Vibrational and electronic entropy of β-cerium and γ-cerium measured by inelastic neutron scattering

Time-of-flight (TOF) inelastic neutron-scattering spectra were measured on β-cerium (double hcp) and γ-cerium (fcc) near the phase-transition temperature. Phonon densities of states (DOS) and crystal-field levels were extracted from the TOF spectra. A softening of the phonon DOS occurs in the transition from β- to γ-cerium, accounting for an increase in vibrational entropy of ΔSvibγ-β=(0.09±0.05)kB/atom. The entropy calculated from the crystal-field levels and a fit to calorimetry data from the literature were significantly larger in β-cerium than in γ-cerium below room temperature, but the difference was found to be negligible at the experimental phase-transition temperature. A contribution to the specific heat from Kondo spin fluctuations was consistent with the quasielastic magnetic scattering, but the difference between phases was negligible. To be consistent with the latent heat of the β-γ transition, the increase in vibrational entropy at the phase transition may be accompanied by a decrease in electronic entropy not associated with the crystal-field splitting or spin fluctuations. At least three sources of entropy need to be considered for the β-γ transition in cerium

Diffusion basis spectrum imaging for identifying pathologies in MS subtypes

Diffusion basis spectrum imaging (DBSI) combines discrete anisotropic diffusion tensors and the spectrum of isotropic diffusion tensors to model the underlying multiple sclerosis (MS) pathologies. We used clinical MS subtypes as a surrogate of underlying pathologies to assess DBSI as a biomarker of pathology in 55 individuals with MS. Restricted isotropic fraction (reflecting cellularity) and fiber fraction (representing apparent axonal density) were the most important DBSI metrics to classify MS using brain white matter lesions. These DBSI metrics outperformed lesion volume. When analyzing the normal-appearing corpus callosum, the most significant DBSI metrics were fiber fraction, radial diffusivity (reflecting myelination), and nonrestricted isotropic fraction (representing edema). This study provides preliminary evidence supporting the ability of DBSI as a potential noninvasive biomarker of MS neuropathology

Analysis of B→a1(1260)(b1(1235))K∗B \to a_1(1260)(b_1(1235))K^* decays in the perturbative QCD approach

Within the framework of perturbative QCD approach, we study the charmless two-body decays $B\to a_1(1260)K^*, b_1(1235)K^*$. Using the decays constants and the light-cone distribution amplitudes for these mesons derived from the QCD sum rule method, we find the following results: (a) Our predictions for the branching ratios are consistent well with the QCDF results within errors, but much larger than the naive factorization approach calculation values. (b) We predict that the anomalous polarizations occurring in the decays $B\to \phi K^*, \rho K^*$ also happen in the decays $B\to a_1K^*$, while do not happen in the decays $B\to b_1K^*$. Here the contributions from the annihilation diagrams play an important role to explain the lager transverse polarizations in the decays $B\to a_1K^*$, while they are not sensitive to the polarizations in decays $B\to b_1K^*$. (c) Our predictions for the direct CP-asymmetries agree well with the QCDF results within errors. The decays $\bar B^0\to b^{+}_1K^{*-}, B^-\to b^{0}_1K^{*-}$ have larger direct CP-asymmetries, which could be measured by the present LHCb experiments.Comment: 12pages, 1figures. arXiv admin note: substantial text overlap with arXiv:1203.5913, arXiv:1203.591

Possible charge inhomogeneities in the CuO2 planes of YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) from pulsed neutron diffraction

The atomic pair distribution functions (PDF) of four powder samples of YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) at 15 K have been measured by means of pulsed neutron diffraction. The PDF is modelled using a full-profile fitting approach to yield structural parameters. In contrast to earlier XAFS work we find no evidence of a split apical oxygen site. However, a slightly improved fit over the average crystallographic model results when the planar Cu(2) site is split along the z-direction. This is interpreted in terms of charge inhomogeneities in the CuO2 planes.Comment: 8 pages, 3 figure

Impact of Subleading Corrections on Hadronic B Decays

We study the subleading corrections originating from the 3-parton (q\bar q g) Fock states of final-state mesons in B decays. The corrections could give significant contributions to decays involving an \omega or \eta^{(\prime)} in the final states. Our results indicate the similarity of \omega K and \omega \pi^- rates, of order 5\times 10^{-6}, consistent with the recent measurements. We obtain a_2(B\to J/\psi K)\approx 0.27+0.05i, in good agreement with data. Without resorting to the unknown singlet annihilation effects, 3-parton Fock state contributions can enhance the branching ratios of K\eta' to the level above 50\times 10^{-6}.Comment: 5 pages, 5 figures, revtex4; some typos corrected, a new figure and a reference added, more explanations for the calculation provided, to appear in Phys. Rev.

Pd/Cu Site Interchange and Non-Fermi-Liquid Behavior in UCu_4Pd

X-ray-absorption fine-structure measurements of the local structure in UCu_4Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with 24 +/- 3% of the Pd atoms occupying nominally Cu sites. A "Kondo disorder" model, based on the effect on the local Kondo temperature T_K of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments.Comment: 4 pages, 3 PostScript figures, to be published in PR

Prospects for Direct CP Violaton in Exclusive and Inclusive Charmless B decays

Within the Standard Model, CP rate asymmetries for $B\to K^-\pi^{+,0}$ could reach 10%. With strong final state phases, they could go up to 20--30%, even for $\bar K^0\pi^-$ mode which would have opposite sign. We can account for $K^-\pi^{+}$, $\bar K^0\pi^-$ and $\phi K$ rate data with new physics enhanced color dipole coupling and destructive interference. Asymmetries could reach 40--60% for $K\pi$ and $\phi K$ modes and are all of the same sign. We are unable to account for $K^-\pi^0$ rate. Our inclusive study supports our exclusive results.Comment: Minor changes, correct a small bug in Fig. 1(b). Version to appear in Phys. Rev. Let

Phenomenological Analysis of D Meson Lifetimes

The QCD-based operator-product-expansion technique is systematically applied to the study of charmed meson lifetimes. We stress that it is crucial to take into account the momentum of the spectator light quark of charmed mesons, otherwise the destructive Pauli-interference effect in $D^+$ decays will lead to a negative decay width for the $D^+$. We have applied the QCD sum rule approach to estimate the hadronic matrix elements of color-singlet and color-octet 4-quark operators relevant to nonleptonic inclusive $D$ decays. The lifetime of $D_s^+$ is found to be longer than that of $D^0$ because the latter receives a constructive $W$-exchange contribution, whereas the hadronic annihilation and leptonic contributions to the former are compensated by the Pauli interference. We obtain the lifetime ratio $\tau(D_s^+)/\tau(D^0)$ $\approx 1.08\pm 0.04$, which is larger than some earlier theoretical estimates, but still smaller than the recent measurements by CLEO and E791.Comment: 14 pages, 3 figure

Local lattice disorder in the geometrically-frustrated spin glass pyrochlore Y2Mo2O7

The geometrically-frustrated spin glass Y2Mo2O7 has been considered widely to be crystallographically ordered with a unique nearest neighbor magnetic exchange interaction, J. To test this assertion, we present x-ray-absorption fine-structure results for the Mo and Y K edges as a function of temperature and compare them to results from a well-ordered pyrochlore, Tl2Mn2O7. We find that the Mo-Mo pair distances are significantly disordered at approximately right angles to the Y-Mo pairs. These results strongly suggest that lattice disorder nucleates the spin-glass phase in this material.Comment: 9 pages, 2 Postscript figures, Phys. Rev. B: Rapid, in pres