29,743 research outputs found
Quantum criticality of the sub-ohmic spin-boson model
We revisit the critical behavior of the sub-ohmic spin-boson model. Analysis
of both the leading and subleading terms in the temperature dependence of the
inverse static local spin susceptibility at the quantum critical point,
calculated using a numerical renormalization-group method, provides evidence
that the quantum critical point is interacting in cases where the
quantum-to-classical mapping would predict mean-field behavior. The subleading
term is shown to be consistent with an w/T scaling of the local dynamical
susceptibility, as is the leading term. The frequency and temperature
dependences of the local spin susceptibility in the strong-coupling
(delocalized) regime are also presented. We attribute the violation of the
quantum-to-classical mapping to a Berry-phase term in a continuum path-integral
representation of the model. This effect connects the behavior discussed here
with its counterparts in models with continuous spin symmetry.Comment: 9 pages, 10 figure
Magnetic anisotropy of YbNi4P2
We report on transport and magnetic measurements between 1.8 and 400 K on
single crystalline YbNi4P2, which was recently reported to be a heavy fermion
system with a low lying ferromagnetic transition at T_C=0.17 K, based on data
from polycrystals. The tetragonal crystal structure of YbNi4P2 presents
quasi-one-dimensional Yb chains along the c direction. Here we show that at
high temperatures, the magnetic anisotropy of YbNi4P2 is dominated by the
crystal electrical field effect with an Ising-type behaviour, which gets more
pronounced towards lower temperatures. The electrical resistivity also reflects
the strong anisotropy of the crystal structure and favours transport along c,
the direction of the Yb chains.Comment: SCES 2011 proceedings, in pres
Top Quark Pair Production and Decay including Spin Effects at Hadron Colliders: Predictions at NLO QCD
Top quark-antiquark () pairs will be produced copiously at the
Tevatron collider and in huge numbers at the LHC. This will make possible
detailed investigations of the properties and interactions of this quark
flavor. The analysis and interpretation of future data requires precise
predictions of the hadronic production of pairs and of their
subsequent decays. In this talk the reactions are considered and results are presented of our calculation
of the dilepton angular distribution at next-to-leading order QCD, keeping the
full dependence on the spins of the intermediate state. The angular
distribution is determined for different choices of reference axes that can be
identified with the and spin axes. While the QCD corrections to
the leading-order distribution turn out to be small in the case of the LHC, we
find them to be sizeable in the case of the Tevatron and find, moreover, the
angular distribution to be sensitive to the parton content of the proton.Comment: Talk given at 3rd Circum-Pan-Pacific Symposium on "High Energy Spin
Physics", Beijing, China, 8-13, 200
The contributions of components to the axial charges of proton and its resonances
We calculate the axial charges of the proton and its resonances in the
framework of the constituent quark model, which is extended to include the
components. If 20% admixtures of the components in
the proton are assumed, the theoretical value for the axial charge in our model
is in good agreement with the empirical value, which can not be well reproduced
in the traditional constituent quark model even though the symmetry breaking or relativistic effect is taken into account. We also
predict an unity axial charge for with 30%
components constrained by the strong and electromagnetic decays.Comment: 4 pages, 4 table
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