J. Kačmarčík

Evidence for Two Superconducting Energy Gaps in MgB2 by Point-Contact Spectroscopy

Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB(2) with the transition temperature T(c) = 39 K. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( Delta(S) = 2.8 meV and Delta(L) = 7 meV) are, respectively, smaller and larger than the expected weak coupling value. Because of the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures, but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk T(c) in the raw data.

Point-contact spectroscopy of MgB2

Abstract Point-contact spectroscopy measurements on magnesium diboride reveal the existence of two superconducting energy gaps closing at the same transition temperature in line with the multi-band model of superconductivity. The sizes of the two gaps ( Δ π =2.8 meV and Δ σ =6.5–7 meV) are respectively smaller and larger than the expected weak coupling value of the one-gap superconductor. The smaller gap is rapidly filled by a small magnetic field of about 1–2 T much lower than the real upper critical field H c2 . The larger gap is closed at the anisotropic H c2 . Above the gap energies reproducible non-linearities are observed at the characteristic phonon energies of MgB 2 .

Low-temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by gas immersion laser doping. The doping concentration c(B) has been varied up to similar to 10 at. % by increasing the number of laser shots to 500. No superconductivity could be observed down to 40 mK for doping level below similar to 2 at. %. The critical temperature T(c) then increased steeply to reach similar to 0.6 K for c(B) similar to 8 at. %. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field mu(o)H(c2) (0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. [E. Bustarret et al., Nature (London) 444, 465 (2006)].

Specific heat measurements of a superconducting NbS2 single crystal in an external magnetic field: Energy gap structure

The heat capacity of a 2H-NbS2 single crystal has been measured by a highly sensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At very low temperatures, data show excitations over an energy gap (2ΔS/kBTc≈2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat Ce can be explained either by the existence of a strongly anisotropic single-energy gap or within a two-gap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient γ shows a strong curvature for both principal-field orientations, parallel (H∥c) and perpendicular (H⊥c) to the c axis of the crystal, resulting in a magnetic field dependence of the superconducting anisotropy. These features are discussed in comparison to the case of MgB2 and to the data obtained by scanning-tunneling spectroscopy. We conclude that the two-gap scenario better describes the gap structure of NbS2 than the anisotropic s -wave model.

Single-gap superconductivity in β − B i 2 Pd

This work was supported by the Slovak projects APVV-0036-11, APVV-14-0605, VEGA 2/0149/16, VEGA 1/0409/15, by the ERDF EU grant under contract No. ITMS26220120005 and the COST action MP1201 as well as by the U.S. Steel Kosice. Sample growth was partially supported by the projects No. FIS2014-54498-R (MINECO, Spain) and No. S2013/MIT-2850 Nanofrontmag-CM (Region of Madrid, Spain) and by the Engineering and Physical Sciences Research Council, UK. E.H. acknowledges support from the Grant No. 560-2012 programa de doctorados en el exterior (COLCIENCIAS)

Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements

Specific heat has been measured down to 600 mK and up to 8 T by highly sensitive ac microcalorimetry on MgCNi3 single crystals with Tc ≈ 7 K. Exponential decay of the electronic specific heat at low temperatures proved that a superconducting energy gap is fully open on the whole Fermi surface, in agreement with our previous magnetic penetration depth measurements on the same crystals. The specific-heat data analysis shows consistently the strong-coupling strength 2 /kBTc ≈ 4. This scenario is supported by the direct measurements of the gap via the point-contact spectroscopy. Moreover, the spectroscopy measurements show a decrease in the critical temperature at the sample surface, which accounts for the observed differences of the superfluid density deduced from the measurements by different techniques.

Interlayer Transport in the Highly Anisotropic Misfit-Layer Superconductor (LaSe)1.14(NbSe2)

The interlayer transport in a two-dimensional superconductor can reveal a peak in the temperature as well as the magnetic field dependence of the resistivity near the superconducting transition. The experiment was performed on the highly anisotropic misfit-layer superconductor [(LaSe)(1.14)](NbSe(2)) with T(c) of 1.2 K. The effect is interpreted within the tunneling mechanism of the charge transport across the Josephson-coupled layers via two parallel channels--the quasiparticles and the Cooper pairs. Similar behavior can be found in the high-T(c) cuprates but there it is inevitably interfering with the anomalous normal state. The upper critical magnetic field can be obtained from the interlayer tunneling conductance.

Two-dimensional behavior of the naturally layered superconductor (LaSe)1.14(NbSe2)

Abstract The three-dimensional to two-dimensional (2D) crossover of the superconducting state is presented for the naturally layered single crystal of (LaSe) 1.14 (NbSe 2 ) with the critical temperature T c of 1.23 K. Namely, the temperature dependence of the upper critical magnetic field B c2 parallel to the layers reveals an upturn at the crossover temperature T * due to the vortex confinement between the superconducting layers. Below this temperature in the 2D regime the angular dependence of B c2 displays a cusp at the field parallel to the layers. In the superconducting state the interlayer transport is accomplished in this system by the tunneling of quasiparticles and Cooper pairs.

Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements

We use sub-Kelvin scanning tunneling spectroscopy to investigate the suppression of superconductivity in homogeneously disordered ultrathin MoC films. We observe that the superconducting state remains spatially homogeneous even on the films of 3 nm thickness. The vortex imaging suggests the global phase coherence in our films. Upon decreasing thickness, when the superconducting transition drops from 8.5 to 1.2 K, the superconducting energy gap follows perfectly Tc. All this is pointing to a two-stage fermionic scenario of the superconductor-insulator transition (SIT) via a metallic state as an alternative to the direct bosonic SIT scenario with a Cooper-pair insulating state evidenced by the last decade STM experiments.

Superconducting properties of laser annealed implanted Si:B epilayers

We report on the superconducting properties of heavily doped silicon epilayers obtained by the implantation of B atoms in silicon wafers and subsequent laser annealing (pulsed laser induced epitaxy). A critical temperature ?250?mK has been obtained for samples with a boron concentration (cB) ranging from 2 to 10 at.%, which were checked by atom probe tomography to be free of any significant boron clustering. The standard dopant implantation technique is therefore an alternative (with respect to gas immersion laser doping) process to induce superconductivity in boron-doped silicon. Superconductivity was not observed with any of the other implanted dopants (P, As, Al) with similar concentrations down to 50?mK.