We investigated the intermediate mixed state of a superconducting niobium sample using very small angle neutron scattering. We show that this state is stabilized through a sequence where a regular ...vortex lattice appears, which then coexists with vortex clusters before vanishing at low temperature. Vortices in clusters have a constant periodicity regardless of the applied field and exhibit a temperature dependence close to the one of the penetration depth. The clusters disappear in the high temperature limit. All the results agree with an explanation in terms of vortex attraction due to non-local effects and indicate a negligible role for pinning. Phase coexistence between the Abrikosov vortex lattice and vortex clusters is reported, showing the first-order nature of the boundary line.
We investigate the in-plane magnetic anisotropy in La0.67Sr0.33MnO3 thin films grown on SrTiO3 (001) substrate using angular dependent room temperature Vectorial Magneto-Optical Kerr Magnetometry. ...The experimental data reveals that the magnetic anisotropy symmetry landscape significantly changes depending upon the strain and thickness. At low film thickness (12 and 25 nm) the dominant uniaxial anisotropy is due to interface effects, step edges due to mis-cut angle of SrTiO3 substrate. At intermediate thickness, the magnetic anisotropy presents a competition between magnetocrystalline (biaxial) and substrate step induced (uniaxial) anisotropy. Depending upon their relative strengths, a profound biaxial or uniaxial or mixed anisotropy is favoured. Above the critical thickness, magnetocrystalline anisotropy dominates all other effects and shows a biaxial anisotropy.
Transition-metal oxides with an ABO3 perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus one expects to unveil exotic phases and properties by acting ...on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr3+V3+O3 compound as a model system, we show that PrVO3 Néel temperature TN can be raised by 40 K with respect to the bulk when grown as thin films. Using advanced experimental techniques, this enhancement is unambiguously ascribed to a tetragonality resulting from the epitaxial compressive strain experienced by the films. First-principles simulations not only confirm experimental results, but they also reveal that the strain promotes an unprecedented orbital ordering of the V3+d electrons, strongly favoring antiferromagnetic interactions. These results show that an accurate control of structural aspects of oxides is the key for unveiling unexpected phases in oxides.
The decay of (19)O(β(-)) and (19)Ne(β(+)) implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. ...Half-lives and branching ratios measured in the two phases are consistent within a 1σ error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for (19)Ne and 400(320) eV for (19)O. Precise determinations of the half-lives were obtained for (19)O, 26.476(9) s, and for (19)Ne, 17.254(5) s.
Pb
1−2
y/3
La
y
Zr
x
Ti
1−
x
O
3 (PLZT) thin films have been prepared “in situ” by multi-target sputtering on Silicon, LaAlO
3 (LAO) and MgO substrates covered with a Pt bottom electrode. The purpose ...was to grow tetragonal PLZT films (Zr/Ti
=
28/72 with different La contents) on these various substrates and to compare their electrical properties. To this aim, Pt was first deposited on the three different substrates to get (1
1
1)Pt/Si, (1
1
1)Pt/LAO and (2
0
0)Pt/MgO. Then PLZT was deposited in a same run on these three kinds of substrates and the influence of La contents and film orientation on electrical properties was investigated. The La content was varied from
y
=
0 to
y
=
32 in order to explore the phase transition between ferroelectric and paraelectric phases as a function of the substrate. For large amount of Lanthanum, relaxor behavior has been observed and studied.
Flux Lines Lattice (FLL) states have been studied using transport measurements and Small Angle Neutron Scattering in low T$_c$ materials. In Pb-In, the bulk dislocations in the FLL do not influence ...the transport properties. In Fe doped NbSe$_{2}$, transport properties can differ after a Field Cooling (FC) or a Zero Field Cooling (ZFC) procedure, as previously reported. The ZFC FLL is found ordered with narrow Bragg Peaks and is linked to a linear V(I) curve and to a superficial critical current. The FC FLL pattern exhibits two Bragg peaks and the corresponding V(I) curve shows a S-shape. This can be explained by the coexistence of two ordered FLL slightly tilted from the applied field direction by different superficial currents. These currents are wiped out when the transport current is increased.
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