Temperature dependence of superconducting gaps in Mg1−xAlxB2 system investigated by SnS-Andreev spectroscopy

Tchesnokov

et al. 2015

J Supercond Nov Magn

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.

supporting

confidence: 60%

mentioning

confidence: 73%

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

Kuzmicheva

Tchesnokov

et al. 2015

J Supercond Nov Magn

“…We did not observe the isotropization of the order parameters (transition to the "dirty limit") until T c ≈ 11 K [36,48,51], which was expected in Refs. [127,128].…”

Section: Tunneling and Andreev Spectroscopy Of Mg 1−x Al X Bmentioning

confidence: 53%

“…In our experiments the crystal lattice of the Mg 1−x Al x B 2 samples formally exhibited both insulator and normal metal properties, and SIS and SnS stacks formed on the steps and terraces of the cryogenic clefts due to the layered structure [36,37,[48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. As such, we got the opportunity to apply four types of spectroscopy to the study of the structure of superconducting order parameters, just as we did with cuprates.…”

Section: Tunneling and Andreev Spectroscopy Of Mg 1−x Al X Bmentioning

confidence: 99%

See 1 more Smart Citation

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

Low Temperature Physics

Kuzmicheva

2016

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

Multigap Superconductivity in GdFeAsO0.88 Evidenced by SnS-Andreev Spectroscopy

Shanygina

Mikheev

et al. 2013

J Supercond Nov Magn

Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy we studied superconducting properties of nearly optimal oxygen-deficient GdFeAsO 0.88 polycrystalline samples (bulk critical temperatures T bulk C = 49÷52 K). Temperature dependences for two superconducting gaps ∆ L,S (T ) (T local C = 48 ÷ 50 K) have been measured in the range from 4.2 to 50 K. The ∆ L,S (T ) dependences were found to deviate from the BCS-like function; this suggests an importance of the k-space (internal) proximity effect between the two condensates.GdFeAsO firstly reported in [1] belongs to oxypnictide high-temperature superconductors [2]. The stoichiometric compound is an antiferromagnetic metal with spin-density-wave (SDW) ground state [3], which can be turned into superconductivity under electron doping. The maximal critical temperatures up to T C = 56 K are achieved by O-deficiency, F introducing in-T.E. Shanygina · A.S. Usol'tsev Lebedev Physical Institute RAS,