Evidence for Weakly Correlated Oxygen Holes in the Highest-T_{c} Cuprate Superconductor HgBa_{2}Ca_{2}Cu_{3}O_{8+δ}.

We study the electronic structure of HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} (Hg1223; T_{c}=134 K) using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Resonant valence band PES across the O K edge and Cu L edge identifies correlation satellites originating in O 2p and Cu 3d two-hole final states, respectively. Analyses using the experimental O 2p and Cu 3d partial density of states show quantitatively different on-site Coulomb energy for the Cu site (U_{dd}=6.5±0.5 eV) and O site (U_{pp}=1.0±0.5 eV). Cu_{2}O_{7}-cluster calculations with nonlocal screening explain the Cu 2p core level PES and Cu L-edge XAS spectra, confirm the U_{dd} and U_{pp} values, and provide evidence for the Zhang-Rice singlet state in Hg1223. In contrast to other hole-doped cuprates and 3d-transition metal oxides, the present results indicate weakly correlated oxygen holes in Hg1223.

Coulomb Correlations Intertwined with Spin and Orbital Excitations in LaCoO_{3}.

We carried out temperature-dependent (20-550 K) measurements of resonant inelastic x-ray scattering on LaCoO_{3} to investigate the evolution of its electronic structure across the spin-state crossover. In combination with charge-transfer multiplet calculations, we accurately quantified the renomalized crystal-field excitation energies and spin-state populations. We show that the screening of the effective on-site Coulomb interaction of 3d electrons is orbital selective and coupled to the spin-state crossover in LaCoO_{3}. The results establish that the gradual spin-state crossover is associated with a relative change of Coulomb energy versus bandwidth, leading to a Mott-type insulator-to-metal transition.

Observation of quadrupole helix chirality and its domain structure in DyFe3(BO3)4.

Resonant X-ray diffraction (RXD) uses X-rays in the vicinity of a specific atomic absorption edge and is a powerful technique for studying symmetry breaking by motifs of various multipole moments, such as electric monopoles (charge), magnetic dipoles (spin) and electric quadrupoles (orbital). Using circularly polarized X-rays, this technique has been developed to verify symmetry breaking effects arising from chirality, the asymmetry of an object upon its mirroring. Chirality plays a crucial role in the emergence of functionalities such as optical rotatory power and multiferroicity. Here we apply spatially resolved RXD to reveal the helix chirality of Dy 4f electric quadrupole orientations and its domain structure in DyFe3(BO3)4, which shows a reversible phase transition into an enantiomorphic space-group pair. The present study provides evidence for a helix chiral motif of quadrupole moments developed in crystallographic helix chirality.

Temperature Dependence of Magnetically Active Charge Excitations in Magnetite across the Verwey Transition.

We study the electronic structure of bulk single crystals and epitaxial films of Fe_{3}O_{4}. Fe 2p core level spectra show clear differences between hard x-ray (HAX) and soft x-ray photoemission spectroscopy (PES). The bulk-sensitive spectra exhibit temperature (T) dependence across the Verwey transition, which is missing in the surface-sensitive spectra. By using an extended impurity Anderson full-multiplet model-and in contrast to an earlier peak assignment-we show that the two distinct Fe species (A and B site) and the charge modulation at the B site are responsible for the newly found double peaks in the main peak above T_{V} and its T-dependent evolution. The Fe 2p HAXPES spectra show a clear magnetic circular dichroism (MCD) in the metallic phase of magnetized 100-nm-thick films. The model calculations also reproduce the MCD and identify the contributions from magnetically distinct A and B sites. Valence band HAXPES shows a finite density of states at E_{F} for the polaronic half metal with a remnant order above T_{V} and a clear gap formation below T_{V}. The results indicate that the Verwey transition is driven by changes in the strongly correlated and magnetically active B-site electronic states, consistent with resistivity and optical spectra.

Existence of orbital order and its fluctuation in superconducting Ba(Fe(1-x)Co(x))2As2 single crystals revealed by x-ray absorption spectroscopy.

We performed temperature dependent x-ray linear dichroism (XLD) experiments on an iron pnictide system, Ba(Fe(1-x)Co(x))2As2 with x=0.00, 0.05, 0.08, and 0.10 to experimentally verify the existence of orbital ordering (OO). Substantial XLD was observed in polarization dependent x-ray absorption spectra of Fe L edges. By exploiting the difference in the temperature dependent behaviors, OO, and structure contributions to XLD could be clearly separated. The observed OO signal indicates different occupation numbers for d(yz) and d(zx) orbitals and supports the existence of ferro-OO. The results are also consistent with the theoretical prediction. Moreover, we find substantial OO signal well above the structural and magnetic transition temperatures, which suggests the existence of strong OO fluctuations up to high temperatures.

Evidence for a cos(4φ) modulation of the superconducting energy gap of optimally doped FeTe(0.6)Se(0.4) single crystals using laser angle-resolved photoemission spectroscopy.

We study the superconducting-gap anisotropy of the Γ-centered hole Fermi surface in optimally doped FeTe(0.6)Se(0.4) (T(c)=14.5 K), using laser-excited angle-resolved photoemission spectroscopy. We observe sharp superconducting (SC) coherence peaks at T=2.5 K. In contrast to earlier angle-resolved photoemission spectroscopy studies but consistent with thermodynamic results, the momentum dependence shows a cos(4φ) modulation of the SC-gap anisotropy. The observed SC-gap anisotropy strongly indicates that the pairing interaction is not a conventional phonon-mediated isotropic one. Instead, the results suggest the importance of second-nearest-neighbor electronic interactions between the iron sites in the framework of s(±)-wave superconductivity.

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films.

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ∼1 and ∼10 nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)↔Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.

The histopathology of the humeral head in glenohumeral osteoarthritis.

Objective: The histopathologic wear patterns in glenohumeral osteoarthritis (GOA) have not been described. The aims of the study were to a) describe the histopathology of humeral head wear patterns in patients with end-stage GOA and b) identify clinical and radiographic parameters that correlate with observed histopathological wear patterns. Methods: Eighteen humeral heads from patients undergoing anatomic total shoulder arthroplasty for end-stage osteoarthritis were divided radially into eight wedge-shaped zones. Each zone was subdivided into central and peripheral regions. Histologic analysis included measurements of cartilage and subchondral bone plate thickness, subchondral bone area, and cartilage structure was scored using the Osteoarthritis Research Society (OARSI) and modified Mankin systems. Clinical variables including patient history, physical exam, functional evaluation, and radiographic assessments were evaluated for correlations with humeral head characteristics. Results: Overall, humeral heads demonstrated a pattern of central and inferior cartilage damage, loss, and subchondral bone changes. However, within the group, composite maps of individual patient wear patterns demonstrated a sub-group of patients with a more focal inferior cartilage lesion. Overall, these more focal inferior lesions were associated with greater pre-operative range of motion (in both upper extremities), higher pre-operative SANE and ASES scores, female sex, non-dominant extremity, concentric wear patterns, and smaller inferior osteophytes. Conclusion: Humeral head cartilage wear patterns in GOA include central and inferior cartilage damage and loss. A histopathological distinction was identified between patients with more focal versus diffuse wear, which may manifest clinically with preservation of function and range of motion, and with less profound radiographical changes.

Out-of-plane nesting driven spin spiral in ultrathin Fe/Cu(001) films.

Epitaxial ultrathin Fe films on fcc Cu(001) exhibit a spin spiral (SS), in contrast to the ferromagnetism of bulk bcc Fe. We study the in-plane (IP) and out-of-plane (OP) Fermi surfaces (FSs) of the SS in 8 monolayer Fe/Cu(001) films using energy-dependent soft-x-ray momentum-resolved photoemission spectroscopy. We show that the SS originates in nested regions confined to OP FSs, which are drastically modified compared to IP FSs. From precise reciprocal-space maps in successive zones, we obtain the associated real space compressive strain of 1.5+/-0.5% along c axis. An autocorrelation analysis quantifies the incommensurate ordering vector q=(2pi/a)(0,0, approximately 0.86), favoring a SS and consistent with magneto-optic Kerr effect experiments. The results reveal the importance of IP and OP FS mapping for ultrathin films.

Strong valence fluctuation in the quantum critical heavy fermion superconductor ß-YbAlB4: a hard x-ray photoemission study.

Electronic structures of the quantum critical superconductor ß-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and ß-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in ß-YbAlB4.

Orbital-dependent modifications of electronic structure across the magnetostructural transition in BaFe2As2.

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magnetostructural transition at T{N} approximately 140 K. A drastic transformation in Fermi surface (FS) shape across T{N} is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at k{z} approximately pi in the low-T antiferromagnetic state are dominated by the Fe3d{zx} orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

Evidence for a correlated insulator to antiferromagnetic metal transition in CrN.

We investigate the electronic structure of chromium nitride (CrN) across the first-order magnetostructural transition at T(N)∼286 K. Resonant photoemission spectroscopy (PES) shows a gap in the 3d partial density of states at the Fermi level and an on-site Coulomb energy U∼4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high-resolution (6 meV) laser PES reveals a clear Fermi edge indicating an antiferromagnetic metal below T(N). Hard x-ray Cr 2p core-level PES shows T-dependent changes across T(N) which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. Electrical resistivity confirms an insulator above T(N) (E(g)∼70 meV) becoming a disordered metal below T(N). Thus, CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magnetostructural transition.

Anomalous state sandwiched between Fermi liquid and charge ordered Mott-insulating phases of Ti4O7.

The Magnéli phase Ti(4)O(7) exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at T(c1) approximately 142 K and T(c2) = 154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray, and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p-3d resonant PES and HX PES show a clear metallic Fermi edge and mixed valency above T(c2). The low temperature phase below T(c1) shows a clear insulating gap of approximately 100 meV. The intermediate phase between T(c1) and T(c2) indicates a pseudogap coexisting with remnant coherent states. HX PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest the existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti(4)O(7).

Fermi surface sheet-dependent superconductivity in 2H-NbSe2.

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectral function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

A versatile system for ultrahigh resolution, low temperature, and polarization dependent laser-angle-resolved photoemission spectroscopy.

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hnu=6.994 eV) as a photon source. With the aim of addressing low energy physics, we show the system performance with angle-integrated PES at the highest energy resolution of 360 mueV and the lowest temperature of 2.9 K. We describe the importance of a multiple-thermal-shield design for achieving the low temperature, which allows a clear measurement of the superconducting gap of tantalum metal with a T(c)=4.5 K. The unique specifications and quality of the laser source (narrow linewidth of 260 mueV, high photon flux), combined with a half-wave plate, facilitates ultrahigh energy and momentum resolution polarization dependent ARPES. We demonstrate the use of s- and p-polarized laser-ARPESs in studying the superconducting gap on bilayer-split bands of a high T(c) cuprate. The unique features of the quasi-continuous-wave vuv laser and low temperature enables ultrahigh-energy and -momentum resolution studies of the spectral function of a solid with large escape depth. We hope the present work helps in defining polarization dependent laser excited angle-resolved photoemission spectroscopy as a frontier tool for the study of electronic structure and properties of materials at the sub-meV energy scale.

Jahn-Teller distortion driven magnetic polarons in magnetite.

The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin-orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons.

Photoemission spectroscopy of the strong-coupling superconducting transitions in lead and niobium

We study the changes in the electronic structure associated with low- T(c) strong-coupling superconducting transitions in Pb and Nb using ultrahigh-resolution (2.3 meV) temperature-dependent (5.3-12.0 K) photoemission spectroscopy. We observe peaks in the density of states on entering the superconducting-phase accompanying gap formation and spectacular redistribution of spectral weight at low energy scales as a function of temperature. The well-known peak-dip feature of the high- T(c) cuprates is seen in Pb, making it a characteristic of strong-coupling superconductivity.

Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity.

Conventional superconductivity follows Bardeen-Cooper-Schrieffer(BCS) theory of electrons-pairing in momentum-space, while superfluidity is the Bose-Einstein condensation(BEC) of atoms paired in real-space. These properties of solid metals and ultra-cold gases, respectively, are connected by the BCS-BEC crossover. Here we investigate the band dispersions in FeTe(0.6)Se(0.4)(Tc = 14.5 K ~ 1.2 meV) in an accessible range below and above the Fermi level(EF) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV (~8 K) above EF at the Γ-point, which shows a sharp superconducting coherence peak with gap formation below Tc. The estimated superconducting gap Δ and Fermi energy [Symbol: see text]F indicate composite superconductivity in an iron-based superconductor, consisting of strong-coupling BEC in the electron band and weak-coupling BCS-like superconductivity in the hole band. The study identifies the possible route to BCS-BEC superconductivity.

Quasiparticles and Fermi liquid behaviour in an organic metal.

Many organic metals display exotic properties such as superconductivity, spin-charge separation and so on and have been described as quasi-one-dimensional Luttinger liquids. However, a genuine Fermi liquid behaviour with quasiparticles and Fermi surfaces have not been reported to date for any organic metal. Here, we report the experimental Fermi surface and band structure of an organic metal (BEDT-TTF)(3)Br(pBIB) obtained using angle-resolved photoelectron spectroscopy, and show its consistency with first-principles band structure calculations. Our results reveal a quasiparticle renormalization at low energy scales (effective mass m*=1.9 m(e)) and ω(2) dependence of the imaginary part of the self energy, limited by a kink at ~50 meV arising from coupling to molecular vibrations. The study unambiguously proves that (BEDT-TTF)(3)Br(pBIB) is a quasi-2D organic Fermi liquid with a Fermi surface consistent with Shubnikov-de Haas results.

Octet-line node structure of superconducting order parameter in KFe2As2.

In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A(1g) superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.