Thermodynamics of a Heavy Ion-Irradiated Superconductor: the Zero-Field Transition

Specific heat measurements show that the introduction of amorphous columnar defects considerably affects the transition from the normal to the superconducting state in zero magnetic field. Experimental results are compared to numerical simulations of the 3D XY model for both the pure system and the system containing random columnar disorder. The numerics reproduce the salient features of experiment, showing in particular that the specific heat peak changes from cusp-like to smoothly rounded when columnar defects are added. By considering the specific heat critical exponent alpha, we argue that such behavior is consistent with recent numerical work [Vestergren et al., PRB 70, 054508 (2004)] showing that the introduction of columnar defects changes the universality class of the transition.Comment: 4 pages, 2 figure

Tunneling Spectroscopy and Vortex Imaging in Boron-Doped Diamond

We present the first scanning tunneling spectroscopy study of single-crystalline boron doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak coupling BCS law with $\Delta(0)/k_B T_c \simeq 1.74$. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.Comment: 4 pages, 4 figures, replaced with revised versio

Comment on "Precision global measurements of London penetration depth in FeTe0.58Se0.42"

International audienceCho et al. [Phys. Rev. B 84, 174502 (2011)] have reported on the temperature dependence of the London penetration depth deduced from tunnel diode oscillator (TDO) measurements in optimally doped Fe(Se,Te) single crystals. According to their analysis, these measurements could suggest a nodeless two-gap pairing symmetry with strong pair-breaking effects. However, to reach this conclusion, the authors fit the temperature dependence of the superfluid density with a two band clean limit model, which is incompatible with the presence of strong pair-breaking effects, deduced from the T n temperature dependence of the London penetration depth below Tc/3. Moreover, they claim that their results are also ruling out the suggestion that surface conditions can significantly affect the TDO data, but this conclusion is based on one very specific damaging process and completely ignores the large dispersion in the previously published TDO data

Superconducting group-IV semiconductors

International audienceWe present recent achievements and predictions in the field of doping-induced superconductivity in column IV-based covalent semiconductors, with a focus on Bdoped diamond and silicon. Despite the amount of experimental and theoretical work produced over the last four years, many open questions and puzzling results remain to be clarified. The nature of the coupling (electronic correlation and/or phonon-mediated), the relationship between the doping concentration and the critical temperature (TC), which determines the prospects for higher transition temperatures, as well as the influence of disorder and dopant homogeneity, are debated issues that will determine the future of the field. We suggest that innovative superconducting devices, combining specific properties of diamond or silicon, and the maturity of semiconductor-based technologies, will soon be developed

AC measurement of heat capacity and magnetocaloric effect for pulsed magnetic fields

International audienceA new calorimeter for measurements of the heat capacity and magnetocaloric effect of small samples in pulsed magnetic fields is discussed for the exploration of thermal and thermodynamic properties at temperatures down to 2 K. We tested the method up to 0H=50 T, but it could be extended to higher fields. For these measurements we used carefully calibrated bare-chip Cernox® and RuO2 thermometers, and we present a comparison of their performances. The monotonic temperature and magnetic field dependences of the magnetoresistance of RuO2 allow thermometry with a precision as good as 4 mK at T=2 K. To test the performance of our calorimeter, heat capacity and magnetocaloric effect for the spin-dimer compound Sr3Cr2O8 and the triangular lattice antiferromagnet RbFe MoO4 2 are presented

Magnetic field dependence of the coherence length and penetration depth of MgB2 single crystals

International audienceWe report on specific heat and Hall probe magnetization measurements in magnesium diboride single crystals. A magnetic field dependence of the coherence length has been deduced from the former assuming that the electronic excitations are localized in field dependent vortex cores in which case is related to the Sommerfeld coefficient = Cp /T T→0 throughout, H /a0 2 a0 being the vortex spacing . The reversible part of the magnetization has been analyzed with a phenomenological Ginzburg-Landau model introducing field dependent parameters i.e., penetration depth and which account for the decreasing contribution of the -band with increasing field. This approach perfectly reproduces the experimental data by combining the field dependence of deduced from Cp 1/ 2 B with an almost linear increase of from 450 Å at low field to 700 Å close to Hc2. These field dependences can then be used to consistently describe the field dependence of the critical current density, small angle neutron scattering form factor, and muon spin relaxation rate

Phase diagram of boron-doped diamond revisited by thickness-dependent transport studies

International audienceWe report on a detailed study of the electronic properties of a series of boron-doped diamond epilayers with dopant concentrations ranging from 1.10^ 20 to 3.10^21 cm −3 and thicknesses (d ⊥) ranging from 2 µm to 8 nm. By using well-defined mesa patterns that minimize the parasitic currents induced by doping inhomogeneities, we have been able to unveil a new phase diagram differing from all previous reports. We show that the onset of superconductivity does actually not coincide with the metal-insulator transition in this system. Moreover a dimensional crossover from 3D to 2D transport properties could be induced by reducing d ⊥ in both the metallic non-superconducting and superconducting epilayers, without any reduction of Tc with d ⊥ in the latter

Doping-induced metal-insulator transition in aluminum-doped 4H silicon carbide

International audienceWe report an experimental determination of the doping-induced metal-insulator transition in aluminum-doped 4H silicon carbide. Low temperature transport measurements down to 360 mK and temperature dependent Raman experiments down to 5 K, together with secondary ion mass spectroscopy profiling, suggest a critical aluminum concentration lying between 6.4 and 8.7 1020 cm−3 for the metal-insulator transition in these epilayers grown by the vapor-liquid-solid technique. Preliminary indications of a superconducting transition in the metallic sample are presented

Silicon Superconducting Quantum Interference Device

We have studied a Superconducting Quantum Interference SQUID device made from a single layer thin film of superconducting silicon. The superconducting layer is obtained by heavily doping a silicon wafer with boron atoms using the Gas Immersion Laser Doping (GILD) technique. The SQUID device is composed of two nano-bridges (Dayem bridges) in a loop and shows magnetic flux modulation at low temperature and low magnetic field. The overall behavior shows very good agreement with numerical simulations based on the Ginzburg-Landau equations.Comment: Published in Applied Physics Letters (August 2015

Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals

International audienceWe report on the Nernst effect, specific heat and transportmeasurements performed in high quality (K,Ba)BiO3 single crystals close to optimal doping (Tc ∼ 31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than kBT . Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates