Polarization Effects in Chargino Production at High Energy γγ\gamma\gamma Colliders

We investigate the chargino production process $\gamma\gamma\rightarrow\tilde{W}^{+}\tilde{W}^{-}$ at high energy $\gamma\gamma$ colliders in the framework of the minimal supersymmetric standard model (MSSM). Here the high energy $\gamma$ beams are obtained by the backward Compton scattering of the laser flush by the electron in the basic linear TeV $ee$ colliders. We consider the polarization of the laser photons as well as the electron beams. Appropriate beam polarization could be effective to enhance the cross section and for us to extract the signal from the dominant background $\gamma\gamma\rightarrow{W}^{+}{W}^{-}$.Comment: 7 pages, latex , 3 figures are available upon reques

Muon spin relaxation and rotation study on the solid solution of the two spin-gap systems (CH3)2CHNH3-CuCl3 and (CH3)2CHNH3-CuBr3

Muon-spin-rotation and relaxation studies have been performed on (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ with $x$=0.85 and 0.95, which are solid solutions of the two isomorphic spin-gap systems (CH$_3$)$_2$CHNH$_3$CuCl$_3$ and (CH$_3$)$_2$CHNH$_3$CuBr$_3$ with different spin gaps. The sample with $x$=0.85 showed a clear muon spin rotation under zero-field below $T_{\rm N}$=11.65K, indicating the existence of a long-range antiferromagnetic order. A critical exponent of the hyperfine field was obtained to be $\beta$=0.33, which agrees with 3D-Ising model. In the other sample with $x$=0.95, an anomalous enhancement of the muon spin relaxation was observed at very low temperatures indicating a critical slowing down due to a magnetic instability of the ground state

On Bose-Einstein condensate inside moving exciton-phonon droplets

We explore a nonlinear field model to describe the interplay between the ability of excitons to be Bose condensed and their interaction with other modes of a crystal. We apply our consideration to the long-living paraexcitons in Cu2O. Taking into account the exciton-phonon interaction and introducing a coherent phonon part of the moving condensate, we solve quasi-stationary equations for the exciton-phonon condensate. These equations support localized solutions, and we discuss the conditions for the inhomogeneous condensate to appear in the crystal. Allowable values of the characteristic width of ballistic condensates are estimated. The stability conditions of the moving condensate are analyzed by use of Landau arguments, and Landau critical parameters appear in the theory. It follows that, under certain conditions, the condensate can move through the crystal as a stable droplet. To separate the coherent and non-coherent parts of the exciton-phonon packet, we suggest to turn off the phonon wind by the changes in design of the 3D crystal and boundary conditions for the moving droplet.Comment: 13 pages, LaTeX, three eps figures are incorporated by epsf. submitted to Phys. Letters

Tunneling and rattling in clathrate crystal

We present tunneling and rattling motions of an off-center guest atom in cage referring to a clathrate crystal La_3Pd_{20}Ge_6. The elastic constant C_{44} of La_3Pd_{20}Ge_6 shows a Debye-type dispersion around 20 K obeying a relaxation time tau = tau_0exp(E/k_{B}T) with an attempt time tau_0 = 2.0*10^{-12} sec and an activation energy E = 197 K. At low temperatures below 3 K down to 20 mK, the C_{44} shows a softening of C_{44} = C_{44}^0(T-T_C^0)/(T-Theta) with T_C^0 = -337.970 mK and Theta = -338.044 mK. These facts are attributed to two different types of the off-center motions with Gamma_5 symmetry in 4a-site cage of La_3Pd_{20}Ge_6, a thermally activated rattling motion over the potential hill and a tunneling motion through the potential hill at low temperatures.Comment: 5 pages, 4 figures, to be published Phys. Rev.

MHD Stability of Boundary Layer Flow over a Moving Flat Plate

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Electronic structure of Ca1−x_{1-x}Srx_xVO3_3: a tale of two energy-scales

We investigate the electronic structure of Ca$_{1-x}$Sr$_x$VO$_3$ using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy-scales, namely the low energy-scale of thermal excitations (~$k_{B}T$) and the high-energy scale related to Coulomb and other electronic interactions.Comment: 4 pages and 3 figures. Europhysics Letters (appearing

Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling

The description of the tunneling of a macroscopic variable in the presence of a bath of localized spins is a subject of great fundamental and practical interest, and is relevant for many solid-state qubit designs. Instead of focusing on the the "central spin" (as is most often done), here we present a detailed study of the dynamics of the nuclear spin bath in the Mn12-ac single-molecule magnet, probed by NMR experiments down to very low temperatures (T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly suppressed with a longitudinal magnetic field. This is consistent with the nuclear relaxation being caused by quantum tunneling of the molecular spin, and we attribute the tunneling fluctuations to the minority of fast-relaxing molecules present in the sample. The transverse nuclear relaxation is also T-independent for T < 0.8 K, and can be explained qualitatively and quantitatively by the dipolar coupling between like nuclei in neighboring molecules. We also show that the isotopic substitution of 1H by 2H leads to a slower nuclear longitudinal relaxation, consistent with the decreased tunneling probability of the molecular spin. Finally, we demonstrate that, even at the lowest temperatures, the nuclear spins remain in thermal equilibrium with the lattice phonons, and we investigate the timescale for their thermal equilibration. After a review of the theory of macroscopic spin tunneling in the presence of a spin bath, we argue that most of our experimental results are consistent with that theory, but the thermalization of the nuclear spins is not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in quantum nanomagnets, plus an extensive review and application of the theor

Effect of Anharmonicity on the Kondo Phenomena of a Magnetic Ion Vibrating in a Confinement Potential

Effect of anharmonicity of a cage potential for a magnetic ion vibrating in a metal is investigated by the numerical renormalization group method. The cage potential is assumed to be one-dimensional and of the double-well type. In the absence of the Coulomb interaction, we find continuous crossover among the three limiting cases: Yu-Anderson-type Kondo regime, the double-well-type Kondo one, and the renormalized Fermi chain one. In the entire parameter space of the double-well potential, the ground state is described by a local Fermi liquid. In the Yu-Anderson-type Kondo regime, a quantum phase transition to the ground state with odd parity takes place passing through the two-channel Kondo fixed point when the Coulomb interaction increases. Therefore, the vibration of a magnetic ion in an oversized cage structure is a promising route to the two-channel Kondo effect.Comment: 6 pages, 3 figures, accepted for JPS