Making tracks: electronic excitation roles in forming swift heavy ion tracks

Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T-c oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self- trapped exciton model of Itoh and Stoneham ( 1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self- trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser- induced time- resolved x- ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials

Developments in mean field density functional theory of simple fluids and charged colloidal suspensions.

This thesis is concerned with the methods of mean field calculations of the properties of soft matter systems. The first part deals with the application of mean field density functional theory to fluid systems containing small numbers of particles. This is relevant to nucleation studies that can be performed using mean field density functional theory (MFDFT), where the critical clusters that constitute the transition states for phase transitions can be very small. It is also relevant for studies of the behaviour of confined fluids such as fluids in nanopores. The problems in applying MFDFT to small systems are investigated, and modifications to improve the accuracy are identified. These principles are tested on a highly simplified model system of attractive hard rods in one dimension. The second part of the thesis investigates the mean field description of interactions in charged colloidal suspensions within the primitive (PM) model. The phase behaviour of these systems is discussed. In particular, the question of whether experimental observations of coexistence between dense and rarefied phases can be accounted for by mean field theory is discussed. A new approximate method for solving the nonlinear mean field Poisson-Boltzmann equation in the limit of dilute suspensions is proposed. This method is applied to the simple case of charged plates, as well as arrays of spherical colloidal particles. For the latter case, comparisons are made between spherical and cubic Wigner-Seitz cell geometries

Thermodynamics of attractive hard rods: A test of mean field density functional theory

Mean field density functional theory (MFDFT) has been employed to calculate the free energy of a pair of attractive hard rods on a ring. The results for homogeneous and optimal inhomogeneous density profiles have been compared with the exact free energy as a test of the approach. We discuss the problems in applying MFDFT to small systems and suggest modifications which allow a reasonably accurate treatment of this particular, rather extreme, case. (C) 2004 American Institute of Physics

Influence of electronic effects on the surface erosion of tungsten

Tungsten is a strong contender for a plasma-facing material in future fusion power plant designs, and the material of choice for the divertor of ITER, due to its high melting point, thermal conductivity, and resistance to sputtering erosion. Sputtering erosion is a major concern for plasma-facing materials because sputtered atoms could enter the plasma and result in cooling. Atomistic modeling, using molecular dynamics, has previously been successful in identifying fundamental mechanisms of surface damage caused by ion bombardment. The damage has been found to be particularly sensitive to the rate of energy dissipation but energy transport is not well described in classical molecular dynamics simulations of metals. We present a methodology for including a realistic description of electronic energy absorption, transport, and redistribution in molecular dynamics simulations of self sputtering. The results for three different 5 keV self-sputtering events are presented for four distinct thermal transport models. The results demonstrate the sensitivity of surface damage to the model used to describe the electronic thermal transport

Development of an electron-temperature-dependent interatomic potential for molecular dynamics simulation of tungsten under electronic excitation

Irradiation of a metal by lasers or swift heavy ions causes the electrons to become excited. In the vicinity of the excitation, an electronic temperature is established within a thermalization time of 10-100 fs, as a result of electron-electron collisions. For short times, corresponding to less than 1 ps after excitation, the resulting electronic temperature may be orders of magnitude higher than the lattice temperature. During this short time, atoms in the metal experience modified interatomic forces as a result of the excited electrons. These forces can lead to ultrafast nonthermal phenomena such as melting, ablation, laser-induced phase transitions, and modified vibrational properties. We develop an electron-temperature-dependent empirical interatomic potential for tungsten that can be used to model such phenomena using classical molecular dynamics simulations. Finite-temperature density functional theory calculations at high electronic temperatures are used to parametrize the model potential

Modelling non-adiabatic processes using correlated electron-ion dynamics

Here we survey the theory and applications of a family of methods (correlated electron-ion dynamics, or CEID) that can be applied to a diverse range of problems involving the non-adiabatic exchange of energy between electrons and nuclei. The simplest method, which is a paradigm for the others, is Ehrenfest Dynamics. This is applied to radiation damage in metals and the evolution of excited states in conjugated polymers. It is unable to reproduce the correct heating of nuclei by current carrying electrons, so we introduce a moment expansion that allows us to restore the spontaneous emission of phonons. Because of the widespread use of Non-Equilibrium Green's Functions for computing electric currents in nanoscale systems, we present a comparison of this formalism with that of CEID with open boundaries. When there is strong coupling between electrons and nuclei, the moment expansion does not converge. We thus conclude with a reworking of the CEID formalism that converges systematically and in a stable manner. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010