Band effective, quasiparticle

More advanced teclmiques take into account quasiparticle corrections to the DFT-LDA eigenvalues. Quasiparticles are a way of conceptualizing the elementary excitations in electronic systems. They can be detennined in band stmcture calculations that properly include the effects of exchange and correlation. In the... 

Fig. 21. Real part of the conductivity of YbFe4St>i2- The symbols on die left axis represent dc values at different temperatures. Below T (fv 50 K), a narrow peak at zero frequency and a gap-like feature at 18 meV gradually develop. Inset Renormalized band structure calculated from die Anderson lattice Hamiltonian. % and f denote bands of free carriers and localized electrons, respectively. At low temperatures a direct gap A opens. The Fermi level, Ep is near die top of die lower band,, resulting in hole-like character and enhanced effective mass of die quasiparticles (Dordevic et al., 2001).

A discussion is given of electron correlations in d- and f-electron systems. In the former case we concentrate on transition metals for which the correlated ground-state wave function can be calculated when a model Hamiltonian is used, i.e. a five-band Hubbard Hamiltonian. Various correlation effects are discussed. In f-electron systems a singlet ground-state forms due to the strong correlations. It is pointed out how quasiparticle excitations can be computed for Ce systems. 

In the LDA, Adolph and Bechstedt [157,158] adopted the approach of Aspnes [116] with a plane-wave-pseudopotential method to determine the dynamic x of the usual IB V semiconductors as well as of SiC polytypes. They emphasized (i) the difficulty to obtain converged Brillouin zone integration and (ii) the relatively good quality of the scissors operator for including quasiparticle effects (from a comparison with the GW approximation, which takes into account wave-vector- and band-dependent shifts). Another implementation of the SOS x —2 ffi, ffi) expressions at the independent-particle level was carried out by Raskheev et al. [159] by using the linearized muffin-tin orbital (LMTO) method in the atomic sphere approximation. They considered... 

The first equation shows that the center of the quasiparticle bands lies at an energy above the original conduction band Fermi level. This should be compared with the position Tg = aT of the Kondo resonance in the impurity model. The second equation simply results from the charge constraint QflN = g, which is now enforced only on the average in contrast to the impurity model. The occupation f(0) is obtained by setting equal to the actual value of 1 IN. The quasiparticle bands are the result of a hybridization with an effective strength K Here r iT) is the average fraction of sites without f occupation,... 

Fig. 47. Schematic quasiparticle density of states N (s) as obtained from the mean-field dispersion, eq. (112). They lead to a hybridization gap centered around , = and two peaks in N e) whose width and separation is also of order The Fermi level (0) is pinned in this region. The temperature dependence of the effective hybridization K, given by the function fl T) = rJ(T)/rJ(0, N = 2) as shown in the inset (Coleman 1987). IF is a slightly renormalized band width [a square DOS of width W has been used for the bare A/,(e)].

Here i/ (z) = d In T(z) /dz is the digamma function and W is the band width of the Lorentzian conduction electron density of states. Furthermore, T) is the effective temperature-dependent coupling strength of the longitudinal sound waves to quasiparticles. It may be written as... 

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