Enhanced Superconductivity in Layered Metallic Films

Strongin, Myron; Kammerer, O. F.; Crow, J. E.; Parks, R. D.; Douglass, D. H.; Jensen, Michael A.

doi:10.1103/physrevlett.21.1320

Cited by 136 publications

(50 citation statements)

References 19 publications

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“…A future challenge is to correlate the changes in the phonon density of states at the nanoscale to changes in the superconducting behaviour. β-Sn is a low temperature superconductor with a bulk superconducting transition temperature (T C ) of 3.7 K. A substantial increase in (T C ) of Sn nanostructures has been experimentally observed [66][67][68] with origins in the subtle interplay of the quantum confinement for the electronic degrees of freedom and a phonon environment 69 . Our work enables further analysis of the role of phonon confinement in nanoscale superconductivity, and its precise account in Sn nanostructures.…”

Section: Resultsmentioning

confidence: 99%

Lattice dynamics in Sn nanoislands and cluster-assembled films

et al. 2017

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To unravel the effects of phonon confinement, the influence of size and morphology on the atomic vibrations is investigated in Sn nano islands and cluster-assembled films. Nuclear resonant inelastic X-ray scattering is used to probe the phonon density of states of the Sn nanostructures which show significant broadening of the features compared to bulk phonon behaviour. Supported by ab initio calculations, the broadening is attributed to phonon scattering and can be described within the damped harmonic oscillator model. Contrary to the expectations based on previous research, the appearance of high-energy modes above the cut-off energy is not observed. From the thermodynamic properties extracted from the phonon density of states, it was found that grain boundary Sn atoms are bound by weaker forces than bulk Sn atoms.

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Section: Resultsmentioning

confidence: 99%

Lattice dynamics in Sn nanoislands and cluster-assembled films

et al. 2017

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“…This effect could be counteracted by the quantum size effect (QSE) arising from the discretization of the electronic energy bands in small particles and leading to a decrease in the effective density of states, N(0), at the Fermi level. 16,17 However, within the Bardeen-Cooper-Shrieffer theory, both ∆(0) and T C are related to N(0)V by the same functional form (V is the effective electron phonon interaction potential). QSEs, therefore, do not lead to a change in 2∆(0)/k B T C with reduction in particle size.…”

Section: Introductionmentioning

confidence: 99%

Competing effects of surface phonon softening and quantum size effects on the superconducting properties of nanostructured Pb

Bose

Galande

Chockalingam

et al. 2009

J. Phys.: Condens. Matter

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The superconducting transition temperature (T C ) in nanostructured Pb remains nearly constant as the particle size is reduced from 65 to 7nm, below which size the superconductivity is lost rather abruptly. In contrast, there is a large enhancement in the upper critical field (H C2 ) in the same size regime. We explore the origin of the unusual robustness of the T C over such a large particle size range in nanostructured Pb, by measuring the temperature dependence of the superconducting energy gap in planar tunnel junctions of Al/Al 2 O 3 /nano-Pb. We show that below 22nm, the electron phonon coupling strength increases monotonically with decreasing particle size, and almost exactly compensates for the quantum size effect, which is expected to suppress T C . # Present address: Max Planck Institute for Solid state Research, Nanoscale Science Department, Stuttgart, Germany. * Electronic Mail: Sangita.Bose@fkf.mpg.de † Electronic Mail: pratap@tifr.res.in ‡ Electronic Mail: pushan@tifr.res.in 2 Superconductivity at reduced length scales has been a subject of intense research over the past few decades. 1,2,3,4,5,6,7,8,9,10,11 Though one may expect changes in the superconducting properties as the system size is reduced below the fundamental length scales such as the coherence length, ξ(T), and the penetration depth, λ L (T), it is now established that there is actually a third length scale that finally defines a zero dimensional superconductor. This is the critical particle diameter (D C ) at which the energy level spacing (δ) arising from the discretization of the energy bands (the "Kubo" gap) equals the superconducting energy gap (∆(0)). Superconductivity is completely destabilized below this length scale. The existence of such an 'Anderson criterion' 2 has been successfully demonstrated in many elemental superconductors such as Al, 3 Sn,5 In, 12 Pb, 6,7 and Nb. 13 However, as the size of the superconductor approaches D C , the behavior of the superconducting transition temperature (T C ) is quite different in different systems: superconductors with a weak electron phonon coupling (In, Al, and Sn) show an increase in T C ; the intermediate coupling superconductorNb shows a gradual, monotonic decrease in T C ; while the T C in the strong coupling superconductor, Pb, shows almost no change.Two competing mechanisms control the T C in nanostructured superconductors. The first arises from the increase in surface to volume ratio with decreasing size. As the surface atoms have a smaller coordination number than the bulk atoms, surface phonons are softer than bulk phonons. This leads to an overall decrease in the phonon frequencies in nanoparticles, 14 resulting in an enhanced electron-phonon coupling strength 15 and a higher T C .Experimentally, an increase in the electron-phonon coupling can be detected by measuring the dimensionless quantity: 2∆(0)/k B T C , which monotonically increases with coupling 3 strength from its value of 3.52 in the weak coupling limit. This effect could be counteracted by the quantum si...

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“…with the earlier work on superconducting oxides interfaces [3][4][5][6] , demonstrates that interface between two different materials provide not only a rich system for studying two-dimensional (2D) superconductivity, but also a potential pathway to high-T c superconductivity [7][8][9][10][11][12] .…”

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confidence: 99%

Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3

Liu

Liu³

et al. 2014

Nature Mater

1,040

843

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Search for superconductors with a T c above the liquid nitrogen temperature (77 K) led to the discovery of a high-T c cuprate with a T c above 130 K over two decades ago 16 . Even though the value of T c is only 26 K in the first Fe-based superconductor, LaFeAsO (ref. 17) with the earlier work on superconducting oxides interfaces [3][4][5][6] , demonstrates that interface between two different materials provide not only a rich system for studying two-dimensional (2D) superconductivity, but also a potential pathway to high-T c superconductivity [7][8][9][10][11][12] .Indeed, recent angle-resolved photoemission spectroscopy (ARPES) experiments on the FeSe/STO system revealed different electronic structure from those of bulk FeSe and possible occurrence of superconductivity around 65 K (refs. 13,14). An ex situ transport measurements performed on FeSe/STO protected by multiple layers of FeTe and amorphous Si overlay revealed a zero-resistance T c of 23.5 K and an onset T c > 40 K (ref. 15). Evidently the addition of protection layers suppresses superconductivity in single-layer FeSe. In this work, we report electrical transport measurements on single-layer films of FeSe grown on Nb-doped SrTiO 3 substrate using an in situ 4-point probe (4PP) technique. We found that superconductivity could be obtained even at a temperature as high as 109 K.Single-layer films of FeSe were grown on Nb-doped SrTiO 3 (001) surface by the same method as reported previously 1 , employing extra Se flux in an MBE system equipped with STM/STS and 4PP capabilities. The growth process was monitored by reflection high-energy electron diffraction (RHEED) (Fig. 1a), which allows the precise control of film growth needed to achieve one unit-cell thickness as exactly as possible. The crystal nature of the films was confirmed by STM imaging at both large and atomic scales, as shown in Figs grown on a conducting substrate, it does mean zero resistance of the film as the film shorts the conducting substrate. However, when the 4PP detects a finite voltage, it may not necessarily mean that the sample is not superconducting. Indeed, the 4PP technique is a powerful tool for investigating superconductivity in films that cannot be taken out of a UHV system or an interface that is not accessible by surface probes.Two typical I-V curves collected at 3 K in C1423 and C1234 are shown in Figs. 2b and 2c, respectively. The data demonstrate explicitly that the film is superconducting, with the critical current (I c ) defined by the current value for which the superconducting top layer can no longer short the conducting substrate with a finite resistance. Even though interpretation of the finite voltage in the superconducting I-V curves are complicated, the essentially zero voltage seen at low currents cannot be resulted from artificial effects of the contact, since all four tips of the 4PP have Ohmic contacts with the sample individually (Supplementary Information). It is also interesting to note that the extracted I c have similar values for both measurement con...

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Enhanced Superconductivity in Layered Metallic Films

Cited by 136 publications

References 19 publications

Lattice dynamics in Sn nanoislands and cluster-assembled films

Lattice dynamics in Sn nanoislands and cluster-assembled films

Competing effects of surface phonon softening and quantum size effects on the superconducting properties of nanostructured Pb

Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3

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