Intrinsic Multiple Andreev Reflections in Layered Th-Doped Sm1−x Th x OFeAs

Abstract

Layered oxypnictide Sm 1−x Th x OFeAs (Sm-1111) is an ideal candidate to be probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy. Using the classical "break-junction" technique, we formed ballistic Andreev arrays of identical S-n-Scontacts, where S is superconductor, and n is a layer of normal metal. For T < T C , the I(V) curve shows an excess-current and a subharmonic gap structure (SGS): a set of sharp dI(V)/dV-dips at positions which depend on the superconducting gap value, the number of junctions in the array, and the natural subharmonic order, thus manifesting the effect of intrinsic multiple Andreev reflections. Here we present the I(V) and dI(V)/dV with up to 4 SGS dips for Andreev arrays formed in optimally doped Sm-1111 with critical temperatures T C ≈ 49 K, as well as in underdoped samples with T C ≈ 37 K. We show that a number of Andreev subharmonics facilitates the determination of the superconducting gap with a better accuracy. In this study, we used polycrystalline Sm 1−x Th x OFeAs samples with nearly optimal Th-doping and T C ≈ 49 K, and underdoped samples with T C ≈ 37 K. The highquality polycrystallites possessing a single superconducting phase were synthesized under high pressure, as detailed in [3,4]. Superconducting properties of Sm-1111 samples were probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy [5], to use this method, we applied the "break-junction" technique [6]. The sample consisting of a thin rectangular plate of 4 × 2 × 0.2 mm 3 was attached to a spring sample holder by a liquid In-Ga alloy. Then the sample holder was cooled down to T = 4.2 K, where a slight mechanical curving produced a cryogenic cleavage, thus creating two superconducting banks separated with a weak link (ScS-contact, c -constriction). In oxypnictides, the constriction usually acts as a thin normal metal, making it possible to observe multiple Andreev reflections in a ballistic 8,9,10]. In a clean classical Andreev mode, it causes a currentvoltage characteristic (CVC) with an excess current at low bias voltages ("foot") and a subharmonic gap structure (SGS) in the dynamic conductance [8,9,10]. SGS presents a series of dI(V)/dV-minima at certain positions V n = 2∆/en, n = 1, 2, . . . The typical diameter of a ballistic SnS-contact in oxypnictides was estimated using Sharvin's formula: a = 10 ÷ 30 nm [11,12].A layered sample exfoliates along the ab-planes with the formation of steps and terraces along the c-direction.

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

“…In our experiments we observed quite the opposite: in both single crystals and polycrystalline samples of similar compounds a typical resistance of ScS contact was reproduced, and the singularities of the dynamic conductance became sharper with increasing m [71]. Note that the position and shape of the singularities along the dI(V)/dV spectra (caused by bulk effects such as gap and phonon singularities) are reproduced when scaling the bias of the electric potential by a natural number m in order to normalize the conductance features to a single junction spectrum [36,51,66,68,69,71,74,75,77], and coincide with the single contact characteristics. Similar data were obtained for single crystals of layered superconductors [30,67,72,76,81].…”

“…[46,47]), magnesium diborides [36,[48][49][50][51][52][53][54][55][56][57][58][59] (for a review see Refs. [60,61]), iron pnictides and chalcogenides [53,[62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77].…”

“…Subsequently the intrinsic multiple Andreev reflections effect is repeatedly observed in other layered materials: cuprates of different families [30,36,37,119], Mg 1−x Al x B 2 [36,48,49,51,52], both single crystal and polycrystalline iron-based superconductors [66][67][68][69]71,72,[74][75][76][77]. Note that the features created by the contribution of base-to-base contacts in the SnS Andreev regime were observed very rarely.…”

“…15 shows the normalized CVC and its derivative dI(V)/dV (at T=4.2K) for a stack (with a number of contacts m=3) in a samarium-based oxypnictide Sm 0.7 Th 0.3 OFeAs [142] optimally doped with thorium, with T c ≈ 52 K. In order to observe multiple Andreev reflections the diameter of SnS contacts, as estimated in Refs. [74,75,77] must be about 2a = 20-60 nm, which is orders of magnitude less than the size of the crystallite in these samples [142] and the typical terrace width (see Fig. 2).…”

Complex Lattice and Charge Inhomogeneity Favoring Quantum Coherence in High-Temperature Superconductors

“…Complexity and phase separation scenarios in complex oxides and functional strongly correlated systems have recently attracted a wide interest [53][54][55][56][57][58][59][60][61][62][63]. Iron-based superconductors have provided a key laboratory showing exotic superconducting phases characterized by inhomogeneity, nematic phases and multiple gaps [64][65][66][67][68][69]. The physics of the BCS-BEC crossover in quantum condensates in particular real space or k space spots widely studied in ultracold gases is now emerging as a generic feature in multiple condensates superconductors [70][71][72][73][74][75][76].…”