Berlin 2015 – scientific programme

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O: Fachverband Oberflächenphysik

O 68: Electronic Structure Theory: Many-Body Effects

O 68.7: Poster

Wednesday, March 18, 2015, 18:15–21:00, Poster A

Acoustic magnons in the long-wavelength limit: resolving the Goldstone violation in many-body perturbation theory — •Mathias C.T.D. Müller, Christoph Friedrich, and Stefan Blügel — Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, 52425 Jülich, Germany

Ferromagnetic materials exhibit a spontaneously broken global rotation symmetry in spin space leading to the appearance of massless quasiparticles (zero gap) in the long-wavelength limit. These magnons are formed by the correlated motion of electron-hole pairs with opposite spins, which we describe from first principles employing the T-matrix formalism in the ladder approximation within the FLAPW method [1]. Due to approximations used in the numerical scheme, the acoustic magnon dispersion exhibits a small but finite gap at Γ. We analyze this violation of the Goldstone mode and discuss possible correction schemes. One of the correction schemes, which involves an adjustment of the Kohn-Sham (KS) exchange splitting, is motivated by the spin-wave solution of the one-band Hubbard model. The new exchange splittings turn out to be closer to experiment. We present corrected magnon spectra for Co, Ni, and Fe. In addition, we discuss an approach that implements the magnetic susceptibility using a renormalized Green function instead of the KS one. The latter, much more expensive approach is expected to fulfill the Goldstone condition without correction.
E. Şaşioğlu et al., Phys. Rev. B 81, 054434 (2010); C. Friedrich et al. Top. Curr. Chem. 347, 259 (2014).