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Single photon emitters can form at the interfaces between layers of WSe2. Crystal structure imperfections in solids often act as efficient carrier trapping centres, which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are well-width or composition fluctuations in semiconductor heterostructures1,2 (resulting in the formation of quantum dots) and coloured centres in wide-bandgap materials such as diamond3,4,5. In the recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials exfoliated on alien substrates. Here, we report comprehensive optical micro-spectroscopy studies of thin layers of tungsten diselenide (WSe2), a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes (transferred onto Si/SiO2 substrates) we discover centres that, at low temperatures, give rise to sharp emission lines (100 μeV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to the two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and have very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities that are relevant for the domain of quantum optoelectronics.

International audience; We report size effects on the charge-density-wave pinning in the quasi-one-dimensional conductor K0.30MoO3 in the temperature range 77 K-160 K. The threshold field is approximately one order of magnitude larger in needle-like samples than in bulk crystals, temperature independent and strongly dependent on the width of the sample. In addition, the nonlinear conductivity shows a maximum near 130 K. The results are discussed in relation with various pinning models and CDW dislocations.

We have synthesized for the first time the metastable compound 1T-CrTe2. We have done its complete structural characterization and measured its magnetization, specific heat and electrical resistivity between 4 and 330 K. We have also performed detailed band structure calculations. We have found that it crystallizes in the CdI2 structure type and that its electrical resistance follows a metallic behaviour below room temperature. Its magnetization and specific heat curves show that the compound has a transition to a ferromagnetic state at TC = 310 K, with the magnetic moments ordered parallel to the basal plane. From the specific heat measurements and the ferromagnetic solutions obtained from our DFT calculations, we conclude that the ferromagnetism is of itinerant nature. Fil: Freitas, Daniele C.. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia. Centro Brasileiro de Pesquisas Fisicas; Brasil Fil: Weht, Ruben Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sulpice, André. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Remenyi, Gyorgy. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Strobel, Pierre. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia Fil: Gay, Frédéric. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Marcus, Jacques. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia Fil: Núñez-Regueiro, Manuel. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia

We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated., Comment: 12 pages, 5 figures

The crystallinity and built-in strain of diamond epilayers grown by MPCVD with boron contents n B varying from 2 x 10 20 to 2 x 10 21 cm -3 as evaluated by SIMS are assessed by HR-XR Diffraction. Both ac transport and ac susceptibility measurements show that the critical concentration for the onset of superconductivity in (001)-oriented single crystalline films is about 5 x 10 20 cm -3 . This is also the critical hole concentration for the metal-non metal transition in this system, as evaluated by two types of theoretical calculations and by low temperature resistivity experiments. We report also preliminary results of point contact spectroscopy on an epilayer with a critical transition temperature T c = 2.1 K, which yield a superconducting gap around 0.3 meV compatible with a conventional BCS-type description of superconductivity in this system.

We have investigated the effect of magnetic field on the low-temperature heat capacity C p of the quasi-1 D compound blue bronze Rb 0.3 MoO 3 , characterized by a charge density wave (CDW) ground state. Below 1 K, C p is dominated by a quasi-linear term, γ T , characteristic of the disordered state of the CDW. Under applied field (between 0 and 2 T), there is an increase of the amplitude γ with H , and simultaneously a downward deviation from the linear regime occurs at an increasing crossover temperature. This could be due to the opening of a (pseudo)-gap in the corresponding spectral density of excitations, which develops with increasing magnetic field.

Using magnetization, magneto-transport and single-crystal specific heat measurements we have determined the superconducting phase diagram of MgB 2 . The superconducting anisotropy γ changes monotonously from a value of around 2 near T c to above 4.5 at 22 K. For H||c a pronounced peak effect in the critical current occurs at the upper critical field. We present a strong evidence for a surface superconducting state for H||c which might account for the wide spread in reported values of the superconducting anisotropy γ.

Quasi-low-dimensional (quasi-low-D) inorganic materials are not only ideally suited for angle resolved photoemission spectroscopy (ARPES) but also they offer a rich ground for studying key concepts for the emerging paradigm of non-Fermi liquid (non-FL) physics. In this article, we discuss the ARPES technique applied to three quasi-low-D inorganic metals: a paradigm Fermi liquid (FL) material TiTe 2 , a well-known quasi-1D charge density wave (CDW) material K 0.3 MoO 3 and a quasi-1D non-CDW material Li 0.9 Mo 6 O 17 . With TiTe 2 , we establish that a many body theoretical interpretation of the ARPES line shape is possible. We also address the fundamental question of how to accurately determine the k F value from ARPES. Both K 0.3 MoO 3 and Li 0.9 Mo 6 O 17 show quasi-1D electronic structures with non-FL line shapes. A CDW gap opening is observed for K 0.3 MoO 3 , whereas no gap is observed for Li 0.9 Mo 6 O 17 . We show, however, that the standard CDW theory, even with strong fluctuations, is not sufficient to describe the non-FL line shapes of K 0.3 MoO 3 . We argue that a Luttinger liquid (LL) model is relevant for both bronzes, but also point out difficulties encountered in comparing data with theory. We interpret this situation to mean that a more complete and realistic theory is necessary to understand these data.

We have studied the resistance of $1T$-CrSe${}_{2}$, as the Cr atoms are substituted by V or Ti. The V replacement leads to a logarithmic increase in the resistance as the temperature is lowered, proportional to the V concentration. While this behavior is consistent with the Kondo effect, the weak dependence of the resistance with magnetic field and the fact that the system has antiferromagnetic order, rule out a Kondo effect due to spin degeneracy. In contrast to the case of V, Ti substitution does not increase the logarithmic term while application of pressure destroys it. Calculations of the electronic structure within the framework of density functional theory, maximally localized Wannier functions, and many-body calculations in a cluster containing a Cr or V atom and its six nearest-neighbor Se atoms, helped to reveal the existence of an orbital Kondo effect due to orbital degeneracy in the V substitutional impurities.

In the blue bronzes A 0.30 MoO 3 ( A = K , Rb ), two regimes for the CDW nonlinear conductivity have been identified: strongly damped CDW motion above ∼ 50 K and CDW motion with almost no damping below ∼ 50 K . In a search for an elastic signature of this CDW behavior, we performed ultrasonic measurements on Rb 0.30 MoO 3 single crystals in the temperature range 4–300 K. Upon cooling, we have observed at ∼ 50 K a large increase of the sound velocity for the longitudinal mode measured along the [ - 2 0 1 ] , [1 0 2] and b directions as well as for the shear mode along the [ - 2 0 1 ] direction. In addition, a maximum in the ultrasonic attenuation is observed around 50 K. The results are discussed in relation with the changes in the CDW rigidity and in the dielectric response.

We present ac susceptibility measurements performed on (K, Ba)BiO3single crystals with different geometries: thick films, bars and hollow cylinders. We show that the hac dependence of the real (χ′) and imaginary (χ′) parts of the ac susceptibility is in very good agreement with Brandt’s numerical calculations (Phys. Rev. B 58, 6523 (1998)) in the modified Bean critical state. Creep effects (at Hdc = 0.3T) are investigated by studying the frequency dependence of the current density deduced from the temperature scans of the ac susceptibility over a large frequency range (0.02 Hz < ω < 2000 HZ). The relaxation rate S = dln(J)/dln(ω) ∼ 4% is temperature independent and very similar to the one usually obtained in high Tc cuprates.

Small Angle Neutron Scattering has been used to observe diffraction from the flux line lattice in the fully isotropic (K, Ba)BiO3high Tcsuperconductor. We show that the flux lines are ordered into a hexagonal lattice at low field (H < He∼ 0.6T ≪ Hg (0) where Hgis the second order vortex glass transition field). The diffracted intensity continuously drops to zero as the field is increased towards He. Those observations are in good agreement with recent theoritical works which suggested that the formation of dislocations becomes favorable above Heleading to a glassy structure. The field Helies close to the onset of the second peak in magnetization measurements indicating that this peak is related to the change in the vortex structure.

Elastic tunneling is used as a powerful direct tool to determine the upper critical field $H_{c2}(T)$ in the high-$T_c$ oxide Ba$_{1-x}$K$_x$BiO$_3$. The temperature dependence of $H_{c2}$ inferred from the tunneling follows the Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A comparison will be made with resistively determined critical field data.

The quasi-one-dimensional charge-density-wave (CDW) compounds , the blue bronze and the platinum chain compound KCP(Br) show a strong deviation from a Debye behaviour of the specific heat at temperatures of several degrees Kelvin. To determine the origin of this specific heat `anomaly' in the case of , we have performed time-of-flight neutron scattering measurements and extracted a generalized phonon density of states (PDOS). The excess over a Debye behaviour at low energies in the generalized PDOS lies at the origin of the observed anomalous specific heat. We have used a simplified dispersion model based on experimental phonon dispersion curves to calculate the low-energy PDOS of the system. The model succeeds in reproducing the observed specific heat anomaly and establishes its lattice phonon origin. Similar calculations of the low-energy PDOS based on available phonon dispersion data for KCP allow us to state that the anomalous behaviour of the specific heat for this compound also has a phonon origin.

The Fermi surfaces and single-particle spectral functions of several low-dimensional materials have been measured as part of an effort to assess the occurrence of non-Fermi-liquid behaviour in non-cuprate materials.

The antiferromagnetic order and structural distortion in the LaFe(As1-xSbx)O system have been investigated by powder neutron diffraction and physical properties measurements. Polycrystalline samples of LaFe(As1-xSbx)O (x, 21 pages, 6 figures

We 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.

The charge-density-wave phase transition of 1T-TiSe2 is studied by angle-resolved photoemission over a wide temperature range. An important chemical-potential shift which strongly evolves with temperature is evidenced. In the framework of the exciton condensate phase, the detailed temperature dependence of the associated order parameter is extracted. Having a mean-field-like behavior at low temperature, it exhibits a nonzero value above the transition, interpreted as the signature of strong excitonic fluctuations, reminiscent of the pseudogap phase of high-temperature superconductors. Integrated intensity around the Fermi level is found to display a trend similar to the measured resistivity and is discussed within the model.

Investigation of relative AC magnetic susceptibility interests for many magnetic transition studies such as superconductor transition. A technique based on mutual or self inductive measure provides a fast and relatively easy (no contact) way to determinate the temperature of any transition affecting the magnetic susceptibility. The half Wheatstone inductive/resistive bridge is used instead of the usual RLC quarter bridge in order to balance the bore inductance of the coil. A comparison between quarter and half bridge measurements illustrates the accuracy of our device.

As shown recently [Proslier et al., Europhys. Lett. 73, 962 (2006)], it is possible to map the superconducting (SC) condensate by measuring locally the Josephson tunneling current. We apply this technique to image the vortex lattice in ${\text{V}}_{3}\text{Si}$ which is used as the simplest example of a spatially varying quantum condensate. The Josephson scanning tunneling microscope (JSTM) maps revealed the vortex lattice with the Josephson effect being present outside the vortices and disappearing progressively toward the vortex core. The characteristic length scales of a vortex observed in the Josephson regime are compared to the ones obtained in the quasiparticle regime. We demonstrate that the JSTM allows a nanometer scale resolution of the SC condensate that may be applied to inhomogeneous phases such as high-${T}_{c}$ superconductors.

High-resolution coherent x-ray diffraction experiment has been performed on the charge density wave (CDW) system K$_{0.3}$MoO$_3$. The $2k_F$ satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the $2k_F$ satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long range order is 1500 times larger than the CDW period itself. This new type of electronic correlation seems inherent to the collective dynamics of electrons in charge density wave systems. Several scenarios are discussed., 4 pages, 3 figures Typos added, references removed

The angular and field dependence of the Sommerfeld coefficient $\ensuremath{\gamma}={\phantom{\ensuremath{\mid}}\mathrm{lim}\phantom{\rule{0.2em}{0ex}}{C}_{\mathit{el}}∕T\ensuremath{\mid}}_{T\ensuremath{\rightarrow}0}$ (${C}_{\mathit{el}}$ being the electronic contribution to the specific heat) has been measured in Al-doped $\mathrm{Mg}{\mathrm{B}}_{2}$ single crystals (for $x=0$, $x=0.1$, and $x\ensuremath{\sim}0.2$). We show that the decomposition previously introduced to describe $\ensuremath{\gamma}(H,\ensuremath{\theta})$ (where $\ensuremath{\theta}$ is the angle between the applied field and the $c$ axis) in pure samples [Phys. Rev. Lett. 98, 137001 (2007)] is well adapted to doped samples: e.g., the contribution of the $\ensuremath{\sigma}$ band to the specific heat is proportional to $B∕{B}_{c2}(\ensuremath{\theta})$ whereas the contribution of the $\ensuremath{\pi}$ band is isotropic but highly nonlinear in field. We hence present the evolution of the coherence lengths of the two bands and corresponding Fermi velocities with doping.

The two-band and/or two-gap superconductivity in aluminum- and carbon-doped $\mathrm{Mg}{\mathrm{B}}_{2}$ has been addressed by the point-contact spectroscopy. Two gaps are preserved in all samples with ${T}_{c}^{\ensuremath{'}}s$ down to $22\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The evolution of two gaps as a function of the critical temperature in the doped systems suggests the dominance of the band-filling effects, but for the increased Al doping, the enhanced interband scattering must also be considered. The extraction of the zero-energy density of states for the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ bands using a simple physical model demonstrates that significant changes in the relative weighting of the intraband scattering are created in the case of the C doping with the scattering in the $\ensuremath{\pi}$ band enhanced faster than in the $\ensuremath{\sigma}$ one. The Al doping does not change the relative weight of the scatterings within the bands.

We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the {100} direction. The system presents a metal-insulator transition (MIT) for a boron concentration (nB) on the order of nc~4.5×1020 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 nBnc) present a superconducting transition at low temperature. The zero-temperature conductivity 0 deduced from fits to the data above the critical temperature (Tc) using a classical quantum interference formula scales as 0(nB/nc−1) with ~1. Large Tc values (0.4 K) have been obtained for boron concentration down to nB/nc~1.1 and Tc surprisingly mimics a (nB/nc−1)1/2 law. Those high Tc values can be explained by a slow decrease of the electron-phonon coupling parameter and a corresponding drop of the Coulomb pseudopotential µ* as nBnc. Original Publication:T. Klein, P. Achatz, J. Kacmarcik, C. Marcenat, F. Gustafsson, J. Marcus, E. Bustarret, J. Pernot, F. Omnes, Bo Sernelius, C. Persson, A. Ferreira da Silva and C. Cytermann, Metal-insulator transition and superconductivity in boron-doped diamond, 2007, Physical Review B. Condensed Matter and Materials Physics, (75), 165313.http://dx.doi.org/10.1103/PhysRevB.75.165313Copyright: American Physical Societyhttp://www.aps.org/

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B--0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis) and gammapi(B,theta)=gammapi(B)=B/Bpi(B). The "critical field" of the pi band Bpi is fully isotropic but field dependent increasing from approximately 0.25 T for Bor =0.1 T up to 3 T approximately Bc2c for B--3 T. Because of the coupling of the two bands, superconductivity survives in the pi band up to 3 T but is totally destroyed above for any orientation of the field.

International audience; The point-contact spectroscopy is used to address the evolution of two superconducting energy gaps in the Al- and C-doped magnesium diboride polycrystals and single crystals with Tc's from 39 to 22 K prepared by different techniques. The obtained evolution of two gaps does not show any anomalous behavior but can be consistently described by the combination of the (prevailing) band filling effect and a (minor) increased interband scattering as proposed by Kortus et al. [Kortus et al., Phys. Rev. Lett. 94 (2005) 027002]. The approaching of two gaps is stronger in the Al-doped systems but interband scattering is still not large enough to merge two gaps. The full merging can expected only for higher dopings with Tc's below 10-15 K. In-magnetic-field measurements are used to analyze the intraband scatterings introduced by these two substitutions. It is shown that the carbon doping introduces significant disorder mainly by decreasing the diffusion coefficient in the p band while the Al substitution leaves the samples in the clean limit.

The lower $({H}_{c1})$ and upper $({H}_{c2})$ critical fields of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ single crystals (for $x=0$, $0.1$, and $\ensuremath{\gtrsim}0.2$) have been deduced from local magnetization and specific heat measurements, respectively. We show that ${H}_{c1}$ and ${H}_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter ${\ensuremath{\Gamma}}_{{H}_{c2}}(0)={H}_{c2}^{ab}(0)∕{H}_{c2}^{c}(0)$ value also decreases from $\ensuremath{\sim}5$ in pure ${\mathrm{MgB}}_{2}$ samples down to $\ensuremath{\sim}1.5$ for $x\ensuremath{\gtrsim}0.2$ whereas ${\ensuremath{\Gamma}}_{{H}_{c1}}(0)={H}_{c1}^{c}(0)∕{H}_{c1}^{ab}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero-field electronic contribution to the specific heat to the two-gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.

Using low-temperature scanning tunneling microscopy under ultrahigh vacuum, high-resolution topographical images were obtained on a cleaved $(\overline{2}01)$ surface of a rubidium blue bronze $({\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_{3})$ single crystal. Only molecular lattice was observed at room temperature. At temperatures of 63 and $78\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, well below the charge density wave (CDW) transition temperature in ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_{3}$, underlying molecular lattice and CDW superlattice were observed simultaneously in topographical constant current images. The amplitude of the CDW modulation is about $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ along $\mathbf{b}$. On the Fourier transform of these images, both main Bragg spots (molecular lattice related) and satellite spots (CDW superlattice related) coexist unambiguously. The average projection of the CDW wave vector on the $(\overline{2}01)$ surface deduced by Fourier transform is consistent with the bulk value obtained previously by other techniques. Our results show clearly that the Peierls phase of the quasi-one-dimensional blue bronze can be studied by STM.

Coherent x-ray diffraction experiments have been performed on high quality crystals of the charge density wave (CDW) system K0.3MoO3. The satellite reflections associated with the CDW have been measured as a function of the 20-microm-diameter beam position. For some positions, regular fringes have been observed. We show that this observation is consistent with the presence of a single CDW dislocation. Beyond charge density wave systems, this experiment shows that coherent x-ray diffraction is a suitable tool to probe topological defects embedded in the bulk.

It is shown that the large effect of heavy ion-irradiation on the thermodynamical properties of the anisotropic superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic K$_{x}$Ba$_{1-x}$BiO$_{3}$ ($x \simeq 0.35$). In both compounds, vortex pinning manifests itself as a sharp angular dependence of the \em equilibrium \rm torque. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, pinning by the defects appears at the temperature $T_{C_{p}}^{max}$ of the specific heat maximum, well above the magnetic irreversibility line $T_{irr}(H)$. In isotropic K$_{x}$Ba$_{1-x}$BiO$_{3}$, the onset of the pinning-related torque anomaly tracks the onset of the specific heat anomaly and the irreversibility line. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, fluctuations of the amplitude of the order parameter (and not vortex line wandering) are ultimately responsible for the vanishing of pinning. In K$_{x}$Ba$_{1-x}$BiO$_{3}$, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the ``Bose glass'' transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered., Comment: 9 pages, 9 figures, resubmitted to PRB 23-09-2005

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.

Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap superconductivity was given with an inverted junction setup, in which a small crystal of MgB 2 was used as the STM tip and 2H-NbSe 2 as the sample. This technique allowed to show that both gaps close at the critical temperature of the bulk material and thus are intimately related to the superconductivity. The experiments performed in the standard N-I-S geometry evidenced for two strongly coupled gaps Δ L =7.0±1.0 meV and Δ S =3.0±1.0 meV at 4.2 K. STS on as-grown c -axis oriented thin films yielded only small gap which confirmed the identification of this gap as originating from 3D-like π-band and, by exclusion, that of the large gap from 2D-like σ-band. The low gap values Δ S =2.2±0.3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to Δ S =2.8±0.3 meV. The c -axis tunneling spectra are better fitted considering anisotropic superconductivity inside the π-band. The issues of our findings are discussed in terms of two-band superconductivity.

The superconducting phase diagram of ${\mathrm{MgB}}_{2}$ was determined from magnetization, magnetotransport, and single-crystal specific-heat measurements. A zero-temperature in-plane coherence length of 9.4 nm is determined. The superconducting anisotropy \ensuremath{\gamma} increases from a value around 2 near ${T}_{c}$ to above 4.5 at 22 K. For $H\ensuremath{\parallel}c$ a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for $H\ensuremath{\parallel}c$ that might account for the widespread in reported values of \ensuremath{\gamma}.

International audience; We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba8@Si–46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I8@Si–46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp3 silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality K, Ba BiO3 single crystals. The temperature dependence of the magnetic field HCp corresponding to the onset of the specific heat anomaly presents a clear positive curvature. HCp is significantly smaller than the field HD for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization presents a strong deviation from the Ginzburg-Landau theory emphasizing the peculiar nature of the superconducting transition in this material.

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound ${\mathrm{MgB}}_{2}$ directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of ${\mathrm{MgB}}_{2}$ is used as a tip in the STM experiment. The ``sample'' is chosen to be a 2H- ${\mathrm{NbSe}}_{2}$ single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages ${V}_{S}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}3.8\mathrm{mV}$ and ${V}_{L}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}7.8\mathrm{mV}$. With rising temperature both peaks disappear at the ${T}_{C}$ of the bulk ${\mathrm{MgB}}_{2}$, a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

Magnetotransport measurements are presented on polycrystalline MgB2 samples. The resistive upper critical magnetic field reveals a temperature dependence with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper critical field is much higher than what is known so far on polycrystals of MgB2 but it is in agreement with recent data obtained on epitaxial MgB2 films. The deviation of Bc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit., 6 pages, incl. 3 figures

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 (DS 2.8 meV and DL 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 Tc in the raw data.

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 the crystalline order. The resulting state of matter has been a longstanding 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 can 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 in type II superconductors 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 transition 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 conductivity of electronic crystals., 9 pages, 4 figures

We report on the evolution of the small and large gap values in Mg 1− x Al x B 2 single crystals (for x = 0, 0.1 and ∼0.2) by specific heat and point contact spectroscopy. The values deduced from the temperature dependence of the electronic contribution to the specific heat are very close to those obtained by point contact spectroscopy. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present and may lead to a merging of the two gaps for T c ∼ 10–15 K.

We report Luttinger liquid line shapes better revealed by new angle resolved photoemission data taken with a much improved angle resolution on a quasi-1-dimensional metal Li$_{0.9}$Mo$_6$O$_{17}$. The new data indicate a larger spinon velocity than our previous lower resolution data indicated., submitted to SCES '01

International audience; We present angle-resolved photoemission experiments on 1T-TiSe(2) at temperatures ranging from 13K to 288 K. The data evidence a dramatic renormalization of the conduction band below 100 K, whose origin can be related to the new potential responsible for the charge density wave phase at low temperature in this system. The renormalization translates into a substantial effective mass reduction of the dominant charge carriers and this observation is thus in opposition to the common belief that strong interactions produce heavier quasiparticles through an increased effective mass

The charge density wave (CDW) nonlinear conductivity of the blue bronzes A(0.30)MoO(3) (A = K, Rb) shows two different regimes depending on the temperature: a strongly damped CDW motion above ∼50 K and a CDW motion with almost no damping below ∼50 K. In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurements on A(0.30)MoO(3) single crystals in the temperature range 4-300 K. In Rb(0.30)MoO(3), at T∼50 K, upon cooling, a large increase of the sound velocity for the longitudinal mode measured along the [Formula: see text], [102] and b directions is observed. The ultrasonic attenuation coefficient shows an increase down to 50 K followed by a plateau. Similar results are found in K(0.30)MoO(3). In V-doped samples, Rb(0.30)(Mo(1-x)V(x))O(3) (x = 0.4%) the anomaly broadens and is shifted towards higher temperatures. The results are discussed in relation to the changes in the CDW rigidity, disorder and dielectric response. A scenario based on a glass transition for the CDW superstructure is proposed.

The magnetic transition of the isostructural single crystals of the (RE) 0.08 MoO 3 series (RE  La, Ce, Gd) has been studied using heat capacity measurements and magnetic susceptibility data between 1.8 and 300 K. The transition was found to be of the same type whatever the rare earth, magnetic or not.

To discuss the nature of superconductivity in the cubic Ba1−xKxBiO3 it is important to understand the role of phonons and their interaction with conduction electrons in both insulating and superconducting phases. In order to provide information on the electron-phonon coupling we performed Raman experiments as a function of temperature in single crystalline samples. The high quality of the spectra allowed us to fit most lines to either Lorentzian or Fano lineshapes and study the dependence of the parameters as function of temperature.

The superconducting energy gap of $\rm Ba_{1-x}K_xBiO_3$ has been measured by tunneling. Despite the fact that the sample was macroscopically single phase with very sharp superconducting transition $T_c$ at 32~$K$, some of the measured tunnel junctions made by point contacts between silver tip and single crystal of $\rm Ba_{1-x}K_xBiO_3$ had lower transition at 20~$K$. Local variation of the potassium concentration as well as oxygen deficiency in $\rm Ba_{1-x}K_xBiO_3$ at the place where the point contact is made can account for the change of $T_c$. The conductance curves of the tunnel junctions reveal the BCS behavior with a small broadening of the superconducting-gap structure. A value of the energy gap scales with $T_c$. The reduced gap amounts to $2\Delta/kT_c = 4\div 4.3$ indicating a medium coupling strength. Temperature dependence of the energy gap follows the BCS prediction., Comment: 2 pages, 3 figures

The tunneling effect in the superconducting Ba 1−x K x BiO 3 , where x=0.4, has been measured in a magnetic field up to 20T. In the BCS-like tunneling conductance we have observed the pair-breaking effect of the field leading to a decrease of the superconducting energy gap. In the applied magnetic field the observed phonon-like anomalies in the tunneling spectra are decreased and smeared out together with the energy-gap structure, pointing to the phonon mediated pairing mechanism for the superconductivity in the system.

Simultaneous magnetotransport and torque magnetization measurements have been performed on superconducting (B, Ka)BiO 3 single crystals. The measurements have been carried out at very low temperatures (0.03 k < T < 2 K) and up to high magnetic fields: (30Tesla ~ Hc2(0)). The resistivity drops below the experimental resolution for a magnetic field HR(T) which is very close to the irreversibility line deduced from torque magnetometry. Close to HR(H) the resistive transition can be well by the vortex-glass scaling formalism1 suggesting that the vortex solid melts into a liquid for H > HR. Our measurements show that the irreversibility line in (K, Ba)BiO3 is closely related to this vortex glass transition line but, surprisingly, a large reversible (i.e. liquid) phase can still be observed at very low temperatures (30 m K ).

Nonlinear electrical conductivity associated with a large noise voltage has been observed in the semiconducting charge-density-wave state of the blue bronze ${\mathrm{K}}_{0.30}$Mo${\mathrm{O}}_{3}$. A well defined and strongly temperature-dependent threshold field for the onset of the nonlinear conductivity is found. Both broadband noise and quasiperiodic noise are detected above the treshold field. These results, including the observation of long relaxation times, are discussed in relation with the current models for charge-density-wave transport.