A. Junod

University of Geneva, Genève, Geneva, Switzerland

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Publications (210)253.72 Total impact

  • G. TRISCONE, A. JUNOD
    ChemInform 01/2010; 28(14).
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    ABSTRACT: The temperature dependence of the resistivity, the magnetic properties and the specific heat were investigated on sintered samples of La1.85Sr0.15CuO4 having zero resistance below 35 K. The crystal structure at 300 K (tetragonal K2NiF4-type) was refined from X-ray powder diffraction data. The d.c. susceptibility shows no indication for the existence of localized Cu2+ moments. The observation of a 60% Meissner effect and a smeared jump at Tc in the specific-heat curve prove the intrinsic character of this superconducting state. The amplitude of this jump is compatible with the DOS estimated from the Pauli susceptibility. With a critical magnetic field slope dHc2/dT|Tc = -2.5 T/K, the orbital critical field is expected to be of the order of 64 T.
    EPL (Europhysics Letters) 07/2007; 3(9):1035. · 2.26 Impact Factor
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    ABSTRACT: YBa2Cu3O7 superconductors with inductive transitions as narrow as 0.45 K above 90 K were synthetized. Samples were characterized by thermogravimetry, differential thermal analysis, X-ray and neutron diffraction. The structure is characterized by a two-dimensional Cu-O network with square-pyramidal and square-planar coordinated Cu atoms. Results show a clear metallic behaviour of the resistivity. An orbital critical field as high as 300 T is extrapolated. Meissner flux expulsion up to 40% is observed. Small amounts of magnetic Cu2+ ions are correlated with the presence of the impurity phase BaCuO2. The Pauli susceptibility and the specific-heat jump at Tc are consistent with γ 2 mJ/(K2 gat) (9 mJ/(K2 mole-Cu)), neglecting all renormalizations.
    EPL (Europhysics Letters) 07/2007; 4(2):247. · 2.26 Impact Factor
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    ABSTRACT: The Tc of superconducting Y2Ba4Cu7O15.32 is measured up to a pressure of 33.7 GPa. After an initial increase from 95 K to 101.7 K a decrease down to 26.6 K is found, with a clear kink at 22 GPa. We demonstrate that Y2Ba4Cu7O15.32 behaves as a multilayer of YBa2Cu3O7 and YBa2Cu4O8 building blocks coupled via the proximity effect. These data are not consistent with the idea that the coupling of two adjacent CuO2 planes is necessary for superconductivity.
    EPL (Europhysics Letters) 07/2007; 20(1):41. · 2.26 Impact Factor
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    ABSTRACT: A new calorimeter has been built with the special purpose to determine the distribution of T<sub>c</sub> in industrial superconducting wires. Specific heat measurements have been carried out on a series of multifilamentary Nb<sub>3</sub>Sn wires, using a long relaxation technique. The advantage of this technique consists in the fact that the measurement is performed in presence of the Cu-Sn matrix, i.e. the filaments are measured under the same stress conditions as under operation, i.e. under the same state of mechanical precompression. In addition, the T<sub>c</sub> distribution is obtained for the whole sample volume, ruling out shielding effects. The deconvolution of the data in the region of the superconducting transition was used for getting the precise distribution of T<sub>c</sub>, which in turn allows a determination of the Sn distribution across the filaments. These data confirm previous TEM measurements showing a Sn gradient inside of the filaments of bronze route processed Nb<sub>3</sub>Sn wires. The T<sub>c</sub> distribution has been determined in Nb<sub>3</sub>Sn wires processed by bronze route, internal Sn and powder-in-tube technique. Based on this information, the various processing parameters can be varied to get narrower T<sub>c</sub> distributions at transition temperatures closer to 18 K.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb3Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found, which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can, however, be restored by a small ac field. In this case, a first-order vortex melting transition with a clear hysteresis is found. In the absence of an ac field, the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.
    Physical review. B, Condensed matter 03/2007; 75(9).
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    ABSTRACT: The mixed-state magnetization M(H||c,T) of a Bi-2212 single crystal has been investigated with high resolution using a SQUID magnetometer. In the high-temperature region (50 K < T < Tc = 80.2 K), we found that the slope ∂M/∂H|T vs. H shows a positive step at Htrans(T) ≈ H0×(1 − T/Tc)n with H0 = 2340 Oe and n = 1.28. This observation is compatible with a first-order phase transition with a distribution of internal fields, and is attributed to the melting of the 3D vortex lattice. The estimated entropy jump is 1 kB/vortex/layer CuO. However, when T is lower than 50 K, we observe radical changes in M(H); the 3D melting line divides into a decoupling line at a temperature-independent field and the onset of the irreversibility.
    EPL (Europhysics Letters) 01/2007; 33(9):701. · 2.26 Impact Factor
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    ABSTRACT: The range of thermal fluctuations in ‘classical’ bulk superconductors is extremely small and especially in low-fields hardly experimentally accessible. With a new type of calorimeter we were able to detect a tiny lambda anomaly in the specific-heat of the superconductor Nb3Sn within a narrow temperature range around the Hc2 line. We show that the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical thermal fluctuations.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2007; 460:149-151.
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    ABSTRACT: The A15 compound Nb3Sn is a well-known technically relevant superconductor with critical temperature Tc ~ 18 K. Recently, a low-temperature anomaly in the specific-heat data on a particularly dense and homogeneous polycrystalline sample has been interpreted in terms of the presence of a second superconducting gap. We performed point-contact spectroscopy on samples of the same batch using both the break-junction and the needle-anvil technique with Pt as a normal-metal counterelectrode. The differential conductance as a function of applied voltage at T ~ 1.5 K shows several characteristic maxima which can be well interpreted under the assumption of two superconducting energy gaps in Nb3Sn.
    09/2006;
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    ABSTRACT: We report the optical properties of high-quality single crystals of low temperature superconductors zirconiumdodecaboride ZrB12 (Tc = 5.95 K) and yttrium hexaboride YB6 (Tc = 7.15 K) in the range 6 meV - 4.6 eV at room temperature. The experimental optical conductivity was extracted from the analysis of the reflectivity in the infrared range and ellipsometry measurement of the dielectric function in the visible range. The electronic band structure of these compounds was calculated by the self-consistent full-potential LMTO method and used to compute the interband part of the optical conductivity and the plasma frequency Ωp. A good agreement was observed between the interband part of the experimental optical conductivities and the band structure calculations. Different methods combining optical spectroscopy, resistivity, specific heat measurements and results of band structure calculations are used to determine the electron-phonon coupling constant. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 08/2006; 3(9):3114 - 3117.
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    ABSTRACT: We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb3Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can however be restored by a small AC field. In this case a first-order vortex melting transition with a clear hysteresis is found. In the absence of an AC field the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.
    05/2006;
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    ABSTRACT: The range of critical thermal fluctuations in classical bulk superconductors is extremely small and especially in low fields hardly experimentally inaccessible. With a new type of calorimeter we have been able to resolve a small lambda anomaly within a narrow temperature range around the Hc2 line. We show that the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical fluctuations. The lower onset of the fluctuation regime shows many characteristics of a continuous solid-to-liquid transition in the vortex matter. It can be driven into a first-order vortex melting transition by a small AC field which helps the vortex matter to reach equilibrium. Comment: 12 pages, 5 figures new extended version, more figures added, more detailed discussion about the vortex melting transition
    Physical Review B 04/2006; · 3.77 Impact Factor
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    ABSTRACT: We found a sharp pronounced peak effect in the magnetization and the isothermal magnetocaloric effect of a single crystal of the classical superconductor Nb3Sn. The construction of a new type of isothermal calorimeter allowed us to study the underlying thermodynamics with an extremely high resolution. A small lambda-type anomaly could be resolved superimposed on the specific heat jump at Tc. It follows scaling laws as expected for the presence of critical fluctuations. We interpret this feature as the first clear observation of the regime of critical fluctuations in a classical superconductor. Simultaneous measurements of the specific heat and the isothermal magnetocaloric effect reveal that the peak effect occurs at the onset of fluctuations below the Hc2 line. No indications of a thermodynamic transition as the origin of the peak effect could be found. Our investigation favours thus rather a scenario in which a vortex lattice looses its topological order more continously due to the increasing strength of thermal fluctuations upon approaching the Hc2 line.
    03/2006;
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    ABSTRACT: The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn . We report measurements of the specific heat, resistivity, magnetic susceptibility, and thermal expansion from 2to300K , using a single crystal with Tc=7.2K . The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity, and expansivity curves are deconvolved to yield approximations of the phonon density of states F(omega) , the spectral electron-phonon scattering function alphatr2F(omega) , and the phonon density of states weighted by the frequency-dependent Grüneisen parameter gammaG(omega)F(omega) , respectively. Lattice vibrations extend to high frequencies >100meV , but a dominant Einstein-like mode at ˜8meV , associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.
    Physical review. B, Condensed matter 01/2006; 73(2).
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    ABSTRACT: The superconducting transition in a single overdoped, detwinned YBa2Cu3Ox (YBCO) crystal is studied using four different probes. Whereas the AC and DC magnetic susceptibilities find a dominant transition at 88 K with a smaller effect near 92 K, the specific heat and electrical resistivity reveal only a single transition at 88 K and 92 K, respectively. Under hydrostatic pressures to 0.60 GPa these two transitions shift in opposite directions, their separation increasing. The present experiments clearly show that the bulk transition lies at 88 K and originates from fully oxygenated YBCO; the 92 K transition likely arises from filamentary superconductivity in a minority optimally doped phase (< 1 %) of YBCO located at or near the crystal surface. Comment: 9 pages, 3 figures
    Physica C Superconductivity 12/2005; · 0.72 Impact Factor
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    ABSTRACT: A recently developed technique for measuring the isothermal magnetocaloric coefficient (MT) is applied to the study of a superconducting NdBa2Cu3O7 single crystal. Results are compared with magnetization (M) and specific heat (C). In the reversible region both C and MT follow the scaling law of the 3D-xy universality class. The anomalies connected with flux-line lattice melting are visible on MT(B) curves as peaks and steps, similar to C(T) curves yet with much smaller background. At lower temperature, in the irreversible region the MT(B) behaviour resembles more that of M(B), exhibiting the 'fishtail' effect. Our results confirm that the peculiarities of the phase diagram known from the high-temperature superconductor YBa2Cu3O7, e.g. vortex melting, dominance of critical fluctuations and absence of a Bc2 critical field line, are a common property of RE-123 systems.
    Journal of Physics Condensed Matter 10/2005; 17(43):6871. · 2.36 Impact Factor
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    ABSTRACT: We measured the specific heat, the magnetization, and the magnetoresistance of a single crystal of ZrB12, which is superconducting below Tc≅6 K. The specific heat in zero field shows a BCS-type superconducting transition. The normal- to superconducting-state transition changes from first order (with a latent heat) to second order (without latent heat) with increasing magnetic field, indicating that the pure compound is a low-κ, type-II/1 superconductor in the classification of Auer and Ullmaier [ Phys. Rev. B 7 136 (1973)]. This behavior is confirmed by magnetization measurements. The H-T phase diagram based on specific-heat and magnetization data yields Hc2(0)=550 G for the bulk upper critical field, whereas the critical field defined by vanishing resistance is a surface critical field Hc3(0)∼1000 G.
    Physical Review B 07/2005; 72(2). · 3.77 Impact Factor
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    ABSTRACT: We developed a new type of AC microcalorimeter based on a modulated-bath technique for measuring the specific heat of small microgram samples in the temperature range from 30-300 K, and tested it in magnetic fields up to 14 T. The device is built from a modified commercial Peltier element. The temperature of its top plate can be modulated periodically by Peltier effect, so that the oscillation is symmetrical about the temperature of the main bath. This avoids the problem of DC offsets which plagues conventional AC calorimeters. The sample is attached to a thin thermocouple cross, acting as a weak thermal link to a platform. The absence of a heater reduces the background heat capacity ('addenda') to a minimum. As an illustrative example of the performance of our device, the specific heat in fields up to 14 T of a small single crystal of the high-temperature superconductor Bi2.12Sr1.71Ca1.22Cu1.95Oy has been determined.
    Review of Scientific Instruments 04/2005; · 1.60 Impact Factor
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    ABSTRACT: The evolution of the specific-heat anomaly in the overdoped range of a single crystal of the high-temperature superconductor YBa2Cu3O7 has been studied under influence of pressure up to 10 GPa, using AC calorimetry in a Bridgman-type pressure cell. We show that the specific-heat jump as well as the bulk Tc are reduced with increasing pressure in accordance with a simple charge-transfer model. This new method enables us through pressure-induced charge transfer to study the doping dependence of the superconducting transition, as well as the evolution of the superconducting condensation energy on a single stoichometric sample without adding atomic disorder. Comment: final version: J. Phys.: Condens. Matter 17 (2005) 4135-4145
    Journal of Physics Condensed Matter 03/2005; · 2.36 Impact Factor
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    ABSTRACT: In an attempt to clarify conflicting published data, we report new measurements of specific heat, resistivity, magnetic susceptibility, and thermal expansivity up to room temperature for the 6 K superconductor ZrB12, using well-characterized single crystals with a residual resistivity ratio >9. The specific heat gives the bulk result 2*delta(0)/(k_B*Tc)=3.7 for the superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for the Cooper pairs. The Sommerfeld constant gamma_n = 0.34 mJ K-2 gat-1 and the magnetic susceptibility xsi = -2.1 x 10^-5 indicate a low density of states at the Fermi level. The Debye temperature theta_D is in the range 1000-1200 K near zero and room temperature, but decreases by a factor of ~2 at ~35 K. The specific heat and resistivity curves are inverted to yield approximations of the phonon density of states F(omega) and the spectral electron-phonon scattering function alpha_tr^2 F(omega), respectively. Both unveil a 15 meV mode, attributed to Zr vibrations in oversized B cages, which gives rise to electron-phonon coupling. The thermal expansivity further shows that this mode is anharmonic, while the vanishingly small discontinuity at Tc establishes that the cell volume is nearly optimal with respect to Tc. Comment: v3: minor changes in the thermal expansion paragraph submitted to Physical Review B
    Physical Review B 02/2005; · 3.77 Impact Factor