Optical evidence of quantum rotor orbital excitations in orthorhombic manganites

Abstract

Abstract. In magnetic compounds with Jahn-Teller (JT) ions (such as Mn 3+ or Cu 2+ ), the ordering of the electron or hole orbitals is associated with cooperative lattice distortions. There the role of JT effect, although widely recognised, is still elusive in the ground state properties. We suggest that, in these materials, there exist elementary excitations whose energy spectrum is described in terms of the total angular momentum eigenstates and is quantised as in quantum rotors found in JT centers. We observed features originating from these excitations in the optical spectra of a model compound LaMnO 3 using ellipsometry technique. They appear clearly as narrow sidebands accompanying the electron transition between the JT split orbitals on neighbouring Mn 3+ ions, strongly influenced by anisotropic spin correlations. We present these results together with new experimental data on photoluminescence and its kinetics found in LaMnO 3 , which lend additional support to the ellipsometry implying the existence of the quantum rotor orbital excitations. We note that the discovered elementary excitations of quantum rotors may play an important role in many unusual properties observed in these materials upon doping, such as hightemperature superconductivity and colossal magnetoresistance.

Lattice-mediated magnetic order melting in TbMnO3

“…Thus, the fine structure reflects these p-d transitions overlapped to the d -d HS absorption band, giving rise to an intermediate regime between the p-d CT and the Mott-Hubbard insulator in the Zaanen-Sawatzky-Allen scheme [39]. A more recent interpretation associates the fine structure with quantum rotor orbital excitations for the e g electron of Mn 3+ ions, disturbed by the lattice anharmonicity [37]. To date there is no clear consensus on the origin of the spectral fine structure.…”

“…To assign the observed structures, we rely on the results reported in literature on orthorhombic manganites [25][26][27][28][29][30][31][32][33][34][35][36][37] and perform ab initio calculations based on Density-Functional Theory (DFT). First, we focus on the main features characterizing the optical spectrum.…”

Jahn–Teller Effect in Orthorhombic Manganites: Ab Initio Hamiltonian and Roto-vibrational Spectrum

“…Model doubly branched “Mexican Hat” APES constructed with experimental JT parameters , (ω b_vib = 500 cm –1 , A = 2.5 × 10 4 cm –1 Å –1 ) for LaMnO 3 (rare earth orthorhombic manganite) complex. The circular rim in contour plot represents the position of minima formed due to the linear E ⊗ e JT interaction.…”

“…Until now, several investigations have been made on the effect of various JT couplings for the vibronic spectra of many transition metal halides, such as, MnF 3 , , CoF 3 , , VF 3 , VF 4 , etc. In LaMnO 3 , along with the usual structural phase transition from the high-temperature pseudocubic perovskite ( Pm 3̅ m ) to the low-symmetry orthorhombic ( Pbnm ) structure at ∼780 K, the JT effect − also stabilizes the A -type antiferromagnetic spin order phase below the Néel temperature, T N ∼ 140 K. Such JT-driven phase transitions in perovskites are actually related to the distortions taking place in octahedral MnO 6 9– (cubic symmetry) units surrounded by positively charged rare-earth ions. Optical ellipsometry study on LaMnO 3 ,,,, reveals the existence of a series of resonant sideband excitations accompanying the intersite metal–metal (d 4 d 4 → d 3 d 5 ) pure electronic transition at 1.96 eV.…”

“…In recent time, Adhikari, Kovaleva, and co-workers extended the linear JT model by incorporating the effect of anharmonicity (i.e., cubic order vibrational distortion) on the centrifugally stabilized excited vibronic state of lanthanum manganite (LaMnO 3 ). Moreover, such model calculations also unveil crucial contribution of lower roto-vibrational levels of excited APES on the origin of high energy satellite transitions (2.2–2.9 eV) in the dielectric function spectra of LaMnO 3 . ,,, However, in the case of detailed investigations of the physical origin of high energy resonant narrow sideband excitations (2.2–2.9 eV) and contribution of the JT effect on the phase transition of LaMnO 3 around the Néel temperature, quantum mechanical calculations based on ab initio constructed APESs would provide further deep insight into this matter. Indeed, it has been assumed that the electron–nuclear coupling in the MnO 6 9– complex is expected to have a close resemblance with that of eight (8) MnO 6 9– units and one (1) La atom in a unit cell of LaMnO 3 crystal.…”