both sides, the induced Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA. *e-mail: shih@physics.utexas.edu.superconducting gap is quite uniform. Similarly, in region γ where the SIC region is near the 'recess'...

Temperature dependence of the superconducting proximity effect quantified by scanning tunneling spectroscopy

“…This technique has been exploited to study and to visualize the proximity effect in Pb films surrounding superconducting Pb nanoislands on Si(111) substrates. [6][7][8] Thermal energy drives a Cooper pair breaking 12 for the induced superconductivity in M, hence inducing a shorter PL for increasing temperature. A 1/T 10,13,14 and 1/ √ T 10,13,14 temperature dependence of the proximity length is predicted for ballistic and diffusive systems, respectively.…”

“…Recent advances in nanofabrication and nanocharacterization renewed the interest in the socalled superconducting proximity effect due to the possibility to examine this effect at nanoscales. [1][2][3][4][5][6][7][8][9] The proximity effect is observed at the junction between a metal (M) and a superconductor (S), and it leads to a depletion of the density of states (DOS) around the Fermi energy in the metal. An induced energy gap in M is therefore observed, and it is the hallmark of the superconducting proximity effect.…”

Quantum particle confined to a thin-layer volume: Non-uniform convergence toward the curved surface

“…For the electromagnetic field, we choose the Coulomb gauge. In V N , we expand the Schrödinger equation (10) and the Coulomb gauge in the following forms…”

“…Recently, some experiments were designed to investigate the geometric effects on the transport in photonic topological crystals [9], on the proximity effects [10] and on the electron states [11]. Both the theoretical and experimental developments have attracted tremendous interest in the generalization of the JKC procedure to discuss a curved system with an electromagnetic field [12-14, 16, 17].…”

Towards spin injection from silicon into topological insulators: Schottky barrier between Si and Bi2Se3

“…The Richardson constant in samples 1 and 2 is approximately 300A/cm 2 /K 2 , three times larger than what is 21,26 ). Because the bandgap of Bi 2 Se 3 is only 0.3 eV < φ B 1 , electrons from the Si conduction band are resonant with bulk conduction band states in the Bi 2 Se 3 even if the Fermi level lies in the gap, hence injection will occur into the spin-textured surface resonance as well as the spin degenerate bulk states.…”