“Break-junction” technique in application to layered superconductors (Review Article)

Abstract

Here presented a systematic study of superconductor-constriction-superconductor contacts realized by a break-junction technique in layered superconductors. Depending on the constriction transparency the tunneling and SnS-Andreev spectroscopies could be used, for the direct determination of values of superconducting gaps, characteristic BCS-ratios and gap temperature dependences in cuprate superconductors, magnesium diboride, novel pnictides and chalcogenides. Basing on these data, one can estimate the gap anisotropy magnitude as well as values of electron-boson coupling constants. We discuss the advantages and difficulties of the break-junction technique and demonstrate this method is powerful enough for high-resolution investigation of optical phonon modes in high-temperature superconducting cuprates and for creating of the contacts with the selective transparence in Mg 1-x Al x B 2 compounds.In memory of Ya. G. Ponomarev, professor, our teacher, warrior, and outstanding experimenter. In memory of S. N. Tchesnokov, mentor, friend, and extraordinary inventor

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

“…IMARE resembles an intrinsic Josephson effect [69] and was observed in cuprates, later in other layered superconductors (for a review, see [58]). When considering an Andreev array as a sequence of m identical SnS-junctions, the position of SGS scales with m: Vn = 2Δ·m/en, n, m =1, 2 .…”

“…Multiple Andreev reflections effect (MARE) spectroscopy is a unique direct probe of the bulk superconducting order parameter [56][57][58]. MARE occurs in ballistic [59] superconductornormal metal-superconductor (SnS) junction which dimension 2a is less than the carrier mean free path l. MARE causes a pronounced excess conductance at low bias voltages (so called "foot"), and a subharmonic gap structure (SGS).…”

“…Angle-dependent in k-space superconducting gap strongly affects the shape of Andreev subharmonics [58,64,65]. Isotropic (s-wave) gap produces high-intensive and symmetrical dynamic conductance dips, while d-wave or fully anisotropic s-wave (having nodes at some angles in k-space) gap make the subharmonics poorly visible and strongly asymmetric.…”

“…The doublet represents two minima connected by an arch, which positions correspond to the higher and lower extremes of the cos(4θ)-type gap angular distribution [58].…”

“…In Ba-122, high-quality SnS-contacts were formed by a "break-junction" technique [58,67]. The single crystal prepared as a thin rectangular plate was attached to a springy sample holder (oriented along the ab-plane) using four pads of In-Ga paste, and then cooled down to T = 4.2 K. In cryogenic environment, a gentle mechanical curving of the sample holder produced a cleavage of the sample, thus creating two superconducting banks separated with a weak link.…”