排序方式: 共有45条查询结果,搜索用时 15 毫秒
41.
S. W. Lovesey 《Zeitschrift für Physik B Condensed Matter》1980,40(1-2):117-126
The structure of annealed and quenched models with localU(1) gauge invariance is studied in terms of the Helmholtz free energy. The first non-trivial, or one-loop, account of fluctuations in the annealed model suggests that spontaneous symmetry breaking occurs in two and three dimensions, through a first-order phase transition. Within the same approximation scheme, the quenched model displays a continuous phase transition. A more complete account of the fluctuations in the annealed model changes the nature of the transition to a continuous one, whereas spontaneous symmetry breaking is then absent with quenched disorder. 相似文献
42.
Stephen W. Lovesey 《Hyperfine Interactions》1992,72(4):389-394
Relaxation rates for positive muons implanted in antiferromagnetic salts are predicted on the basis of calculations performed
with a coupled-mode theory of critical fluctuations in isotropic and uniaxial magnets. Anisotropy suppresses fluctuations
transmitted by the isotropic (hyperfine) relaxation mechanism. The dipolar mechanism leads to a relaxation rate which increases
on lowering the temperature to the critical point; the predicted form, proportional to the critical correlation length, yields
the same temperature dependence as for the NMR linewidth. For a magnet with isotropic spin interactions, both contributions
(hyperfine and dipolar) to the relaxation rate diverge with a temperature dependence given by the square-root of the correlation
length. 相似文献
43.
E. Balcar S. W. Lovesey J. M. Loveluck 《Zeitschrift für Physik B Condensed Matter》1984,54(3):195-199
Low temperature data are given for the spectra of longitudinal and transverse correlation functions of an antiferromagnetic classical heisenberg chain. The one dimensional magnet subjected to large external fields is investigated by means of a computer simulation calculation. The results are tied in with the static properties and compared with theoretical predictions. 相似文献
44.
We have demonstrated that resonant diffraction experiments using the circularly polarized X-ray beam absolutely determines the crystal chirality. Both berlinite and quartz crystals belonging to space group P3221 show higher azimuth-constant intensity for the negative (?) helicity beam than that for the the positive (+) helicity beam for space-group forbidden reflection 001. The relation is opposite for quartz crystal belonging to space group P3121. Theoretical calculation shows that this relation completely agrees with the experimental findings for the enantiomorphic space-group pair P3121 and P3221. This method is applicable to chiral motifs that occur in biomolecules, liquid crystals, ferroelectrics, and multiferroics, etc. 相似文献
45.
The low-temperature ordered state of neptunium dioxide (NpO(2)) remains enigmatic. After decades of experimental and theoretical efforts, long-range order of a time-odd (magnetic) high-order atomic multipole moment is now generally considered to be the fundamental order parameter, the most likely candidate being a magnetic triakontadipole (rank 5). To date, however, direct experimental observation of the primary order parameter remains outstanding. In the light of new experimental findings, we re-examine the effect of crystal symmetry on the atomic multipoles and the resulting x-ray resonant scattering signature. Our simulations use the crystallographic point group ?3m (D(3d)), because corresponding magnetic groups ?3m', ?3'm', and ?3'm are shown by us to be at odds with a wealth of experimental results. In addition to the previously observed (secondary) quadrupole order, we derive expressions for higher-order multipoles that might be observed in future experiments. In particular, magnetic octupole moments are predicted to contribute to Np M(2,3) and L(2,3) resonant scattering via E2–E2 events. The Lorentzian-squared lineshape observed at the M(4) resonance is shown to be the result of the anisotropy of the 3p(3/2) core levels. Quantitative comparison of our calculations to the measured data yields a core–hole width Γ = 2.60(7) eV and a core-state exchange energy [absolute value]ε(1/2)[absolute value] = 0.76(2) eV. 相似文献