Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB2 with the transition temperature Tc = 39 K. By means of Andreev reflection evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps (∆S = 2.8 meV and ∆L = 7 meV) are respectively smaller and larger than the expected weak coupling value. Due to 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 Tc in the raw data.PACS numbers: 74.50.+r, 74.60.Ec, Two decades of the boom in the field of superconductivity has recently been boosted by the surprising discovery of superconductivity in MgB 2 [1]. In contrast to the cuprates, the first tunneling [2][3][4] and point-contact [5,6] spectroscopy measurements have unequivocally shown that this system is a s-wave superconductor and isotope effects [7,8] have pointed towards a phonon mechanism. However, the size of the superconducting energy gap has remained unclear. We report here on experimental support for the multiband model of superconductivity recently proposed by Liu et al. [9] thus showing that MgB 2 belongs to an original class of superconductors in which two distinct 2D and 3D Fermi surfaces contribute to superconductivity. Indeed, our point-contact spectroscopy experiments clearly show the existence of two distinct superconducting gaps with ∆ S (0) = 2.8 meV and ∆ L (0) = 7 meV. Both gaps close near to the bulk transition temperature T c = 39 K. Our measurements in magnetic field show directly in the raw data the presence of two superconducting gaps at all temperatures up to the same bulk transition T c indicating that the two gaps are inherent to the superconductivity in MgB 2 .Although quite scattered, the first spectroscopy measurements [2-6,10] yielded to superconducting gap values surprisingly smaller than the BCS weak coupling limit 2∆/kT c = 3.52. Moreover simultaneous topographic imaging and quasiparticle density of states mapping [11] revealed substantial inhomogeneities at the surface of the sample as well as a large scattering of the energy gap values measured at different parts of the polycrystalline sample (with ∆ ranging from 3 to 7.5 meV). This energy gap distribution can be caused by sample inhomogeneities. However, Giubileo et al. also observed a superposition of two gaps (∆ S (0) = 3.9 meV and ∆ L (0) = 7.5 meV) in some of their local tunneling spectra. The same inhomogeneity argument could of course also explain such a superposition but a much more attractive scenario would be a two-gap model. Such a model has been first developed by Suhl et al. [12] in the case of overlapping s-an d-bands in conventional superconductors (such as V, Nb, Ta). Experimental evidence for the existence of two band superconductivity was obtained by tunneling spectroscopy in Nb-doped SrTiO 3 [13]. A similar model has been recently proposed by Liu et al. for MgB 2 . It ...

The three central phenomena of cuprate superconductors are linked by a common doping p*, where the enigmatic pseudogap phase ends, around which the superconducting phase forms a dome, and at which the resistivity exhibits an anomalous linear dependence on temperature as T → 0 (ref. 1). However, the

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of crystalline order. The resulting state of matter has been a long-standing puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into 'crystallites'. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The 'lattice' of vortices that contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron-diffraction data of the vortex lattice provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transitions and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover, the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and the conductivity of electronic crystals.

We 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 (T C ), 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. 1) IntroductionIt was probably the discovery of a superconducting transition around 40 K in the rather simple MgB 2 compound [1] that revived the interest for a specific class of superconducting materials, belonging to the so-called covalent metals [2] . These superconducting covalent systems (see box 1), including B-doped diamond [3] , silicon [4] , and silicon carbide [5,6] , Ba-doped silicon clathrates [7,8] , alkali-doped fullerenes [9,10] and the CaC 6 or YbC 6 intercalated graphites [11,12] , share the specificity of involving at least one relatively light element and of preserving strongly directional covalent bonds in their metallic state. The implications of this covalent character are important in superconductors in which Cooper pairs are coupled through phonons. The use of perturbation theory to study the renormalisation of the electron-electron repulsion by the electron-phonon interaction leads to the so-called Eliashberg equations [13] and to the celebrated McMillan formula [13] relating, in an approximate way, the superconducting transition temperature T C to an average phonon frequency ω ln , the electron-phonon coupling parameter λ ep and the screened and retarded Coulomb repulsion parameter µ*: Clearly, low atomic masses lead to high frequency phonon modes, which may enhance the ω ln prefactor and thus T C . This is the basis of the so-called isotope effect. Furthermore, strong covalent bonding will lead both to large phonon frequencies and a large electron-phonon coupling potential V ep =λ ep /N(E F ), with N(E F ) the density of states at the Fermi level, also contributing to enhance T C . Even within a phonon-mediated coupling scenario, these criteria do not necessarily warrant a large T C since increasing λ ep may also lead to a lattice instability, and a large electron-phonon potential V ep may be impaired by a low density N(E F ). However, these simple considerations, as well as more elaborate surveys and predictions of a larger T C [14][15][16] , have provided much incentive to study this class of materials.The most familiar covalent systems are certainly diamond and silicon. The former can be considered as the prototype insula...

We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the ͕100͖ direction. The system presents a metal-insulator transition ͑MIT͒ for a boron concentration ͑n B ͒ on the order of n c ϳ 4.5ϫ 10 20 cm −3 , in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for n B Ͻ n c with a characteristic hopping temperature T 0 strongly reduced due to the proximity of the MIT. All metallic samples ͑i.e., for n B Ͼ n c ͒ present a superconducting transition at low temperature. The zero-temperature conductivity 0 deduced from fits to the data above the critical temperature ͑T c ͒ using a classical quantum interference formula scales as 0 ϰ ͑n B / n c −1͒ with ϳ 1. Large T c values ͑ജ0.4 K͒ have been obtained for boron concentration down to n B / n c ϳ 1.1 and T c surprisingly mimics a ͑n B / n c −1͒ 1/2 law. Those high T c values can be explained by a slow decrease of the electron-phonon coupling parameter and a corresponding drop of the Coulomb pseudopotential * as n B → n c .

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Homoepitaxial diamond layers doped with boron in the 10(20)-10(21) cm(-3) range are shown to be type II superconductors with sharp transitions (approximately 0.2 K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5-7 10(20) cm(-3). The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.

Granular aluminum (grAl) is a promising high kinetic inductance material for detectors, amplifiers, and qubits. Here we model the grAl structure, consisting of pure aluminum grains separated by thin aluminum oxide barriers, as a network of Josephson junctions, and we calculate the dispersion relation and nonlinearity (self-Kerr and cross-Kerr coefficients). To experimentally study the electrodynamics of grAl thin films, we measure microwave resonators with open-boundary conditions and test the theoretical predictions in two limits. For low frequencies, we use standard microwave reflection measurements in a low-loss environment. The measured low-frequency modes are in agreement with our dispersion relation model, and we observe self-Kerr coefficients within an order of magnitude from our calculation starting from the grAl microstructure. Using a high-frequency setup, we measure the plasma frequency of the film around 70 GHz, in agreement with the analytical prediction.