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تسجيل مستخدم جديدType-1.5 superconductivity in multiband systems: magnetic response, broken symmetries and microscopic theory. A brief overview
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A conventional superconductor is described by a single complex order parameter field which has two fundamental length scales, the magnetic field penetration depth lambda and the coherence length xi. Their ratio kappa determines the response of a superconductor to an external field, sorting them into two categories as follows; type-I when kappa <1/sqrt{2} and type-II when kappa >1/sqrt{2} . We overview here multicomponent systems which can possess three or more fundamental length scales and allow a separate type-1.5 superconducting state when, e.g. in two-component case xi_1<sqrt{2}lambda<xi_2. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges. As a consequence the system should form an additional Semi-Meissner state which properties we discuss below. In that state vortices form clusters in low magnetic fields. Inside the cluster one of the component is depleted and the superconductor-to-normal interface has negative energy. In contrast the current in second component is mostly concentrated on the clusters boundary, making the energy of this interface positive. Here we briefly overview recent developments in Ginzburg-Landau and microscopic descriptions of this state.
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In contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single parameter kappa and are divided in type-I kappa<1/sqrt{2} and type-II kappa>1/sqrt{2} classes, two-component systems in general po
ssess three fundamental length scales and have been shown to possess a separate type-1.5 superconducting state. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges, and therefore should form clusters in low magnetic fields. In this work we investigate the appearance of type-1.5 superconductivity and the interpretation of the fundamental length scales in the case of two bands with substantial interband couplings such as intrinsic Josephson coupling, mixed gradient coupling and density-density interactions. We show that in the presence of substantial intercomponent interactions of the above types the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields.
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.
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.
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.
We report point contact Andreev Reflection (PCAR) measurements on a high-quality single crystal of the non-centrosymmetric superconductor Re6Zr. We observe that the PCAR spectra can be fitted by taking two isotropic superconducting gaps with Delta_1
~ 0.79 meV and Delta_2 ~ 0.22 meV respectively, suggesting that there are at least two bands which contribute to superconductivity. Combined with the observation of time reversal symmetry breaking at the superconducting transition from muon spin relaxation measurements (Phys. Rev. Lett. 112, 107002 (2014)), our results imply an unconventional superconducting order in this compound: A multiband singlet state that breaks time reversal symmetry or a triplet state dominated by interband pairing.
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