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1.
Nanotechnology ; 23(47): 475709, 2012 Nov 30.
Article in English | MEDLINE | ID: mdl-23117292

ABSTRACT

Arrays of vertically aligned Si-nanopillars, with average diameters of 100 nm and 5 µm length, have been prepared by wet chemical etching of crystalline silicon in a special manner. Samples with smooth- and porous-walled nanopillars have been studied. In the case of the latter, Si-nanocrystals, passivated with SiO(x), decorating the surface of the nanopillars are identified by the means of TEM and FTIR. When excited by UV-blue light, the porous-walled Si-nanopillars are found to have a strong broad visible emission band around 1.8 eV with a nearly perfect Gaussian shape, µs luminescence lifetimes, minor emission polarization and a non-monotonic temperature dependence of luminescence. The Si-nanocrystal surface is found to be responsible for the luminescence. The red-shift of the emission maximum and the luminescence quenching induced by oxidation in UV-ozone confirm this assumption. A model of luminescence involving UV photon absorption by Si-nanocrystals with subsequent exciton radiative recombination on defect sites in SiO(x) covering Si-nanocrystals has been proposed. Possible applications of the nanopillar arrays are discussed.

2.
Nat Mater ; 11(4): 329-36, 2012 Feb 12.
Article in English | MEDLINE | ID: mdl-22327747

ABSTRACT

Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa(2)Cu(3)O(7) matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions.

3.
Phys Rev Lett ; 102(2): 027203, 2009 Jan 16.
Article in English | MEDLINE | ID: mdl-19257315

ABSTRACT

Time-resolved measurements of the magnetization reversal in single crystals of Mn12Ac in pulsed magnetic fields, at magnetic field sweep rates from 1.5 kT/s up to 7 kT/s, suggest a new process that cannot be scaled onto a deflagrationlike propagation driven by heat diffusion. The sweep rate dependence of the propagation velocity, increasing from a few 100 m/s up to the speed of sound in Mn12Ac, indicates the existence of two new regimes at the highest sweep rates, with a transition around 4 kT/s that can be understood as a magnetic deflagration-to-detonation transition.

4.
Phys Rev Lett ; 100(5): 056405, 2008 Feb 08.
Article in English | MEDLINE | ID: mdl-18352402

ABSTRACT

We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO(4). Contrary to spectroscopic and magnetic methods, x-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean-field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.

5.
Phys Rev Lett ; 84(17): 4018, 2000 Apr 24.
Article in English | MEDLINE | ID: mdl-11019267
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