Tuesday, January 05, 2010

Phys. Rev. B

Identification of individual ^{13} C isotopes of nitrogen-vacancy center in diamond by combining the polarization studies of nuclear spins and first-principles calculations

from Recent Articles in Phys. Rev. B We determine the charge- and spin-density distributions of nitrogen-vacancy center in diamond for both the ground and excited states by ab initio supercell calculations yielding very good agreement with the experiment. We correctly determine the polarization of 15N nuclear spin in the level anticrossing (LAC) mechanism. We show that LAC together with the accurate ab initio data can be used to identify the individual 13C nuclei around the defect that can also reveal the dominant component of the precession vector responsible for the decoherence of the addressed nuclear-spin qubit.

Nutation versus angular-dependent NQR spectroscopy and impact of underdoping on charge inhomogeneities in YBa_{2} Cu_{3} O_{y}

Author(s): Rinat Ofer and Amit Keren
We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient η and the tilt angle α of the main NQR principal axis ẑ from the crystallographic axis ĉ. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa2Cu3Oy fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by α≃20°±5°. This occurs with no change to η. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen.

Nuclear quadrupole resonance and x-ray investigation of the structure of Na_{2/3} CoO_{2}

from Recent Articles in Phys. Rev. B We have synthesized various samples of the x=2/3 phase of sodium cobaltate NaxCoO2 and performed x-ray powder diffractions spectra to compare the diffraction with the structure proposed previously from NMR and nuclear quadrupole resonance (NQR) experiments [H. Alloul, I. R. Mukhamedshin, T. A. Platova, and A. V. Dooglav, EPL 85, 47006 (2009)]. Rietveld analyses of the data are found in perfect agreement with those and confirm the concentration x=2/3 obtained in the synthesis procedure. They even give indications on the atomic displacements of Na inside the unit cell. The detailed NQR data allow us to identify the NQR transitions and electric field gradient parameters for four cobalt sites and three Na sites. The spin-lattice and spin-spin relaxation rates are found much smaller for the nonmagnetic Co3+ sites than for the magnetic sites on which the holes are delocalized. The atomic ordering of the Na layers is therefore at the source of this ordered distribution of cobalt charges. The method used here to resolve the Na ordering and the subsequent Co charge order can be used valuably for similar structural determinations for various phases with x>0.45 for which Na ordering has been established.

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