Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

The nature of the pseudogap in high transition temperature (high-T c ) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-T c superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-T c superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe 2 (As 1-x P x ) 2 . The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry.The pseudogap (PG) observed in the normal state of the high-T c copper oxide superconductors remains a mysterious state of matter [1][2][3]. It has been attributed to several mechanisms such as a precursor pairing [4][5][6] and a novel form of spin/charge ordering [7][8][9]. Nearly a quarter-century after the discovery of high-T c superconductivity [10], the PG phase is still extensively debated in the literature and no consensus has been reached regarding its origin. In order to gain further insights into the relationship between the high-T c superconductivity and the PG, the exploration of the PG phase in other high-T c superconductors is highly desired.Iron-pnictides [11] are another class of high-T c superconductors whose typical phase diagram is shown in Fig. 1(a). The parent compound shows stripe-type antiferromagnetic (AF) ordering at T N accompanying a lattice distortion from tetragonal to orthorhombic structure at T s [12]. In contrast to the parent cuprate which is an AF insulator, the parent pnictide is an AF metal. The electronic structure derived from multiple Fe 3d orbitals [13], consists typically of disconnected hole and electron Fermi surfaces (FSs), which undergo an electronic reconstruction across T N and T s [14,15]. In addition to spin fluctuations derived from the nesting between the disconnected FSs [16,17], orbital fluctuations are also a candidate for a driving force of electron pairing in iron-pnictides [18,19] AsP122 system shows quasi-two dimensional FSs above T N,s as shown in Fig. 2(a). Three hole-FSs (, , ) and two electron-FSs (, ) exist around the BZ center and the BZ corner, respectively ( Fig.2 (b)). SubstitutingAs ions by isovalent P ion causes reduction of the pnictogen height without changing the carrier density. As P ion concentration is increased, the warping of the -hole In order to investigate the T-dependence of the fine electronic structure in the multi-orbital system, it is required to clearly separate the multiple band dispersions.For this purpose, we chose p-polarization...