The Finite-UU Impurity Anderson Model in the presence of an external magnetic field

We have investigated effects of an external magnetic field in the impurity Anderson model with a finite on-site Coulomb repulsion $U$. Large $N_f$ expansion is employed in the slave boson representation, by taking into account $f^0$, $f^1$, and $f^2$ subspaces. To evaluate the vertex function for the ``empty state boson" self-energy, we have devised two approximations which reduce much computational efforts without losing general features of the model. It is found that the Kondo temperature is reduced by the presence of a magnetic field, and that at low field and at low temperature, the field dependence of both the Kondo temperature and the impurity magnetization exhibits a scaling behavior with high accuracy. Further, some interesting features are found in the field dependence of the impurity magnetization at finite temperature, the physical implications of which are discussed in terms of the renormalized Kondo temperature.Comment: 18 pages Revtex, 7 Postscript figures, To appear in Phys.Rev.

Electronic structure of YbB6_{6}: Is it a Topological Insulator or not?

To resolve the controversial issue of the topological nature of the electronic structure of YbB$_{6}$, we have made a combined study using density functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES). Accurate determination of the low energy band topology in DFT requires the use of modified Becke-Johnson exchange potential incorporating the spin-orbit coupling and the on-site Coulomb interaction $U$ of Yb $4f$ electrons as large as 7 eV. We have double-checked the DFT result with the more precise GW band calculation. ARPES is done with the non-polar (110) surface termination to avoid band bending and quantum well confinement that have confused ARPES spectra taken on the polar (001) surface termination. Thereby we show definitively that YbB$_{6}$ has a topologically trivial B 2$p$-Yb 5$d$ semiconductor band gap, and hence is a non-Kondo non-topological insulator (TI). In agreement with theory, ARPES shows pure divalency for Yb and a $p$-$d$ band gap of 0.3 eV, which clearly rules out both of the previous scenarios of $f$-$d$ band inversion Kondo TI and $p$-$d$ band inversion non-Kondo TI. We have also examined the pressure-dependent electronic structure of YbB$_{6}$, and found that the high pressure phase is not a Kondo TI but a \emph{p}-\emph{d} overlap semimetal.Comment: The main text is 6 pages with 4 figures, and the supplementary information contains 6 figures. 11 pages, 10 figures in total To be appeared in Phys. Rev. Lett. (Online publication is around March 16 if no delays.

Effects of the Spin-Orbit Coupling and the Superconductivity in simple-cubic alpha-Polonium

We have investigated the mechanism of stabilizing the simple-cubic (SC) structure in polonium (alpha- Po), based on the phonon dispersion calculations using the first-principles all-electron band method. We have demonstrated that the stable SC structure results from the suppression of the Peierls instability due to the strong spin-orbit coupling (SOC) in alpha-Po. Further, we have explored the possible superconductivity in alpha-Po, and predicted that it becomes a superconductor with Tc ~ 4 K. The transverse soft phonon mode at q ~ 2/3 R, which is greatly influenced by the SOC, plays an important role both in the structural stability and the superconductivity in alpha-Po. We have discussed effects of the SOC and the volume variation on the phonon dispersions and superconducting properties of alpha-Po.Comment: 5pages, 5figure

Gauge/String-Gravity Duality and Froissart Bound

The gauge/string-gravity duality correspondence opened renewed hope and possibility to address some of the fundamental and non-perturbative QCD problems in particle physics, such as hadron spectrum and Regge behavior of the scattering amplitude at high energies. One of the most fundamental and long-standing problem is the high energy behavior of total cross-sections. According to a series of exhaustive tests by the COMPETE group, (1). total cross-sections have a universal Heisenberg behavior in energy corresponding to the maximal energy behavior allowed by the Froissart bound, i.e., $A + B ln^2 (s/s_0)$ with $B \sim 0.32 mb$ and $s_0 \sim 34.41 GeV^2$ for all reactions, and (2). the factorization relation among $\sigma_{pp, even}, \sigma_{\gamma p}, and \sigma_{\gamma \gamma}$ is well satisfied by experiments. I discuss the recent interesting application of the gauge/string-gravity duality of $AdS/CFT$ correspondence with a deformed background metric so as to break the conformal symmetry that can lead to the Heisenberg behavior of rising total cross-sections, and present some preliminary results on the high energy QCD from Planckian scattering in $AdS$ and black-hole production.Comment: 10 pages, Presented to the Coral Gables Conference 2003, Launching of BelleE\'poque in High Energy Physics and Cosmology, 17 - 21 December 2003, Fort Lauderdale, Florid

Nuclear Modification to Parton Distribution Functions and Parton Saturation

We introduce a generalized definition of parton distribution functions (PDFs) for a more consistent all-order treatment of power corrections. We present a new set of modified DGLAP evolution equations for nuclear PDFs, and show that the resummed $\alpha_s A^{1/3}/Q^2$-type of leading nuclear size enhanced power corrections significantly slow down the growth of gluon density at small-$x$. We discuss the relation between the calculated power corrections and the saturation phenomena.Comment: 4 pages, to appear in the proceedings of QM200