27 research outputs found
q-Symmetries in DNLS-AL chains and exact solutions of quantum dimers
Dynamical symmetries of Hamiltonians quantized models of discrete non-linear
Schroedinger chain (DNLS) and of Ablowitz-Ladik chain (AL) are studied. It is
shown that for -sites the dynamical algebra of DNLS Hamilton operator is
given by the algebra, while the respective symmetry for the AL case is
the quantum algebra su_q(n). The q-deformation of the dynamical symmetry in the
AL model is due to the non-canonical oscillator-like structure of the raising
and lowering operators at each site.
Invariants of motions are found in terms of Casimir central elements of su(n)
and su_q(n) algebra generators, for the DNLS and QAL cases respectively.
Utilizing the representation theory of the symmetry algebras we specialize to
the quantum dimer case and formulate the eigenvalue problem of each dimer
as a non-linear (q)-spin model. Analytic investigations of the ensuing
three-term non-linear recurrence relations are carried out and the respective
orthonormal and complete eigenvector bases are determined.
The quantum manifestation of the classical self-trapping in the QDNLS-dimer
and its absence in the QAL-dimer, is analysed by studying the asymptotic
attraction and repulsion respectively, of the energy levels versus the strength
of non-linearity. Our treatment predicts for the QDNLS-dimer, a
phase-transition like behaviour in the rate of change of the logarithm of
eigenenergy differences, for values of the non-linearity parameter near the
classical bifurcation point.Comment: Latex, 19pp, 4 figures. Submitted for publicatio
Tight-binding parameters for charge transfer along DNA
We systematically examine all the tight-binding parameters pertinent to
charge transfer along DNA. The molecular structure of the four DNA bases
(adenine, thymine, cytosine, and guanine) is investigated by using the linear
combination of atomic orbitals method with a recently introduced
parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are
discussed and then used for calculating the corresponding wavefunctions of the
two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO
and LUMO energies of the bases are in good agreement with available
experimental values. Our results are then used for estimating the complete set
of charge transfer parameters between neighboring bases and also between
successive base-pairs, considering all possible combinations between them, for
both electrons and holes. The calculated microscopic quantities can be used in
mesoscopic theoretical models of electron or hole transfer along the DNA double
helix, as they provide the necessary parameters for a tight-binding
phenomenological description based on the molecular overlap. We find that
usually the hopping parameters for holes are higher in magnitude compared to
the ones for electrons, which probably indicates that hole transport along DNA
is more favorable than electron transport. Our findings are also compared with
existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table
Who Said or What Said? Estimating Ideological Bias in Views Among Economists
There exists a long-standing debate about the influence of ideology in economics. Surprisingly, however, there is no concrete empirical evidence to examine this critical issue. Using an online randomized controlled experiment involving economists in 19 countries, we examine the effect of ideological bias on views among economists. Participants were asked to evaluate statements from prominent economists on different topics, while source attribution for each statement was randomized without participants’ knowledge. For each statement, participants either received a mainstream source, an ideologically different less-/non-mainstream source, or no source. We find that changing source attributions from mainstream to less-/non-mainstream, or removing them, significantly reduces economists’ reported agreement with statements. Using a model of Bayesian updating we examine two competing hypotheses as potential explanations for these results: unbiased Bayesian updating versus ideologically-biased Bayesian updating. While we find no evidence in support of unbiased updating, our results are consistent with biased Bayesian updating. More specifically, we find that changing/removing sources (1) has no impact on economists’ reported confidence with their evaluations; (2) similarly affects experts/non-experts in relevant areas; and (3) affects those at the far right of the political spectrum much more significantly than those at the far left. Finally, we find significant heterogeneity in our results by gender, country, PhD completion country, research area, and undergraduate major, with patterns consistent with the existence of ideological bias