A. Sulpice

Macroscopic quantum tunneling in molecular magnets

Abstract Our present understanding of the phenomenon of quantum tunneling of magnetization (QTM), is reviewed in the light of the experiments performed on the molecular complex Mn 12 -ac. This system, in which QTM was clearly shown for the first time, consists of molecules with mesoscopic spins in dipolar interactions. Both single and many-molecule effects are essential for the observation of QTM (crystal field, hyperfine and dipolar interactions), which allows one to make a link between mesoscopic physics and magnetism.

Thermodynamic and transport properties of UPt3

In the normal phase of UPt3, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT2 dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromTc to 146 mK (the lowest experimental temperature), a largeT2 contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.

Superconductivity of La2CuxO4 − y

Abstract Bulk superconductivity is present in La 2 Cu x O 4 − y for 0.98 ⩽ x ⩽ 1.04 and for an y value which is strongly reduced by annealing under high pressure of oxygen. Two superconducting temperatures are observed T 2 and T c ; the first one, T 2 = 37 K, corresponds to the resistivity fall; the second one, T c , corresponds to the appearance of diamagnetism. T 2 and the critical fields of samples stay unchanged whatever the normal state resistivity is, even if it varies by a factor 40000 from a semi conducting to a metallic state. Then a conventional mechanism of pairing is unconceivable. The existence of a bump in the resistivity at T = 220 K of samples which undergo a superconducting transition suggest a possible correlation between coherence effects among copper spins and superconductivity.

High Resistivity and Diamagnetism in AlPdMn Icosahedral Phase

We present here the first results on transport and magnetic properties in the new stable icosahedral Al1-x-yPdxMny phases (x/y = 22/7.5, 21/8.5, 19/10). These melt-spun samples are of high structural quality even in the as-quenched state and present high-resistivity values which depend strongly on the nominal composition and structural quality with ρ4K ~ 10 000 μΩ cm in annealed Al70.5Pd22Mn7.5 and 1600 μΩ cm in as-quenched Al71Pd19Mn10. Moreover the system becomes more diamagnetic as its resistivity increases.

The superconducting “copper/carbonate cuprates” An electron microscopy study

Abstract By means of electron microscopy and diffraction together with EDX and EELS, we have confirmed the main characteristics of the structure of the recently discovered HTSC family of “copper/carbonate cuprates”. These can be obtained at high pressure and have critical temperatures of the order of 117 K. It consists of a copper/carbonate ordered variant of the tetragonal “single X-O layered” cuprate structure. The unit cell is given by a =2 a t , b = b t and c =2 c t , where subindex t corresponds to the tetragonal single layer cell.

Critical field, Hc2, and critical current in YBa2Cu3O7, up to 20 Tesla

Abstract The upper critical field Hc 2 of two sintered YBa 2 Cu 3 O 7 samples has been obtained up to 20 T from resistivity measurements. An upward curvature of Hc 2 (T) is observed close to T c (≈ 92 K), then, above ≈ 5 T, its variation becomes linear with dHc 2 /dT ≈ −3T/K. The critical current, J c , has been determined from the magnetic hysteresis cycles up to 18 T. Large values of J c (140 000 A/cm 2 ) are calculated at 1.5 K. J c decreases exponentially with T down to ≈ 1100 A/cm 2 around 77 K. PACS numbers 74.70 rv, 74.10 + v

BIQUADRATIC INTERACTIONS AND SPIN-PEIERLS TRANSITION IN THE SPIN-1 CHAIN LIVGE2O6

The magnetic susceptibility of a new one-dimensional S=1 system, the vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is consistent with a gapless spectrum above this temperature. We propose that this behavior is due to the presence of a significant biquadratic exchange interaction, a suggestion supported by quantum chemistry calculations that take into account the quasidegeneracy of the t2g levels.

Thermodynamic phase diagram of Fe(Se0.5Te0.5) single crystals in fields up to 28 tesla

We report on specific heat

Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO

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Optimization of texturing process by hot pressing of Bi-Pb-Sr-Ca-Cu-O (2223) ceramics

, transport, Hall probe, and penetration depth measurements performed on