M. Decroux

Ferroelectric field effect in ultrathin SrRuO3 films

We report the observation of a ferroelectric field effect in the conducting oxide SrRuO3 using Pb(Zr0.52Ti0.48)O3/SrRuO3 epitaxial heterostructures. Upon reversing the polarization of the ferroelectric Pb(Zr0.52Ti0.48)O3 layer, we measured a 9% change in the resistance of a nominally 30 A SrRuO3 film at room temperature. This change was nonvolatile for a period of several days. Conductivity measurements taken between 4.2 and 300 K are consistent with n-type conduction throughout this temperature range. Hall effect measurements also yield n-type conduction, with n≈2×1022 electrons/cm3, and furthermore allow us to understand quantitatively the magnitude of the observed resistivity change.

Ferroelectric Field Effect in Epitaxial Thin Film Oxide SrCuO2/Pb(Zr0.52Ti0.48)O3 Heterostructures

A ferroelectric field effect in epitaxial thin film SrCuO2/Pb(Zr0.52Ti0.48)O3 heterostructures was observed. A 3.5 percent change in the resistance of a 40 angstrom SrCuO2 layer (a parent high-temperature superconducting compound) was measured when the polarization field of the Pb(Zr0.52Ti0.48)O3 layer was reversed by the application of a pulse of small voltage (<5 volts). This effect, both reversible and nonvolatile, is attributed to the electric field-induced charge at the interface of SrCuO2 and Pb(Zr0.52Ti0.48)O3. This completely epitaxial thin film approach shows the possibility of making nonvolatile, low-voltage ferroelectric field effect devices for both applications and fundamental studies of field-induced doping in novel compounds like SrCuO2.

New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na, In, K, TI), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, TI)

Abstract We present a new series of ternary chalcogenides, derived from divalent molybdenum: M2Mo6X6. These compounds crystallize in a hexagonal lattice with a ≈ 9 A, c ≈ 4.5 A, and space group P6 3 m . The compounds are characterized by clusters (Mo3)1∞ in the form of linear chains, resulting from a linear condensation of Mo6 octahedral clusters. The (Mo3)1∞ clusters are well separated from each other, with the shortest MoMo intercluster distance larger than 6 A. The resulting pseudo-one-dimensional structures show remarkable anisotropy of physical properties.

Critical currents in Ag sheathed tapes of the 2223-phase in (Bi,Pb)-Sr-Ca-Cu-O

The critical current density of Ag-sheathed (Bi,Pb)-Sr-Ca-Cu-O tapes with thicknesses below 0.2 mm exhibiting the 2223 phase has been systematically studied for various compositions and oxygen contents of the surrounding atmosphere. Various sequences of the final thermochemical treatment leading to a textured structure were also examined. Conditions for obtaining a high degree of texturing in Ag-sheathed tapes were found, yielding J/sub c/ values up to 17.500 A/cm/sup 2/ at 77 K, 0 T. A major factor for the improvement of J/sub c/ by the present cold deformation approach was the thickness of the 2223 layer, which should be smaller than 20 mu m. Due to the large number of parameters involved in the optimization of this system and to the complex formation conditions, a substantial further increase of J/sub c/ in the 2223 phase is still expected. This is particularly important in view of the use of 2223 tapes at 4.2 K and at high magnetic fields.

High critical current densities in Bi(2223)/Ag tapes

Bi(2223)/Ag monofilamentary tapes have been characterized by measurements of jc vs. B and vs. theta , the angle between the applied magnetic field and the c axis. High degree of texturing, high density and Bi(2223) single phase conditions were found to be the major effects leading to high jc values. A further condition for reaching the highest jc values is a very low overall carbon content, the presence of carbon at the grain boundaries acting as a strong limiting factor. The highest value of jc in the present work is 4.104 A/cm2 at 77K,0T, corresponding to more than 2.105 A/cm2 at 4.2K,0T.

Transport and inductive critical current densities in superconducting MgB2

Abstract The critical current density of four MgB 2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current–voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magneto-resistivity, indicative of a flux-flow regime.

A new pseudo-one-dimensional superconductor: Tℓ2Mo6Se6

Abstract The properties of a new superconducting pseudo-one-dimensional compound, Tl2Mo6Se6 are presented. The crystal structure of this material contains linear chains in the form of infinite stacks of Mo6 clusters. The anisotropy in the normal state resistivity is found to be of the order of 103 and the parallel upper critical field is 26 times higher than the perpendicular one.

In situ thin films of YBa2Cu3O7 prepared by single target DC-magnetron sputtering

Abstract We have prepared in situ thin films of YBa 2 Cu 3 O 7 by single target dc reactive magnetron sputtering. By a delicate adjustment of the oxygen partial pressure as well as the total sputtering pressure, we can grow stoichiometric films using a stoichiometric target. This method gives highly reproducible results and the films have optimum superconducting properties without the need for any post-annealing treatment. The films are highly textured with the c -axis perpendicular to the plane of the substrate for films grown on unoriented ZrO 2 and a mixed c -axis and a -axis orientation on (100) SrTiO 3 . Zero resistances are obtained between 85 and 88 K with onsets between 90 and 91 K. The residual resistivity ratios R(RT) R(100 K) have values between 2.5 and 3 and the resistivity at 100 K is 60 μΩ cm for films on SrTiO 3 . SEM photos reveal much smoother surfaces than post-annealed films. We also obtained RHEED streaks, indicative of an oriented atomically smooth surface, on a 250–300 A thick layer on SrTio 3 .

Superconducting wires of pbmo6s8

Abstract Using a gas diffusion technique, we have produced PbMo 6 S 8 in the form of wires. The critical temperature of these wires is strongly dependent on the reaction conditions. Optimal conditions give a T c of 14.4 K ( ΔT c ≈ 0.25 K ). The lower limit for the critical current density at 4.2 K is 4.10 4 A cm −2 in a field of 4.2 T.

Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process

Manufacturing superconducting circuits out of ultrathin films is a challenging task when it comes to pattern complex compounds, which are likely to be deteriorated by the patterning process. With the purpose of developing high-Tc superconducting photon detectors, we designed a novel route to pattern ultrathin YBCO films down to the nanometric scale. We believe that our method, based on a specific use of a focused ion beam, consists in locally implanting Ga 3+ ions and/or defects instead of etching the film. This protocol could be of interest to engineer high-Tc superconducting devices (SQUIDS, SIS/SIN junctions, Josephson junctions), as well as to treat other sensitive compounds.