Ya. G. Ponomarev

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

Ya. G. Ponomarev, S. A. Kuzmichev, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, N. Z. Timergaleev, A. V. Yarigin, M. V. Lomonosov Moscow State University, Department of Physics, 119899 Moscow, Russia, E. G. Maksimov, S. I. Krasnosvobodtsev and A. V. Varlashkin, P. N. Lebedev Physics Institute, RAS, Moscow, Russia, M. A. Hein, G. Müller, H. Piel, Bergische Universität Wuppertal, Fachbereich Physik, D-42097 Wuppertal, Germany, L. G. Sevastyanova, O. V. Kravchenko, K. P. Burdina, B. M. Bulychev, M. V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow, Russia.

Evidence for strong electron-phonon interaction from inelastic tunneling of cooper pairs in c-direction in bi2sr2cacu2o8 break junctions

A reproducible fine structure at subgap voltages in the I(U)-characteristics of Bi2Sr2CaCu2O8 break junctions has been observed and investigated. The structure is detectable only in the presence of an a.c. Josephson current. The position of the dips, composing the structure in the dI/dU-characteristics, is independent of the gap parameter Δ, the temperature T and the geometry of the contacts. The overall form of the fine structure is in good agreement with the Raman scattering spectra of the phonon modes in this material. We attribute this structure to an inelastic (phonon assisted) tunneling of Cooper pairs, which is accompanied by the emission of coherent Raman-active optical phonons at resonance voltages Ures=ℏωphon/2e. These results hint for strong electron–phonon interaction in this material.

Two-gap structure in Yb(Y)Ba2Cu3O7−x single crystals

Tunneling effects in YbBa2Cu3O7−x (YbBCO) and YbBa2Cu3O7−x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the “large” gap is in the range of 24–28 meV for YBCO (Tc=85–90 K) and 29–30 meV for YbBCO (Tc=87–90 K). A variation of the value of the “small” gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.

Investigation of the gapless state in bismuth-antimony alloys

The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi1−xSbx by pressure-induced band inversion. The magnetoresistance properties of the alloys have been investigated both in weak magnetic fields (μH « 1) and in strong fields (H≤75 kOe) at liquidhelium temperatures in the Sb concentration interval 0.06≤x≤0.15 and pressure interval 1 bar ≤p<20 kbar. At pressuresp close to the pressurepk at which the gapless state is realized a “semiconductor-semimetal-semiconductor” transition is detected inBi1−xSbx alloys withx=0.070 and 0.071. The rates of change of the gap εgL before and after inversion are determined: −(2.5±0.5)×10−6 eV/bar and (1.5±0.5)×10−6 eV/bar, respectively. A reduction in the carrier effective mass as εgL → 0 is observed down to values of ∼ 10−4m0. It is shown that as εgL → 0 the carrier mobilities in the alloys increase abruptly, the effect being a maximum in the purest alloys, where forT=4.2 K the mobility along the binary axis attains the record-high value of ∼ 3×108 cm2/V · sec.

Structure and properties of BSCCO-whiskers grown from amorphous precursors with foreign dopants

Abstract The process of BSCCO whisker growth from varying Bi 2 Sr 2 Ca 3 Cu 5 Al x M y O z ( M = Li, Mg,Pb, Y, Zr, V, Nb, Mo) compositions of amorphous precursor have been investigated. The Al 2 O 3 ZrO 2 double oxide favours the whisker growth even if the glass precursor contains a crystalline portion. The whiskers grown from a 2235 plate and from 2235AlZr plate had a similar cation composition, but in the second case the Sr and Cu content in the crystal tends to be lower. The Al and Zr additives are distributed uniformly inside the plates with foreign framework fabrication. The variation of whisker structure parameters and superconducting properties have been investigated. The tunnelling characteristics for break junctions of whiskers are discussed and a model of BSCCO whisker growth is presented.

Josephson effect and single-particle tunneling in YBa2Cu3O7−x and YbBa2Cu3O7−x single-crystal break junctions

Abstract We have studied the current-voltage characteristics of break junctions in single crystals of YBa 2 Cu 3 O 7− x and YbBa 2 Cu 3 O 7− x at helium temperature. For a YBa 2 Cu 3 O 7− x Josephson junction Fiske resonances in low magnetic fields have been observed in the I ( V ) characteristics, thus proving the tunneling nature of the contact. The reduced velocity of the electromagnetic wave in the contact area has been calculated to be c / c 0 ∼ 0.018 ( c is the Swihart velocity). The ratio of the barrier thickness d 1 to the relative dielectric constant of the barrier ϵ was found to be d I / ϵ ∼ 0.09 nm. In the quasiparticle tunneling regime, the YbBa 2 Cu 3 O 7− x break junctions showed in some cases I ( V ) characteristics with a relatively small excess current at “sub-gap” voltages and a steep rise of the current at the “gap” voltage V g = 2 Δ / e . The “BCS-like” form of the observed I ( V ) characteristics seems to be in favor of s wave superconductivity.

Investigation of a superconducting Mg1−xAlxB2 system by tunneling and microjunction (Andreev) spectroscopies

A detailed investigation of multiband superconductivity in the Mg 1−xAlxB2 system was carried out by the methods of tunneling and Andreev spectroscopy. Temperature dependences of the superconducting gaps and their variation upon an increase in the degree of disorder and the Al concentration were studied. It is shown that the experimentally observed dependences cannot be explained in detail within the framework of the presently available microscopic theories.

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO0.88F0.12FeAs samples with bulk critical temperature Tc = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are ΔL = 10.5 ± 2 meV and Δs = 2.3 ± 0.4 meV, respectively.

Leggett’s mode in Mg1−x AlxB2

A detailed investigation of multiband superconductivity and Leggett’s mode in the Mg1−xAlxB2 (0 ≤ x ≤ 0.45) system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps Δσ and Δπ and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett’s mode energy ε0 upon the values of the gaps Δσ and Δπ has been derived.

Quasiparticle tunneling in the c-direction in stacks of Bi2Sr2CaCu2O8+δ S–I–S junctions and the symmetry of the superconducting order parameter

Abstract A clear gap structure has been observed in the current–voltage characteristics (CVC) of stacks of S–I–S junctions obtained in Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO) crystals using a break junction technique. Due to the high resistance of the stacks, the range of bias voltages could be extended well above the gap voltage V gn without significant overheating of the contacts. For samples with a critical temperature T c =84–89 K and 2 Δ / kT c =6.7±0.3, the value of V gn corresponding to a steep rise of the quasiparticle current I qp is well-described by the relation V gn = n (2 Δ / e ) with a gap parameter Δ (4.2 K)=(25±1) meV. The presence of a sharp gap feature in characteristics of stacks points to an essential contribution of s-wave pairing to the symmetry of the order parameter in BSCCO.