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Type-1.5 superconductivity in two-band systems

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 نشر من قبل Egor Babaev
 تاريخ النشر 2010
  مجال البحث فيزياء
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In the usual Ginzburg-Landau theory the critical value of the ratio of two fundamental length scales in the thery $kappa_c=1/sqrt{2}$ separates regimes of type-I and type-II superconductivity. The latter regime possess thermodynamically stable vortex excitations which interact with each other repulsively and tend to form vortex lattices. It was shown in [5] that this dichotomy in broken in U(1)xU(1) Ginzburg-Landau models which possess three fundamental length scales which results in the existence of a distinct phase with vortex excitations which interact attractively at large length scales and repulsively at shorter distances. Here we briefly review these results in particular discussing the role of interband Josephson coupling and the case where only one band is superconducting while superconductivity in another band is induced by interband proximity effect. The report is partially based on E. Babaev, J. Carlstrom, J. M. Speight arXiv:0910.1607.



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In general a superconducting state breaks multiple symmetries and, therefore, is characterized by several different coherence lengths $xi_i$, $i=1,...,N$. Moreover in multiband material even superconducting states that break only a single symmetry ar e nonetheless described, under certain conditions by multi-component theories with multiple coherence lengths. As a result of that there can appear a state where some coherence lengths are larger and some are smaller than the magnetic field penetration length $lambda$: $xi_1leq xi_2... < sqrt{2}lambda<xi_Mleq...xi_N$. That state was recently termed type-1.5 superconductivity. This breakdown of type-1/type-2 dichotomy is rather generic near a phase transition between superconducting states with different symmetries. The examples include the transitions between $U(1)$ and $U(1)times U(1)$ states or between $U(1)$ and $U(1)times Z_2$ states. The later example is realized in systems that feature transition between s-wave and $s+is$ states. The extra fundamental length scales have many physical consequences. In particular in these regimes vortices can attract one another at long range but repel at shorter ranges. Such a system can form vortex clusters in low magnetic fields. The vortex clustering in the type-1.5 regime gives rise to many physical effects, ranging from macroscopic phase separation in domains of different broken symmetries, to unusual transport properties.
196 - Egor Babaev , Mihail Silaev 2012
Usual superconductors are classified into two categories as follows: type-1 when the ratio of the magnetic field penetration length (lambda) to coherence length (xi) with Ginzburg-Landau parameter kappa=lambda/xi <1/sqrt{2} and type-2 when kappa >1/s qrt{2}. The boundary case kappa =1/sqrt{2} is also considered to be a special situation, frequently termed as Bogomolnyi limit. Here we discuss multicomponent systems which can possess three or more fundamental length scales and allow a separate superconducting state, which was recently termed type-1.5. In that state a system has the following hierarchy of coherence and penetration lengths xi_1<sqrt{2}lambda<xi_2. We also briefly overview the works on single-component regime $kappa approx 1/sqrt{2}$ and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.
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