Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Probing superconductivity in the 2D limit using resistivity noise
R.Koushik 1 , Mintu Mondal 2 , John Jesudasan 2 , Pratap Raychaudhuri 2 , Aveek Bid 1 and Arindam Ghosh 1
1
Department of Physics, Indian Institute of Science, Bangalore 560012, India
2
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
Superconductivity in low dimensions has invoked lot of interest in recent times with the advent of new materials
like topological superconductors, electron gas at oxide interface etc. In 2D, the superconducting transition is
known to occur via Berezenskii Kosterlitz Thouless (BKT) phase transition. Conventional techniques like
resistivity measurements, superfluid density measurements, IV characteristics are generally used to study the
nature of transition (BKT or BCS). But their sensitivity is limited as inhomogeneity can smear out signatures of
BKT transition.
In this work, we use the higher order statistics of resistivity fluctuations to address the nature of superconducting
transition in ultra thin films of Niobium Nitride. Our technique involves detecting non-Gaussian component (NGC)
in fluctuations which are sensitive to long- range correlations in the system. The measure of NGC is given by
second spectrum (equivalent to kurtosis). We find strong non-Gaussian fluctuations closer to the transition
temperature (T KT) in films exhibiting BKT transition which monotonically decrease with increase in temperature
and reduces to background level as the mean field temperature is approached (T BCS). The NGC is completely
absent in bulk films. We attribute the NGC to the presence of long-range interaction between vortices which
naturally occur in a BKT transition whereas the Ginzburg Landau (GL) fluctuations occuring in bulk films are
Gaussian in nature.
Our experiments underline a new method to identify the characteristic temperature scales of phase fluctuations
in the superconducting state, that can be useful to probe other low dimensional superconductors as well.
Key words: Non Gaussian, Kosterlitz-Thouless transition
References
1. R. Koushik et al., (under preparation)
2. Mintu Mondal et al., Phys. Rev. Lett 107, 217003 (2011)