Spin state and phase competition in TbBaCo_{2}O_{5.5} and the lanthanide series LnBaCo_{2}O_{5+\delta} (0<=\delta<=1)

A clear physics picture of TbBaCo$_{2}$O$_{5.5}$ is revealed on the basis of density functional theory calculations. An antiferromagnetic (AFM) superexchange coupling between the almost high-spin Co$^{3+}$ ions competes with a ferromagnetic (FM) interaction mediated by both p-d exchange and double exchange, being responsible for the observed AFM-FM transition. And the metal-insulator transition is accompanied by an xy/xz orbital-ordering transition. Moreover, this picture can be generalized to the whole lanthanide series, and it is predicted that a few room-temperature magnetoresistance materials could be found in LnBa$_{1-x}$A$_{x}$Co$_{2}$O$_{5+\delta}$ (Ln=Ho,Er,Tm,Yb,Lu; A=Sr,Ca,Mg).Comment: 13 pages, 2 figures; to be published in Phys. Rev. B on 1st Sept. Title and Bylines are added to the revised versio

Evidence for orbital ordering in LaCoO3

We present powder and single crystal X-ray diffraction data as evidence for a monoclinic distortion in the low spin (S=0) and intermediate spin state (S=1) of LaCoO3. The alternation of short and long bonds in the ab plane indicates the presence of eg orbital ordering induced by a cooperative Jahn-Teller distortion. We observe an increase of the Jahn-Teller distortion with temperature in agreement with a thermally activated behavior of the Co3+ ions from a low-spin ground state to an intermediate-spin excited state.Comment: Accepted to Phys. Rev.

Evidence for a Low-Spin to Intermediate-Spin State Transition in LaCoO3

We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO$_3$ 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 to an intermediate-spin state without (100K - 500K) and with (>500K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500K to (probably local) Jahn-Teller distortions of the CoO$_6$ octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.Comment: an error in the scaling factor of Eq.(4) and consequently 2 values of table I have been corrected. The conclusions of the paper remain unchanged. See also: C. Zobel et al. Phys. Rev. B 71, 019902 (2005) and J. Baier et al. Phys. Rev. B 71, 014443 (2005

ELECTRONIC-STRUCTURE OF TETRAHEDRAL IRON(III) SULFUR CLUSTERS IN ALKALINE THIOFERRATES - AN X-RAY-ABSORPTION STUDY

X-ray absorption spectra of Na5FeS4 and KFeS2 containing separate and interconnected edge-shared FeS45- tetrahedra, respectively, are reported and interpreted in terms of an S = 5/2 ground state for Fe3+ in both compounds. A rather small value of the ligand-to-metal charge transfer energy Delta(eff) was indicated by the comparison of the experimental Fe 2p XAS spectrum with atomic multiplet calculations, reflecting an almost 1 : 1 admixture of d(5) and d(6)L (L hole on the ligand) character in the ground state for Na5FeS4. The broadening of the main peak toward higher energies when going from Na5FeS4 to KFeS2 is attributed to symmetry lowering from T-d to D-2d. Ligand-field parameterization schemes, such as the angular overlap model, are found to be quantitatively not applicable for the systems under consideration. The phase diagram of d(5) ions and its dependence on the crystal-field splitting (10Dq) acid the ligand-to-metal charge transfer energy (Delta(eff)) are discussed and it is shown that an intermediate spin state (S = 3/2) can become stable for negative Delta(eff) and moderate values of 10Dq and metal-ligand hybridization. This presents another limit for the ligand-field approach and the Tanabe-Sugano diagrams, where only S = 5/2 and S = 1/2 ground states are predicted. For the Fe3+ in a tetrahedral field the theory indicates that it is unlikely that the S = 3/2 and S = 1/2 states become the ground state since the gain in energy due to ligand-ligand coupling and the relatively small charge transfer energy (both tending to lower energetically the S = 3/2 state) are not sufficient to overcome the high exchange stabilization of the high-spin (S = 5/2) state. (C) 1995 Academic Press, Inc

Diffusion tensor imaging and chemical shift imaging assessment of heterogeneity in low grade glioma under temozolomide chemotherapy.

Item does not contain fulltextDiffusion tensor imaging and multiple voxel magnetic resonance spectroscopy were performed in the MRI follow-up of a patient with a glioma treated with temozolomide chemotherapy. Tumor shrinkage was paralleled by reductions in choline level and by increases in apparent diffusion coefficient indicating decreased cellularity. Within the tumor, choline level and apparent diffusion coefficient showed a significant inverse correlation (P < 0.01). Fractional anisotropy distribution in the tumor correlated positively with N-acetyl aspartate level (P < 0.001), indicating that these parameters reflect (remaining) axonal structure. Tumor lactate level, also found to decrease under therapy, did not correlate with any other parameter

Quantum chemical study of Co 3+ spin states in LaCoO 3

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO 3. The main concern is to calculate the energy level ordering of different spin states of Co 3+ , which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T=5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S=0) ground state is separated from the first excited high-spin (HS, S=2) state by a gap >100 meV, while the intermediate-spin (IS, S=1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co 3+ ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200 $\div$ 300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010