Lamellar phase separation and dynamic competition in La0.23Ca0.77MnO3

We report the coexistence of lamellar charge-ordered (CO) and charge-disordered (CD) domains, and their dynamical behavior, in La0.23Ca0.77MnO3. Using high resolution transmission electron microscopy (TEM), we show that below Tcd~170K a CD-monoclinic phase forms within the established CO-orthorhombic matrix. The CD phase has a sheet-like morphology, perpendicular to the q vector of the CO superlattice (a axis of the Pnma structure). For temperatures between 64K and 130K, both the TEM and resistivity experiments show a dynamic competition between the two phases: at constant T, the CD phase slowly advances over the CO one. This slow dynamics appears to be linked to the magnetic transitions occurring in this compound, suggesting important magnetoelastic effects.Comment: 4 pages, 4 figure

Thermodynamics of lattice QCD with 2 flavours of colour-sextet quarks: A model of walking/conformal Technicolor

QCD with two flavours of massless colour-sextet quarks is considered as a model for conformal/walking Technicolor. If this theory possess an infrared fixed point, as indicated by 2-loop perturbation theory, it is a conformal(unparticle) field theory. If, on the other hand, a chiral condensate forms on the weak-coupling side of this would-be fixed point, the theory remains confining. The only difference between such a theory and regular QCD is that there is a range of momentum scales over which the coupling constant runs very slowly (walks). In this first analysis, we simulate the lattice version of QCD with two flavours of staggered quarks at finite temperatures on lattices of temporal extent $N_t=4$ and 6. The deconfinement and chiral-symmetry restoration couplings give us a measure of the scales associated with confinement and chiral-symmetry breaking. We find that, in contrast to what is seen with fundamental quarks, these transition couplings are very different. $\beta=6/g^2$ for each of these transitions increases significantly from $N_t=4$ and $N_t=6$ as expected for the finite temperature transitions of an asymptotically-free theory. This suggests a walking rather than a conformal behaviour, in contrast to what is observed with Wilson quarks. In contrast to what is found for fundamental quarks, the deconfined phase exhibits states in which the Polyakov loop is oriented in the directions of all three cube roots of unity. At very weak coupling the states with complex Polyakov loops undergo a transition to a state with a real, negative Polyakov loop.Comment: 21 pages, 9 figures, Revtex with postscript figures. One extra reference was added; text is unchanged. Corrected typographical erro

Nucleon Sigma Term and In-medium Quark Condensate in the Modified Quark-Meson Coupling Model

We evaluate the nucleon sigma term and in-medium quark condensate in the modified quark-meson coupling model which features a density-dependent bag constant. We obtain a nucleon sigma term consistent with its empirical value, which requires a significant reduction of the bag constant in the nuclear medium similar to those found in the previous works. The resulting in-medium quark condensate at low densities agrees well with the model independent linear order result. At higher densities, the magnitude of the in-medium quark condensate tends to increase, indicating no tendency toward chiral symmetry restoration.Comment: 9 pages, modified version to be publishe

SUSY Dark Matter In Light Of CDMS/XENON Limits

In this talk we briefly review the current CDMS/XENON constraints on the neutralino dark matter in three popular supersymmetric models: the minimal (MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM). The constraints from the dark matter relic density and various collider experiments are also taken into account. The conclusion is that for each model the current CDMS/XENON limits can readily exclude a large part of the parameter space allowed by other constraints and the future SuperCDMS or XENON100 can cover most of the allowed parameter space. The implication for the Higgs search at the LHC is also discussed. It is found that in the currently allowed parameter space the MSSM charged Higgs boson is quite unlikely to be discovered at the LHC while the neutral Higgs bosons $H$ and $A$ may be accessible at the LHC in the parameter space with a large $\mu$ parameter.Comment: talk given at 2nd International Workshop on Dark Matter, Dark Energy and Matter-Antimatter Asymmetry, Nov 5-6, 2010, Hsinchu, Taiwan (to appear in Int. J. Mod. Phys. D

Measurement of the interaction strength in a Bose-Fermi mixture with 87Rb and 40K

A quantum degenerate, dilute gas mixture of bosonic and fermionic atoms was produced using 87Rb and 40K. The onset of degeneracy was confirmed by observing the spatial distribution of the gases after time-of-flight expansion. Further, the magnitude of the interspecies scattering length between the doubly spin polarized states of 87Rb and 40K, |a_RbK|, was determined from cross-dimensional thermal relaxation. The uncertainty in this collision measurement was greatly reduced by taking the ratio of interspecies and intraspecies relaxation rates, yielding |a_RbK| = 250 +/- 30 a_0, which is a lower value than what was reported in [M. Modugno et al., Phys. Rev. A 68, 043626 (2003)]. Using the value for |a_RbK| reported here, current T=0 theory would predict a threshold for mechanical instability that is inconsistent with the experimentally observed onset for sudden loss of fermions in [G. Modugno et al., Science 297, 2240 (2002)].Comment: RevTeX4 + 4 eps figures; Replaced with published versio