Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L2,3 edge we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spinstate system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.PACS numbers: 71.28.+d, 71.70.Ch, 78.70.Dm LaCoO 3 shows a gradual non-magnetic to magnetic transition with temperature, which has been interpreted originally four decades ago as a gradual population of high spin (HS, t 4 2g e 2 g , S = 2) excited states starting from a low spin (LS, t 6 2g , S = 0) ground state [1,2,3,4,5,6,7,8]. This interpretation continued to be the starting point for experiments carried out up to roughly the first half of the 1990's [9,10,11,12]. All this changed with the theoretical work in 1996 by Korotin et al., who proposed on the basis of local density approximation + Hubbard U (LDA+U) band structure calculations, that the excited states are of the intermediate spin (IS, t 5 2g e 1 g , S = 1) type [13]. Since then many more studies have been carried out on LaCoO 3 with the majority of them [14,15,16,17,18,19,20,21,22,23,24,25,26,27] claiming to have proven the presence of this IS mechanism. In fact, this LDA+U work is so influential [28] that it forms the basis of most explanations for the fascinating properties of the recently synthesized layered cobaltate materials, which show giant magneto resistance as well as metal-insulator and ferroferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [29,30].In this paper we critically re-examine the spin state issue in LaCoO 3 . There has been several attempts made since 1996 in order to revive the LS-HS scenario [31,32,33,34,35], but these were overwhelmed by the above mentioned flurry of studies claiming the IS mechanism [14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Moreover, a new investigation using inelastic neutron scattering (INS) has recently appeared in Phys. Rev. Lett.[36] making again the claim that the spin state transition involves the IS states. Here we used soft xray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the Co-L 2,3 edge and we revealed that the spin state transition in LaCoO 3 can be well described by a LS ground state and a triply degenerate HS excited state, and that an inhomogeneous mixed-spinstate system is formed. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility and electron spin resonance (ESR) data, and provide support for an alternative interpretation of the INS [37]. C...
The central quantity of density functional theory is the so-called exchange-correlation functional. This quantity encompasses all non-trivial many-body effects of the ground-state and has to be approximated in any practical application of the theory. For the past 50 years, hundreds of such approximations have appeared, with many successfully persisting in the electronic structure community and literature. Here, we present a library that contains routines to evaluate many of these functionals (around 180) and their derivatives.
Using Co-L2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La1.5Sr0.5CoO4 involves high spin (S=3/2) Co 2+ and low spin (S=0) Co 3+ ions. This provides evidence for the spin blockade phenomenon as a source for the extremely insulating nature of the La2−xSrxCoO4 series. The associated e 2 g and e 0 g orbital occupation accounts for the large contrast in the Co-O bond lengths, and in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the non-magnetic (S=0) Co 3+ ions. From the identification of the bands we infer that La1.5Sr0.5CoO4 is a narrow band material.PACS numbers: 71.28.+d, 78.70.Dm Considerable research effort has been put in cobaltate materials during the last decade in search for new phenomena and extraordinary properties. A key aspect of cobaltates that distinguish them from e.g. the manganates and cuprates [1], is the spin state degree of freedom of the Co 3+/III ions: it can be low spin (LS, S=0), high spin (HS, S=2) and even intermediate spin (IS, S=1) [2,3]. This aspect comes on top of the orbital, spin (up/down) and charge degrees of freedom that already make the manganates and cuprates so exciting. Indeed, numerous cobaltate systems have been discovered with properties that include giant magneto resistance [4,5], superconductivity [6] and ferro-ferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [7,8,9,10,11,12,13,14]. A new and exciting aspect in here is the recognition that the so-called spin blockade mechanism could be present and responsible for several of those unusual properties [15]. If true, this would open up new research opportunities since one could envision exploiting explicitly this mechanism in materials design.Here we focus on the La 2−x Sr x CoO 4 system, which shows quite peculiar transport and magnetic properties [16,17,18,19,20,21,22,23,24,25]. This material is extremely insulating for a very wide range of x values with anomalously high activation energies for conductivity, very much unlike the Mn, Ni, or Cu compounds [1,18,26]. The commensurate antiferromagnetic (AF) state remains stable up to a surprisingly high value of x=0.3 [24,25]. Charge ordering (CO) and spin ordering (SO) at half doping involve quite different transition temperatures, namely T CO ∼ 750 K and T SO ≤ 30 K, respectively. This constitutes a ratio of 25, which is extraordinary since it is an order of magnitude larger than in the Mn and Ni materials [1,21,27].It was already reported that the SO in the La 1.5 Sr 0.5 CoO 4 composition involves non-magnetic Co 3+ ions with the claim that these Co 3+ ions are in the IS state and become non-magnetic due to strong planar anisotropy driven quenching of the spin angular momentum below the T SO [21,22]. Here we go one step further. Using soft x-ray absorption spectroscopy (XAS) we are able to show unambiguously that the Co 3+ ions are in the LS (S=0) state, both below and above T SO . Together with the verification...
In this article we present a generalization of the electron localization function (ELF) that can be used to analyze time-dependent processes. The time-dependent ELF allows the time-resolved observation of the formation, the modulation, and the breaking of chemical bonds, and can thus provide a visual understanding of complex reactions involving the dynamics of excited electrons. We illustrate the usefulness of the time-dependent ELF by two examples: the pi-pi* transition induced by a laser field, and the destruction of bonds and formation of lone pairs in a scattering process.Comment: 4 pages, 3 figures; related information (including movies) can be found at http://www.physik.fu-berlin.de/~ag-gross/tdelf
The origin of both the Ising chain magnetism and ferroelectricity in Ca3CoMnO6 is studied by ab initio electronic structure calculations and x-ray absorption spectroscopy. We find that Ca3CoMnO6 has the alternate trigonal prismatic Co 2+ and octahedral Mn 4+ sites in the spin chain. Both the Co 2+ and Mn 4+ are in the high spin state. In addition, the Co 2+ has a huge orbital moment of 1.7 µB which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed ↑↑↓↓ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.PACS numbers: 78.70.Dm, 71.27.+a Among a variety of multiferroic materials discovered so far [1,2], ferroelectric Ising chain magnet Ca 3 CoMnO 6 is quite unique, because the ferroelectricity (FE) is driven by exchange striction in a collinear Ising spin chain consisting of the charge ordered transition-metal ions [3]. The spin chain has the alternate trigonal prismatic and octahedral sites [3,4]. Special ↑↑↓↓ antiferromagnetic (AF) structure is detected in Ca 3 CoMnO 6 below T N ≈13 K by neutron diffraction. However, the measured magnetic moment of 0.66 µ B /Co and 1.93 µ B /Mn is much smaller than the expected one of the normal high-spin (HS) Co 2+ (S=3/2) and Mn 4+ (S=3/2). This led Choi et al. to a conclusion that the Co 2+ is (surprisingly) in a low-spin (LS) state [3]. In contrast, the effective magnetic moment of µ eff =5.8-6.0 µ B per formula unit (f.u.), extracted from magnetic susceptibility measurements above T N [4,5], suggests that both Co 2+ and Mn 4+ are in a HS state. Thus there is an apparent controversy between those data, and the problem concerning the spin state of, in particular, Co 2+ ions seems to be still unsolved. Another important question is to understand the nature of the Ising magnetism and of the resulting exchange striction, which are apparently crucial for the appearance of FE in Ca 3 CoMnO 6 . To this end, we carried out ab initio electronic structure calculations and x-ray absorption spectroscopy (XAS). We address the important issues including the Co/Mn site preference, their charge/spin/orbital states, the origin of the Ising magnetism, and the exchange striction.Our ab initio calculations were performed by using the full-potential augmented plane waves plus local orbital method (Wien2k code) [6]. We took the experimental centrosymmetric structure data of the rhombohedral lattice (R-3c) which has in a hexagonal setting the lattice constant a=9.1314Å and c=10.5817Å [4,7]. The calculations were done for different magnetic structures (↑↓↑↓, ↑↑↓↓, ↑↓↓↓, and ↑↑↑↑ orderings in the Co-Mn-CoMn chains). To study the exchange striction effect, we investigated the effect of internal atomic relaxation allowing the inversion symmetry to be broken in the ↑↑↓↓ spin chain, as discussed below. The muffin-tin sphere radii are chosen to be 2.4, 2.1 and 1.4 Bohr for ...
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