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...

Spin correlations in La2-xSrxCoO4 (0.3 < or = x < or = 0.6) have been studied by neutron scattering. The commensurate antiferromagnetic order of La2CoO4 persists in a very short range up to a Sr content of x = 0.3, whereas small amounts of Sr suppress commensurate antiferromagnetism in cuprates and in nickelates. La2-xSrxCoO4 with x > 0.3 exhibits incommensurate spin ordering with the modulation closely following the amount of doping. These incommensurate phases strongly resemble the stripe phases observed in cuprates and nickelates, but incommensurate magnetic ordering appears only at larger Sr content in the cobaltates due to a reduced charge mobility.

We present a study of the thermal conductivity κ and the thermopower S of single crystals of La1−xSrxCoO3 with 0 ≤ x ≤ 0.3. For all Sr concentrations La1−xSrxCoO3 has rather low κ values. For the insulators (x < 0.18) this arises from a suppression of the phonon thermal conductivity by lattice disorder due to temperature-and/or doping-induced spin-state transitions of the Co ions. For larger x, the heat transport by phonons remains low, but an additional contribution from mobile charge carriers causes a moderate increase of κ. The thermopower of the low-doped crystals is positive and shows a pronounced maximum as a function of temperature. With increasing x, this maximum strongly broadens and its magnitude decreases. For the highest Sr content (x = 0.3) S becomes even negative in the intermediate temperature range. From S, κ, and the electrical resistivity ρ we derive the thermoelectric figure of merit Z = S 2 / κρ. For intermediate Sr concentrations we find notably large values of Z indicating that Co-based materials could be promising candidates for thermoelectric cooling.

The experimental values of the thermal boundary resistance occurring at interfaces between two solids at sub-ambient temperatures are reviewed. New data are presented in the temperature range from 4 K to 300 K for the thermal resistance between different metals (Cu, stainless steel), interlayered by various cryogenic bonding agents (Apiezon-N, Cryocon grease, In and InGa), or mechanically connected (dry) contacts. Depending on the contact materials, the thermal conductance varies between 10 2 W m −2 K −1 and 5 × 10 4 W m −2 K −1 at room temperature, and decreases approximately linearly by one order of magnitude between 200 K and 20 K. Our experimental data agree well with the data reported in the literature for the temperature range below 4 K and measurements near room temperature.

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