Observation of bound quasiparticle states in thin Au islands by scanning tunneling microscopy

Tessmer, S. H.; Lyding, Joseph W.

doi:10.1103/physrevlett.70.3135

Cited by 35 publications

(22 citation statements)

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“…Scanning tunneling microscopy (STM) was also used to study this proximity effects in ballistic N metals (i.e. L ≪ ξ N , l N where l N is the elastic mean free path in N) [8,9]. In these experiments the induced gap crucially depends on the thickness of the normal system.…”

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confidence: 99%

Spatially resolved spectroscopy on superconducting proximity nanostructures

2001

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We investigated the local density of states (LDOS) of a normal metal (N) in good electrical contact with a superconductor (S) as a function of the distance x to the NS interface. The sample consists of a pattern of alternate stripes of Au and Nb made by UV-lithography. We used a low temperature scanning tunneling microscope to record simultaneously dI/dV (V, x) curves and the topographic profile z(x). Nearby the NS interface, all the spectra show a dip near the Fermi energy but depending on the geometry, different behaviours can be distinguished. First, when the characteristic size of the normal metal L is much larger than the coherence length ξN ≃ hDN /2∆, the spectral extension of the dip decreases from ∆ at the NS interface to zero at distances x ≫ ξN . Second, when L is comparable to ξN the apparent gap in the LDOS is space-independent and related to the Thouless energy. A normal metal in good metallic contact with a superconductor can acquire some superconducting properties and reciprocally the superconductivity can be affected by the normal metal vicinity. This phenomenon is known as the proximity effect. It has been first extensively studied in the late 1960's using the Ginzburg-Landau theory which describes macroscopic superconducting systems near their transition temperature T c [1,2]. Recent technical and material developments renewed the interest in the physics of this effect, especially at the mesoscopic scale. Simultaneously a more comprehensive understanding of the proximity effect in the diffusive regime based on the theory of non-equilibrium superconductivity has emerged [3,6]. For instance, predictions on the spatial dependence of the local density of states (LDOS) of a proximity structure are made. At very low temperature, when the characteristic size L of the normal metal becomes smaller than the thermal length L T = hD N /2πk B T (D N is the diffusion constant in the normal metal, k B the Boltzman constant, ∆ the superconducting gap), the variation of the LDOS of a NS structure is predicted to depend on the ratio L/ξ N with ξ N = hD N /2∆ the coherence length [5,6]. When L ≫ ξ N , the superconducting correlations extend to distances larger than ξ N leading to a depression of the electronic density of states around E F , the Fermi energy. Experimental tests of this theory were obtained by Guéron et al. [7]. They used nanofabricated tunnel junctions to probe the electronic density of states n(E, x) at three distinct positions along a normal wire in contact with a superconductor. Scanning tunneling microscopy (STM) was also used to study this proximity effects in ballistic N metals (i.e. L ≪ ξ N , l N where l N is the elastic mean free path in N) [8,9]. In these experiments the induced gap crucially depends on the thickness of the normal system. The shape of the spectra is explained within the de Gennes-Saint James bound states model [11]. In a diffusive sample where l N ≪ L ≤ ξ N , a gap whose value depends on the Thouless energy, E T h =hD/L 2 , is predicted [5,12]. STM experiments on small N ...

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confidence: 99%

Spatially resolved spectroscopy on superconducting proximity nanostructures

2001

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“…normal reflections) may give rise to quasi-bound states [23,24] that manifest themselves as conductance resonances. Transport resonances linked to similar multilayer configuration were observed experimentally in all-metal structures [25,26,27], thus providing elegant evidence of quasi-particle coherent dynamics in SN systems. A qualitative sketch of the energy-band diagram of our SNIN engineered structure is depicted in Fig.…”

Section: Snin Engineered Junctions: De Gennes-saint-james Resonanmentioning

confidence: 74%

Coherent Transport in Nb/δ-Doped-GaAs Hybrid Microstructures

2003

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Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is presented. The structures fabrication procedure is based on δ-doped layers grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap layer to protect the active semiconductor layers during ex situ transfer. The superconductor is then sputter deposited in situ after thermal desorption of the protective layer. Two types of structures in particular will be discussed, i.e., a reference junction and the engineered one that contains an additional insulating AlGaAs barrier inserted during the growth in the semiconductor. This latter configuration may give rise to controlled interference effects and realizes the model introduced by de Gennes and Saint-James in 1963. While both structures show reflectionless tunneling-dominated transport, only the engineered junction shows additionally a lowtemperature single marked resonance peaks superimposed to the characteristic Andreev-dominated subgap conductance. The analysis of coherent magnetotransport in both microstructures is successfully performed within the random matrix theory of Andreev transport and ballistic effects are included by directly solving the Bogoliubov-de Gennes equations. The impact of junction morphology on reflectionless tunneling and the application of the employed fabrication technique to the realization of complex semiconductor-superconductor systems are furthermore discussed.

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“…Some high spatial resolution tunneling spectroscopy measurements of the PE have already been performed [6][7][8]. However, in these works tunneling occurred through (or near) very thin N islands deposited on top of a bulk superconductor [6], or in granular samples where adjacent grains (425) …”

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“…The dashed line is the theoretical fit for the bottom curve, and indeed is not a perfect fit. These latter issues require a more thorough theoretical treatment [5,6] which will be pursued subsequently. Nevertheless, a general picture may be already obtained.…”

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confidence: 99%

“…The gap's nature cannot be easily determined, in particular not in N, since it seems too large to be attributed exclusively to a local pair potential. Therefore, it may be also due to a combination of the following: (1) Leakage of Cooper pairs and quasiparticles from S to N. (2) A single quasiparticle bound state of energy close to the superconducting gap of NbTi [6].…”

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STM Studies of the Proximity Effect in Superconducting Wires with Artificial Pinning Centers

et al. 1998

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Scanning tunneling microscopy and spectroscopy were applied to study the proximity effect with nanometer spatial resolution. The measurements were conducted on novel superconducting wires, which consist of an ordered array of submicron diameter normal metal filaments (Cu, Ni) embedded in a superconducting (NbTi) matrix. Spatially resolved information about the local density of states is obtained by taking I-V curves simultaneously with topographic images. Our data provide direct evidence on the interplay between the normal and superconducting constituents in the vicinity of the interface between them.PACS numbers: 74.50.+r, 73.40.Gk, 61.16.Ch The mutual effect of a normal metal (N) in good electrical contact with a superconductor (S), a phenomenon known as the proximity effect (PE), has been studied for over three decades [1,2]. A new interest in the subject has arisen lately, because of technological advancements that allow to conduct experiments which reveal novel mesoscopic effects [3][4][5]. One of the main points of interest is the microscopic variations of the superconducting condensate and the pair potential near the N-S boundary, and the way they influence the local density of states (DOS). Most of the experiments that probe the PE dealt so far with macroscopic properties (resistivity, magnetization, etc.), averaging out local information. Tunneling spectroscopy has also been extensively employed, but using relatively large area tunnel junctions where tunneling was performed perpendicular to the N-S interface [2].Local studies of electronic properties were made possible by the development of scanning tunneling microscopy (STM). Some high spatial resolution tunneling spectroscopy measurements of the PE have already been performed [6][7][8]. However, in these works tunneling occurred through (or near) very thin N islands deposited on top of a bulk superconductor [6], or in granular samples where adjacent grains (425)

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Observation of bound quasiparticle states in thin Au islands by scanning tunneling microscopy

Cited by 35 publications

References 13 publications

Spatially resolved spectroscopy on superconducting proximity nanostructures

Spatially resolved spectroscopy on superconducting proximity nanostructures

Coherent Transport in Nb/δ-Doped-GaAs Hybrid Microstructures

STM Studies of the Proximity Effect in Superconducting Wires with Artificial Pinning Centers

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