Orbital Ordering inLa0.5Sr1.5MnO<

Abstract

We found that the conventional model of orbital ordering of 3x 2 − r 2 /3y 2 − r 2 type in the eg states of La0.5Sr1.5MnO4 is incompatible with measurements of linear dichroism in the Mn 2p-edge x-ray absorption, whereas these eg states exhibit predominantly cross-type orbital ordering of x 2 − z 2 /y 2 − z 2 . LDA+U band-structure calculations reveal that such a cross-type orbital ordering results from a combined effect of antiferromagnetic structure, Jahn-Teller distortion, and on-site Coulomb interactions.PACS numbers: 75.47. Lx, 75.30.Mb, 71.28.+d, 78.70.Dm Orbital ordering, which manifests itself in the spatial distribution of the outermost valence electrons, is an important topic in current research of transition-metal oxides, as the magnetic and transport properties are closely related to the orbital and charge degrees of freedom [1]. In particular, charge-orbital ordering of half-doped manganites has attracted much attention [2,3,4,5,6,7,8]. The mechanism of charge-orbital ordering is being hotly debated [9,10,11,12,13,14,15]. To observe orbital ordering directly is a difficult task. Experimental results of resonant x-ray scattering (RXS) at the Mn K-edge in La 0.5 Sr 1.5 MnO 4 and LaMnO 3 show removal of degeneracy between 4p x and 4p y ; these observations have been argued to be direct evidence of orbital ordering [16,17]. However, the origin of RXS at Mn K-edge is controversial. Orbital ordering in transition-metal oxides is typically accompanied by Jahn-Teller lattice distortion. Calculations based on a local-density approximation including on-site Coulomb interactions (LDA+U) [18,19] and multiple scattering theory [20] indicate that RXS measurements pertain mainly to Jahn-Teller distortion, instead of directly observing 3d orbital ordering. Multiplet calculations have shown that one can use linear dichroism (LD) in soft x-ray absorption spectroscopy (XAS) to identify the orbital character of 3d states in orbitalordered manganites [21].Half-doped manganites such as La 0.5 Sr 1.5 MnO 4 exhibit CE-type antiferromagnetic (AFM) ordering and charge-orbital ordering [16,22,23]. They have a zigzag magnetic structure in which the magnetic moments of Mn on the chain form a ferromagnetic coupling, but AFM coupling between the zigzag chains. Below the charge-ordering (CO) temperature T CO =217 K, the valence of La 0.5 Sr 1.5 MnO 4 orders in an alternating pattern with two distinct sites identified as Mn 3+ and Mn 4+ [22,23]. The e g electrons on the nominal Mn 3+ sites of La 0.5 Sr 1.5 MnO 4 are believed to exhibit an orbital ordering of 3x 2 − r 2 /3y 2 − r 2 , in which occupied d 3x 2 −r 2 and d 3y 2 −r 2 orbitals are alternately arranged at two sublattices in the ab plane [7]. However, d 3x 2 −r 2 and d x 2 −z 2 (d 3y 2 −r 2 and d y 2 −z 2 ) orbitals might be mixed, because orbitals of these two types have the same spatial symmetry with respect to the MnO 2 plane. To clarify the nature of orbital ordering in La 0.5 Sr 1.5 MnO 4 is essential to reveal the origin of orbital ordering in half-doped manganites...

Doping dependence of spin and orbital correlations in layered manganites

“…These findings, i.e., the relatively small difference in the density at bridge and corner sites and the occupation of the directional orbitals, are in contrast to some experimental results. 51,52 However, the OO similar to our findings was also reported, 16,53,54 and one expects that rather extreme charge modulation, with holes at corner sites and e g electrons in bridge positions, should be excluded as then the FM double exchange which stabilizes the CE phase would be lost. Therefore, the charge order with alternating Mn 3+ and Mn 4+ ions has been challenged, and intermediate valence picture with only small density variations has been suggested both in experimental 55 and in theoretical studies.…”

Orbital ordering inLa0.5Sr1.5MnO4studied by model Hartree-Fock calculation

“…1(a). More recently, however, from the studies of liner dichroism in the Mn 2p-edge x-ray absorption [9] and resonant soft x-ray scattering at the Mn 2p edge [10] of the La 0.5 Sr 1.5 MnO 4 , where T CO ≃ 217 K and T N ≃ 110 K [11], it was demonstrated that e g electrons exhibit predominantly cross-type (z 2 − x 2 /y 2 − z 2 ) orbital ordering as shown in Fig. 1(b).…”

“…More recently, however, from the studies of liner dichroism in the Mn 2p-edge x-ray absorption [9] and resonant soft x-ray scattering at the Mn 2p edge [10] of the La 0.5 Sr 1.5 MnO 4 , where T CO ≃ 217 K and T N ≃ 110 K [11], it was demonstrated that e g electrons exhibit predominantly cross-type (z 2 − x 2 /y 2 − z 2 ) orbital ordering as shown in Fig. 1(b).In order to understand the underlying mechanism of the cross-type orbital ordering, we have studied orbital ordering in La 0.5 Sr 1.5 MnO 4 in the presence of lattice dis- Rod-type (3x 2 − r 2 /3y 2 − r 2 ) orbital ordering; (b) Cross-type (z 2 − x 2 /y 2 − z 2 ) orbital ordering [9]. Also shown is the CEtype AFM ordering by arrows.…”

“…In order to understand the underlying mechanism of the cross-type orbital ordering, we have studied orbital ordering in La 0.5 Sr 1.5 MnO 4 in the presence of lattice dis- Rod-type (3x 2 − r 2 /3y 2 − r 2 ) orbital ordering; (b) Cross-type (z 2 − x 2 /y 2 − z 2 ) orbital ordering [9]. Also shown is the CEtype AFM ordering by arrows.…”

High-pressure and high-temperature synthesis and physical properties of Ca2CrO4 solid

“…[3][4][5] In the strongly correlated electronic systems, spin, charge and orbital coupled each other which leading to very interesting phenomenon. 6 Sr 2 VO 4 with 3d 1 electronic configuration and antiferromagnetic ground state shows an apparently orbital ordering transition at T c about 100K, while muon spin rotation and relaxation (µ + SR) measurement shows that static long-range antiferromagnetic order occurred below T N =8K.…”