4,468 research outputs found
Spin-ladders with spin gaps: A description of a class of cuprates
We investigate the magnetic properties of the Cu-O planes in stoichiometric
SrCuO (n=3,5,7,...) which consist of CuO double chains
periodically intergrown within the CuO planes. The double chains break up
the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with
rungs and legs and described by
the usual antiferromagnetic coupling J inside each ladder and a weak and
frustrated interladder coupling J. The resulting lattice is a new
two-dimensional trellis lattice. We first examine the spin excitation spectra
of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless
spectra when is even and is odd ( corresponding to n=5,9,...) and a
gapped spectra when is odd and is even (corresponding to
n=3,7,...). We use the bond operator representation of quantum
spins in a mean field treatment with self-energy corrections and obtain a spin
gap of for the simplest single rung ladder (n=3), in
agreement with numerical estimates.Comment: 21 pages, 5 figures upon request, REVTEX, ETH-TH/93-3
R-values in Low Energy e^+e^- Annihilation
This presentation briefly summarizes the recent measurements of R-values in
low energy e^+e^- annihilation. The new experiments aimed at reducing the
uncertainties in R-values and performed with the upgraded Beijing Spectrometer
(BESII) at Beijing Electron Positron Collider (BEPC) in Beijing and with CMD-2
and SND at VEEP-2M in Novosibirsk are reviewed and discussed.Comment: 17 pages, 10 figures, invited presentation at the XIX International
Symposium on Lepton and Photon Interactions at High Energy, Stanford
University, August 199
Left bundle branch block causes relative but not absolute septal underperfusion during exercise
Aims Left bundle branch block (LBBB) often causes septal perfusion defects in radionuclide myocardial perfusion imaging using exercise (Ex) but rarely using vasodilator stress. We studied whether this is due to an underlying structural disease inherent to spontaneous LBBB or whether it is also found in temporary LBBB induced by right ventricular pacing (PM) indicating a functional rather than a structural alteration. Methods and results Regional myocardial blood flow (MBF) at rest and at Ex was measured with(15)O-H(2)O and PET in 10 age-matched healthy volunteers (controls), 10 LBBB patients and 10 PM patients with right ventricular pacing off and on (PM off and PM on). Although at Ex septal MBF tended to be higher in LBBB than in controls (3.04 +/- 1.18 vs. 2.27 +/- 0.72 mL/min/g; P= ns), the ratio septal/lateral MBF was 19% lower in LBBB than in controls (P < 0.05). Similarly, switching PM on at Ex decreased the ratio septal/lateral MBF by 17% (P < 0.005). Conclusion The apparent septal perfusion defect in LBBB is mainly due to a relative lateral hyperperfusion rather than to an absolute septal flow decrease. This pattern seems to be reversibly inducible by right ventricular pacing, suggesting a functional rather than a structural alteratio
Bias-Dependent Generation and Quenching of Defects in Pentacene
We describe a defect in pentacene single crystals that is created by bias
stress and persists at room temperature for an hour in the dark but only
seconds with 420nm illumination. The defect gives rise to a hole trap at Ev +
0.38eV and causes metastable transport effects at room temperature. Creation
and decay rates of the hole trap have a 0.67eV activation energy with a small
(108 s-1) prefactor, suggesting that atomic motion plays a key role in the
generation and quenching process.Comment: 10 pages, 3 figure
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