A. Forget

Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields

Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100μCcm−2, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.

Influence of parasitic phases on the properties of BiFeO3 epitaxial thin films

We have explored the influence of deposition pressure and temperature on the growth of BiFeO3 thin films by pulsed laser deposition onto (001)-oriented SrTiO3 substrates. Single-phase BiFeO3 films are obtained in a region close to 10−2mbar and 580°C. In nonoptimal conditions, x-ray diffraction reveals the presence of Fe oxides or of Bi2O3. We address the influence of these parasitic phases on the magnetic and electrical properties of the films and show that films with Fe2O3 systematically exhibit a ferromagnetic behavior, while single-phase films have a low bulklike magnetic moment. Conductive-tip atomic force microscopy mappings also indicate that Bi2O3 conductive outgrowths create shortcuts through the BiFeO3 films, thus preventing their practical use as ferroelectric elements in functional heterostructures.

The crystal field and exchange interactions in Yb2Ti2O7

In the pyrochlore-structure compounds R2Ti2O7, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb2Ti2O7, we combine 170Yb Mossbauer spectroscopy, 172Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb3+ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb3+-Yb3+ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature (<1 K) properties of Yb2Ti2O7 in a separate publication.

Synthesis, crystal structure and magnetic properties of superconducting single crystals of HgBa2CuO4+δ

Abstract Single crystals of HgBa 2 CuO 4+δ of submillimetric sizes were grown with the same one step, low pressure, gold amalgamation technique used to obtain single crystals of HgBa 2 Ca 2 Cu 3 O 8+δ . Remarkable superconducting properties are displayed by the samples which are optimally doped as grown. The sharpness of the transition profiles of the magnetic susceptibility, its anisotropy dependence and the volume fraction exhibiting the Meissner effect exceed the values obtained with the best crystal samples of Hg-1223. X-rays show that no substitutional defects have been found in the mercury plane, in particular no mixed occupancy of copper at the mercury site. The interstitial oxygen content at ( 1 2 , 1 2 , 0 ) δ = 0.066 ± 0.008 is about one third that observed in optimally doped Hg-1223, resulting in an identical doping level per CuO 2 plane in both compounds.

Thermodynamic signature of a phase transition in the pseudogap phase of YBa2Cu3Ox high-TC superconductor

We present here high-precision magnetisation measurements in polycrystalline YBa2Cu3Ox samples, with oxygen content ranging from x=6.19 to x=7.00. By analysing the temperature derivative of the susceptibility, we found in the underdoped superconducting samples a singular point at a temperature corresponding to T mag, the temperature below which polarised neutrons experiments have evidenced a symmetry breaking. We believe that this is a thermodynamic indication for the existence of a phase transition in the pseudogap state of underdoped YBa2Cu3Ox.

Evolution of the gaps through the cuprate phase-diagram

The actual physical origin of the gap at the antinodes and a clear identification of the superconducting gap are fundamental open issues in the physics of high-

Self-organization processes in highly epitaxial La2/3Sr1/3MnO3 thin films grown on SrTiO3 (001) substrates

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Spin dynamics and magnetic order in magnetically frustrated Tb2Sn2O7

superconductors. Here, we present a systematic electronic Raman scattering study of a mercury-based single layer cuprate as a function of both doping level and temperature. On the deeply overdoped side, we show that the antinodal gap is a true superconducting gap. In contrast, on the underdoped side, our results reveal the existence of a breakpoint close to optimal doping, below which the antinodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal states is distinctly different on each side of this breakpoint.

Point defects, thermodynamic and superconducting properties of the non-stoichiometric HgBa2CuO4+δ phase

We report on the growth of highly epitaxial La2/3Sr1/3MnO3 thin films on SrTiO3 (001) oriented substrates by rf magnetron sputtering. It is shown that structural strain caused by lattice mismatch between film and substrate and kinetics growing instabilities may offer unique opportunities to control the film morphologies. By increasing growth rate, the surface morphology undergoes a dramatic transformation: from very smooth surface through arrays of mounds or antidots to hatched morphology. All nano-objects form long-range ordered arrays running in the steps direction defined by the miscut angle of underlying substrate. These self-assembled structures are of high interest envisaging a new route for the fabrication of oxide-based magnetoelectronic devices by using a bottom-up approach.

Incommensurate spin density wave in Co-doped BaFe2As2

We report a study of the geometrically frustrated magnetic material Tb2Sn2O7 by the positive muon-spin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb2Sn2O7: the Tb3+ magnetic moment characteristic fluctuation time is approximately 10(-10) s. The strong effect of the magnetic field on the muon-spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy.