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We investigate the spectral function of a hole moving in the
orbital-ordered ferromagnetic planes of LaMnO3, and show that it
depends critically on the type of orbital ordering, and on the type of
excitations on which it scatters. First we consider a single plane with
a hole interacting strongly with the excitations of eg orbitals
(orbitons), and solve this problem in a self-consistent Born
approximation. The hole-orbiton scattering leads to new type of
quasiparticles with a dispersion on the orbiton energy scale and with
strongly enhanced mass and reduced spectral weight [1]. Polarization of
orbitals around the hole can lead to a very narrow quasiparticle band
and to large incoherent spectral weight, indicating a confinement of
holes in lightly doped LaMnO3 insulator, while the electron-phonon
interaction generates vibrational side-bands which result from
the hole-lattice coupling [2]. We consider also the inter-planar
hole-magnon scattering in the A-type antiferromagnetic phase of
LaMnO3, and show that the classical concept of a ferromagnetic
polaron does not apply - the spectra are then dominated by the
incoherent spectral weight at high energies [3]. Finally, an interplay
between the spin and orbital degrees of freedom and the crossover from
localized orbital polarons to a metallic behavior is shown on an
example of a one-dimensional chain partly filled by eg electrons
coupled to core t2g spins [4]. Altogether, these results predict
a large redistribution of spectral weight with respect to the bands
found in local density approximation (LDA) or in LDA+U. Taking the
complexity of the problem, more ARPES experiments are needed to
establish the role played by various excitations in the manganites.
[1] J. van der Brink, P. Horsch, and A. M. Oles, Phys. Rev. Lett. 85, 5174 (2000). [2] J. Bala et al., Phys. Rev. B 65, 134420 (2002), Phys. Rev. B 65, 184414 (2002). [3] J. Bala, G. A. Sawatzky, A. M. Oles, and A. Macridin, Phys. Rev. Lett. 87, 067204 (2001). [4] M. Daghofer, A. M. Oles, and W. von der Linden, Phys. Rev. B 70, 184430 (2005). |