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One way to induce insulator-to-metal transitions in the spin-orbit Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, in contrast to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below ${E}_{F}$ coexisting with the metallic ones that we assign to nonbonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control either the charge transfer or the transition threshold.

Hybridization between $f$ electrons and conduction bands ($c$-$f$ hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn$_5$ heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of $T=6$ K. The used photon energy of 122 eV corresponds to Ce $4d$-$4f$ resonance. Calculations carried out with relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining $f$-electron character when approaching Fermi energy. We have also observed a considerable variation of $f$-electron spectral weight at $E_F$, which is normally determined by both matrix element effects and wave vector dependent $c$-$f$ hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of $4f$-electron enhanced spectral intensity at $E_F$ around $\bar{\Gamma}$ and $\bar{M}$ points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of $4f$-electron density for a range of values of the parameters used in the model., Comment: 15 pages, 9 figures

${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is characterized by a large spin-orbit coupling, which gives rise to bands with strongly entangled spin and orbital characters, called ${\mathrm{J}}_{1/2}$ and ${\mathrm{J}}_{3/2}$. We use light-polarization dependent ARPES to study directly the orbital character of these bands and fully map out their dispersion. We observe bands in very good agreement with our cluster dynamical mean-field theory calculations. We show that the ${\mathrm{J}}_{1/2}$ band, the closest to the Fermi level ${E}_{F}$, is dominated by ${d}_{xz}$ character along ${k}_{x}$ and ${d}_{yz}$ along ${k}_{y}$. This is actually in agreement with an isotropic ${\mathrm{J}}_{1/2}$ character on average, but this large orbital dependence in $\mathbf{k}$ space was mostly overlooked before. It gives rise to strong modulations of the ARPES intensity that we explain and carefully take into account to compare dispersions in equivalent directions of the Brillouin zone. Although the latter dispersions look different at first, suggesting possible symmetry breakings, they are found essentially similar, once corrected for these intensity variations. In particular, the pseudogaplike features close to the $X$ point appearing in the nearly metallic 15% Rh-doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ strongly depend on experimental conditions. We reveal that there is nevertheless an energy scale of 30 meV below which spectral weight is suppressed, independent of the experimental conditions, which gives a reliable basis to analyze this behavior. We suggest it is caused by disorder.

In ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ and ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, correlations, magnetism, and spin-orbit coupling compete on similar energy scales, creating a new context to study metal-insulator transitions (MIT). We use here angle-resolved photoemission to investigate the MIT as a function of hole and electron doping in ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, obtained respectively by Ir/Rh and Sr/La substitutions. We show that there is a clear reduction as a function of doping of the gap between a lower and upper band on both sides of the Fermi level, from 0.2 to 0.05 eV. Although these two bands have a counterpart in band structure calculations, they are characterized by a very different degree of coherence. The upper band exhibits clear quasiparticle peaks, while the lower band is very broad and loses weight as a function of doping. Moreover, their ARPES spectral weights obey different periodicities, reinforcing the idea of their different nature. We argue that a very similar situation occurs in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ and conclude that the physics of the two families is essentially the same.

Author(s): Kuo, CT; Lin, SC; Conti, G; Pi, ST; Moreschini, L; Bostwick, A; Meyer-Ilse, J; Gullikson, E; Kortright, JB; Nemsak, S; Rault, JE; Le Fevre, P; Bertran, F; Santander-Syro, AF; Vartanyants, IA; Pickett, WE; Saint-Martin, R; Taleb-Ibrahimi, A; Fadley, CS | Abstract: A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi2Sr2CaCu2O8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.

$\mathrm{LaCr}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{LCO})/\mathrm{SrTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{STO})$ heterojunctions are intriguing due to a polar discontinuity along $[001]$, exhibiting two distinct and controllable charged interface structures [${(\mathrm{LaO})}^{+}/{(\mathrm{Ti}{\mathrm{O}}_{2})}^{0}$ and ${(\mathrm{SrO})}^{0}/{(\mathrm{Cr}{\mathrm{O}}_{2})}^{\ensuremath{-}}$] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged ${(\mathrm{LaO})}^{+}/{(\mathrm{Ti}{\mathrm{O}}_{2})}^{0}$ intermediate layer at the $\mathrm{LC}{\mathrm{O}}_{\mathrm{top}}/\mathrm{ST}{\mathrm{O}}_{\mathrm{bottom}}$ interface and a negatively charged ${(\mathrm{SrO})}^{0}/{(\mathrm{Cr}{\mathrm{O}}_{2})}^{\ensuremath{-}}$ intermediate layer at the $\mathrm{ST}{\mathrm{O}}_{\mathrm{top}}/\mathrm{LC}{\mathrm{O}}_{\mathrm{bottom}}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies.

${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is the archetype of the spin-orbit Mott insulator, but the nature of the metallic states that may emerge from this type of insulator is still not very well known. We study with angle-resolved photoemission the insulator-to-metal transition observed in ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{O}}_{4}$ when Ir is substituted by Rh ($0.02lxl0.35$). The originality of the Rh doping is that Ir and Rh, which are formally isovalent, adopt different charge states, a rather unusual and inhomogeneous situation. We show that the evolution to the metallic state can be essentially understood as a shift of the Fermi level into the lower Hubbard band of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. The Mott gap appears quite insensitive to the introduction of up to $\ensuremath{\sim}20%$ holes in this band. The metallic phase, which forms for $xg0.07$, is not a Fermi liquid. It is characterized by the absence of quasiparticles, unrenormalized band dispersion compared to calculations, and an $\ensuremath{\sim}30\text{\ensuremath{-}}\mathrm{meV}$ pseudogap on the entire Fermi surface.

The electronic structure of the reentrant superconductor $\mathrm{Eu}({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}$)${}_{2}{\mathrm{As}}_{2}$ (${T}_{c}=22$ K) with coexisting ferromagnetic order (${T}_{M}=18$ K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of $\mathrm{Eu}{({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14})}_{2}{\mathrm{As}}_{2}$. The near-${E}_{F}$ Fe-$3d$-derived band dispersions near the $\mathrm{\ensuremath{\Gamma}}$ and $X$ high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at $T=5$ K, we estimate an essentially isotropic $s$-wave gap ($\mathrm{\ensuremath{\Delta}}\ensuremath{\sim}5.25\ifmmode\pm\else\textpm\fi{}0.25$ meV), indicative of strong-coupling superconductivity with $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\simeq}5.8$. The gap gets closed at temperatures $T\ensuremath{\ge}10$ K, and this is attributed to the resistive phase which sets in at ${T}_{M}=18$ K due to the ${\mathrm{Eu}}^{2+}$-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and −0.51eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV.

Members of the Mo${}_{x}$W${}_{1-x}$Te${}_{2}$ series are predicted to be Weyl semimetals, hosting type-II Weyl fermions, which have yet to be experimentally realized and which are unusual because they strongly violate Lorentz invariance. Crucially, the Weyl points in this system are predicted to sit above the Fermi level. Here, the authors show that for a type-II Weyl cone, although not for a type-I Weyl cone, if the Weyl point is above the Fermi level, then it's necessary to see the band structure above the Fermi level to observe a topological Fermi arc. The authors also discover that pump-probe angle-resolved photoemission beautifully displays the unoccupied band structure in Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Their work sets the stage for demonstrating that this system is the first type-II Weyl semimetal, as well as the first tunable Weyl semimetal.

We present energy filtered electron emission spectromicroscopy with spatial and wave-vector resolution on few-layer epitaxial graphene on SiC$(000\overline{1})$ grown by furnace annealing. Low-energy electron microscopy shows that more than 80% of the sample is covered by 2--3 graphene layers. C1s spectromicroscopy provides an independent measurement of the graphene thickness distribution map. The work function, measured by photoelectron emission microscopy (PEEM), varies across the surface from 4.34 to 4.50 eV according to both the graphene thickness and the graphene-SiC interface chemical state. At least two SiC surface chemical states (i.e., two different SiC surface structures) are present at the graphene/SiC interface. Charge transfer occurs at each graphene/SiC interface. $k$-space PEEM gives 3D maps of the $|{\mathrm{k\ifmmode \bar{}\else \={}\fi{}}}_{\ensuremath{\parallel}}|$ $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ band dispersion in micron-scale regions showing that the Dirac point shifts as a function of graphene thickness. Bragg diffraction of the Dirac cones via the superlattice formed by the commensurately rotated graphene sheets is observed. The experiments underline the importance of lateral and spectroscopic resolution on the scale of future electronic devices in order to precisely characterize the transport properties and band alignments.