Proximity-Induced Superconductivity in Nanowires: Minigap State and Differential Magnetoresistance Oscillations

Wang, Jian; Shi, Chuntai; Tian, Mingliang; Zhang, Qi; Kumar, Nitesh; Jain, J. K.; Chan, Moses H. W.

doi:10.1103/physrevlett.102.247003

Cited by 70 publications

(77 citation statements)

References 34 publications

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“…Recently, exotic proximity effects have been observed in various systems, including superconductor-metallic nanowires [5][6][7] and graphenesuperconductor structures 4 . However, it is still not clear how superconducting order propagates spatially in a heterogeneous superconductor system.…”

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“…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][6][7] and graphenesuperconductor structures 4 .…”

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

et al. 2012

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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'...

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

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“…8 The investigation of proximity- induced superconductivity in metallic nanowires contacted by FIB has also been recently performed. 9 The advantages of this technique are the deposition of the material of practically any shape and size, without any mask, and the good quality of the interface created. The main disadvantage is the Ga contamination of about 250 nm around the deposited wires.…”

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Geometry-related magnetic interference patterns in longSNSJosephson junctions

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We have measured the critical current dependence on the magnetic flux of two long SNS junctions differing by the normal wire geometry. The samples are made by a Au wire connected to W contacts, via focused ion beam assisted deposition. We could tune the magnetic pattern from the monotonic Gaussian-like decay of a quasi-one-dimensional (1D) normal wire to the Fraunhofer-like pattern of a square normal wire. We explain the monotonic limit with a semiclassical 1D model, and we fit both field dependencies with numerical simulations of the two-dimensional Usadel equations. Furthermore, we observe both integer and fractional Shapiro steps. The magnetic flux dependence of the integer steps reproduces as expected that of the critical current I c , while fractional steps decay slower with the flux than I c .

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“…It has been proposed that surface-state electrons are strongly affected by the inversion asymmetry of the surface, generating large Rashba couplings [6]. Proximity-induced superconductivity has been studied for heavy metals like Ag, Au, and Pb [7][8][9][10] and electrostatic control of superconductivity (SC) has been reported for proximityinduced SC generated by metallic tin nanoparticles on graphene [11] and for electron accumulation at the surface of semiconducting transition-metal dichalcogenides [12,13]. Recently, magnetoresistance measurements on nanoplates of the In-doped topological superconductor candidate SnTe revealed the role of surface states despite the high bulk carrier density [14].…”

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Two-band superconductivity of bulk and surface states in Ag thin films on Nb

et al. 2016

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We use epitaxial strain to spatially tune the bottom of the surface-state band ESS of Ag(111) islands on Nb(110). Bulk and surface-state contributions to the Ag(111) local density of states (LDOS) can be separated with scanning tunneling spectroscopy. For thick islands (≈ 20 nm), the Ag surface states are decoupled from the Ag bulk states and the superconductive gap induced by proximity to Nb is due to bulk states only. However, for thin islands (3-4 nm), surface-state electrons develop superconducting correlations as identified by a complete energy gap in the LDOS when ESS is smaller than but close to the Fermi level. The induced superconductivity in this case is of two-band nature and appears to occur when the surface-state wave function reaches down to the Ag/Nb interface.PACS numbers: 73.20. At, 74.45.+c, 74.55.+v, 74.78.Na With the discovery of topological insulators [1][2][3], topologically protected surface states have become one of the most challenging topics in today's condensed-matter physics. In bulk insulating materials, a nontrivial band inversion leads to conducting surface states. A conventional s-wave superconductor in proximity to a topological insulator [4] might carry exotic superconducting states with non-Abelian Majorana states [5]. It has been proposed that surface-state electrons are strongly affected by the inversion asymmetry of the surface, generating large Rashba couplings [6]. Proximity-induced superconductivity has been studied for heavy metals like Ag, Au, and Pb [7][8][9][10] and electrostatic control of superconductivity (SC) has been reported for proximityinduced SC generated by metallic tin nanoparticles on graphene [11] and for electron accumulation at the surface of semiconducting transition-metal dichalcogenides [12,13]. Recently, magnetoresistance measurements on nanoplates of the In-doped topological superconductor candidate SnTe revealed the role of surface states despite the high bulk carrier density [14]. Here we report on the interplay between bulk and surface states of Ag with regard to SC induced by proximity to superconducting Nb. We apply epitaxial strain to tune the energy of the surface-state band, allowing to disentangle the relative roles of bulk and surface states in superconducting Ag.The role of surface states of a bulk superconductor that do not hybridize with bulk states has become a new twist with the discovery of topologically protected surface states. Ginzburg already considered SC of electrons in two-dimensional (2D) surface states [15]. However, for simple conventional SC, this situation is hard to find in nature. The range of materials can be considerably expanded by considering proximity-induced SC. Thus, we have investigated (111)-oriented Ag nano-islands on a superconducting Nb (110) single crystal (T c = 9.1 K) to address this question. The Ag(111)/Nb(110) system fulfills two prerequisites: First of all, Ag islands of less than ≈ 100 nm thickness become fully penetrated by the superconductive order parameter [16]. Secondly, the Ag(111) surface ...

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Proximity-Induced Superconductivity in Nanowires: Minigap State and Differential Magnetoresistance Oscillations

Cited by 70 publications

References 34 publications

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

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

Geometry-related magnetic interference patterns in longSNSJosephson junctions

Two-band superconductivity of bulk and surface states in Ag thin films on Nb

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