Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice.1 The electron nematicity has been recently observed in the iron-pnictide 2-6 and cuprate 7-9 hightemperature superconductors. Whether such a tendency of electrons to self-organise unidirectionally has a common feature in these superconductors is, however, a highly controversial issue. In the cuprates, the nematicity has been suggested as a possible source of the pseudogap phase, 7-9 whilst in the iron-pnictides, it has been commonly associated with the tetragonalto-orthorhombic structural phase transition at T s . Here, we provide the first thermodynamic evidence in BaFe 2 (As 1−x P x ) 2 that the nematicity develops well above the structural transition and persists to the nonmagnetic superconducting regime, resulting in a new phase diagram strikingly similar to the pseudogap phase diagram in the cuprates.9,10 Our highly sensitive magnetic anisotropy measurements using microcantilever torque-magnetometry under in-plane field rotation reveal pronounced two-fold oscillations, which break the tetragonal symmetry. Combined with complementary high-resolution synchrotron X-ray and resistivity measurements, our results consistently identify two distinct temperaturesone at T * , signifying a true nematic transition, and the other at T s (< T * ), which we show to be not a true phase transition, but rather what we refer to as a "meta-nematic transition", in analogy to the well-known metamagnetic transition in the theory of magnetism. Our observation of the extended nematic phase above the superconducting dome establishes that the nematicity has primarily an electronic origin, inherent in the normal state of high-temperature superconductors.In the iron pnictides, the antiferromagnetic transition is closely intertwined with the structural phase transition from tetragonal (T) to orthorhombic (O) crystal symmetry. Although recent experiments, including neutron scattering, 2 ARPES, 3,11 STM, 4 and transport measurements, 5,6 have provided evidence for electronic anisotropy, these measurements were carried out either in the low-temperature orthorhombic phase, 2,4,11 where the crystal lattice structure has already broken C 4 symmetry, or in the tetragonal phase under uniaxial strain 3,5,6 that also breaks this symmetry. Therefore, the question remains open whether the electronic anisotropy can exist above the structural transition without an external driving force, including under the superconducting (SC) dome. In the past, the nematic transition in the pnictides has been associated either with the orbital ordering, [12][13][14][15][16][17][18] or with the spontaneous breaking of the Z 2 Ising symmetry between two collinear magnetic ordering wave-vectors Q = (π, 0) and (0, π).19-22 Therefore determining the nature of the nematicity is a key to understanding the microscopic origin of the lattice and magnetic...

A lightly doped manganite La 0.88 Sr 0.12 MnO 3 exhibits a phase transition at T OO 145 K from a ferromagnetic metal (T C 172 K) to a novel ferromagnetic insulator. We identify that the key parameter in the transition is the orbital degree of freedom in e g electrons. By utilizing the resonant x-ray scattering, orbital ordering is directly detected below T OO , in spite of a significant diminution of the cooperative Jahn-Teller distortion. The experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present studies uncover a crucial role of the orbital degree of freedom in the metal-insulator transition in lightly doped manganites.[S0031-9007(99)09213-3]

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T c shows a striking enhancement. The completed phase diagram uncovers that high-T c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T c remains low near the nematic critical point.