URu2Si2 is a uranium compound that exhibits a so-called ‘hidden-order’ transition at ~17.5 K. However, the order parameter of this second-order transition as well as many of its microscopic properties remain unclarified despite considerable research. One of the key questions in this regard concerns the type of spontaneous symmetry breaking occurring at the transition; although rotational symmetry breaking has been detected, it is not clear whether another type of symmetry breaking also occurs. Another key question concerns the property of Fermi-surface gapping in the momentum space. Here we address these key questions by a momentum-dependent observation of electronic states at the transition employing ultrahigh-resolution three-dimensional angle-resolved photoemission spectroscopy. Our results provide compelling evidence of the spontaneous breaking of the lattice's translational symmetry and particle-hole asymmetric gapping of a heavy quasiparticle pocket at the transition.

The electronic structure of nearly optimally doped novel superconductor LaO 1−x F x BiS 2 (x = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (E F ), which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near E F shows a squarelike distribution around the (Z) point in addition to electronlike Fermi-surface (FS) sheets around the X(R) point, indicating that FS of LaO 0.54 F 0.46 BiS 2 is in close proximity to the theoretically predicted topological change.

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (T c ). In the early days, based on a weak-coupling viewpoint, the nesting between hole-and electron-like Fermi surfaces (FSs) leading to the so-called s± state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in K x Fe 2−y Se 2 , which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole-and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

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The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr 2 IrO 4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds. DOI: 10.1103/PhysRevB.96.041106 Unconventional physics of superconductivity near the metal-insulator transition in strongly correlated Mott insulators has been one of the major themes in a variety of systems, such as cuprates, iron-based compounds, heavyelectron systems, and organic materials [1]. Recently, much attention has been given to 5d-electron systems in which the magnitude of spin-orbit coupling is comparable to the transfer-integral and Coulomb repulsion energies, and this interplay may produce possible novel phases. Sr 2 IrO 4 is a good example of such a system for which the electronic states can be well described by considering spin-orbit coupling as well as Coulomb repulsion energy U [2,3].Sr 2 IrO 4 is an antiferromagnetic insulator with T N = 240 K, and is isostructural to one of the parent compounds of cuprate superconductors, namely, La 2 CuO 4 [4]. Similar to the cuprates, the electronic structure is highly two dimensional, as revealed by angle-resolved photoelectron spectroscopy (ARPES) [5,6]. Unlike cuprates, to date, Sr 2 IrO 4 has not shown superconductivity, although a possible emergence of superconductivity in this system has been theoretically predicted by carrier doping [7][8][9][10]. On the other hand, a d-wave gapped state and Fermi arc behavior have been observed in both the bulk [11] and surface [12][13][14] electronic structures of doped Sr 2 IrO 4 , similar to the cuprates. Such a similarity is puzzling and raises several questions, but this is merely due to the lack of momentum-resolved data in a wide range of doping, especially in the deeply underdoped regime. This is indeed crucial to explore if this anisotropic gap has the same origin as the pseudogap in cuprate superconductors and if the gap is related to superconductivity.To address these unsettled issues, we have studied how this d-wave gapped state evolves by doping in lightly doped Sr 2−x La x IrO 4 (x = 0, 0.04, 0.08) using ARPES. We have observed a dispersive in-gap state that evolves by carrier doping and coexists with the lower Hubbard band (LHB) seen (Fig. S1).ARPES experiments were performed at the 1-squared beamline of BESSY II, using a Scienta-Omicron R8000 analyzer. Circularly polarized light with hν = 100 eV was used to excite the photoelectrons. Clean surfaces for measurements were obtained by in situ cleaving ...

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Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-Tc). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-Tc materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with Tc = 45 K, which is the highest Tc found for the AEFe2As2 bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole- and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole- and electron-doped 122-type high-Tc superconductors.

We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO 2 . We used CrO 2 epitaxial films on TiO 2 (100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO 2 . In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (E F ) with an energy gap of 0.5 eV below E F was observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state closes. These results are consistent with resistivity measurements and magnetic hysteresis curves of the fabricated CrO 2 film, constituting spectroscopic evidence for the half-metallicity of CrO 2 at low temperature and reducing the spin polarization at room temperature. We also discuss the electron correlation effects of Cr 3d.