2,862,288 research outputs found
Impurity Energy Level Within The Haldane Gap
An impurity bond in a periodic 1D antiferromagnetic, spin 1 chain with
exchange is considered. Using the numerical density matrix renormalization
group method, we find an impurity energy level in the Haldane gap,
corresponding to a bound state near the impurity bond. When the level
changes gradually from the edge of the Haldane gap to the ground state energy
as the deviation changes from 0 to 1. It seems that there is
no threshold. Yet, there is a threshold when . The impurity level
appears only when the deviation is greater than ,
which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4
figure
The spin-half Heisenberg antiferromagnet on two Archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond
We investigate the ground state of the two-dimensional Heisenberg
antiferromagnet on two Archimedean lattices, namely, the maple-leaf and bounce
lattices as well as a generalized - model interpolating between both
systems by varying from (bounce limit) to (maple-leaf
limit) and beyond. We use the coupled cluster method to high orders of
approximation and also exact diagonalization of finite-sized lattices to
discuss the ground-state magnetic long-range order based on data for the
ground-state energy, the magnetic order parameter, the spin-spin correlation
functions as well as the pitch angle between neighboring spins. Our results
indicate that the "pure" bounce () and maple-leaf () Heisenberg
antiferromagnets are magnetically ordered, however, with a sublattice
magnetization drastically reduced by frustration and quantum fluctuations. We
found that magnetic long-range order is present in a wide parameter range and that the magnetic order parameter varies only
weakly with . At a direct first-order transition to
a quantum orthogonal-dimer singlet ground state without magnetic long-range
order takes place. The orthogonal-dimer state is the exact ground state in this
large- regime, and so our model has similarities to the Shastry-Sutherland
model. Finally, we use the exact diagonalization to investigate the
magnetization curve. We a find a 1/3 magnetization plateau for and another one at 2/3 of saturation emerging only at large .Comment: 9 pages, 10 figure
Majorana neutrinos and lepton-number-violating signals in top-quark and W-boson rare decays
We discuss rare lepton-number-violating top-quark and W-boson four-body
decays to final states containing a same-charge lepton pair, of the same or of
different flavors: t -> b W- li^+ lj^+ and W+ -> J J' li^+ lj^+, where i \ne j
or i=j and J J' stands for two light jets originating from a (u-bar d) or a
(c-bar s) pair. These \Delta L=2 decays are forbidden in the Standard Model and
may be mediated by exchanges of Majorana neutrinos. We adopt a model
independent approach for the Majorana neutrinos mixing pattern and calculate
the branching ratios (BR) for these decays. We find, for example, that for O(1)
mixings between heavy and light Majorana neutrinos (not likely but not ruled
out) and if at least one of the heavy Majorana neutrinos has a mass of ~100
GeV, then the BR's for these decays are: BR(t -> b li^+ lj^+ W-) ~ 10^{-4} and
BR(W+ -> li^+ lj^+ J J') ~ 10^{-7} if m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J
J') ~ BR(W+ -> li^+ lj^+ J J') ~ 0.01 if m_N < 50 GeV. Taking into account the
present limits on the neutrino mixing parameters, we obtain more realistic
values for these BR's: BR(t -> b li^+ lj^+ W-) ~ 10^{-6} and BR(W+ -> li^+ lj^+
J J') ~ 10^{-10} for m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J J') ~ BR(W+ ->
li^+ lj^+ J J') ~ 10^{-6} for m_N < 50 GeV.Comment: latex, 7 pages, 2 figures. V2 as published in PL
Impurity state in Haldane gap for S=1 Heisenberg antiferromagnetic chain with bond doping
Using a new impurity density matrix renormalization group scheme, we
establish a reliable picture of how the low lying energy levels of a
Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond
doping. A new impurity state gradually occurs in the Haldane gap as ,
while it appears only if with as . The
system is non-perturbative as . This explains the
appearance of a new state in the Haldane gap in a recent experiment on
YCaBaNiO [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].Comment: 4 pages of uuencoded gzip'd postscrip
Universality in the partially anisotropic three-dimensional Ising lattice
Using transfer-matrix extended phenomenological renormalization-group methods
the critical properties of spin-1/2 Ising model on a simple-cubic lattice with
partly anisotropic coupling strengths are studied.
Universality of both fundamental critical exponents and is
confirmed. It is shown that the critical finite-size scaling amplitude ratios
, , and
are independent of the lattice anisotropy
parameter . By this for the last above invariant of the
three-dimensional Ising universality class we give the first quantitative
estimate: (shape , periodic boundary
conditions in both transverse directions).Comment: 11 pages in latex; no figure
Supersymmetry of Tensionless Rotating Strings in AdS_5 x S^5, and Nearly-BPS Operators
It is shown that a class of rotating strings in AdS_5 x S^5 with SO(6)
angular momenta (J,J',J') preserve 1/8-supersymmetry for large J,J', in which
limit they are effectively tensionless; when J=0, supersymmetry is enhanced to
1/4. These results imply that recent checks of the AdS/CFT correspondence
actually test a nearly-BPS sector.Comment: 12 pages, no figures; v2: new section on CFT operators and new
references added, discussion section and acknowledgements modified, abstract
rephrashe
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