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	STUDIA PHYSICA - Ediţia nr.1 din 2007

	Articol:	CORRELATION EFFECTS AND THE ELECTRONIC STRUCTURE OF HALF-METALLIC FEROMAGNETS. Autori: L. CHIONCEL.


	Rezumat: Most theoretical efforts for understanding half-metallic ferromagnets (HMF) are supported by first-principles calculations, based on density-functional theory (DFT). In fact the very discovery of HMF was due to such calculations [1]. DFT calculations are usually based on the Local Spin Density Approximation (LSDA) or the Generalized Gradient Approximation (GGA). These approximations have been proved very successful to interpret or even predict material properties in many cases, but they fail notably in the case of strongly-correlated electron systems. For such systems the so-called LSDA+U (or GGA+U) method is used to describe static correlations, whereas dynamical correlations can be approached within the LSDA+DMFT (Dynamical Mean-Field Theory). An important dynamical many-electron feature of half-metallic ferromagnets is the appearance of non-quasiparticle states [2,3] which can contribute essentially to the tunneling transport in heterostructures containing HMF [4,5].The origin of these states is connected with “spin-polaron” processes: the spin-down low-energy electron excitations, which are forbidden for HMF in the one-particle picture, turn out to be possible as superpositions of spin-up electron excitations and virtual magnons. The density of these non-quasiparticle states vanishes at the Fermi level but increases drastically at the energy scale of the order of a characteristic magnon frequency m, giving an important contribution in the temperature dependence of the conductivity due to the interference with impurity scattering [3].




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