G. Bonfait

Magnetic field dependence of the metal-insulator transition in (PER)2Pt(mnt)2 and (PER)2Au(mnt)2

Abstract Magnetoresistance measurements in the quasi-one-dimensional conductors (Perylene) 2 M(mnt) 2 , M=Pt and Au, in the range 2 to 300K and under magnetic fields up to 8T are reported. For the first time, a well defined metal-insulator transition was observed in the gold compound. At zero field, this transition occurs at 12.0K and is shifted down to 11.5K at 8 Tesla while in the Pt compound the transition at Tc=8.2K under zero field is shifted down to 7.3K at 8T. The field dependence of Tc in both compounds is consistent with theoretical predictions for a Peierls transition.

UFe6Ge6: a new ternary magnetic compound

Abstract The ternary intermetallic magnetic compound UFe6Ge6 was studied by single crystal X-ray diffraction and 57Fe Mossbauer spectroscopy; the magnetic properties were also investigated. UFe6Ge6 was found to melt congruently at c. 950 °C and to crystallize in space group P6/mmm, with cell parameters a = 5.126 8(4) A , c = 4.050 7(5) A , V = 92.204 7(13) A 3 , Z = 0.5 and a YCo6Ge6 type structure that was solved by single crystal X-ray diffraction to a final R = 0.059 (Rw = 0.071). Magnetization measurements suggest two transitions: one at 322(5) K reminiscent of an antiferromagnetic ordering and another ferromagnetic at 230(10) K. According to 57Fe Mossbauer spectroscopy there is a transition at 322(5) K associated with the ordering of the Fe moments which do not undergo any noticeable reordering at lower temperatures. Thus the transition at 230 K is tentatively attributed to the ordering of the U-sublattice.

The Peierls transition under high magnetic field

Abstract The dependence on magnetic field of the Peierls transition has been clearly measured on the quasi-one-dimensional compound Per 2 [Au(mnt) 2 ]. A quadratic dependence of the transition temperature has been found, in agreement with the theoretical predictions.

CUSTOMIZABLE GAS‐GAP HEAT SWITCH

The so‐called gas gap heat switch, in which the pressure is managed by a coupled small cryopump having no moving parts, is known to be a very reliable and simple heat switch.Mechanical design improvements can lead to optimized ON or OFF characteristics of a gas‐gap heat switch. Their ON conductance characteristics are mainly determined by the gas properties and the gap geometry. However, their operational temperature range is limited by the gas‐sorbent pair adsorption characteristics. Traditionally the gas chosen is helium, since it is the best conductive one below 100 K, and the sorbent used is activated charcoal. Such a switch is limited to be used at cold end temperatures below ∼15 K.In order to obtain a customizable device working at the whole range below 100 K, a gas gap heat switch was studied and extensively characterized. Hydrogen, Neon and Nitrogen were used as conducting gas, under different sorption conditions. A thermal model was built in order to determine the ON and OFF conductances over eac...

Magnetic properties of lanthanum nickelates

Abstract Electric and magnetic measurements were carried out for La n+1 Ni n O 3n+1 , n=2, 3, samples before and after oxidation to elucidate the mechanism underlying the phase transition known in these compounds.

The (Per)2M(mnt)2 series: The interaction of 1D conduction electrons with localised spin chains

Abstract A survey of the (Perylene) 2 M(mnt) 2 series, with M = Cu, Ni, Co, Fe, Pd, Au, Pt and mnt=maleonitriledithiolate, is presented. The role of the coexistence of itinerant electrons and arrays of localised magnetic moments (for M=Ni, Fe, Pd and Pt) in the phase transitions undergone by these compounds is discussed.

Material for resistive barriers in Bi-2223/Ag tapes

The reactivities of several oxide materials (OM) in direct contact with BSCCO powder has been tested at a temperature of approximately 845 °C in air. The OM such as BaZrO3, SrCO3, MgO and ZrO2 showing little or no reactivity with BSCCO were mixed (10 wt%) with a BSCCO precursor powder and used for monocore tapes made by a standard powder-in-tube technique. The microstructure of the BSCCO+OM cores was analysed by SEM and XRD and the transport current properties - critical current, pinning force and resistance up to 16 T - were measured as a function of the magnetic field for various orientations with respect to the ab plane. The OM used influenced the electrical properties of the Bi-2223 phase in different ways. This is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation as well as the Bi-2223 grain growth and grain connectivity. Submicrometre commercial SrCO3 powder was evaluated as the best material from all those tested, for resistive barriers in Bi-2223/Ag tapes.

CDW state and superconductivity in the quasi-two-dimensional monophosphate tungsten bronze P4W14O50

Abstract The monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2 m are quasi two-dimensional conductors which show electronic instabilities of the Peierls type. The m = 7 member of the series P 4 W 14 O 50 , undergoes two successive Peierls transitions at T P 1 = 188 K and T P 2 = 60 K. Weak localisation effects are observed in the low temperature charge density wave state. Resistivity, magnetoresistance and a.c. susceptibility measurements establish that a superconducting transition takes place at temperatures lower than approximately T c = 0.3 K. This indicates that superconductivity is related to parts of the Fermi surface left over after the Peierls gap openings.

High field magnetoresistance of UFe4Al8

Abstract Magnetoresistance measurements up to 16 T on UFe4Al8 single crystal are reported. The results show a strong anisotropic magnetoresistance. The normal magnetoresistance is always negative. No new phase transition has been detected up to 16 T.

Magnetic properties of a UFe4 Al8 single crystal

Abstract Magnetisation measurements on a single crystal of UFe 4 Al 8 show the high anisotropic properties of this compound and that the a, b plane is an easy plane for the magnetisation. The ZFC and FC magnetisation curves are explained by the temperature variation of the coercive field.