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High-temperature superconductivity in cuprates arises from an electronic state that remains poorly understood. We report the observation of a related electronic state in a noncuprate material, strontium iridate (Sr2IrO4), in which the distinct cuprate fermiology is largely reproduced. Upon surface electron doping through in situ deposition of alkali-metal atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments of zero-energy states, known as Fermi arcs, and a gap as large as 80 millielectron volts. Its evolution toward a normal metal phase with a closed Fermi surface as a function of doping and temperature parallels that in the cuprates. Our result suggests that Sr2IrO4 is a useful model system for comparison to the cuprates.

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A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction 1 . Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs' 2 . This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity 3 or by invoking orders which are natural competitors of d-wave superconductors 4,5 . Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 . Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state. La 1.2 Sr 1.8 Mn 2 O 7 (LSMO) is a prototypical bilayer manganite that exhibits the colossal magnetoresistance (CMR) effect-the extremely large drop in resistivity induced by application of a magnetic field near the Curie temperature (T C ) 6,7 . The CMR effect exploits a metalinsulator transition between a low-temperature ferromagnetic-metallic ground state and a high-temperature paramagnetic-insulating phase. The nature of the ferromagnetic-metallic ground state in LSMO remains highly controversial. On the one hand, the underlying Fermi surface and band structure have clear resemblance to those of the copper oxides 8 (Lin, H., Sahrakorpi, S., Barbiellini, B. & Bansil, A., personal communication on full potential band structure and Fermi surface computations for x ¼ 0.4). On the other hand, previous angle-resolved photoemission (ARPES) investigations revealed no quasiparticle peak, a suppression of spectral weight at the Fermi level (E F ) (the pseudogap), and an unusually light effective mass, a factor of two lighter than the calculated band structure value. This last observation is puzzling given the general expectation of strong interactions in the manganites. Moreover, the value for the inplane conductivity calculated with the ARPES parameters is nearly one order of magnitude higher than that measured by transport 9-11 .Our ARPES experiments resolve the controversy of the lowtemperature ferromagnetic-metallic groundstate of LSMO by demonstrating that its electronic structure is strikingly similar to that found in th...

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