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Fundamental study of unconventional superconductors

Calculations within the Eliashberg theory

The BCS theory describes well conventional superconductors in which the pairing is mediated by phonons and the electron-phonon coupling is rather weak.
In most unconventional superconductors, the electron-boson coupling is strong and this has a series of consequences that cannot be explained within the BCS theory. The Eliashberg theory is more general than BCS and allows describing both weak-coupling and strong-coupling superconductors.  Moreover, it can be adapted to describe (at least in a semi-phenomenological way) systems in which the coupling is mediated by any kind of bosonic excitations.

We use the Eliashberg theory to:
  • test the consistency of the experimentally-determined critical temperature and gap amplitudes within a given microscopic model for the superconducting pairing
  • determine the coupling strength and the dominant coupling channels (in multiband superconductors)
  • calculate the density of states as a function of the electron-boson spectral function and of the symmetry of the order parameter
  • analyze the strong electron-boson interaction (EBI) features in the point-contact spectra (from tunnelling to Andreev regime)
  • calculate the energy-dependent complex gaps that can be inserted into the BTK model to fit the EBI in experimental point-contact spectra
  • predict the effect of charge doping or chamical doping on the superconducting properties (critical temperature, gaps, critical field, etc)
  • predict or analyze the effect of disorder or impurities (either magnetic or non-magnetic) on various superconducting properties (Tc, gap, critical fields, etc)
  • calculate several quantities of experimental interest to be compared with results in literature.
In particular, three- or four-band effective models, with dominant repulsive interband coupling, have been developed to describe iron-based compounds of the 1111, 122, 111 families.

Gaps as a function of Mn concentration in MgB2
Experimental (symbol) and theoretical (lines) behaviour of the gaps of Mn-doped MgB2 as a function of the critical temperature
Tc in PuCoGa
Effect of disorder induced by self-irradiation: Critical temperature as a function of the residual resisitivity in PuCoGa5 (symbols) compared to the predictions of Eliashberg theory for s-wave and d-wave order parameter.
Doping dependence of the gaps in Ba(FeCo)2As2

Gaps and critical temperature in Ba(Fe,Co)2As2, as a function of the Co content. Comparison of experimental values and predictions of the three-band Eliashberg model.


LaTEST - Department of Applied Science and Technology, Politecnico di Torino
corso Duca degli Abruzzi 24, 10129 Torino (TO) - Italy
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