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 Co 3+ ions from a low-spin ground state to an intermediate-spin excited state.The study of orbital degrees of freedom in transition metal (TM) oxides has gained prominent interest. Novel techniques such as Resonant X-ray Scattering and X-Ray Absorption Spectroscopy give direct information about orbital occupancy. It is realized that the orbital moments are as important as the spin moments to understand the electronic properties. Prominent examples in which the orbital degrees of freedom determine the electronic properties are the metal-insulator transitions in V 2 O 3 [1] and doped LaM nO 3 [2]. Here, spin-and orbital-induced transitions are intimately related. In the perovkites, metallicity and orbital ordering are mutually exclusive because of the large Jahn-Teller (JT) splitting of the e g orbitals [3]. Furthermore, novel ground states can be obtained such as an orbital liquid [4,5].Considerable interest in the perovskite-type LaCoO 3 originates from the puzzling nature of two transitions in this compound and the vicinity to a metal-insulator transition. The ground state of LaCoO 3 is a nonmagnetic insulator and there is no long range magnetic order at all temperatures. At low temperatures, the magnetic susceptibility increases exponentially with temperature exhibiting a maximum near 100K. At higher temperatures, a second anomaly is observed around 500K which is accompanied by a semiconductor to metal transition. The maximum at 100K was ascribed initially to a change of the spin state in the Co 3+ ions i.e. a transition from a low-spin (LS) nonmagnetic ground state (t 6 2g , S = 0) to a high-spin (HS) state (t 4 2g e 2 g , S = 2) [6,7,8,9]. In the more recent literature [10,11,12,13,14], new scenarios involving an intermediate-spin state (IS) (t 5 2g e 1 g , S = 1) have been proposed. Using LDA+U calculations, Korotin et al. [15] proposed the stabilization of the IS state due to the large hybridization between the Co-e g and O-2p levels. Due to the partially filled e g level, the IS state is JT active. The degeneracy of the e g orbitals of Co 3+ ions in the LS state is expected to be lifted in the IS state by a JT distortion.All structural studies, based on powder X-ray and neutron diffraction experiments are consistently interpreted in rhombohedral R3c symmetry and no structural transitions are reported in the temperature interval 4.2K − 1248K [16,17,18,19]. A cooperative JT distortion is incompatible with this space group. The rhombohedral distortion of the parent cubic perovskite structure consists of a deformation along the body diagonal, and preserves only one Co ...

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All solid-state tunnel spectroscopy experiments performed on single-crystal Fe/MgO/Fe magnetic tunnel junctions show sharp features at 0.2 and 1.1 V. These peaks are observed on the electrical differential conductivity only in the antiparallel magnetic configuration and only for the voltage sign corresponding to the injection of electrons toward the bottom electrode. They are attributed to the conductivity of two different resonant states of the Fe͑001͒/MgO bottom interface. The analysis of the attenuation of these peaks as a function of the insulator thickness provides information on their symmetry. Nowadays, magnetic tunnel junctions ͑MTJs͒ benefit on a strong scientific interest 1 motivated both by their high potential for applications in sensor and storage devices and by the complex fundamental physics of spin dependent tunneling. Theoretical models of tunneling in single-crystal devices 2-4 have been successfully confronted to experimental observations of giant high tunneling magnetoresistance ͑TMR͒ in single-crystal MTJs involving MgO tunnel barrier. 5-8 In these systems, the large TMR effects are determined by the different tunneling mechanisms and symmetry-related decay rates of the Bloch waves for the majority-and the minorityspin channels within the barrier. According to Butler's theory, 2 along the k ʈ = 0 propagation direction, the transport properties of an epitaxial Fe/MgO/Fe stack are governed by the ⌬ i symmetry bulk density of states ͑DOS͒ of the Fe electrodes and their different decay rates into the barrier. For the parallel configuration the transport is governed by the majority ⌬ 1 states which have the lowest decay rate into the barrier. For the antiparallel configuration, there are no ⌬ 1 states available at the Fermi level, the conductance being dominated by the ⌬ 5 states. Moreover, the electronic properties at the metal/barrier interface have been often proven to be important in determining the TMR sign and amplitude. 9-11 Surface states may appear at the ferromagnet/insulator interface through the breakdown of the translational symmetry. 12,13 When coupled to the bulk, they become interfacial resonant states ͑IRSs͒ and are predicted to strongly affect and even dominate the tunnel transmission. 2,4,[14][15][16] Experimentally, the surface states have been already studied for the Fe͑001͒/vacuum system below 17 and above 18 the Fermi level. However, at the Fe͑001͒/MgO interface, the nature of the interfacial electronic structure and its role on the spin dependent transport through the MgO barrier are expected to be more complex. Even in a perfect system, the chemical bondings at the Fe/MgO interface affect the Fe͑001͒ electronic structure and modify the propagation by tunneling of existent IRS. Moreover, the cubic symmetry of the insulator will determine the tunneling attenuation of the IRS as a function of its symmetry. A direct analysis of the Fe͑001͒ surface states covered by MgO above and under the Fermi level ͑E F ͒ is difficult by standard scanning tunneling spectroscopy ͑unoccupied s...