30 research outputs found
Gapless points of dimerized quantum spin chains: analytical and numerical studies
We study the locations of the gapless points which occur for quantum spin
chains of finite length (with a twisted boundary condition) at particular
values of the nearest neighbor dimerization, as a function of the spin S and
the number of sites. For strong dimerization and large values of S, a tunneling
calculation reproduces the same results as those obtained from more involved
field theoretic methods using the non-linear sigma-model approach. A different
analytical calculation of the matrix element between the two Neel states gives
a set of gapless points; for strong dimerization, these differ significantly
from the tunneling values. Finally, the exact diagonalization method for a
finite number of sites yields a set of gapless points which are in good
agreement with the Neel state calculations for all values of the dimerization,
but the agreement with the tunneling values is not very good even for large S.
This raises questions about possible corrections to the tunneling results.Comment: Revtex4, 10 pages including 5 figure
Dynamical freezing and switching in periodically driven bilayer graphene
A class of integrable models, such as the one-dimensional transverse-field
Ising model, respond nonmonotonically to a periodic drive with respect to the
driving parameters and freezes almost absolutely for certain combinations of
the latter. In this paper, we go beyond the two-band structure of the
Ising-like models studied previously and ask whether such unusual nonmonotonic
response and near-absolute freezing occur in integrable systems with a higher
number of bands. To this end, we consider a tight-binding model for bilayer
graphene subjected to an interlayer potential difference. We find that when the
potential is driven periodically, the system responds nonmonotonically to
variations in the driving amplitude and frequency and shows near
absolute freezing for certain values of . However, the freezing
occurs only in the presence of a constant bias in the driving, i.e., when . When , the freezing is switched off for all
values of . We support our numerical results with analytical
calculations based on a rotating wave approximation. We also give a proposal to
realize the driven bilayer system via ultracold atoms in an optical lattice,
where the driving can be implemented by shaking the lattice.Comment: 12 pages, 6 figure
Periodically driven three-dimensional Kitaev model
We study the dynamics of a three-dimensional generalization of Kitaev's
honeycomb lattice spin model (defined on the hyperhoneycomb lattice) subjected
to a harmonic driving of , one of the three types of spin-couplings in the
Hamiltonian. Using numerical solutions supported by analytical calculations
based on a rotating wave approximation, we find that the system responds
nonmonotonically to variations in the frequency (while keeping the
driving amplitude fixed) and undergoes dynamical freezing, where at
specific values of , it gets almost completely locked in the initial
state throughout the evolution. However, this freezing occurs only when a
constant bias is present in the driving, i.e., when , with . Consequently, the bias acts as a switch that triggers the
freezing phenomenon. Dynamical freezing has been previously observed in other
integrable systems, such as the one-dimensional transverse-field Ising model.Comment: 10 pages, 4 figure
Exactly solvable Kitaev model in three dimensions
We introduce a spin-1/2 model in three dimensions which is a generalization
of the well-known Kitaev model on a honeycomb lattice. Following Kitaev, we
solve the model exactly by mapping it to a theory of non-interacting fermions
in the background of a static Z_2 gauge field. The phase diagram consists of a
gapped phase and a gapless one, similar to the two-dimensional case.
Interestingly, unlike in the two-dimensional model, in the gapless phase the
gap vanishes on a contour in the k space. Furthermore, we show that the flux
excitations of the gauge field, due to some local constraints, form loop like
structures; such loops exist on a lattice formed by the plaquettes in the
original lattice and is topologically equivalent to the pyrochlore lattice.
Finally, we derive a low-energy effective Hamiltonian that can be used to study
the properties of the excitations in the gapped phase.Comment: 9 pages, 7 figures; published version; a new section and more
references adde
Spin-1 chain with spin-1/2 excitations in the bulk
We present a spin-1 chain with a Hamiltonian which has three exactly solvable
ground states. Two of these are fully dimerized, analogous to the
Majumdar-Ghosh (MG) states of a spin-1/2 chain, while the third is of the
Affleck-Kennedy-Lieb-Tasaki (AKLT) type. We use variational and numerical
methods to study the low-energy excitations which interpolate between these
ground states in different ways. In particular, there is a spin-1/2 excitation
which interpolates between the MG and AKLT ground states; this is the lowest
excitation of the system and it has a surprisingly small gap. We discuss
generalizations of our model of spin fractionalization to higher spin chains
and higher dimensions.Comment: 7 pages including 4 figures; this is the published version of the
pape
General topological features and instanton vacuum in quantum Hall and spin liquids
We introduce the concept of super universality in quantum Hall and spin
liquids which has emerged from previous studies. It states that all the
fundamental features of the quantum Hall effect are generically displayed as
general topological features of the parameter in nonlinear sigma
models in two dimensions. To establish super universality in spin liquids we
revisit the mapping by Haldane who argued that the anti ferromagnetic
Heisenberg spin chain is effectively described by the O(3) nonlinear sigma
model with a term. By combining the path integral representation for
the dimerized spin chain with renormalization group decimation
techniques we generalise the Haldane approach to include a more complicated
theory, the fermionic rotor chain, involving four different renormalization
group parameters. We show how the renormalization group calculation technique
can be used to lay the bridge between the fermionic rotor chain and the sigma
model. As an integral and fundamental aspect of the mapping we establish the
topological significance of the dangling spin at the edge of the chain which is
in all respects identical to the massless chiral edge excitations in quantum
Hall liquids. We consider various different geometries of the spin chain and
show that for each of the different geometries correspond to a topologically
equivalent quantum Hall liquid.Comment: Title changed, Section 2 and Appendix expanded, an error in the
expression for theta correcte
Generalised Shastry-Sutherland Models in three and higher dimensions
We construct Heisenberg anti-ferromagnetic models in arbitrary dimensions
that have isotropic valence bond crystals (VBC) as their exact ground states.
The d=2 model is the Shastry-Sutherland model. In the 3-d case we show that it
is possible to have a lattice structure, analogous to that of SrCu_2(BO_3)_2,
where the stronger bonds are associated with shorter bond lengths. A dimer mean
field theory becomes exact at d -> infinity and a systematic 1/d expansion can
be developed about it. We study the Neel-VBC transition at large d and find
that the transition is first order in even but second order in odd dimensions.Comment: Published version; slightly expande
Effects of Experimental Sarcocystis neurona
Sarcocystis neurona is the most common cause of Equine Protozoal Myeloencephalitis (EPM), affecting 0.5–1% horses in the United States during their lifetimes. The objective of this study was to evaluate the equine immune responses in an experimentally induced Sarcocystis neurona infection model. Neurologic parameters were recorded prior to and throughout the 70-day study by blinded investigators. Recombinant SnSAG1 ELISA for serum and CSF were used to confirm and track disease progression. All experimentally infected horses displayed neurologic signs after infection. Neutrophils, monocytes, and lymphocytes from infected horses displayed significantly delayed apoptosis at some time points. Cell proliferation was significantly increased in S. neurona-infected horses when stimulated nonspecifically with PMA/I but significantly decreased when stimulated with S. neurona compared to controls. Collectively, our results suggest that horses experimentally infected with S. neurona manifest impaired antigen specific response to S. neurona, which could be a function of altered antigen presentation, lack of antigen recognition, or both