118,701 research outputs found
Evidence for very strong electron-phonon coupling in YBa_{2}Cu_{3}O_{6}
From the observed oxygen-isotope shift of the mid-infrared two-magnon
absorption peak of YBaCuO, we evaluate the oxygen-isotope
effect on the in-plane antiferromagnetic exchange energy . The exchange
energy in YBaCuO is found to decrease by about 0.9% upon
replacing O by O, which is slightly larger than that (0.6%) in
LaCuO. From the oxygen-isotope effects, we determine the lower
limit of the polaron binding energy, which is about 1.7 eV for
YBaCuO and 1.5 eV for LaCuO, in quantitative
agreement with angle-resolved photoemission data, optical conductivity data,
and the parameter-free theoretical estimate. The large polaron binding energies
in the insulating parent compounds suggest that electron-phonon coupling should
also be strong in doped superconducting cuprates and may play an essential role
in high-temperature superconductivity.Comment: 4 pages, 1 figur
Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation
The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth
in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by
muon-spin rotation. A pronounced OIE on was observed
with a relative isotope shift of
=-5(2)% for x =0.3 and -9(2)% for
x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane
effective mass . The OIE exponents of T_{c} and of
exhibit a relation that appears to be generic for
cuprate superconductors.Comment: 4 pages, 4 eps figures, RevTex
Speed of Meridional Flows and Magnetic Flux Transport on the Sun
We use the magnetic butterfly diagram to determine the speed of the magnetic
flux transport on the solar surface towards the poles. The manifestation of the
flux transport is clearly visible as elongated structures extended from the
sunspot belt to the polar regions. The slopes of these structures are measured
and interpreted as meridional magnetic flux transport speed. Comparison with
the time-distance helioseismology measurements of the mean speed of the
meridional flows at the depth of 3.5--12 Mm shows a generally good agreement,
but the speeds of the flux transport and the meridional flow are significantly
different in areas occupied by the magnetic field. The local circulation flows
around active regions, especially the strong equatorward flows on the
equatorial side of active regions affect the mean velocity profile derived by
helioseismology, but do not influence the magnetic flux transport. The results
show that the mean longitudinally averaged meridional flow measurements by
helioseismology may not be used directly in solar dynamo models for describing
the magnetic flux transport, and that it is necessary to take into account the
longitudinal structure of these flows.Comment: 4 pages, 3 figures, accepted in ApJ Letter
Possible isotope effect on the resonance peak formation in high-T cuprates
Starting from the three-band Hubbard Hamiltonian we derive an effective
model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient which can be further tested
experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004);
in pres
Representation theory of the stabilizer subgroup of the point at infinity in Diff(S^1)
The group Diff(S^1) of the orientation preserving diffeomorphisms of the
circle S^1 plays an important role in conformal field theory. We consider a
subgroup B_0 of Diff(S^1) whose elements stabilize "the point of infinity".
This subgroup is of interest for the actual physical theory living on the
punctured circle, or the real line. We investigate the unique central extension
K of the Lie algebra of that group. We determine the first and second
cohomologies, its ideal structure and the automorphism group. We define a
generalization of Verma modules and determine when these representations are
irreducible. Its endomorphism semigroup is investigated and some unitary
representations of the group which do not extend to Diff(S^1) are constructed.Comment: 34 pages, no figur
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