Characterisation of Anderson localisation using distributions

We examine the use of distributions in numerical treatments of Anderson localisation and supply evidence that treating exponential localisation on Bethe lattices recovers the overall picture known from hypercubic lattices in 3d.Comment: 2 pages, 2 figures, submitted to SCES'0

Phase diagram of the excitonic insulator

Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in $\rm TmSe_{0.45}Te_{0.55}$, we developed a scheme to quantitatively construct, for generic two-band models, the phase diagram of an excitonic insulator. As a first application of our approach, we calculated the phase diagram for an effective mass two-band model with long-range Coulomb interaction. The shielded potential approximation is used to derive a generalized gap equation controlling for positive (negative) energy gaps the transition from a semi-conducting (semi-metallic) phase to an insulating phase. Numerical results,obtained within the quasi-static approximation, show a steeple-like phase diagram in contrast to long-standing expectations.Comment: 2 pages, 1 figure, SCES'05, accepted for publication in Physica

Anderson localization of polaron states

Using the vanishing of the typical polaron tunneling rate as an indicator of the breakdown of itinerancy, we study the localization of polaron states in a generic model for a disordered polaronic material. We find that extremely small disorder causes an Anderson localization of small polaron states. However, the ratio between the critical disorder strength needed to localize all states in the polaron band and the renormalized bandwidth is not necessarily smaller than for a bare electron.Comment: 4 pages, 3 figure

Absorption spectrum of a weakly n-doped semiconductor quantum well

We calculate, as a function of temperature and conduction band electron density, the optical absorption of a weakly n-doped, idealized semiconductor quantum well. In particular, we focus on the absorption band due to the formation of a charged exciton. We conceptualize the charged exciton as an itinerant excitation intimately linked to the dynamical response of itinerant conduction band electrons to the appearance of the photo-generated valence band hole. Numerical results for the absorption in the vicinity of the exciton line are presented and the spectral weights associated with, respectively, the charged exciton band and the exciton line are analyzed in detail. We find, in qualitative agreement with experimental data, that the spectral weight of the charged exciton grows with increasing conduction band electron density and/or decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure

Resonant charge transfer at dielectric surfaces

We report on the theoretical description of secondary electron emission due to resonant charge transfer occurring during the collision of metastable  N2(3Σ+u)  molecules with dielectric surfaces. The emission is described as a two step process consisting of electron capture to form an intermediate shape resonance  N2-(2Πg) and subsequent electron emission by decay of this ion, either due to its natural life time or its interaction with the surface. The electron capture is modeled using the Keldysh Green’s function technique and the negative ion decay is described by a combination of the Keldysh technique and a rate equation approach. We find the resonant capture of electrons to be very efficient and the natural decay to be clearly dominating over the surface-induced decay. Secondary electron emission coefficients are calculated for Al2O3, MgO, SiO2, and diamond at several kinetic energies of the projectile. With the exception of MgO the coefficients turn out to be of the order of 10-1 over the whole range of kinetic energies. This rather large value is a direct consequence of the shape resonance acting as a relay state for electron emission

Bound state formation and the nature of the excitonic insulator phase in the extended Falicov-Kimball model

Motivated by the possibility of pressure-induced exciton condensation in intermediate-valence Tm[Se,Te] compounds, we study the Falicov-Kimball model extended by a finite f-hole valence bandwidth. Calculating the Frenkel-type exciton propagator we obtain excitonic bound states above a characteristic value of the local interband Coulomb attraction. Depending on the system parameters coherence between c and f states may be established at low temperatures, leading to an excitonic insulator phase. We find strong evidence that the excitonic insulator typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate (BEC) of preformed excitons (strong-coupling regime), which points toward a BCS-BEC transition scenario as Coulomb correlations increase