We report on specific heat and magnetotransport measurements performed on superconducting Cu10%TiSe2 single crystals. We show that superconductivity persists in transport measurements up to magnetic fields H-R well above the upper critical field H-c2 deduced from the calorimetric measurements. Surprisingly this "surface" superconductivity is present for all magnetic field orientations, either parallel or perpendicular to the layers. For H parallel to ab, the temperature dependence of the H-R/H-c2 ratio can be well reproduced by solving the Ginzburg-Landau equations in presence of a surface layer with reduced superconducting properties. Unexpectedly this temperature dependence does not depend on the field orientation. (C) 2016 Elsevier B.V. All rights reserved.

We study the Cr1-xRex phase diagram finding that its phase transition temperature towards an antiferromagnetic order T-N follows a quantum [(x(c) - x)/x(c)](psi) law, with psi = 1/2, from the quantum critical point (QCP) at x(c) = 0.25 up to T-N approximate to 600 K. We compare this system to others in order to understand why this elemental material is affected by the QCP up to such unusually high temperatures. We determine a general criterion for the crossover, as a function of an external parameter such as concentration, from the region controlled solely by thermal fluctuations to that where quantum effects become observable. The properties of materials with low coherence lengths will thus be altered far away from the QCP.

Good understanding of how properties can be tuned by changing the size of material is a basic prerequisite for production of new materials with designed superior properties. Systems with charge density wave (CDW) as a type of coupled electronic-lattice instability (usually found in low dimensional materials) are especially interesting due to their exceptional properties such as giant dielectric constant, nonlinear transport, memory effects, unusual electro-mechanical and thermoelectric properties, all of conceptual importance in various thin film applications. On the other hand, CDW films open the door for studying of meso- and micro-scale aspects of CDW physics caused by finite size effects. Our previously produced thin granular films of CDW prototype system K0.3MoO3 (blue bronze) did not show evidence of CDW condensation in the electrical transport measurements, and femtosecond time-resolved spectroscopy was established as the most appropriate method for characterization of CDW ground state in those nanocrystalline grains. However, the new films prepared by improved pulsed laser deposition (PLD) set-up in optimal conditions and characterized by various standard methods such as GI-XRD, electric transport, TOF-ERDA, AFM and UV–vis spectroscopy exhibit better properties. Electrical resistance decreased by three orders of magnitude and an indication of the Peierls transition is found in films with the best texture, which means that we have achieved the first requirement for testing of other interesting CDW properties related to the size effect (and possible applications of these blue bronze films).

We have performed electrical resistivity measurements under pressures up to 20GPa between 1 and 300K on monocrystalline hexagonal Rb0.19WO3. For pressures lower than ∼5GPa, we observe a decrease of the metallic-like resistivity at room temperature as well as a small decrease of Tc. At this pressure, the resistivity starts to increase slowly up to 10GPa accompanied by a sharper decrease of Tc.The resistivity curves above 10GPa denote an activated behaviour and a Tc lower than 3K indicating that there is a phase transition that takes place gradually between 5 and 10GPa. We interpret our measurements as the result of structural transformations under high pressure.

We have investigated nonlinear characteristics and broad band noise in the quasi-one dimensional charge-density-wave conductors K0.30MoO3 and K0.30Mo(1-x)WxO3 (x=0.001 and x=0.002) When the amplitude of a bipolar rectangular voltage pulse excitation is modulated by adding a white noise perturbation, the average pulse amplitude is smaller than the unperturbed one and shows a non monotonous behaviour when the pulse amplitude is increased. Our findings suggest that coupling between internal broad band noise and external white noise may lead to an increase of phase coherence of the charge-density-wave.

We have investigated the combined effects of a multiplicative noise and nonlinear conductivity on the charge density wave (CDW) dynamics of the quasi-one dimensional conductor Rb0.30MoO3. When the amplitude of a bipolar rectangular voltage pulse excitation is modulated by a multiplicative Gaussian white noise perturbation, the average pulse amplitude is smaller than the unperturbed one and shows a non monotonous behaviour when the pulse amplitude is increased. We analyze the transient response in terms of an increase of phase coherence of the charge-density-wave.

Thin granular films of charge density wave (CDW) system K0.3MoO3 were prepared by pulsed laser deposition and investigated by various standard characterization methods such as GI-XRD, electric transport, TOF-ERDA, AFM and UV–visible spectroscopy. While all these methods indicate that the thin films consist of nanometer grains of K0.3MoO3, it is only the non-destructive femtosecond time-resolved spectroscopy (fsTRS) that demonstrates the charge density wave nature of the ground state and therefore proves directly the presence of K0.3MoO3. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films with respect to single crystals, which is attributed to the granularity and crystal growth morphology. Our results establish fsTRS technique as the essential tool for the detection and characterization of complex ground states in nano-sized systems.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals Thierry Klein, Z. Pribulova, Raoul Piquerel, Hervé Cercellier, Jacques Marcus, C. Marcenat

Stochastic resonance (SR) in nonlinear systems is a counterintuitive concept in which a weak periodic signal and noise cooperate and give rise to a maximum in the signal-to-noise ratio at the output of the system when the noise is tuned to a certain value. The spatial coupling of a large number of oscillators showing SR may have a constructive effect and leads to the so-called array enhanced stochastic resonance (AESR). We discuss the possible application of SR and AESR concepts to charge density wave (CDW) dynamics in quasi-one-dimensional conductors. We show in a preliminary experiment that the addition of noise can modify the behavior of the CDW in the quasi-one-dimensional conductor K 0.30 MoO 3 .

The antiferromagnetic (AFM) order and structural distortion in the LaFe(As${}_{1\ensuremath{-}\mathrm{x}}$Sb${}_{\mathrm{x}}$)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As${}_{1\ensuremath{-}\mathrm{x}}$Sb${}_{\mathrm{x}}$)O ($x$ 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed.

Two key parameters were analyzed in Si/SiO/Pt/Co/AlOx: the oxidation time of the Al layer resulting in AlOx, and the ex situ annealing temperatures varied in the 15 and 55 s and 20, 300, and 450 °C ranges, respectively. For intermediate annealing temperatures (∼300 °C), the quantitative analysis of specular reflectometry data shows that the progressive oxidation of layers by increasing the oxidation time goes along with an improvement of the homogeneity of the alumina layer. This outcome casts new light on the temperature dependence of magnetic properties of the samples. The remarkable temperature variation of the coercive field, extracted from extraordinary Hall effects in the 5–300 K range, is associated with structural change due to Co–oxygen bondings, which leads to strong pinning of Co spins in the low temperature regime.

We present the synthesis and the tunneling spectroscopy study of superconducting FeSe0.5Te0.5 (Tc = 14 K), SmFeAsO0.85 (Tc = 54 K) and SmFeAsO0.9F0.1 (Tc = 45 K). The samples were characterized by Rietveld refinement of x-ray diffraction patterns and transport as well as temperature-dependent magnetic measurements. Tunneling experiments on FeSe0.5Te0.5 revealed a single superconducting gap ∼ 1 meV in BCS-like tunneling conductance spectra. In SmFeAsO0.85 and SmFeAsO0.9F0.1, however, more complex spectra were observed, characterized by two gap-like structures at ∼ 4 and ∼ 10 meV. These spectra are qualitatively understood assuming a two-band superconductor with a 's ±' order parameter. We show that, depending on the sign relation between the pairing amplitudes in the two bands, the interband quasiparticle scattering has a crucial effect on the shape of the tunneling spectra. On the other hand, single-gap spectra found in FeSe0.5Te0.5 are more compatible with a disorder-induced 's '-wave gap, due to the Se–Te substitution.

We present angle-resolved photoemission experiments on 1T-TiSe2 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. Copyright (C) EPLA, 2010

We present recent results obtained using angle-resolved photoemission spectroscopy performed on 1T-TiSe2. Emphasis is put on the peculiarity of the bandstructure of TiSe2 compared to other transition metal dichalcogenides, which suggests that this system is an excellent candidate for the realization of the excitonic insulator phase. This exotic phase is discussed in relation to the BCS theory, and its spectroscopic signature is computed via a model adapted to the particular bandstructure of 1T-TiSe2. A comparison between photoemission intensity maps calculated with the spectral function derived for this model and experimental results is shown, giving strong support for the exciton condensate phase as the origin of the charge density wave transition observed in 1T-TiSe2. The temperature-dependent order parameter characterizing the exciton condensate phase is discussed, both on a theoretical and an experimental basis, as well as the chemical potential shift occurring in this system. Finally, the transport properties of 1T-TiSe2 are analyzed in the light of the photoemission results.

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 behaviour at low temperature, it exhibits a non-zero value above the transition, interpreted as the signature of strong excitonic fluctuations, reminiscent of the pseudo-gap 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.

We present angle-resolved photoemission experiments on 1T-TiSe2 at temperatures ranging from 13K to 288K. The data evidence a dramatic renormalization of the conduction band below 100K, whose origin can be explained with the exciton condensate phase model. The renormalization translates into a substantial effective mass reduction of the dominant charge carriers and can be directly related to the low temperature downturn of the resistivity of 1T-TiSe2. This observation is 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.

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. (C) 2009 Elsevier B.V. All rights reserved.