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Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe 0.56 Se 0.44 , monolayer FeSe grown on SrTiO 3 and K 0.76 Fe 1.72 Se 2 . We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the d xy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the d xy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors. A proper theoretical description for unconventional superconductivity in iron-based compounds remains elusive. Here, the authors, to capture the electron correlation strength and the role of Fermi surfaces, report ARPES measurements of three iron chalcogenide superconductors to establish universal features.

For materials that harbour a continuous phase transition, the susceptibility of the material to various fields can be used to understand the nature of the fluctuating order and hence the nature of the ordered state. Here we use anisotropic biaxial strain to probe the nematic susceptibility of URu 2 Si 2 , a heavy fermion material for which the nature of the low temperature ‘hidden order’ state has defied comprehensive understanding for over 30 years. Our measurements reveal that the fluctuating order has a nematic component, confirming reports of twofold anisotropy in the broken symmetry state and strongly constraining theoretical models of the hidden-order phase. The heavy fermion material URu 2 Si 2 exhibits a hidden-order phase transition that remains poorly understood. Using differential elastoresistance measurements, Riggs et al . show that this phase has a nematic component and that it spontaneously breaks fourfold lattice symmetry.

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped Ba 1− x K x Fe 2 As 2 , we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials. Whether superconductivity coexists or competes with other types of order in unconventional superconductors is a question that has been hotly contested. An ARPES study reported by Yi et al. suggest that superconductivity and spin-density wave orders coexist and compete dynamically in Ba 1− x K x Fe 2 As 2 .

In addition to a bulk energy gap, topological insulators accommodate a conducting, linearly dispersed Dirac surface state. This state is predicted to become massive if time reversal symmetry is broken, and to become insulating if the Fermi energy is positioned inside both the surface and bulk gaps. We introduced magnetic dopants into the three-dimensional topological insulator dibismuth triselenide (Bi₂Se₃) to break the time reversal symmetry and further position the Fermi energy inside the gaps by simultaneous magnetic and charge doping. The resulting insulating massive Dirac fermion state, which we observed by angle-resolved photoemission, paves the way for studying a range of topological phenomena relevant to both condensed matter and particle physics.

The phase diagram of BaPb 1− x Bi x O 3 exhibits a superconducting dome in the proximity of a charge density wave phase. For the superconducting compositions, the material coexists as two structural polymorphs. Here we show, via high-resolution transmission electron microscopy, that the structural dimorphism is accommodated in the form of partially disordered nanoscale stripes. Identification of the morphology of the nanoscale structural phase separation enables determination of the associated length scales, which we compare with the Ginzburg–Landau coherence length. We find that the maximum T c occurs when the superconducting coherence length matches the width of the partially disordered stripes, implying a connection between the structural phase separation and the shape of the superconducting dome. In BaPb 1− x Bi x O 3 a superconducting dome emerges in the proximity to a charge density wave phase. Here, the authors show with transmission electron microscopy measurements that for superconducting compositions of this allow, the structural dimorphism is accommodated in partially disordered nanoscale stripes.

Superconductivity: 'itinerant' oxypnictides The discovery of superconductivity in the iron-based layered compounds known as iron oxypnictides has renewed interest in high-temperature superconductivity. Two distinct classes of theories about the nature of the ground state of the oxypnictides have been put forward, characterized by contrasting underlying band structures. Such a controversy is partly due to the lack of conclusive experimental information on the electronic structures. Now Lu et al . report angle-resolved photoemission spectroscopy (ARPES) of an iron oxypnictide, LaOFeP, with a pretty high critical temperature of T c = 5.9 K. Their results favour an 'itinerant' ground state, over one resembling the 'Mott insulator' state found in copper oxide superconductors. Angle-resolved photoemission spectroscopy (ARPES) of LaOFeP ( T c = 5.9 K) is reported. These results favour the itinerant ground state, albeit with band renormalization. In addition, the data reveal important differences between these and copper-based superconductors. The recent discovery of superconductivity in the iron oxypnictide family of compounds 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K (refs 10 , 11 ) seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach 12 , 13 , 14 , 15 , 16 , 17 , and an itinerant ground state in the weak-coupling approach 18 , 19 , 20 , 21 , 22 . The first approach stresses on-site correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, T c = 5.9 K), the first-reported iron-based superconductor 2 . Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.

Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe{sub 3}, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe{sub 3}, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi₂Te₃ with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Our results establish that Bi₂Te₃ is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi₂Te₃ also points to promising potential for high-temperature spintronics applications.

Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe₃, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe₃, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.