Tuesday, October 21, 2008

Physical Review Letters - Up to Vol 101 No 16

93Nb NMR Spin Echo Spectroscopy in Single Crystal NbSe3
S. Suh, W. G. Clark, P. Monceau, R. E. Thorne, and S. E. Brown
Phys. Rev. Lett. 101, 136407 (2008)
We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E=(0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60° for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.

Boroxol Rings in Liquid and Vitreous B2O3 from First Principles
Guillaume Ferlat, Thibault Charpentier, Ari Paavo Seitsonen, Akira Takada, Michele Lazzeri, Laurent Cormier, Georges Calas, and Francesco Mauri
Phys. Rev. Lett. 101, 065504 (2008)
We investigate the structural and vibrational properties of glassy B2O3 using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction f for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, 11B and 17O NMR data). We show that only the boroxol-rich model (f=75%) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about 6 kcal/(mol boroxol). Finally, the liquid state is explored in the 2000–4000 K range and a reduction of f to 10%–20% is obtained.

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