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

Dependence of the superconducting transition temperature on the doping level in single crystalline diamond films.

Homoepitaxial diamond layers doped with boron in the 10^20-10^21 /cm3 range are shown to be type II superconductors with sharp transitions (~0.2K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity is about 5-7 10^20 /cm3, close to the metal-insulator transition. The H-T phase diagram has been obtained from transport and a.c. susceptibility measurements down to 300mK. These results bring new quantitative constraints on the theoretical models proposed for superconductivity in diamond

Unveiling the double-peak structure of quantum oscillations in the specific heat

Quantum oscillation phenomenon is an essential tool to understand the electronic structure of quantum matter. Here we report the first systematic study of quantum oscillations in the electronic specific heat $C_{el}$ in natural graphite. We show that the crossing of a single spin Landau level and the Fermi energy give rise to a double-peak structure, in striking contrast to the single peak expected from Lifshitz-Kosevich theory. Intriguingly, the double-peak structure is predicted by the kernel term for $C_{el}/T$ in the free electron theory. The $C_{el}/T$ represents a spectroscopic tuning fork of width 4.8 $k_B T$ which can be tuned at will to resonance. Using a coincidence method, the double-peak structure can be used to accurately determine the Lande $g$-factor of quantum materials. More generally, the tuning fork can be used to reveal any peak in fermionic density of states tuned by magnetic field, such as Lifshitz transition in heavy-fermion compounds.Comment: 22 pages, 5 figure

s-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi3 single crystals

International audienceThe magnetic penetration depth $\lambda$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $\delta\lambda$(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $\lambda(0)=$230$\,$nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease of the critical temperature at the sample surface

Strongly dissimilar vortex-liquid regimes in single-crystalline NdFeAs(O,F) and (Ba,K)Fe2As2: A comparative study

International audienceThe extent of the vortex-liquid state in underdoped single crystals of the oxypnictide superconductors NdFeAs(O,F) and (Ba,K)Fe2As2 is investigated using specific heat (C-p) and Hall-probe magnetization experiments. In both materials, the vortex liquid lies entirely in the regime where the three-dimensional lowest Landau-level (3D-LLL) approximation is valid and both systems present a very small shift in the specific heat anomaly with increasing field. The irreversibility line, defined as the onset of diamagnetic response, is very rapidly shifted toward lower temperatures in NdFeAs(O,F) but remains close to the C-p anomaly in (Ba,K)Fe2As2. These measurements strongly suggest that a vortex-liquid phase occupies a large portion of the mixed-state phase diagram of NdFeAs(O,F) but not in (Ba,K)Fe2As2. This difference can be attributed to different Ginzburg numbers Gi, the latter being about 100 times larger in NdFeAs(O,F) than in (Ba,K)Fe2As2. The angular dependence of the upper critical field, derived from 3D-LLL scaling of the irreversibility lines, presents deviations from the standard 3D effective-mass model in both materials with an anisotropy being about three times smaller in (Ba,K)Fe2As2 (gamma similar to 2.5) than in Nd(F,O)FeAs (gamma similar to 7.5)

Superconducting properties of laser annealed implanted Si:B epilayers

4 pagesInternational audienceWe 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

Anomalous Anisotropy in Superconducting Nanodiamond Films Induced by Crystallite Geometry

status: publishe

Bosonic Confinement and Coherence in Disordered Nanodiamond Arrays

In the presence of disorder, superconductivity exhibits short-range characteristics linked to localized Cooper pairs which are responsible for anomalous phase transitions and the emergence of quantum states such as the bosonic insulating state. Complementary to well-studied homogeneously disordered superconductors, superconductor-normal hybrid arrays provide tunable realizations of the degree of granular disorder for studying anomalous quantum phase transitions. Here, we investigate the superconductor-bosonic dirty metal transition in disordered nanodiamond arrays as a function of the dispersion of intergrain spacing, which ranges from angstroms to micrometers. By monitoring the evolved superconducting gaps and diminished coherence peaks in the single-quasiparticle density of states, we link the destruction of the superconducting state and the emergence of bosonic dirty metallic state to breaking of the global phase coherence and persistence of the localized Cooper pairs. The observed resistive bosonic phase transitions are well modeled using a series-parallel circuit in the framework of bosonic confinement and coherence.status: publishe

Superconducting Ferromagnetic Nanodiamond

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature Tc ∼ 3 K and a Curie temperature TCurie > 400 K. In spite of the high TCurie, our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.status: publishe