Strongly anharmonic collective modes in a coupled electron-phonon-spin problem

• We solve for the finite temperature collective mode dynamics in the Holstein-double-exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice, the system transits from an undistorted ferromagnetic metal at low temperature to a structurally distorted paramagnetic insulator at high temperature, through a short-range charge-ordered (CO) phase near the ferromagnetic crossover at T-FM. The small amplitude harmonic phonons at low temperature cross over to large amplitude dynamics around 0.5T(FM) as spin disorder promotes formation of short-range correlated polarons. The CO background in turn leads to the softening of phonons near momentum q = (pi, pi) with temperature, while strong anharmonicity close to the magnetic transition leads to unusually large damping. The most remarkable phonon feature, however, arises from the rare thermal tunneling of CO domains, generating a momentum selective spectral weight at very low energy. The spin dynamics is similar to that of a nearest-neighbor Heisenberg model with sharp spin waves at low T, transiting to diffusive modes near T-FM. We compare our results to inelastic neutron data in the manganites and suggest how the singular low-energy features can be probed.

Subject headings

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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