2009/07/29 by N. N. Kovaleva, A.M. Oleś, Andrzej M. Oles +6
Chemistry · Materials Science · Physics and Astronomy · #Advanced Condensed Matter Physics #Antiferromagnetism #Chemistry #Condensed matter physics #Crystal structure #Crystallography #Energy (signal processing) #Magnetic and transport properties of perovskites and related materials #Multiferroics and related materials #Order (exchange) #Orthorhombic crystal system #Physics #Quantum mechanics #Spin (aerodynamics) #Thermodynamics #cond-mat.str-el
paper · pdf · doi:10.1103/physrevb.81.235130
10 pages, 14 figures
arxiv created 2009/07/29 · openalex publication_date 2010/06/25 · arxiv updated 2015/05/13 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO3 crystal in the spectral range 1.2--6.0 eV at temperatures 20\ensuremath≤T\ensuremath≤300 K. The complex dielectric response along b and c axes of the Pbnm orthorhombic unit cell, \stackrel\ifmmode \else \~\fi\ensuremathεb(\ensuremathν) and \stackrel\ifmmode \else \~\fi\ensuremathεc(\ensuremathν), is highly anisotropic over the spectral range covered in the experiment. The difference between \stackrel\ifmmode \else \~\fi\ensuremathεb(\ensuremathν) and \stackrel\ifmmode \else \~\fi\ensuremathεc(\ensuremathν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A-type antiferromagnetic long-range order at TN=139.6 K. In this study we focus on the analysis of excitations observed at high energy (\ensuremath∼4--5 eV) and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at \ensuremath∼2 eV. We used a classical dispersion analysis to quantitatively determine the temperature-dependent optical spectral-weights shifts between low- and high-energy optical bands. Based on the observation of a pronounced spectral-weight transfer between both features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d4d4\ensuremath\rightleftharpoonsd3d5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral-weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn eg electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between eg orbitals in LaMnO3, as well as to extract experimental information concerning the type of orbital order in LaMnO3. This study identifies the lowest-energy optical transition at \ensuremath∼2 eV as an intersite d\text\ensuremath-d transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.