Fluctuation spectroscopy as a probe of granular superconducting diamond films

Abstract

We present resistance versus temperature data for a series of boron-doped nanocrystalline diamond films whose grain size is varied by changing the film thickness. Upon extracting the fluctuation conductivity near to the critical temperature we observe three distinct scaling regions -3D intragrain, quasi-0D, and 3D intergrain -in confirmation of the prediction of Lerner, Varlamov and Vinokur. The location of the dimensional crossovers between these scaling regions allows us to determine the tunnelling energy and the Thouless energy for each film. This is a demonstration of the use of fluctuation spectroscopy to determine the properties of a superconducting granular system. Tunable granular materials offer rich physical systems with which to study the interplay between electron correlations and the mesoscopic effects of disorder. The occurrence of the metal-insulator and superconductorinsulator transitions appear to be strongly linked to granularity, be it structural or pertaining to variations of the order parameter [1][2][3]. There are also clear theoretical predictions for the signature of granularity in the transport properties of disordered superconductors close to the superconducting transition [4][5][6][7]. Boron doped nanocrystalline diamond (BNCD) provides a suitable tuneable material in which to explore these theoretical predictions. Superconductivity was first observed in high-pressure, high-temperature fabricated boron doped diamond in 2004 [8]. The phenomenon was quickly demonstrated in both single-crystalline [9] and polycrystalline [10] diamond synthesised by chemical vapour deposition (CVD). While superconductivity in doped semiconductor materials [11,12] is an active area of research, and although nanocrystalline diamond retains many of the desirable mechanical properties of single-crystalline material [13], a sometimes overlooked property in the study of superconductivity in polycrystalline boron-doped diamond is the physical granularity itself.A clear experimental signature of superconducting granular systems, as pointed out by Lerner, Varlamov and Vinokur [5] (henceforth referred to as LVV), is that there are three distinct temperature regimes in the vicinity of the critical temperature (T c ), distinguished by the magnitude of the temperature-dependent GinzburgLandau coherence length. At temperatures immediately above T c , short-lived Cooper pairs act as charge carriers, modifying the conductivity. The principal modification close to T c is the Aslamazov-Larkin pair contribution to the conductivity -the so-called paraconductivity [14]. When the coherence length is much larger than the typical grain size, the granularity is not seen by a Cooper pair and the system behaves as a 3D superconductor with paraconductivity taking the well-known form ∝ −1/2 , where = (T − T c )/T c is the reduced temperature. When the coherence length is comparable to the typical grain size, each grain acts as its own 0D superconductor for which the paraconductivity is expected to be ∝ −2 . However, in addition, the...

Possible observation of the Berezinskii-Kosterlitz-Thouless transition in boron-doped diamond films

“…31 SB is very thankful to Prof. M. Nesládek (Hasselt University) for providing the samples and conducting stimulating discussions. NRF (SA) and CSIR-NLC are acknowledged for the financial support.…”

Observation of a superconducting glass state in granular superconducting diamond

“…We are able to make a very precise determination of T c in these films by fitting to the resistance data using our understanding of the fluctuation conductivity. In a previous work 18 , we have shown that the fluctuation contribution to the conductivity - that is the conductivity due to finite-lifetime Cooper pairs above T c - exhibits a dimensional cross-over into a quasi-zero-dimensional regime. In the approach to the transition, this diverges as σ fl ~ ( T − T c ) −3 , in confirmation of a prediction by Lerner et al .…”

“…This glassy dynamics can be attributed to frustration in a system of weakly coupled superconducting clusters, each of which acts as an individual spin 2 . The BNCD films studied here have a granular morphology 15 – 18 similar to sintered powders, and are close to a metal-insulator transition 19 – 21 , so that the observation of glassy superconductivity is perhaps not surprising. Nonetheless, this is a novel observation in BNCD films, made possible by detailed analysis of dimensional crossover in fluctuation conductivity 18 – fluctuation spectroscopy –which allows us to probe material properties that are governed by the morphological granularity of our films.…”

Superconductor-insulator transition driven by pressure-tuned intergrain coupling in nanodiamond films

“…As a result, it becomes more difficult to establish a percolative path for the Cooper pairs, making the condensation and thus the dissipationless flow of Cooper pairs impossible. In this case, a quasi-zero-dimensional state of Cooper pairs, which has been revealed for granular superconducting diamond films by so-called fluctuation spectroscopy [31], can set in and lead to the appearance of an overall insulating phase with localized Cooper pairs. Note that our picture, as illustrated in Fig.…”