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Early Research
Current Research
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Early Research

Most of my research to-date has involved the theoretical study of various electronic properties of model liquids. The emphasis has been on understanding phenomena appropriate to a large range of experimental systems, rather than studying specific systems. The phenomena in question are traditionally associated with the solid phase, but are in fact equally relevant to an understanding of the liquid phase.

Electronic DoS of Liquid Metals

A large number of elements are metallic in the liquid phase. In addition to those elements traditionally classed as metals, a number of "non-metals" also possess a metallic liquid phase, e.g. Si, Ge, Te. The metallic nature of the liquid phase in these elements implies the existence of extended electronic states. While the electronic structure of the so-called simple liquid metals (alkali metals, Mg, Al, etc.) can be accomodated within the nearly-free electron model, describing the electronic structure of strongly-scattering liquid metals is a far more difficult problem. Although my work was done principally with liquid metals in mind, the same kind of ideas are applicable to amorphous solids and the impurity bands of doped semiconductors (e.g. P-doped Si). Furthermore, extended electronic states are also likely to be important for corresponding insulating states, e.g. in the solid phase of Si, or as produced by expanding alkali metals along the liquid-gas coexistence curve.

The principal challenge in constructing a theoretical framework for strongly-scattering liquid metals, is to include in the theory a correct treatment of the liquid-like disorder. I have mostly used Green function techniques, leading to integral equation theories analogous to those used in classical liquid state physics. I have studied the density of electronic states (publications [1]-[5], [8]), a quantity which is fundamental to a description of the electronic states of liquid metals.

Dr. Kahl's group in Vienna and Dr. Lomba's group in Madrid have extended the work on the density of states problem, see publication [17] for a recent application to liquid Si. Workers in Rostock and Catania are applying the techniques to the problem of the electrical conductivity of a spatially disordered system. Finally, a group in Lvov have just announced plans to extend the density of states work to binary and inhomogeneous liquids.

Anderson Localisation

I have also considered the Anderson localisation of electronic states arising from the disorder inherent in the liquid (publications [6] and [7]). Such a mechanism probably plays a role in the metal-insulator transition observed in expanded fluid mercury.

Frenkel excitonic insulator

In subsequent work, I studied the postulated Frenkel excitonic insulator phase for a model solid phase (publication [9]) and for a liquid solute/solvent system ([10] and [11]). Such a phase leads to a dipolar atomic ground state which has been observed in a number of computer simulations, and which may be responsible for a number of unusual experimental observations.

Frenkel exciton bands

On arriving in Vienna, I started a study of the evolution of Frenkel exciton bands in fluids, such as are observed in low-density rare gases, metal vapours, and molecular fluids. Results for both a linear (publication [12]) and a non-linear (publication [14]) integral equation theory have been published. Related work has been done recently by Enrique Lomba and coworkers of the Fluid Theory Group at the Instituto de Quimica Fisica Rocasolano, Madrid.

Structure of Liquid Metals

In a recent project, I have combined earlier calculations of the electronic structure of liquids with a technique (based on the Tight-Binding Bond theory, see A. P. Sutton et. al., J. Phys. C, 21, 35, (1988)) for obtaining the atomic structure from the electronic structure, resulting in a self-consistent theory for the atomic and electronic structures of a liquid metal (publication [13]). Such a theory should be applicable to strongly-scattering materials for which the traditional nearly-free electron approach fails. The method is based on alternate calculations of the atomic and electronic structures, and the iteration was found to converge rapidly.

The valence electrons of the model liquid metal (described in a tight-binding formalism) yield a density and temperature-dependent contribution to the effective pair potential characterising the system. The atomic structure of the liquid arises from a competition between this contribution and a reference repulsive pair potential. The contribution to the effective pair potential due to the valence electrons increases with increasing strength of the electronic matrix elements and with decreasing density. Note that in contrast to the NFE description of liquid metals, the present formulation includes no structure-independent volume term.

Tight-Binding Molecular Dynamics

In recent years, Tight-Binding Molecular Dynamics (TBMD) has grown to be a popular technique for studying a variety of materials. In a TBMD simulation (see C. Z. Wang et. al., Phys. Rev. B, 39, 8586, (1989)), the electronic structure is described by a parameterised tight-binding Hamiltonian from which Hellmann-Feynman forces are calculated at each time step. The latter are supplemented by forces derived from a repulsive pair potential. In exchange for a limited amount of parameterisation, one can study much larger systems than is at present possible with ab initio methods.

To-date, the TBMD method has been used to study a wide range of covalent materials, for example the liquid and amorphous group IV elements, silicon clusters, liquid and amorphous GaAs, and fullerenes. Together with Marcel Rassinger, I have worked on a TBMD modelling of the hydrogenated surfaces of diamond, a topic also studied in Vienna by Juergen Furthmueller using ab initio techniques. Predictions for the reconstructed surfaces for a variety of hydrogen coverages, and some results on the surface dynamics, are described in publications [15] and [16].

The text of a poster presented at the 7th International Workshop on Computational Condensed Matter Physics, ICTP Trieste, Jan. 1995 can be found here.


m.d.winn@dl.ac.uk
Last modified: Mon Jun 30 12:21:06 BST 1997