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Scientific Report 2007-2009
Condensed matter physics and biophysics
C2. Strongly Correlated Superconductivity
The origin of high-temperature superconductivity is
one of the most elusive topics in modern solid-state
physics. Superconductivity appears with highest critical
temperatures in “strongly correlated” materials, and
in particular by doping a Mott insulator, a state in which
the carriers are localized by the mutual repulsive interactions.
This is particularly surprising because superconductivity
is associated to the formation of a coherent
state of “Cooper pairs” in which the fermions are paired
by an effective attractive interaction.
So, how can pairing be favoured by strong repulsion?
The continuous advances of material science and experimental
research are helping us to answer the question,
through the design of new superconducting materials
and an unprecedented accuracy in the investigation of
their physics. These studies have shown that copper oxides
are the most spectacular members of a wider class
of strongly correlated superconductors including heavy
fermion and organic molecular compounds.
During the last few years we have shown that trivalent
fulleride superconductors of generic formula A 3 C 60 (A
being an alkali-metal atom) belong to the same family
[1], despite the fact that the pairing mechanism is the
conventional electron-phonon coupling and the pair wave
function has an isotropic s-wave symmetry.
In particular we have shown that a phononic pairing
and correlations are not incompatible in fullerides, and
indeed they can cooperate to provide high critical temperatures.
The key observation is that phononic pairing
of fullerides involves orbital and spin degrees of freedom,
which are still active when charge fluctuations are frozen
by the strong correlations and the system is approaching
the Mott insulating state. As a consequence, an unrenormalized
attraction is effective between heavy quasiparticles
leading to an enhancement of superconductivity
(with respect to a system with the same attraction and
no repulsion).
Our approach predicted a first-order transition between
an s-wave superconductor and an antiferromagnet
as a function of pressure [1], and a bell-shaped superconducting
region which reminds of the doping dependence
of T c in the copper oxides. These effects have been recently
experimentally observed in a new expanded fulleride,
Cs 3 C 60 with A15 structure, providing a crucial
support to our theory. Further predictions of our approach
include a superconducting transition which is associated
to a gain of kinetic energy (as opposed to the
standard BCS state, which is stabilized by potential energy
gain) and a pseudogapped normal state [2].
Besides the remarkable success in describing the
physics of expanded fullerides, this “Strongly correlated
superconductivity” scenario that we briefly described has
a more general validity. We expect indeed that different
pairing mechanisms that involve spin or orbital degrees
of freedom can coexist and even be favoured by strong
Figure 1: Theoretical and experimental phase diagram for
Cs 3C 60
repulsion. This is for example the case of superexchange
interactions in the cuprates.
The surprising result that phonon-driven superconductivity
can be favoured by repulsion depends crucially
on the symmetry of the electron-phonon interaction.
On the other hand in a model in which both repulsion
and attraction are associated to the charge degrees
of freedom [3,4] the two terms are competitive. In
this case we have demonstrated that electron-phonon
interaction is strongly reduced in correlated states, even
if antiferromagnetic correlations revive its effect. Generically
the depression is stronger at large transferred
momentum than at small momentum. Interestingly,
while phonon effects are reduced in the low-energy
properties associated to quasiparticle motion, they are
still present in the high-energy physics.
References
1. M. Capone et al., Rev. Mod. Phys. 81, 943 (2009).
2. M. Schirò et al., Phys. Rev. B 77, 104522 (2008).
3. A. Di Ciolo et al. Phys. Rev. B 79, 085101 (2009).
4. P. Barone et al. Europhys. Lett. 79, 47003 (2007).
Authors
C. Castellani, M. Capone 3 , M. Grilli, J. Lorenzana 3
http://theprestige.phys.uniroma1.it/clc/
Sapienza Università di Roma 55 Dipartimento di Fisica