State Of Electron System Research Articles

Hot Electron Cooling in Silicon Nanoclusters via Landau-Zener Nonadiabatic Molecular Dynamics: Size Dependence and Role of Surface Termination.

We develop a new express methodology for modeling excited-state dynamics occurring in dense manifolds of electronic states in atomistic systems. The approach leverages a modified Landau-Zener formula, the neglect of a back-reaction approximation, and the highly efficient density functional tight-binding method. We study the hot electron dynamics in a series of H- and F-terminated silicon nanocrystals (NCs) containing up to several hundred atoms. We explain the slower electron cooling dynamics in F-terminated NCs by the larger energy gaps between the adjacent electronic states in these systems as well as their slower fluctuations. We conclude that both the mass and chemical identity of the surface termination groups equally influence the electron dynamics, on average. However, the mass effect becomes dominant for higher-energy excitations. We find that the electron decay dynamics in F-terminated NCs has a greater sensitivity to the mass of the surface ligands than do the H-terminated NCs and explain this observation by the details of the electron-phonon coupling in the systems. We find that in the H-terminated NCs, electronic transitions in the cooling process occur predominantly between the surface states, whereas in F-terminated Si NCs, both surface and NC core states are coupled to the nuclear vibrations. We find that electron energy relaxation is accelerated in larger NCs and attribute this effect to the higher densities of states and smaller energy gaps in these systems.

Metastable and spin-polarized states in electron systems with localized electron–electron interaction

We study the formation of spontaneous spin polarization in inhomogeneous electron systems with pair interaction localized in a small region that is not separated by a barrier from surrounding gas of non-interacting electrons. Such a system is interesting as a minimal model of a quantum point contact in which the electron–electron interaction is strong in a small constriction coupled to electron reservoirs without barriers. Based on the analysis of the grand potential within the self-consistent field approximation, we find that the formation of the polarized state strongly differs from the Bloch or Stoner transition in homogeneous interacting systems. The main difference is that a metastable state appears in the critical point in addition to the globally stable state, so that when the interaction parameter exceeds a critical value, two states coexist. One state has spin polarization and the other is unpolarized. Another feature is that the spin polarization increases continuously with the interaction parameter and has a square-root singularity in the critical point. We study the critical conditions and the grand potentials of the polarized and unpolarized states for one-dimensional and two-dimensional models in the case of extremely small size of the interaction region.

Nonadiabatic couplings from time-dependent density functional theory: Formulation by the Kohn-Sham derivative matrix within density functional perturbation theory

We present an improved method for accurately calculating nonadiabatic couplings (NACs) between the ground and excited states of electronic systems, using time-dependent density functional theory (TDDFT) within the plane-wave pseudopotential framework. Based on our previous work that the TDDFT formulation of NACs by the Kohn-Sham derivative matrix elements can avoid the accuracy problem caused by the nonlocal pseudopotentials, we extend the evaluation of these matrix elements to the analytical scheme, achieved by density functional perturbation theory (DFPT). As DFPT has been a standard implementation in many ab initio codes, our scheme requires little effort in the coding. Extensive calculations of NACs near various Jahn-Teller or Renner-Teller intersections show the good accuracy of our results. A comparison of our present and previous work has also clarified an important fact in the TDDFT computation of NACs, regarding the evaluation of the nuclear derivative of the many-body Hamiltonian after mapping to the Kohn-Sham system.

A thermodynamic framework for a system with itinerant-electron magnetism

Thermodynamic fluctuation among many electronic states in systems with ‘itinerant-electron’magnetism is addressed with a first-principles formulation of the Helmholtz energy. The Ceγ–α phase transition is used as an illustrative case. ThekBln(2J+1) form for the magnetic entropy that is commonly found in the literature is replaced with anexpression that is derived from configurational fluctuations among nonmagnetic,ferromagnetic, and antiferromagnetic electronic states. Predicted first- and second-ordermagnetic phase transitions are in close accord with experiment. The mixture of states leadsto a Schottky anomaly in the Ce specific heat.

ELECTRONIC STATES IN SYSTEMS WITH RECONSTRUCTED OVERLAYERS

The electronic states in systems with reconstructed overlayers have been treated by Y. Muda and T. Hanawa. By means of a different method which is very effective for one-dimensional chain problems and through a simpler derivation the same results are obtained.

Electronic states for surfaces of muffin-tin potentials

Exact expressions are given from which the electronic states of systems consisting of a finite number of planes of non-overlapping muffin-tin potentials can be determined. It is shown that no general solution can be given in three dimensions, contrary to earlier claims in the literature. By using a single plane wave approximation, which is valid only in restricted circumstances, it is shown that an analogy can be found with the one-dimensional results of Shockley.