R E S E A R C H P R O P O S A L L F 6 1

SYNCHROTRON RADIATION SPECTROSCOPIES AND THE PHYSICS OF STRONGLY ORRELATED ELECTRON SYSTEMS, INCLUDING LOW-DIMENSIONAL SYSTEMS AND NANOSTRUCTURED MATERIALS.


The aim of this project is the theoretical analysis of strongly correlated electron systems, like the manganites, the cuprates and the transition metal oxides, including low-dimensional systems and nanostructured materials.

The enormous recent progress both in the art of sample preparation and in the measurement techniques has produced a wealth of high quality data on thermodynamic, transport and spectroscopic properties which often challenge the simple textbook interpretation. This is particularly true for materials showing evidence for strong electronic and magnetic correlation such as the materials mentioned above.

At the same time new non perturbative methods have been developed for dealing with strongly correlated systems, such as the Density Matrix Renormalization Group, the Dynamical Mean Field Theory (DMFT) or the extension of the Lanczos technique to finite frequences and temperatures.

In order to make progress in the description of such systems, these techniques and in general methods borrowed from field theory and statistical mechanics will be used, when appropriate. At the same time in-depth theoretical studies of significant synchrotron radiation spectroscopies (absorption, dichroism, elastic and inelastic resonant x-ray scattering, photoemission, etc.) that can shed light on
charge and magnetic correlations in strongly interacting electronic systems will be carried out.

We intend to analyze in particular spin and interaction effects on the electrical and thermal transport properties in low -dimensional electron systems. Also, channeling of radiation through nanotubes will be studied theoretically, with the help of Monte Carlo simulations.

Finally the theory underlying the multiple scattering programs used to analyse such spectroscopies will be improved and applications be made to obtain structural and electronic information in condensed matter systems, with an eye also to systems of biological relevance.

Recently the group of Prof. G. Falcone of the Universita' della Calabrai (CS) has joined our research activity by bringing their expertise on the theoretical study of ion impact on solid surfaces to study electronic properties of
nanostructural material.

Research in the first half of 20005 has developed along the following lines:


1) Interpretation of x-ray spectroscopies in parity and time-reversal breaking materials.

We have provided a link between several electromagnetic multipole moments and the measurable quantities of some x-ray spectroscopies, like resonant x-ray scattering or dichroism in absorption.


2) Description of the anomalous electronic properties in titanium and vanadium spinels

We have obtained a complete description of the electronic ground-state properties of titanium and vanadium spinels. We have found that the occurence of an orbital ordering can modulate the spin exchange and the actual ground-state then depends drastically on the average filling of the ion.


3) Carbon nanotubes

We have overviewed the definitions, properties and applications of Carbon Nanotubes (CNTs) with an eye to applications. We have discussed the effects of a strong magnetic field in Quantum Wires. Recent experiments about the low temperature behaviour of a Single Wall Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero bias conductance and allowed us to investigate the crucial role that the long range nature of the Coulomb interaction plays in the correlated electronic transport through a SWCNT with two intramolecular tunneling barriers.


4) Channeling in nanostructures

We have developed methods for probing the capability of the channeling technique to produce micrometric or nanometric sized beams using micro-structured crystals or nanotubes.
Also, coherent bremsstrahlung of high energy electrons moving in a three-dimensional imperfect periodic lattice consisting of a complicated system of atoms has been considered.


5) Ion impact on Carbon Nanotubes

We have begun to study the effect of ion impact on Carbon
Nanotubes, following recent studies on the electronic structure of nanoclusters grown on crystal substrates. The basic idea is that the electronic states of the studied cluster form narrow bands within the electronic structure of the substrate. some of these bands may lie above the Fermi level and tunnel to the projectilelevel (G. F. Liu, Z. Sroubek, and J. A. Yarmoff, Phys. Rev. Lett. 92,
216801 (2004)).

See details and publications on the joint activity reports.