Andreev spectroscopy of FeSe: Evidence for two-gap superconductivity
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RDER, DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Andreev Spectroscopy of FeSe: Evidence for TwoGap Superconductivity1 Ya. G. Ponomareva, S. A. Kuzmicheva, M. G. Mikheeva, M. V. Sudakovaa, S. N. Tchesnokova, T. E. Shanyginaa, O. S. Volkovaa, A. N. Vasilieva, and Th. Wolfb a
Low Temperature Physics and Superconductivity Department, Moscow State University, Moscow, 119991 Russia b Karlsruher Institut für Technologie, Institut fur Festkörperphysik D76021, Karlsruhe, Germany email: [email protected] Received December 21, 2010
Abstract—Currentvoltage characteristics (CVCs) of Andreev superconductor–constriction–superconduc tor (ScS) contacts in polycrystalline samples of FeSe with the critical temperature TC = (12 ± 1) K have been measured using the breakjunction technique. In Sharvintype nanocontacts, two sets of subharmonic gap structures were detected due to multiple Andreev reflections, indicating the existence of two nodeless super conducting gaps ΔL = (2.75 ± 0.3) meV and ΔS = (0.8 ± 0.2) meV. Wellshaped CVCs for stacks of Andreev contacts with up to five contacts were observed due to the layered structure of FeSe (the intrinsic multiple Andreev reflections effect). An additional fine structure in the CVCs of Andreev ScS nanocontacts is attrib uted to the existence of a Leggett mode. A linear relation between the superconducting gap ΔL and the mag netic resonance energy Emagres ≈ 2ΔL is found to be valid for layered iron pnictides. DOI: 10.1134/S1063776111080097 1
Among a wide range of ironbased superconduc tors [1, 2], the binary compound FeSe has the simplest crystal structure and a moderate critical temperature TC = (8–13) K [3, 4]. The transition temperature TC can be increased to 37 K by applying external pressure [5, 6] or totally suppressed by tensile strain [7]. It is generally accepted that superconductivity exists only in tetragonal phase [3, 8, 9]. At ambientpressure FeSe is not magnetically ordered, but its magnetic proper ties become more complicated as the external pressure increases or Se is substituted by Te [10, 11]. A layered structure is a common feature of iron based superconductors [12]. In the case of FeSe, the Fermi surface consists of two intersecting cylindrical electron Fermi surfaces at the Brillouin zone corner and two concentric hole cylinders at the zone center, similar to FeAsbased multiband superconductors [13]. It can be expected that FeSe also belongs to the class of multiband superconductors, which have been studied intensively since the original theoretical papers [14, 15]. Initially, a twogap model was proposed by Kresin and Wolf for cuprate superconductors [16, 17]. Exper imental evidence for a twoband superconductivity in YBaCu3O7 – δ came from surface impedance measure ments [18] and tunneling spectroscopy [19, 20]. Stud ies of multigap superconductivity intensified after the discovery of superconductivity in MgB2 (2001) [21] 1 The article is published in the original.
and later in iron pnictides (2008) [22] (see [2, 23] for the details).
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