177,320 research outputs found
Heisenberg antiferromagnet on Cayley trees: low-energy spectrum and even/odd site imbalance
To understand the role of local sublattice imbalance in low-energy spectra of
s=1/2 quantum antiferromagnets, we study the s=1/2 quantum nearest neighbor
Heisenberg antiferromagnet on the coordination 3 Cayley tree. We perform
many-body calculations using an implementation of the density matrix
renormalization group (DMRG) technique for generic tree graphs. We discover
that the bond-centered Cayley tree has a quasidegenerate set of a low-lying
tower of states and an "anomalous" singlet-triplet finite-size gap scaling. For
understanding the construction of the first excited state from the many-body
ground state, we consider a wave function ansatz given by the single-mode
approximation, which yields a high overlap with the DMRG wave function.
Observing the ground-state entanglement spectrum leads us to a picture of the
low-energy degrees of freedom being "giant spins" arising out of sublattice
imbalance, which helps us analytically understand the scaling of the
finite-size spin gap. The Schwinger-boson mean-field theory has been
generalized to nonuniform lattices, and ground states have been found which are
spatially inhomogeneous in the mean-field parameters.Comment: 19 pages, 12 figures, 6 tables. Changes made to manuscript after
referee suggestions: parts reorganized, clarified discussion on Fibonacci
tree, typos correcte
Dynamics of a two-level system under the simultaneous influence of a spin bath and a boson bath
We study dynamics of a two-level system coupled simultaneously to a pair of
dissimilar reservoirs, namely, a spin bath and a boson bath, which are
connected via finite interbath coupling. It is found that the steady-state
energy transfer in the two-level system increases with its coupling to the spin
bath while optimal transfer occurs at intermediate coupling in the transient
process. If the two-level system is strongly coupled to the spin bath, the
population transfer is unidirectional barring minor population oscillations of
minute amplitudes. If the spin bath is viewed as an atomic ensemble, robust
generation of macroscopic superposition states exists against parameter
variations of the two-level system and the boson bath.Comment: 12 pages, 4 figure
High-order Harmonic Generation and Dynamic Localization in a driven two-level system, a non-perturbative solution using the Floquet-Green formalism
We apply the Floquet-Green operator formalism to the case of a
harmonically-driven two-level system. We derive exact expressions for the
quasi-energies and the components of the Floquet eigenstates with the use of
continued fractions. We study the avoided crossings structure of the
quasi-energies as a function of the strength of the driving field and give an
interpretation in terms of resonant multi-photon processes. From the Floquet
eigenstates we obtain the time-evolution operator. Using this operator we study
Dynamic Localization and High-order Harmonic Generation in the non-perturbative
regime
Effect of selection on ancestry: an exactly soluble case and its phenomenological generalization
We consider a family of models describing the evolution under selection of a
population whose dynamics can be related to the propagation of noisy traveling
waves. For one particular model, that we shall call the exponential model, the
properties of the traveling wave front can be calculated exactly, as well as
the statistics of the genealogy of the population. One striking result is that,
for this particular model, the genealogical trees have the same statistics as
the trees of replicas in the Parisi mean-field theory of spin glasses. We also
find that in the exponential model, the coalescence times along these trees
grow like the logarithm of the population size. A phenomenological picture of
the propagation of wave fronts that we introduced in a previous work, as well
as our numerical data, suggest that these statistics remain valid for a larger
class of models, while the coalescence times grow like the cube of the
logarithm of the population size.Comment: 26 page
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