Abstract
Materials, consisting of Jahn-Teller (JT) ions, such as cuprates and manganites, display many outstanding properties, including high temperature superconductivity and colossal magnetoresistance. There, the role of JT effect, although widely recognized, is still elusive. Here we show that these materials have vibronic excitations, related to local deformations rotating around JT ions in the dynamic limit, arising from linear electron-vibrational coupling in the “Mexican hat” potential profile. Their energy depends on total angular momentum, which is quantized, as in quantum rotors. We found them in the representative JT compound of orthorhombic manganites, LaMnO3. Since the “Mexican hat” potential energy surface is double-valued, they show up near the ground and excited states of JT ions. Recently, by using spectroscopic ellipsometry technique, we showed that they appear in the excited state in the form of sidebands, accompanying the electron transition between the JT split orbitals at neighboring Mn3+ ions. Here, by using Raman scattering technique, we show that they also exist near the ground state. The found quantum rotor excitations may play an important role in many unusual properties observed in these materials.
Highlights
Orbital degeneracy, for example, the twofold degeneracy of e symmetry electronic orbitals of Cu2+(3d9) or Mn3+ (3d4) ions in cubic symmetry, leads to peculiar behaviour when there is linear electron-lattice coupling
As follows from the diagram, the E energy level is associated with the ground state with the quantum number of the angular momentum around the potential trough j = ±1/2
In our previous detailed temperature Raman study of the untwinned LaMnO 3 crystal [9], we find that the temperature coefficient of the full width at half maximum (FWHM) of the anti-symmetric JT Ag mode is about 0.07 ± 0.01 cm−1/K, whereas for the symmetric JT B1g mode it is about 0.04 ± 0.01 cm−1/K
Summary
For example, the twofold degeneracy of e symmetry electronic orbitals of Cu2+(3d9) or Mn3+ (3d4) ions in cubic symmetry, leads to peculiar behaviour when there is linear electron-lattice coupling.
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