Insulator-to-Semiconductor-to-Conductor Phase-Like Transition of DNA Chains.

Using a tight-binding model, we study the transport of charge carriers through DNA molecular wires. In double-stranded DNA chains, according to Chargaff's rules, only Adenine-Thymine (AT) and Cytosine-Guanine (CG) pairs are allowed. In our model, a decimation procedure allows us to represent each pair of bases by a single site with one localized electronic state. We consider chains of different lengths with only AT (CG) sites, and ordered and disordered chains with both types of sites. Disordered chains may include short range correlation. Additionally, hydration is considered in the form of a change of the site energy. We find a conductor-to-semiconductor-to-insulator transition as a function of the three effects taken into account: chain size, intrinsic disorder of CG and AT pairs, and hydration. This model predicts that an appropriate choice of chain size and concentration of AT pairs can be used to tailor the electrical behavior of DNA strands.

Quantum time scales and the classical limit: analytic results for some simple systems.

We set up a semiclassical approximation which helps us clarify by means of several simple examples the rich variety of time scale in the quantum domain. The underlying structure of quantum and classical mechanics is so completly different that it is naive to expect to reach a classical regime by counting powers of the quantum scale variant Planck's over 2pi. We show although it is possible to define a time scale for nonclassical phenomena, but it is impossible to characterize quantum dynamics through a unique time scale, such as Ehrenfest's time. We use simple systems to critically discuss and illustrate these features of the quantum-classical limit.