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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.

The manifestation of Weyl fermions in strongly correlated electron systems is of particular interest. We report evidence for Weyl fermions in the heavy fermion semimetal YbPtBi from electronic structure calculations, angle-resolved photoemission spectroscopy, magnetotransport and calorimetric measurements. At elevated temperatures where 4 f- electrons are localized, there are triply degenerate points, yielding Weyl nodes in applied magnetic fields. These are revealed by a contribution from the chiral anomaly in the magnetotransport, which at low temperatures becomes negligible due to the influence of electronic correlations. Instead, Weyl fermions are inferred from the topological Hall effect, which provides evidence for a Berry curvature, and a cubic temperature dependence of the specific heat, as expected from the linear dispersion near the Weyl nodes. The results suggest that YbPtBi is a Weyl heavy fermion semimetal, where the Kondo interaction renormalizes the bands hosting Weyl points. These findings open up an opportunity to explore the interplay between topology and strong electronic correlations. Weyl fermions are evidenced in weakly correlated electron systems, but whether they survive strong electron correlations remains obscure. Here, Guo et al. report evidence of the chiral anomaly, topological Hall effect and a cubic temperature dependence of specific heat, suggesting existence of Weyl fermions in a heavy fermion semimetal YbPtBi.

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realized in Ca 2 RuO 4 . Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca 2 RuO 4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund’s coupling J =0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d xy orbital due to c -axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund’s coupling that together generate the Mott insulating state of Ca 2 RuO 4 . Detailed knowledge of the low-energy electronic structure is required to understand the Mott insulating phase of Ca 2 RuO 4 . Here, Sutter et al . provide directly the experimental band structure of the paramagnetic insulating phase of Ca 2 RuO 4 and unveil the electronic origin of its Mott phase.

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca RuO . Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide-using angle-resolved photoemission electron spectroscopy-the band structure of the paramagnetic insulating phase of Ca RuO and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca RuO .

In copper-oxide superconductors, charge carriers must be added to the insulating ‘parent’ compound before superconductivity appears. Exactly how the dopants affect the crystalline surface and evolving Fermi surface is now clear. Central to the understanding of high-temperature superconductivity is the evolution of the electronic structure as doping alters the density of charge carriers in the CuO 2 planes. Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal’s Fermi surface 1 , 2 , 3 . Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi 2 Sr 2 CaCu 2 O 8+ δ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs 4 , 5 , 6 . However, in high magnetic field, quantum oscillations 7 at low temperatures in YBa 2 Cu 3 O 6.5 indicate the existence of small Fermi surface pockets 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 . Reconciling these two phenomena through ARPES studies of YBa 2 Cu 3 O 7− δ (YBCO) has been hampered by the surface sensitivity of the technique 19 , 20 , 21 . Here, we show that this difficulty stems from the polarity and resulting self-doping of the YBCO surface. Through in situ deposition of potassium atoms on cleaved YBCO, we can continuously control the surface doping and follow the evolution of the Fermi surface from the overdoped to the underdoped regime. The present approach opens the door to systematic studies of high-temperature superconductors, such as creating new electron-doped superconductors from insulating parent compounds.

Spin and charge are inseparable traits of an electron, but in one-dimensional solids, theory predicts their separation into collective modes—as independent excitation quanta (or particles) called spinons and holons. Experimentalists have long sought to verify this effect. Angle-resolved photoemission (ARPES) should provide the most direct evidence of spin–charge separation, as the single quasiparticle peak splits into a spinon–holon two-peak-like structure. Despite extensive ARPES experiments, the unambiguous observation of the two-peak structure has remained elusive. Here we report ARPES data from SrCuO 2 , made possible by recent technological developments, that unequivocally show the spinon–holon two-peak structure and their distinct dispersions. The spinon and holon branches are found to have energy scales of ∼0.43 and 1.3 eV, respectively, which are in quantitative agreement with the theoretical predictions.

A central question in the study of high-temperature superconductivity is whether this phenomenon is linked to the doped antiferromagnetic Mott insulator or whether it emerges from a Fermi-liquid state across the whole cuprate phase diagram. Discriminating between these orthogonal cases hinges on the quantitative determination of the elusive quasiparticle strength Z as a function of hole-doping p , from the heavily overdoped to the deeply underdoped regime. Here we show, by means of angle-resolved photoemission spectroscopy and an in situ doping technique, that the electronic structure of the overdoped metal (0.24≤ p ≤0.37) is in remarkable agreement with density functional theory and Fermi-liquid-like descriptions. However, below p ≈0.10–0.15, we observe the loss of nodal quasiparticle integrity. This marks a clear departure from Fermi-liquid behaviour and a more rapid than expected crossover to Mott physics, indicating that the physical properties of underdoped cuprates are dominated by incoherent excitations. Quantitative measurements that establish the existence and evolution of quasiparticles across the whole phase diagram of a cuprate superconductor help to distinguish the many theoretical models for high-temperature superconductivity.

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba2IrO4 (Ba-214), an antiferromagnetic (TN = 230 K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO6 octahedra that is present in its sister compound Sr2IrO4 (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above TN, and favor a Mott over a Slater scenario for this compound.

In non-magnetic materials the combination of inversion symmetry breaking (ISB) and spin-orbit coupling (SOC) determines the spin polarization of the band structure. However, a local spin polarization can also arise in centrosymmetric crystals containing ISB subunits. This is namely the case for the nodal-line semimetal ZrSiTe where, by combining spin- and angle-resolved photoelectron spectroscopy with ab initio band structure calculations, we reveal a complex spin polarization. In the bulk, the valence and conduction bands exhibit opposite spin orientations in two spatially separated two-dimensional ZrTe sectors within the unit cell, yielding no net polarization. We also observe spin-polarized surface states that are well separated in energy and momentum from the bulk bands. A layer-by-layer analysis of the spin polarization allows us to unveil the complex evolution of the signal in the bulk states near the surface, thus bringing the intertwined nature of surface and bulk effects to the fore. Local inversion symmetry breaking in centrosymmetric materials can lead to large spin polarization of the electronic band structure in separate sectors of the unit cell. Here, the authors reveal such hidden spin polarisation in ZrSiTe using spin and angle resolved photoemission spectroscopy in combination with ab initio band structure calculations and investigate the resultant spin polarised bulk and surface properties