Disorder, Superconducting fluctuations and Metal-Insulator crossover in high Tc cuprates
F. Rullier-Albenque, IRAMIS-SPEC, CEA Saclay, 91191 Gif-sur-Yvette, France
It is well known that
disorder has profound effects on strongly correlated materials. In order to
separate intrinsic properties from those induced by disorder in high Tc cuprates, we have undertaken
systematic studies of the effect of controlled disorder on the transport
properties of optimally doped YBCO7 and underdoped YBCO6.6, which
are known to be very clean compounds. We have used electron irradiation at low
temperature to create point defects in the CuO2 planes of these
compounds. Those have been found for long to behave quite similarly as Zn
substitution on the Cu planar site [1]
The Nernst effect has been recently
shown to be a very sensitive probe of superconducting fluctuations surviving in
the normal state of high-Tc cuprates. We shall present data which
have allowed us to demonstrate that the analysis of the transverse magnetoresistance
(MR) in fields up to 60T is also a reliable means to detect the (H,T) range where superconductivity
survives above Tc: the normal state MR is restored above a threshold field Hc′(T), which is found to vanish at Tc′>Tc [2]. When Tc is decreased by disorder, we found that the
fluctuation range probed by the Nernst effect or by magnetoresistance is much
less depressed than Tc itself [2] while –lnT upturns of the resistivity similar
to those observed in pure " low-Tc" cuprates appear. This
suggests that specific disorder is responsible for the extended range of Nernst
signal found in the "low-Tc" cuprate families and is at the origin of
their depressed Tc values. Furthermore our results show that the transition from the Mott
insulator to the metallic superconductor is mostly governed by non generic
disorder. We therefore propose a realistic phase diagram, including disorder,
in which the superconducting state might reach the antiferromagnetic phase in
the clean limit [3].
[1] F. Rullier-Albenque et al., Europhys. Lett. 50, 81 (2000)
[2] F. Rullier-Albenque et al., Phys. Rev. Lett. 99, 027003 (2007)
[3] F. Rullier-Albenque et al., Europhys. Lett. 81, 37008 (2008)