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Scientific Report 2007-2009
Condensed matter physics and biophysics
C1. Superconductivity in low-dimensional materials
Large part of the experimental and theoretical research
on new superconducting (SC) materials focuses
nowadays on systems that are effectively low dimensional.
To this category belong layered materials
with weakly-coupled planes, like the high-temperature
cuprate superconductors, as well as confined 2D structures,
like thin films or conducting layers at the interface
of artificial heterostructures. Besides the low dimensionality,
a common characteristic of many of these systems
is the low superfluid density, i.e. the energy scale which
controls the phase fluctuations of the SC order parameter.
Under these conditions, the SC transition could
share a Berezinsky-Kosterlitz-Thouless (BKT) character,
where vortex excitations play a crucial role.
The BKT transition has in principle very specific signatures,
both below and above the SC transition temperature
T BKT . For example the superfluid density J s
vanishes with an “universal” jump approaching T BKT
from below, while the SC correlation length ξ(T ), that
is probed by several quantities like paraconductivity, diamagnetism
or Nerst effect, should diverge exponentially
as T → T BKT from above. This form would be in marked
contrast with the typical power-law behavior of ξ(T ) due
to Ginzburg-Landau (GL) SC fluctuations, where modulus
and phase fluctuate simultaneously.
The observation of clear signatures of BKT physics
in these new systems is still debated. In cuprates
contradicting conclusions emerge from different probes:
while Nerst effect and diamagnetism point towards a
relevance of BKT physics, no clear signature of the
would be universal jump of the superfluid density has
been reported. Moreover paraconductivity in underdoped
compounds provides evidence of a more traditional
Aslamazov-Larkin GL-behavior [1]. This scenario
calls for a deepr theoretical investigation of the additional
effects that can affect the “universal” character
of the BKT transition, mainly related to the quasi-2D
structure and/or disorder. We have investigated these
issues [2-3] using the sine-Gordon description of the BKT
transition, which allows us to go beyond standard results
derived in the XY model. First we discussed the role
played by the energy µ of the vortex core in a weaklycoupled
layered system. Here two ratios are relevant:
µ/J s , which fixes the temperature where vortices would
like to unbind, driving the J s to zero, and J ⊥ /J s , where
J ⊥ is the interlayer Josephson coupling, which tends to
keep J s finite. While in the XY model µ/J s has a fixed
value, we showed that in the more general case the behavior
of the system strongly depend on the ratio µ/J s ,
the increasing of which effectively enhances J ⊥ /J s [2].
As a consequence, even though the transition can ultimatively
have a BKT character (i.e. vortex excitations
are relevant) the jump of the superfluid-density J s is reduced
and smoothed, and does not occur any more at the
“universal” temperature. A similar ’non-universal’ µ/J s
dependence of the BKT transition was found in the analysis
of the magnetic-field effect [3]. Indeed, we showed
that the standard linear scaling of the field-induced diamagnetism
M ≃ −ξ 2 (T )H above T c is restricted to a
range of fields that decreases as µ/J s increases, accounting
thus for the persistent non-linear effects reported in
experiments in high-T c superconductors.
R/R N
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
T(K)
Data
Hom
Inhom
0.15 0.2 0.25 0.3 0.35 0.4
Figure 1: Resistivity in the presence of GL+BKT fluctuations,
in the absence (Hom) and in the presence (Inhom)
of inhomoegenity, compared to experimental data in SC heterostructures.
[4]
Interestingly, a revised approach to the BKT transition
can be necessary also in the case of low-temperature
superconductivity, as it is shown by our recent analysis
of 2D superconducting heterostructures [4]. Our work
shows that the contribution of SC BKT fluctuations
to the conductivity cannot be computed neglecting the
prominent role of the intrinsic sample inhomogeneity
(see Fig. 1). This result could give new insight also on
the nature of the superconductor-insulator transition,
that in these systems can be induced in field-effect
devices, which represent a promising candidate for
future technological applications.
References
1. S. Caprara et al., Phys. Rev. B 79, 024506 (2009).
2. L. Benfatto et al., Phys. Rev. Lett. 98, 117008 (2007).
3. L. Benfatto et al., Phys. Rev. Lett. 99, 207002 (2007).
4. L. Benfatto et al., Phys. Rev. B 80, 214506 (2009).
Authors
L. Benfatto 3 , S. Caprara, C. Castellani, M. Grilli
http://theprestige.phys.uniroma1.it/clc/
Sapienza Università di Roma 54 Dipartimento di Fisica