41,839 research outputs found
Strong Correlations and Magnetic Frustration in the High Tc Iron Pnictides
We consider the iron pnictides in terms of a proximity to a Mott insulator.
The superexchange interactions contain competing nearest-neighbor and
next-nearest-neighbor components. In the undoped parent compound, these
frustrated interactions lead to a two-sublattice collinear antiferromagnet
(each sublattice forming a Neel ordering), with a reduced magnitude for the
ordered moment. Electron or hole doping, together with the frustration effect,
suppresses the magnetic ordering and allows a superconducting state. The
exchange interactions favor a d-wave superconducting order parameter; in the
notation appropriate for the Fe square lattice, its orbital symmetry is
. A number of existing and future experiments are discussed in light of
the theoretical considerations.Comment: (v2) 4+ pages, 4 figures, discussions on several points expanded;
references added. To appear in Phys. Rev. Let
Spin properties of top quark pairs produced at hadron colliders
We discuss the spin properties of top quark pairs produced at hadron
colliders at next-to-leading order in the coupling constant alpha_s of the
strong interaction. Specifically we present, for some decay channels, results
for differential angular distributions that are sensitive to t tbar spin
correlations.Comment: Invited talk given by A. Brandenburg at the Cracow epiphany
conference on heavy flavours, 3 - 6 January 2003, Cracow, Polan
Investigation of Top quark spin correlations at hadron collider
We report on our results about hadronic production at NLO QCD
including spin effects, especially on spin correlations.Comment: talk given at the 32nd International Conference on High Energy
Physics (ICHEP'04), Beijing, China, 16-22 Aug. 200
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
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