Destruction of Superconductivity in Disordered Near-Monolayer Films

Strongin, Myron; Thompson, Richard S.; Kammerer, O. F.; Crow, J. E.

doi:10.1103/physrevb.1.1078

Cited by 398 publications

(294 citation statements)

References 29 publications

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“…The development of thin film growth methods in the late 1960s allowed Cohen and Abeles [11] to demonstrate an increase in T c with decreasing thickness in aluminum films in a study that pioneered the currently ongoing research of thin superconducting films. This enhancement of T c , which is still not completely understood, was later confirmed by Strongin et al [12], who also reported the more common behavior of T c -its suppression with reduced film thickness. Strongin et al empirically examined different scaling options for the observed suppression of T c in lead and suggested that T c scales with R s better than it does with the other parameters, such as the film thickness.…”

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confidence: 51%

“…This observation began the race to understanding the superconductor insulator phase transition, which is believed by many researchers to be of a quantum nature [5]. To date, although much data for thin film superconductivity have been accumulated [11,12,[15][16][17][18][19][20][21][22][23][24][25][26][27][28], and the local disorder has already been observed directly [33], the mechanisms governing the collective behavior close to the superconducting-to-insulating transition, or near the onset of superconductivity have remained elusive. That is, a model equivalent to the BCS, but that is valid for thin films, is still missing.…”

mentioning

confidence: 99%

“…As opposed to these three models, which suggest that T c depends merely on R s , competing models, such as the proximity effect [7] and the quantum size effect [10] theories, suggest that T c depends on d only, with no direct dependence on R s . Nevertheless, none of these models is sufficient to explain the entirety of the accumulated experimental data [11,12,[15][16][17][18][19][20][21][22][23][24][25][26][27][28], despite the long-standing attempt to do so either through a direct mathematical derivation as in the above model, or with the aid of empirical universal laws [29].…”

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Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition

et al. 2014

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Thin superconducting films form a unique platform for geometrically confined, strongly interacting electrons. They allow an inherent competition between disorder and superconductivity, which in turn enables the intriguing superconducting-to-insulating transition and is believed to facilitate the comprehension of high-T c superconductivity. Furthermore, understanding thin film superconductivity is technologically essential, e.g., for photodetectors and quantum computers. Consequently, the absence of established universal relationships between critical temperature (T c ), film thickness (d), and sheet resistance (R s ) hinders both our understanding of the onset of the superconductivity and the development of miniaturized superconducting devices. We report that in thin films, superconductivity scales as dT c (R s ). We demonstrated this scaling by analyzing the data published over the past 46 years for different materials (and facilitated this database for further analysis). Moreover, we experimentally confirmed the discovered scaling for NbN films, quantified it with a power law, explored its possible origin, and demonstrated its usefulness for nanometer-length-scale superconducting film-based devices. Relationships between low-temperature and normal-state properties are crucial for understanding superconductivity. For instance, the Bardeen-Cooper-Schrieffer theory (BCS) successfully associates the normal-to-superconducting transition temperature, T c , with material parameters, such as the Debye temperature ( D ) and the density of states at the Fermi level [N (0)]. Hence, the BCS model allows us to infer superconducting characteristics (i.e., T c ) from properties measured at higher temperatures [1]. In the BCS framework, superconductivity occurs when attractive phonon-mediated electron-electron interactions overcome the Coulomb repulsion, giving rise to paired electrons (Cooper pairs) with a binding energy gap:. Moreover, within a superconductor, all Cooper pairs are coupled, giving rise to a collective electron interaction. Such a collective state is described by a complex global order parameter with real amplitude ( ) and phase (ϕ): = e iϕ .Because superconductivity relies on a collective electron behavior, the onset of superconductivity occurs when the number of participating electrons is just enough to be considered collective, i.e., at the nanoscale [2-5]. Thus, it is known that the superconductivity-disorder interplay varies in thin films and is effectively tuned with the film thickness (d) or with the disorder in the system, which is represented by sheet resistance of the film at the normal state (R s ) [6][7][8][9][10]. The mechanism of superconductivity in thin films has been investigated since the 1930s [6] increase in T c with decreasing thickness in aluminum films in a study that pioneered the currently ongoing research of thin superconducting films. This enhancement of T c , which is still not completely understood, was later confirmed by Strongin et al. [12], who also reported the more common be...

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confidence: 51%

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confidence: 99%

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confidence: 99%

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Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition

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“…Adding material results in a decrease of the average distance between the islands leading to an exponential drop of the resistance with thickness. On the other hand, when the substrate is pre-coated by a thin layer of amorphous Ge or Sb the deposited film is electrically continuous at a thickness of 1-2 monolayers of material (∼5Å) [17,18] after which the Ag film obeys the usual R ✷ ∝ 1/d dependence, where d is the sample thickness. These films are referred to as "uniform films".…”

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Zero-bias anomaly in a two-dimensional granular insulator

et al. 2013

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We compare tunneling density of states (TDOS) into two ultrathin Ag films, one uniform and one granular, for different degrees of disorder. The uniform film shows a crossover from Altshuler-Aronov (AA) zero bias anomaly to Efros Shklovskii (ES) like Coulomb gap as the disorder is increased. The granular film, on the other hand, exhibits AA behavior even deeply in the insulating regime. We analyze the data and find that granularity introduces a new regime for the TDOS. While the conductivity is dominated by hopping between clusters of grains and is thus insulating, the TDOS probes the properties of an individual cluster which is "metallic".PACS numbers: 73.40. Rw; 72.80.Ng; 72.20.Ee; The metal-insulator transition (MIT) in disordered electronic systems renders their transport properties one of the most exciting topics in condensed matter physics. The effects of Coulomb interaction in the vicinity of the MIT are particularly strong, and despite major theoretical and experimental efforts are not yet fully understood [2]. Much of the work is focused on 2D metallic films. While infinite 2D systems are insulators [1], samples shorter than the localization length ξ exhibits metallic behavior. This enables to utilize thin dirty metals to study both metallic and insulating regimes.For weakly disordered metallic films with relatively high conductivity the interplay between electron-electron interaction and disorder has a pronounced influence on the electronic properties, as shown by Altshuler and Aronov (AA) [3]. Both conductivity and the tunneling density of states (TDOS) are significantly modified (relatively to the non-interacting values) and acquire temperature dependent corrections. The theory predicts the depletion of the TDOS around the Fermi level, a phenomenon that became known as zero bias anomaly (ZBA), which was indeed observed experimentally in disordered metals [4].For strongly disordered systems, electronic states are localized, and transport at low temperatures is achieved via variable range hopping (VRH). For non-interacting electrons there is no gap in the TDOS at the Fermi level and the electric conductivity is given by Mott's law [5]. Electron interactions open a Coulomb gap at the Fermi energy, affecting the TDOS and the electric conductivity as shown by Efros and Shklovskii (ES) [6]. The Coulomb gap in the 2D density of states is given byand the conductivity as a function of temperature is where κ is a dielectric constant and T 0 = e 2 /κξ. Though the ZBA and the Coulomb gap both result from the long range Coulomb interaction and lead to a depletion of the TDOS at the Fermi energy, a unified theory of both effects is still absent [8].Most theoretical and experimental works deal with homogeneously disordered systems in which the degree of disorder controls the vicinity to the MIT. A different system that may provide a natural and convenient arena for experimental and theoretical study of strongly interacting systems is a granular metal [9] i.e. a disordered array of metallic islands with typical size ...

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“…In order to circumvent these problems we prepared the granular films using the technique of quench condensation, i.e. evaporation on a cryocooled substrate within the measurement probe under ultra high vacuum conditions [14,15,16,17]. This technique enables the growth of ultra-clean stable Ni grains with an excellent control over the inter-grain distance and the film resistance [18,19].…”

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Zero field resistance dip in magnetic tunnel junctions employing a granular electrode

Barness

Frydman

2005

Phys. Rev. B

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We present magnetoresistance (MR) measurements performed on magnetic tunnel junctions in which one of the electrodes is a granular ferromagnetic film. These junctions exhibit a zero field resistance dip. The dip magnitude depends on the size of the grains. We interpret these results as a consequence of the orange peel effect between the continuous ferromagnetic film and the magnetic grains. The coupling is found to be much stronger than that between continuous ferromagnetic layers. PACS: 75.70.Ak, 75.47.De, 75.75.+a GMR (giant magnetoresistance) and TMR (tunneling magnetoresistance) devices are primary candidates in future magneto-electronic applications and media [1,2,3,4,5]. The ability to create arrays of magnetic junctions on micro sized areas can enhance storage size drastically and enable the production of non volatile ultra-dense RAM chips. Granular ferromagnets have a promising potential to act as a further step in this direction since a single junction may be able support numerous bits, considerably increasing the possible storage densities.All applications based on GMR and TMR effects require high quality multilayers constructed of thin ferromagnetic and non-magnetic films. The performance of the devices depend strongly on the morphological and structural properties of the films as well as their physical characteristics. Among the crucial factors is the interlayer coupling between two ferromagnetic layers separated by a non-magnetic spacer. This coupling may be a sum of several different mechanisms, of which three appear to be dominant. The first is pinhole coupling, which results from structural defects in the spacer and may destroy MR effects altogether. The second is the RKKY interaction which oscillates with the spacer thickness. This coupling is due to indirect exchange mechanism and applies only to conductive barriers (GMR multilayers). The third mechanism is the Néel Coupling [6], also named Orange Peel Effect (OPE), which applies both to conducting and to insulating spacers. This coupling utilizes the surface waviness of correlated layers to produce ferromagnetic interaction between ferromagnetic layers that could otherwise be antiferromagnetically coupled. The mechanism is based on the fact that the waviness of the magnetic film creates dipoles on the surface. A second layer with correlated waviness placed on top and separated by a non-magnetic spacer, experiences similar moment orientation due to dipole-dipole interaction as illustrated in figure 1a. Such ferromagnetic coupling reduces the GMR signal which requires antiferromagnetic orientation at low fields. Hence, a lot of effort is invested in an attempt to minimize this coupling in order to improve the performance of GMR/TMR elements [7,8,9,10,11,12]. The basic Néel model was derived for two infinitely thick magnetic layers separated by a non magnetic spacer [6]. Kools et al [13] extended the theory to included the finite size of the magnetic layers and obtained the following expression for the coupling strength:where h and λ are the a...

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Destruction of Superconductivity in Disordered Near-Monolayer Films

Cited by 398 publications

References 29 publications

Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition

Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition

Zero-bias anomaly in a two-dimensional granular insulator

Zero field resistance dip in magnetic tunnel junctions employing a granular electrode

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