61,040 research outputs found

### The Finite-$U$ 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 YbB$_{6}$: 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

- …