The valence photoemission spectra of alkali metals exhibit multiple plasmon satellite structure. The calculated spectral functions within the GW approximation show only one plasmon satellite at too large binding energy. In this Letter we use the cumulant expansion approach to obtain the spectral functions of Na and Al from ab initio calculations including the effects of band structure. The GW spectral functions are dramatically improved and the positions of the multiple plasmon satellites are in very good agreement with experiment while their intensities cannot be explained from intrinsic effects only. [S0031-9007(96)01170-2] PACS numbers: 73.20.Mf, 71.45.Gm, 79.60.Bm The GW approximation (GWA) [1,2] has by now become a standard theory for electronic structure calculations beyond the local density approximation (LDA) [3,4]. It is well known that the GWA well reproduces quasiparticle energies in a wide range of systems from simple systems (simple metals, semiconductors [5,6]) to highly correlated systems (Ni, NiO) [7,8]. There is, however, a problem with the spectral functions in the GWA concerning the satellite structures. It is known, for instance, that the plasmon satellite in the core electron case is positioned too low, 1.5v p (plasmon energy) below the quasiparticle peak rather than v p [9]. Moreover, low energy satellite structures in strongly correlated systems are too weak (or almost missing) and their energies are overestimated [10]. It is clear that a theory beyond the GWA (vertex corrections) is needed to improve the satellite description. An important set of vertex corrections is obtained from the so-called cumulant expansion where one makes a diagrammatic expansion directly in the Green function rather than in the self-energy [11][12][13][14][15]. Previous attempts of including vertex corrections have been mainly concerned with their effects on the quasiparticle energies [16 -18]. As far as we know, the effects of vertex corrections on the satellite structure have not been studied before for real systems.The cumulant expansion has been shown to be very suitable for systems which can be mapped into a polaron Hamiltonian consisting of electrons interacting with bosons (plasmons). Thus, for a core electron interacting with a plasmon field, the first order cumulant expansion already gives the exact result. The physics behind extending this approximation to valence electrons was discussed in detail by Hedin [13]. Two new important developments are, however, made here. References [13,14] dealt with an homogeneous electron gas, while here we treat real systems using a priori band structure calculations based on the linear muffin-tin orbital method [19].While the cumulant expansion has been used to study model systems, applications to real systems are still lacking. For calculational details of the screened interaction and the self-energy, we refer to Refs. [7,20]. The second development is the finding that the satellite contributions to the photoelectron spectrum can be written as a series of terms close...
In photoemission spectra of strongly correlated systems one usually observes a satellite structure below the main peak. Description of such satellite structures in the commonly used GW approximation has been found to be insufficient. To account for these satellite structures that originate from shortrange correlations, we have developed a T -matrix formalism for performing ab initio calculations on real systems. The method is applied to Ni and we obtain a satellite structure below the Fermi level as well as a reduced exchange splitting. We also found a new interesting satellite structure above the Fermi level, which can be ascribed to particle-particle scattering. [S0031-9007(98)05337-X]
A Green's function formalism for calculating spin-wave excitations is developed for practical calculations and tested for real solids. The mapping to the Heisenberg Hamiltonian commonly used in spin-wave calculations is avoided, making the formalism suitable for both localized and itinerant magnetic systems. To test the formalism, we have calculated the spin-wave spectra and dispersions of ferromagnetic Fe and Ni. The results prove to be in very good agreement with experiment and some novel features are predicted.First-principles studies of the electronic structure of materials have been primarily concentrated on the charge degree of freedom whereas the spin degree of freedom is principally frozen. The energy scale of spin fluctuations ͑a few tens of meV͒ is indeed much smaller than a typical energy scale of charge excitations which is of the order of a few eV. However, transport and thermal phenomena such as specific heat and resistivity are well-known examples which lie in the low-energy regime. 1 More recently, the study of spin excitations has gained a lot of interest due to the discovery of high-temperature superconductivity. There is compelling evidence that spin fluctuations are the mediator of the attractive interaction. 2,3 Despite the importance of spin excitations, first-principles calculations using realistic energy bands and wave functions are rather rare. This is partly due to a lack of theoretical frameworks and partly due to a lack of efficient numerical schemes and the large numerical effort required. The most common technique for calculating spin-wave dispersions is the frozen magnon method where the problem is mapped to the celebrated Heisenberg Hamiltonian 4 and the parameters in the model are obtained from realistic calculations. Several shortcomings are readily apparent: The method does not give the spin-wave spectra so that lifetime and multiple branches are not accessible. Moreover, applications of the method to itinerant electrons are difficult to justify. Spin-wave excitation spectra are also attainable using time-dependent density functional theory ͑TD DFT͒. 5 Although the method is formally exact, the calculated spectra may depend significantly on the quality of the exchange-correlation potential. Thus, a simple approximation may not be adequate, and not surprisingly, the spin-wave dispersions calculated within the local density approximation 6-9 ͑LDA͒ are often in large discrepancy with experiment. A new method for calculating spinwave spectra was also proposed recently but the applicability of the method has not been demonstrated. 10 In this report, we develop a recently proposed formalism for calculating spin-wave excitations into a practical scheme and demonstrate its applicability by calculating the spinwave spectra and dispersions of ferromagnetic Fe and Ni. The spin-wave spectra of these materials are rather complex and they should provide a stringent test of the method.The experimental spin-wave dispersion of Ni ͑Ref. 11͒ shows a number of interesting features which have not be...
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