We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co 3+ ions with increasing temperature from a low-spin ground state (t ) without (100 K -500 K) and with (> 500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.Transition-metal oxides have fascinating physical properties as e.g. high-temperature superconductivity in the cuprates or colossal magnetoresistance in the manganites. Their properties are often governed by a complex interplay of charge, magnetic, structural, and orbital degrees of freedom. Moreover, for a given oxidation state some transition metals display different spin states as it is the case in various cobalt oxides. Quite recently a class of layered cobalt compounds with the chemical composition REBaCo 2 O 5+δ (RE = rare earth) has attracted considerable interest. These compounds show a broad variety of ordering phenomena and other transitions, e.g. (antiferro-and/or ferro-) magnetic order, charge and/or orbital order, metal-insulator transitions or spin-state transitions [1,2,3,4,5,6,7,8,9]. For TlSr 2 CoO 5 it has been proposed that a metal-insulator transition is driven by a spin disproportionation, which consists of an alternating ordering of Co The occurrence of Co 3+ in different spin states is known since the 1950s from LaCoO 3 [12, 13], which transforms with increasing temperature from a non-magnetic insulator to a paramagnetic insulator around 100 K and shows an insulator-to-metal transition around 500 K. But even for this rather simple pseudo-cubic perovskite the nature of these transitions is still unclear. The ground state is usually attributed to the low-spin configuration (LS: t 6 2g e 0 g ; S = 0) and the paramagnetic behavior above 100 K to the thermal population of an excited state. However, the question whether the excited state has to be identified with the HS or the IS state is subject of controversial discussions. Early publications often assume a LS/HS scenario [14,15,16]. In order to explain the insulating nature up to 500 K an ordering of LS and HS Co 3+ ions has been proposed which vanishes at the insulatorto-metal transition [17,18]. Yet the presence of a HS configuration below 400 K has been questioned on the basis of X-ray absorption and photoemission experiments [19]. Alternative descriptions of LaCoO 3 favoring a LS/IS scenario [20,21,22,23,24] are mainly based on the results of LDA+U calculations [25], which propose that due to a strong hybridization between Co-e g levels and O-2p levels the IS state is lower in energy than the HS state. Within this scenario the occurrence of orbital order and its melting have been proposed in order to e...
We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.
The crystal and magnetic structure of La1−xSr1+xMnO4 (0 ≤ x ≤ 0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.
Orbital ordering (OO) in the layered perovskite La0.5Sr1.5MnO4 has been investigated using the enhanced sensitivity of soft x-ray resonant diffraction at the Mn L edges. The energy dependence of an OO diffraction peak over the L(2,3) edges is compared to ligand-field calculations allowing a distinction between the influences of Jahn-Teller distortions and spin correlations. The energy dependence of the diffraction peak at the Mn L1 edge is remarkably different from that observed at the Mn K edge.
Collective orbital excitations in orbitally ordered YVO3 and HoVO3 E Benckiser, R Rückamp, T Möller et al. Abstract. The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO 3 (R = La, Sm and Y), LaMnO 3 , Y 2 BaNiO 5 , CaCu 2 O 3 and K 4 Cu 4 OCl 10 , ranging from early to late transition-metal ions, from t 2g to e g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO 3 , we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yields good agreement, demonstrating that the coupling to the lattice is important for a 7 Author to whom any correspondence should be addressed. quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g. the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular, in case of the orbital excitations at ≈0.25 eV in RTiO 3 . Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing. Contents
One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.
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