9,509 research outputs found
Spin Polarisability of the Nucleon in the Heavy Baryon Effective Field Theory
We have constructed a heavy baryon effective field theory with photon as an
external field in accordance with the symmetry requirements similar to the
heavy quark effective field theory. By treating the heavy baryon and
anti-baryon equally on the same footing in the effective field theory, we have
calculated the spin polarisabilities of the nucleon at
third order and at fourth-order of the spin-dependent Compton scattering. At
leading order (LO), our results agree with the corresponding results of the
heavy baryon chiral perturbation theory, at the next-to-leading order(NLO) the
results show a large correction to the ones in the heavy baryon chiral
perturbation theory due to baryon-antibaryon coupling terms. The low energy
theorem is satisfied both at LO and at NLO. The contributions arising from the
heavy baryon-antibaryon vertex were found to be significant and the results of
the polarisabilities obtained from our theory is much closer to the
experimental data.Comment: 21pages, title changed, minimal correction
Renormalization of the Cabibbo-Kobayashi-Maskawa Quark Mixing Matrix
We have investigated the present renormalization prescriptions of
Cabibbo-Kobayashi-Maskawa (CKM) matrix. When considering the prescription which
is formulated with reference to the case of zero mixing we find it doesn't
satisfy the unitary condition of the bare CKM matrix. After added a delicate
patch this problem can be solved at one-loop level. In this paper We generalize
this prescription to all loop levels and keep the unitarity of the bare CKM
matrix, simultaneously make the amplitude of an arbitrary physical process
involving quark mixing convergent and gauge independent. We also find that in
order to keep the CKM counterterms gauge independent the unitarity of the bare
CKM matrix must be preserved.Comment: has been revised, 8 pages, 1 figur
Multiple stellar populations in the Galactic globular cluster NGC 6752
We have carried out high-precision photometry on a large number of archival
HST images of the Galactic globular cluster NGC 6752, to search for signs of
multiple stellar populations. We find a broadened main sequence, and
demonstrate that this broadening cannot be attributed either to binaries or to
photometric errors. There is also some indication of a main-sequence split. No
significant spread could be found along the subgiant branch, however.
Ground-based photometry reveals that in the U vs. (U-B) color-magnitude
diagram the red-giant branch exhibits a clear color spread, which we have been
able to correlate with variations in Na and O abundances. In particular the
Na-rich, O-poor stars identified by Carretta et al. (2007) define a sequence on
the red side of the red-giant branch, while Na-poor, O-rich stars populate a
bluer, more dispersed portion of the red-giant branch.Comment: 31 pages, 12 figures; Accepted for Publication in the Astrophysical
Journa
Optical conductivity in the normal state fullerene superconductors
We calculate the optical conductivity, , in the normal state
fullerene superconductors by self-consistently including the impurity
scatterings, the electron-phonon and electron-electron Coulomb interactions.
The finite bandwidth of the fullerenes is explicitely considered, and the
vertex corection is included Nambu in calculating the renormalized
Green's function. is obtained by calculating the
current-current correlation function with the renormalized Green's function in
the Matsubara frequency and then performing analytic continuation to the real
frequency at finite temperature. The Drude weight in is
strongly suppressed due to the interactions and transfered to the mid-infrared
region around and above 0.06 eV which is somewhat less pronounced and much
broader compared with the expermental observation by DeGiorgi .Comment: 6 pages, 4 figures. To be published in Physical Review B, July 1
A New Method of Probing the Phonon Mechanism in Superconductors including MgB
Weak localization has a strong influence on both the normal and
superconducting properties of metals. In particular, since weak localization
leads to the decoupling of electrons and phonons, the temperature dependence of
resistance (i.e., ) is decreasing with increasing disorder, as
manifested by Mooij's empirical rule. In addition, Testardi's universal
correlation of (i.e., ) and the resistance ratio (i.e.,
) follows. This understanding provides a new means to probe the
phonon mechanism in superconductors including MgB. The merits of this
method are its applicability to any superconductors and its reliability because
the McMillan's electron-phonon coupling constant and
change in a broad range, from finite values to zero, due to weak localization.
Karkin et al's preliminary data of irradiated MgB show the Testardi
correlation, indicating that the dominant pairing mechanism in MgB is the
phonon-mediated interaction.Comment: 9 pages, latex, 3 figure
Coulomb suppression of NMR coherence peak in fullerene superconductors
The suppressed NMR coherence peak in the fullerene superconductors is
explained in terms of the dampings in the superconducting state induced by the
Coulomb interaction between conduction electrons. The Coulomb interaction,
modelled in terms of the onsite Hubbard repulsion, is incorporated into the
Eliashberg theory of superconductivity with its frequency dependence considered
self-consistently at all temperatures. The vertex correction is also included
via the method of Nambu. The frequency dependent Coulomb interaction induces
the substantial dampings in the superconducting state and, consequently,
suppresses the anticipated NMR coherence peak of fullerene superconductors as
found experimentally.Comment: 4 pages, Revtex, and 2 figures. Revised and final version to appear
in Phys. Rev. Lett. (1998
Unusual T_c variation with hole concentration in Bi_2Sr_{2-x}La_xCuO_{6+\delta}
We have investigated the variation with the hole concentration in
the La-doped Bi 2201 system, BiSrLaCuO. It is
found that the Bi 2201 system does not follow the systematics in and
observed in other high- cuprate superconductors (HTSC's). The vs
characteristics are quite similar to what observed in Zn-doped HTSC's. An
exceptionally large residual resistivity component in the inplane resistivity
indicates that strong potential scatterers of charge carriers reside in CuO
planes and are responsible for the unusual variation with , as in the
Zn-doped systems. However, contrary to the Zn-doped HTSC's, the strong scatter
in the Bi 2201 system is possibly a vacancy in the Cu site.Comment: RevTeX, 3 figures, to be published in the Physical Review
Sommerfeld's quantum condition of action and the spectra of Schwarzschild black hole
If the situation of quantum gravity nowadays is nearly the same as that of
the quantum mechanics in it's early time of Bohr and Sommerfeld, then a first
step study of the quantum gravity from Sommerfeld's quantum condition of action
might be helpful. In this short paper the spectra of Schwarzschild black
hole(SBH) in quasi-classical approach of quantum mechanics is given. We find
the quantum of area, the quantum of entropy and the Hawking evaporation will
cease as the black hole reaches its ground state.Comment: 7 pages, no figures, submitted to Classical and Quantum Gravit
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