13,354 research outputs found
Pathology of Schwinger boson mean field theory for Heisenberg spin models
We have re-analyze the Schwinger boson mean field theory (SBMFT) for
Heisenberg spin models on the cubic lattice. We find that the second order
phase transition point for magnetic ordering previously reported corresponds to
a local maximum of the free energy functional. For both ferromagnetic and
antiferromagnetic Heisenberg models with spin , where ,
the mean field transitions are first order from the magnetically long-ranged
ordered phase to the completely uncorrelated phase. In addition to erroneously
giving a first order transition for magnetic ordering, the mean field theory
does not include a phase with finite short-range correlation, thus negating one
of the prime advantages of SBMFT. The relevance of these pathologies to other
situations beyond the cubic lattice is discussed.Comment: 15 pages including 6 postscript figure
Theory for spin and orbital orderings in high temperature phase in
Motivated by the recent neutron diffraction experiment on , we
consider a microscopic model where each ion is occupied by two 3d
electrons of parallel spins with two fold degenerate orbital configurations.
The mean field classical solutions of the spin-orbital superexchange model
predicts an antiferro-orbital ordering at a higher temperature followed by a
C-type antiferromagnetic spin ordering at a lower temperature. Our results are
qualitatively consistent with the observed orbital phase transition at and the spin phase transition at in .Comment: 7 pages, 3 figures and 2 tables. Accepted to be published in PR
3d Modularity
We find and propose an explanation for a large variety of modularity-related
symmetries in problems of 3-manifold topology and physics of 3d
theories where such structures a priori are not manifest. These modular
structures include: mock modular forms, Weil
representations, quantum modular forms, non-semisimple modular tensor
categories, and chiral algebras of logarithmic CFTs.Comment: 119 pages, 10 figures and 20 table
Low Scale Non-universal, Non-anomalous U(1)'_F in a Minimal Supersymmetric Standard Model
We propose a non-universal U(1)'_F symmetry combined with the Minimal
Supersymmetric Standard Model. All anomaly cancellation conditions are
satisfied without exotic fields other than three right-handed neutrinos.
Because our model allows all three generations of chiral superfields to have
different U(1)'_F charges, upon the breaking of the U(1)'_F symmetry at a low
scale, realistic masses and mixing angles in both the quark and lepton sectors
are obtained. In our model, neutrinos are predicted to be Dirac fermions and
their mass ordering is of the inverted hierarchy type. The U(1)'_F charges of
the chiral super-fields also naturally suppress the mu term and automatically
forbid baryon number and lepton number violating operators. While all
flavor-changing neutral current constraints in the down quark and charged
lepton sectors can be satisfied, we find that constraint from D0-D0bar turns
out to be much more stringent than the constraints from the precision
electroweak data.Comment: 21 pages, 2 figures; v2: discussion on sparticle mass spectrum
included, 27 pages, 2 figure
Mott scattering at the interface between a metal and a topological insulator
We compute the spin-active scattering matrix and the local spectrum at the
interface between a metal and a three-dimensional topological band insulator.
We show that there exists a critical incident angle at which complete (100%)
spin flip reflection occurs and the spin rotation angle jumps by . We
discuss the origin of this phenomena, and systematically study the dependence
of spin-flip and spin-conserving scattering amplitudes on the interface
transparency and metal Fermi surface parameters. The interface spectrum
contains a well-defined Dirac cone in the tunneling limit, and smoothly evolves
into a continuum of metal induced gap states for good contacts. We also
investigate the complex band structure of BiSe.Comment: published versio
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