9,334 research outputs found
Photoluminescence and spectral switching of single CdSe/ZnS colloidal nanocrystals in poly(methyl methacrylate)
Emission from single CdSe nanocrystals in PMMA was investigated. A fraction
of the nanocrystals exhibiting switching between two energy states, which have
similar total intensities, but distinctly different spectra were observed. We
found that the spectral shift characteristic frequency increases with the pump
power. By using the dynamic shift in the spectral position of emission peaks,
we were able to correlate peaks from the same nanocrystal. The measured
correlation is consistent with assignment of low energy lines to phonon
replicas.Comment: 5 pages, 4 figure
Neutron-Proton Correlations in an Exactly Solvable Model
We examine isovector and isoscalar neutron-proton correlations in an exactly
solvable model based on the algebra SO(8). We look particularly closely at
Gamow-Teller strength and double beta decay, both to isolate the effects of the
two kinds of pairing and to test two approximation schemes: the renormalized
neutron-proton QRPA (RQRPA) and generalized BCS theory. When isoscalar pairing
correlations become strong enough a phase transition occurs and the dependence
of the Gamow-Teller beta+ strength on isospin changes in a dramatic and
unfamiliar way, actually increasing as neutrons are added to an N=Z core.
Renormalization eliminates the well-known instabilities that plague the QRPA as
the phase transition is approached, but only by unnaturally suppressing the
isoscalar correlations. Generalized BCS theory, on the other hand, reproduces
the Gamow-Teller strength more accurately in the isoscalar phase than in the
usual isovector phase, even though its predictions for energies are equally
good everywhere. It also mixes T=0 and T=1 pairing, but only on the isoscalar
side of the phase transition.Comment: 13 pages + 11 postscript figures, in RevTe
Tailoring of motional states in double-well potentials by time-dependent processes
We show that the vibrational state tailoring method developed for molecular
systems can be applied for cold atoms in optical lattices. The original method
is based on a three-level model interacting with two strong laser pulses in a
counterintuitive sequence [M. Rodriguez et al., Phys. Rev. A 62, 053413
(2000)]. Here we outline the conditions for achieving similar dynamics with
single time-dependent potential surfaces. It is shown that guided switching
between diabatic and adiabatic evolution has an essential role in this system.
We also show that efficient and precise tailoring of motional states in optical
lattices can be achieved, for instance, simply by superimposing two lattices
and moving them with respect to each other.Comment: 9 pages, 11 figures, 25 references; accepted to PRA; v2: minor
explanatory remarks added & typos correcte
Quantum integrable system with two color components in two dimensions
The Davey-Stewartson 1(DS1) system[9] is an integrable model in two
dimensions. A quantum DS1 system with 2 colour-components in two dimensions has
been formulated. This two-dimensional problem has been reduced to two
one-dimensional many-body problems with 2 colour-components. The solutions of
the two-dimensional problem under consideration has been constructed from the
resulting problems in one dimensions. For latters with the -function
interactions and being solved by the Bethe ansatz, we introduce symmetrical and
antisymmetrical Young operators of the permutation group and obtain the exact
solutions for the quantum DS1 system. The application of the solusions is
discussed.Comment: 14 pages, LaTeX fil
Discrete Razumikhin-type technique and stability of the Euler-Maruyama method to stochastic functional differential equations
A discrete stochastic Razumikhin-type theorem is established to investigate whether the Euler--Maruyama (EM) scheme can reproduce the moment exponential stability of exact solutions of stochastic functional differential equations (SFDEs). In addition, the Chebyshev inequality and the Borel-Cantelli lemma are applied to show the almost sure stability of the EM approximate solutions of SFDEs. To show our idea clearly, these results are used to discuss stability of numerical solutions of two classes of special SFDEs, including stochastic delay differential equations (SDDEs) with variable delay and stochastically perturbed equations
Boson mappings and four-particle correlations in algebraic neutron-proton pairing models
Neutron-proton pairing correlations are studied within the context of two
solvable models, one based on the algebra SO(5) and the other on the algebra
SO(8). Boson-mapping techniques are applied to these models and shown to
provide a convenient methodological tool both for solving such problems and for
gaining useful insight into general features of pairing. We first focus on the
SO(5) model, which involves generalized T=1 pairing. Neither boson mean-field
methods nor fermion-pair approximations are able to describe in detail
neutron-proton pairing in this model. The analysis suggests, however, that the
boson Hamiltonian obtained from a mapping of the fermion Hamiltonian contains a
pairing force between bosons, pointing to the importance of boson-boson (or
equivalently four-fermion) correlations with isospin T=0 and spin S=0. These
correlations are investigated by carrying out a second boson mapping. Closed
forms for the fermion wave functions are given in terms of the fermion-pair
operators. Similar techniques are applied -- albeit in less detail -- to the
SO(8) model, involving a competition between T=1 and T=0 pairing. Conclusions
similar to those of the SO(5) analysis are reached regarding the importance of
four-particle correlations in systems involving neutron-proton pairing.Comment: 31 pages, Latex, 3 Postscript figures, uses epsf.sty, submitted to
Physical Review
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