Biomedical subjects

Raffaele Resta

Publications and source records attributed to Raffaele Resta.

5 recordsLinked to original sources

PubMed2006

Polarization fluctuations in insulators and metals: new and old theories merge.

The ground-state fluctuation of polarization P is finite in insulators and divergent in metals, owing to the SWM sum rule [I. Souza, T. Wilkens, and R. M. Martin, Phys. Rev. B 62, 1666 (2000)]. This is a virtue of periodic (i.e., transverse) boundary conditions. I show that within any other boundary conditions the P fluctuation is finite even in metals, and a generalized sum rule applies. The boundary-condition dependence is a pure correlation effect, not present at the independent-particle level. In the longitudinal case inverted triangle x P = -rho, and one equivalently addresses charge fluctuations: the generalized sum rule reduces then to a well-known result of the many-body theory.

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PubMed2006

Kohn's theory of the insulating state: a quantum-chemistry viewpoint.

The qualitative difference between insulators and conductors not only manifests itself in the excitation spectra but also--according to Kohn's theory [Phys. Rev. 133, A171 (1964)]--in a different organization of the electrons in their ground state: the wave function is localized in insulators and delocalized in conductors. Such localization, however, is hidden in a rather subtle way in the many-body wave function. The theory has been substantially revisited and extended in modern times, invariably within a periodic-boundary-condition framework, i.e., ideally addressing an infinite condensed system. Here we show how the localization/delocalization of the many-body wave function shows up when considering either three-dimensional clusters of increasing size or quasi-one-dimensional systems (linear polymers, nanotubes, and nanowires) of increasing length, within the ordinary "open" boundary conditions adopted for finite systems. We also show that the theory, when specialized to uncorrelated wave functions, has a very close relationship with Boy's theory of localization [Rev. Mod. Phys. 32, 296 (1960)]: the Boys orbitals in the bulk of the sample behave in a qualitatively different way in insulating versus conducting cases.

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PubMed2005

Electron localization in the quantum Hall regime.

The theory of the insulating state discriminates between insulators and metals by means of a localization tensor, which is finite in insulators and divergent in metals. In absence of time-reversal symmetry, this same tensor acquires an off-diagonal imaginary part, proportional to the dc transverse conductivity, leading to quantization of the latter in two-dimensional systems. I provide evidence that electron localization--in the above sense--is the common cause for both vanishing of the dc longitudinal conductivity and quantization of the transverse one in quantum Hall fluids.

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PubMed2005

Intermolecular dynamical charge fluctuations in water: a signature of the H-bond network.

We report a simulation of deuterated water using a Car-Parrinello approach based on maximally localized Wannier functions. This provides local information on the dynamics of the hydrogen-bond network and on the origin of the low-frequency infrared activity. The oscillator strength of the translational modes, peaked around approximately 200 cm-1, is anisotropic and originates from intermolecular--not intramolecular--charge fluctuations. These fluctuations are a signature of a tetrahedral hydrogen-bonding environment.

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PubMed2005

Longitudinal polarizability of long polymeric chains: quasi-one-dimensional electrostatics as the origin of slow convergence.

The longitudinal linear polarizability alpha(N) of a stereoregular oligomer of size N is proportional to N in the large-N limit, provided the system is nonconducting in that limit. It has long been known that the convergence of alpha(N)/N to the asymptotic alpha(infinity) value is slow. We show that the leading term in the difference between alpha(N)/N and alpha(infinity) is of the order of 1/N. The difference [alpha(N)-alpha(N-1)], as well as alpha(center)(N) (when computationally accessible), also converge to alpha(infinity), but faster, the leading term being of the order of 1/N(2). We also present evidence that in these cases the power law convergence behavior is due to quasi-one-dimensional electrostatics, with one exception. Specifically, in molecular systems the difference between alpha(N)/N and alpha(infinity) has not just one but two sources of the O(1/N) term, with one being due to the aforementioned Coulomb interactions, and the second due to the short ranged exponentially decaying perturbations on chain ends. The major role of electrostatics in the convergence of the remainders is demonstrated by means of a Clausius-Mossotti-type classical model. The conclusions derived from the model are also shown to be applicable in molecular systems, by means of test-case ab initio calculations on linear stacks of H(2) molecules, and on polyacetylene chains. The implications of the modern theory of polarization for extended systems are also discussed.

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