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In the last years several experiments in cuprate superconductors have been interpreted as signature of vortex-antivortex phase fluctuations[1-4], which are usually expected within the Kosterlitz-Thouless (KT) theory for
two-dimensional superconductors. Even though the quasi-2D nature of cuprates and the weakness of the Josephson interlayer coupling can justify
this interpretation, still the experimental scenario seems more complex than what expected within the standard KT theory. For example, the lack of the universal jump of the superfluid density[5] or the persistence of
non-linear behavior in the field-induced magnetization above Tc[1,3] have been the subject of intense debate on the occurrence or not of KT
physics in cuprates. To solve this puzzle it is necessary to understand what remains of the 'universal' KT physics when 'relevant' perturbations act on the systems, as for example the interlayer coupling or the magnetic field. Here I will review our recent progresses on these issues: using a powerful mapping onto the sine-Gordon model we were able to show that the
presence of Josephson coupling between layers[6,8] or an external magnetic field[7] can partly spoil the universal character of the KT transition of
pure 2D systems. In particular, the vortex-core energy plays a crucial role in determining the typical energy scale where KT fluctuations are
visible. On this respect, it is particularly useful the analysis of the superfluid-density behavior in ultra-thin films[4]. Indeed, we could show not only a linear scaling of the vortex-core energy with the critical temperature, but also that the experimental data can be fully explained only by taking into account the intrinsic inhomogeneity of the system[8], which strongly resembles that observed by tunneling spectroscopy.
[1] L.Li et al., Europhys. Lett. 72, 451 (2005) [2] Y. Wang et al. Phys. Rev. B 73, 024510 (2006) [3] L.Li et al, Nat. Phys. 3, 311 (2007) [4] I. Hetel et al, Nat. Phys. 3, 700 (2007) [5] D. Broun et al., cond-mat/0509223 [6] L.Benfatto, C.Castellani and T.Giamarchi, Phys. Rev. Lett. 98, 117008 (2007) [7] L.Benfatto, C.Castellani and T.Giamarchi Phys. Rev. Lett. 99, 207002 (2007) [8] L.Benfatto, C.Castellani and T.Giamarchi Phys. Rev. B 77, 100506(R) (2008) |
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