Temperature-dependent band structure of a ferromagnetic semiconductor film

Abstract

The electronic quasiparticle spectrum of a ferromagnetic film is investigated within the framework of the sf model. Starting from the exact solvable case of a single electron in an otherwise empty conduction band being exchange coupled to a ferromagnetically saturated localized spin system we extend the theory to finite temperatures. Our approach is a moment-conserving decoupling procedure for suitable defined Green functions. The theory for finite temperatures evolves continuously from the exact limiting case. The restriction to zero conduction band occupation may be regarded as a proper model description for ferromagnetic semiconductors such as EuO and EuS. Evaluating the theory for a simple cubic film cut parallel to the ͑100͒ crystal plane, we find some marked correlation effects which depend on the spin of the test electron, on the exchange coupling, and on the temperature of the local-moment system.

Prediction of a Surface State and a Related Surface Insulator-Metal Transition for the (100) Surface of Stochiometric EuO

“…The spin-↑ densities of state for T = 0 ( S z /S = 1) are, except for constant energy shift, those obtained within the band-structure calculations, which is due to the lack of possibility for the spin-↑ electron to exchange its spin with the system of perfectly aligned local moments. However, the spin-↓ electron can flip its spin even for T = 0 by emitting a magnon and forming a polaron-like quasiparticle [8]. Hence, the spin-↓ spectra for T = 0 already exhibit correlation effects and differ from those of the band-structure calculations.…”

“…The many-body problem that arises with the Hamiltonian (1) is far from being trivial and a full solution is lacking even for the case of the bulk. In a previous paper we have presented an approximate treatment of the special case of a single electron in an otherwise empty conduction band [8]. The presented approach applies to the situation in a model-type ferromagnetic semiconductor film containing a single-nondegenerated s-like band.…”

“…Due to the empty conduction bands we are considering throughout the whole paper, the Hamiltonian (1) can be split into an electronic part, H s + H sf , and a magnetic part, H f , which can be solved separately [8]. For the magnetic subsystem H f we employ an approach based on the random phase approximation (RPA) which has been described in detail in [6].…”

“…For the electronic part a moment-conserving decoupling approximation (MCDA) for suitably defined Green functions is employed [8]. In the set of equations which constitute the solution for the electronic subsystem, there appear f-spin correlation functions which have to be evaluated within the magnetic subsystem (Eq.…”

Temperature dependence of surface magnetization in local-moment systems

“…Kurz et al 16 studied the magnetism and the electronic structure of Gd(0001) surface on the basis of density functional theory. Schiller et al 17,18 investigated the temperature-dependent band structure of ferromagnetic semiconductor films and surfaces in the s-f model 19 . Using a moment-conserving decoupling procedure for suitable defined Green's functions, the layer-dependent magnetizations and the LDOS were studied at different temperatures.…”