Forum DQMP

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Date(s) - 31/03/2015
13 h 00 min - 14 h 00 min

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Forum DQMP
Tuesday, 31 March 2015 – 13h00
Auditoire Stückelberg

Ecole de physique
Coffee and tea will be available from
12h50 at the entrance of the Auditoire

Unconventional superconductivity in nanoscale aluminum grains
Nimrod Bachar (group of Prof. Van der Marel)
The superconducting transition temperature of coupled nano-sized Al grains, also known as granular
Al, is increased by factor of 3 from that of the bulk [1]. It was shown recently that this enhanced
superconductivity coexists with magnetic properties and increasing electronic interactions [2,3] which
cannot be explained by conventional theories. In this talk I will show the experimental results of the
dynamical THz conductivity of granular Al thin films. The energy gap 2Δ inferred by THz spectroscopy
leads to a ratio 2Δ/kT which exceeds the BCS value. While 2Δ remains fairly unaffected by grain
decoupling, the superfluid density is reduced by more than one order of magnitude and the superfluid
stiffness is reduced below 2Δ for high resistivity films. We find a low-energy excess absorption that can
be understood in terms of phase collective modes [4]. Altogether, we show that superconductivity in
granular Al is probably of the unconventional type.
[1] R. Cohen and B. Abeles, Phys. Rev. 168, 444–450 (1968).
[2] N. Bachar et al., Phys. Rev. B 87, 214512 (2013).
[3] N. Bachar et al., Phys. Rev. B 91, 041123(R) (2015).
[4] T. Cea et al., Phys. Rev. B 89, 174506 (2014).

Proximity effects in a superconductor-normal metal bilayer
system: a dynamical mean field study
Naushad Ahmad Kamar (group of Prof. Giamarchi)
In this talk, I will be talking about the proximity of a superconductor to a normal metal. The system,
represented by a bilayer attractive Hubbard model, is investigated using layer-dynamical mean field
theory and iterated perturbation theory for superconductivity as an impurity solver. The bilayer system
comprises a superconducting and a normal metallic layer, connected by an inter-planar hopping (t⊥). It
is found that superconductivity is induced in the normal layer for small t⊥. With increasing inter-layer
hopping, the bilayer system undergoes two transitions: a first order transition to a normal metallic
phase, and subsequently a continuous crossover to a band insulator.

Forum Committee : L. Foini, C. Lichtensteiger, N. Ubrig (23.03.2015)

 

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