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The anomalous superconducting state of granular aluminum: an experimental point of view

The anomalous superconducting state of granular aluminum: an experimental point of view

Dienstag, 16.01.2018, 15.45 Uhr


Seminarraum 3-1, Physikhochhaus


Dr. Nimrod Bachar
Department of Quantum Matter Physics, University of Geneva, Switzerland

Abstract: Granular aluminum consists of nanometer scale particles
coupled through a thin insulating layer. It shows a superconducting
temperature which can be enhanced to more than 3 times of bulk Al Tc
[1]. The superconducting properties of this system can be tuned by
carefully controlling the grain size and the coupling between the grains.
The superconducting temperature forms a dome shape with maximum
Tc inversely proportional to the grain size [2,3]. This stands in
contradiction to conventional theories of superconductivity and raises
important questions on the bare existence of SC in small metallic
particles. Recent experimental studies based on electrical transport [3],
thermal transport [4] and THz spectroscopy [5] explored the peculiar
superconducting state of this system by showing the evolution of several
important properties such as the critical temperature, the energy gap,
the superfluid stiffness, the critical magnetic field and superconducting
fluctuations as a function of the grain coupling. I will review our main
experimental results showing that the phase diagram is shaped by a
crossover from an amplitude to phase driven superconductivity. I will
also present several of the current theoretical studies aiming to explain
the enhancement of Tc in granular Al. Our recent results will not only
help to shed light on the anomalous superconducting state in granular Al
but also will aid in adapting this material to future applications.

[1] B. Abeles and R.W. Cohen, Phys. Rev. Lett. 17 (1966).
[2] G. Deutscher et al., J. Vac. Sci. Tech. 10 (1973);
G. Deutscher et al., J. Low Temp. Phys. 10 (1973).
[3] N. Bachar et al., Phys. Rev. B 87 (2013); N. Bachar et al., Phys Rev.
B91 (2015).

[4] S. Lerer, N. Bachar et al., Phys. Rev. B. 90 (2014).
[5] U. S. Pracht, N. Bachar et al., Phys. Rev. B 93 (2016).