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Hybridization of Bogoliubov quasiparticles (BQPs) between the CuO_{2} layers in the triple-layer cuprate high-temperature superconductor Bi_{2}Sr_{2}Cu_{2}Cu_{3}O_{10+δ} is studied by angle-resolved photoemission spectroscopy (ARPES). In the superconducting state, an anticrossing gap opens between the outer- and inner-BQP bands, which we attribute primarily to interlayer single-particle hopping with possible contributions from interlayer Cooper pairing. We find that the d-wave superconducting gap of both BQP bands smoothly develops with momentum without an abrupt jump in contrast to a previous ARPES study. Hybridization between the BQPs also gradually increases in going from the off nodal to the antinodal region, which is explained by the momentum dependence of the interlayer single-particle hopping. As possible mechanisms for the enhancement of the superconducting transition temperature, the hybridization between the BQPs as well as the combination of phonon modes of the triple CuO_{2} layers and spin fluctuations represented by a four-well model are discussed.

Insulator-to-metal transition in Ca$_{2}$RuO$_{4}$ has drawn keen attention because of its sensitivity to various stimulation and its potential controllability. Here, we report a direct observation of Fermi surface, which emerges upon introducing excess oxygen into an insulating Ca$_{2}$RuO$_{4}$, by using angle-resolved photoemission spectroscopy. Comparison between energy distribution curves shows that the Mott insulating gap is closed by eV-scale spectral-weight transfer with excess oxygen. Momentum-space mapping exhibits two square-shaped sheets of the Fermi surface. One is a hole-like $\alpha$ sheet around the corner of a tetragonal Brillouin zone, and the other is an electron-like $\beta$ sheet around the $\Gamma$ point. The electron occupancies of the $\alpha$ and $\beta$ bands are determined to be $n_{\alpha}=1.6$ and $n_{\beta}=0.6$, respectively. Our result indicates that the insulator-to-metal transition occurs selectively in $d_{xz}$ and $d_{yz}$ bands and not yet in $d_{xy}$ band. This orbital selectivity is most likely explained in terms of the energy level of $d_{xy}$, which is deeper for Ca$_{2}$RuO$_{4+\delta}$ than for Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$. Consequently, we found substantial differences from the Fermi surface of other ruthenates, shedding light on a unique role of excess oxygen among the metallization methods of Ca$_{2}$RuO$_{4}$., Comment: 9 pages, 6 figures

Conventional superconductivity is caused by electron-phonon coupling. The discovery of high-temperature superconductors raised the question of whether such strong electron-phonon coupling is realized in cuprates. Strong coupling with some collective excitation mode has been indicated by a dispersion "kink". However, there is intensive debate regarding whether the relevant coupling mode is a magnetic resonance mode or an oxygen buckling phonon mode. This ambiguity is a consequence of the energy of the main prominent kink. Here, we show a new landscape of dispersion kinks. We report that heavily overdoping a Bi

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ reveals a distinct oxygen isotope shift near the electron-boson coupling ``kink'' in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, and are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches.

We report substantial advance of our low-energy angle-resolved photoemission study of nodal quasiparticles in Bi2Sr2CaCu2 O 8 + δ . The new data cover the samples from underdoped down to heavily underdoped levels. We also present the nodal Fermi velocities that determined by using an excitation-photon energy of h ν = 7.0 eV over a wide doping range. The consistency between the results with h ν = 8.1 and 7.0 eV allows us to rule out the effect of photoemission matrix elements. In comparison with the data previously reported, the nodal effective mass increases by a factor of ∼ 1.5 in going from optimally doped to heavily underdoped levels. We find a rapid enhancement of the nodal quasiparticle mass at low doping levels near the superconductor-to-insulator transition. The effective coupling spectrum, λ ( ω ) , is extracted directly from the energy derivatives of the quasiparticle dispersion and scattering rate, as a causal function of the mass enhancement factor. A steplike increase in Re λ ( ω ) around ∼ 65 meV is demonstrated clearly by the Kramers-Kronig transform of Im λ ( ω ) . To extract the low-energy renormalization effect, we calculated a simple model for the electron-boson interaction. This model reveals that the contribution of the renormalization at | ω | ≤ 15 meV to the quasiparticle mass is larger than that around 65 meV in underdoped samples.

We have developed a laser-based scanning angle-resolved photoemission spectroscopy system (µ-ARPES) equipped with a high precision 6-axis control system, realizing not only high-resolution photoemission spectroscopy in energy and momentum, but also spatial resolution of a µm scale. This enables our µ-ARPES system to probe fine details of intrinsic electronic states near the Fermi level such as the superconducting gaps and lifetime broadening.

Combining Angle resolved photoelectron spectroscopy (ARPES) and a $\mu$-focused Laser, we have performed scanning ARPES microscopy measurements of the domain population within the nematic phase of FeSe single crystals. We are able to demonstrate a variation of the domain population density on a scale of a few 10 $\mu$m while constraining the upper limit of the single domain size to less than 5 $\mu m$. This experiment serves as a demonstration of how combining the advantages of high resolution Laser ARPES and an ultimate control over the spatial dimension can improve investigations of materials by reducing the cross contamination of spectral features of different domains.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique in materials science, as it can directly probe electronic states inside solids in energy (E) and momentum (k) space. As an advanced technique, spatially-resolved ARPES using a well-focused light source (high-resolution ARPES microscopy) has recently attracted growing interests because of its capability to obtain local electronic information at micro- or nano-metric length scales. However, there exist several technical challenges to guarantee high precision in determining translational and rotational positions in reasonable measurement time. Here we present two methods of obtaining k-space mapping and real-space imaging in high-resolution ARPES microscopy. One method is for k-space mapping measurements that enables us to keep a target position on a sample surface during sample rotation by compensating rotation-induced displacements (tracing acquisition method). Another method is for real-space imaging measurements that significantly reduces total acquisition time (scanning acquisition method). We provide several examples of these methods that clearly indicate higher accuracy in k-space mapping as well as higher efficiency in real-space imaging, and thus improved throughput of high-resolution APRES microscopy.

Conventional superconductivity is caused by electron-phonon coupling. The discovery of high-temperature superconductors raised the question of whether such strong electron-phonon coupling is realized in cuprates. Strong coupling with some collective excitation mode has been indicated by a dispersion “kink”. However, there is intensive debate regarding whether the relevant coupling mode is a magnetic resonance mode or an oxygen buckling phonon mode. This ambiguity is a consequence of the energy of the main prominent kink. Here, we show a new landscape of dispersion kinks. We report that heavily overdoping a Bi2Sr2CaCu2O8+δ superconductor results in a decline of the conventional main kink and a rise of another sharp kink, along with substantial energy shifts of both. Notably, the latter kink can be ascribed only to an oxygen-breathing phonon. Hence, the multiple phonon branches provide a consistent account of our data set on the multiple kinks. Our results suggest that strong electron-phonon coupling and its dramatic change should be incorporated into or reconciled with scenarios for the evolution of high-Tc superconductivity.

We report a high-resolution laser-based angle-resolved photoemission spectroscopy (laser-ARPES) study of single crystals of FeSe, focusing on the temperature-dependence of the hole-like bands around the ${\rm \Gamma}$ point. As the system cools through the tetragonal-orthorhombic "nematic" structural transition at 90~K, the splitting of the $d_{xz}$/$d_{yz}$ bands is observed to increase by a magnitude of 13 meV. Moreover, the onset of a $\sim$10 meV downward shift of the $d_{xy}$ band is also at 90~K. These measurements provide clarity on the nature, magnitude and temperature-dependence of the band shifts at the ${\rm \Gamma}$ point in the nematic phase of FeSe.

In high-Tc cuprate superconductors, the d-wave-like superconducting gap appears to depart from the standard proportionality with critical temperature Tc, and shows different doping dependences between the nodal and antinodal regions of momentum space. The failure of the attempts so far to formulate these momentum-resolved gaps with Tc has posed questions to their roles in electron pairing and their mutual relationship. Here, we report a simple proportional relation between nodal gap and critical temperature in superconducting Bi2Sr2CaCu2O8+δ and Bi2Sr1.6La0.4CuO6+δ. Using low-energy synchrotron radiation on angle-resolved photoemission spectroscopy, we observed the strongly curved gap profile in the underdoped cuprates, and found that a nodal-gap component 2∆N at the nodal limit of the gap slope closely follows 8.5kBTc and concomitantly related to the antinodal-gap energy ∆ * through a reduction factor of square-root superfluid density ρs. These empirical formulae provide a new gap-based paradigm on the principle governing Tc in strong-coupling superconductors.

We investigate the k z dispersion in the hole Fermi surfaces (FSs) and the superconducting (SC) gaps of BaFe2(As0.65P0.35)2 by employing bulk-sensitive angle-resolved photoemission spectroscopy (ARPES) with low energy photons. We find that this material exhibits a comparable magnitude of SC gaps for three-hole FSs at k z ∼ π . The comparable magnitude of the SC gap is observed also at k z ∼ 0. Since each FS sheet has a distinct orbital character, these observations are an indication of the orbital-independent SC gaps in the hole FSs.

We analyze the excitation of secondary electrons by hard-X-rays in subsurface layers. By studying core-excited photoelectron lines and their plasmon satellites in photoemission spectra, we show how electrons excited by hard-X-rays can carry information from deep regions deep within the sample to the surface. It is believed that the decay of high-energy photoelectrons via plasmon-loss is strongly related to the production of secondary electrons. For high-energy electrons, however, the momentum transfer to plasmons is small compared to the electron’s initial momentum, so the lateral position on the surface from which the secondary electrons are emitted is close to that of the atom initially excited by the hard-X-rays. This explains why the spatial resolution of hard-X-ray photoelectron emission microscope (HX-PEEM) images is good even if the buried interface is covered by a film with a thickness many times the inelastic mean free path of the primary electrons. This argument explains well recent HX-PEEM results.

We have re-examined the oxygen isotope effect in high-temperature superconductors Bi 2 Sr 2 CaCu 2 O 8+ δ using high-precision angle-resolved photoemission spectroscopy with low-energy photons in order to elucidate the origin of the kink. The results taken with a much improved energy and momentum resolution showed the absence of the reported unusual oxygen isotope effect.

We have performed an angle-resolved photoemission study of the nodal quasi-particle spectra of the high-Tc cuprate tri-layer Bi2Sr2Ca2Cu3O10+d (Tc~ 110 K). The spectral weight Z of the nodal quasi-particle increases with decreasing temperature across the Tc. Such a temperature dependence is qualitatively similar to that of the coherence peak intensity in the anti nodal region of various high-Tc cuprates although the nodal spectral weight remains finite and large above Tc. We attribute this observation to the reduction of electron correlation strength in going from the normal metallic state to the superconducting state, a characteristic behavior of a superconductor with strong electron correlation., 13 pages, 5 figures

We have obtained the Cr 3d-like energy states, which located in the band gap of GaN by means of resonant photoemission spectroscopy. In the difference spectrum between the valence band photoemission spectra of non-doped GaN and that of the Ga0.937Cr0.063N, we observed the new energy state, in band gap, consists of Cr 3d-like and N 2p-like component by strong hybridization.

The shift of the chemical potential as a function of band filling provides useful insight into the electronic structure of solids. In this article, we describe how the chemical potential shift can be measured using core-level photoemission spectroscopy and what information can be obtained from the measured chemical potential shift, in particular on strongly correlated systems. We present results on various transition-metal oxides, which include materials showing typical Fermi-liquid behavior, pseudogap behavior, charge–density wave formation, ‘stripe’ formation, etc.

We report results of the angle-resolved photoemission experiments on a quasi-one-dimensional halogen-bridged complex [Ni(chxn) 2 Br]Br 2 ( chxn = 1R, 2R-cyclohexanediamine), which is a one-dimensional Heisenberg spin chain with S = 1/2 and J ~ 3600 K . The expected two dispersions, originated with spin–charge separations as have been observed in other quasi-one-dimensional electron systems like Sr 2 CuO 3, are not clearly observed in this compound. Instead, only one 'band" having about 0.5 eV energy dispersion is found in the half of the Brillouin zone.

We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As(1-x)P(x))2 (x = 0.30, Tc = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s ± gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.

The nature of the pseudogap in high transition temperature (high-Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-Tc superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-Tc superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe2(As1-xPx)2. The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry., Comment: 14 pages, 10 figures

We have performed angle-resolved photoemission spectroscopy study of the electronic structure of the tri-layer high- T c cuprate superconductor Bi 2 Sr 2 Ca 2 Cu 3 O 10 + δ and observed separate band dispersions and Fermi surfaces corresponding to the outer and inner CuO2 planes. From the band dispersions, the c-axis hopping parameter t ⊥ between the outer and inner CuO2 planes is estimated to be ∼56 meV.

Using angle-resolved photoemission spectroscopy (ARPES) with low-energy tunable photons, we studied the oxygen isotope effect in optimally doped Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212). We found the oxygen isotope shift in the real part of the electron self-energies [ReΣ( ω )s] along the nodal direction derived not only from the momentum distribution curves (MDCs) but also from the energy distribution curves (EDCs). Present results indicate straightforwardly the coupling between the nodal electrons and the phonons.

Using angle-resolved photoemission spectroscopy (ARPES), we have observed the band structure, Fermi surface and their doping dependences in La2-xSrxCuO4. The results reveal that the Fermi surface undergoes a dramatic change: it is hole-like and centered at k=(pi,pi) in underdoped (x = 0.1) and optimally doped (x = 0.15) samples as in other cuprates while it is electron-like and centered at (0,0) in heavily overdoped (x = 0.3) ones. The peak in the ARPES spectra near (pi/2,pi/2) is broad and weak unlike in other cuprates. In the underdoped and optimally doped samples, a superconducting gap (Delta = 10 - 15 meV) is observed on the Fermi surface near (pi,0)., 5 pages, 4 EPS figures, uses epsf.sty

We successfull treated a patient with bilateral pulmonary arteriovenous fistulas (PANT) with Rendu-Osler-Weber disease by surgery and pulmonary artery embolization. A 56-year-old woman was referred to the hospital because of bilateral abnormal shadows in a chest radiograph. Chest CT and pulmonary angiography revealed pulmonary arteriovenous fistulas located in the right middle lobe (2 PAVFs, 2cm and 10cm in diameter respectively) and left lower lobe (1PAVF, 5cm in diameter). The patient had a history of recurrent epistaxis and showed telangiectasia of lips and tongue. She was diagnosed as Rendu-Osler-Weber disease with bilateral pulmonary arteriovenous fistulas. Right middle lobectomy and selective left pulmonary artery embolization was performed. In the treatment of bilateral multiple arteriovenous fistulas of the lung like this case, more effective therapeutic outcome can be expected by selecting either or both surgery or pulmonary artery embolization for each fistula.

The electronic structure of the T*-phase cuprate superconductor SmLa 0.85 Sr 0.15 CuO 4 - δ has been studied by angle-resolved photoemission spectroscopy (ARPES). While a clear Fermi surface has been observed near ( π / 2 , π / 2 ) , the electronic band is lowered to ∼ - 180 meV around ( π , 0 ) due to the pseudogap formation. These features are quite similar to the ARPES result of Ca 1.9 Na 0.1 CuO 2 Cl 2 rather than La 2 - x Sr x CuO 4 . We have also observed a distinct “kink” in the nodal band dispersion as in all the other hole-doped cuprates.

The electronic band structures of 1T-TiTe2 and Fe0.25TiTe2 have been clarified by angle-resolved resonant-photoemission spectroscopy. Upon Fe intercalation, while most of the band structure is unchanged, two new flat bands appear at the Fermi level and at E∼−0.5 eV. Constant-initial-state spectra indicate that these states are derived from both Ti 3d and Fe 3d states. While the Fermi wave number of the electron pocket around M point is reduced by Fe intercalation, those of hole pockets around Γ point are almost unchanged, suggesting that the additional electrons are accommodated by the new flat band at −0.5 eV.

Angle-resolved photoemission spectra of 1T-TaS2 have been measured to reveal the electronic structure of valence bands in the incommensurate (IC) and commensurate (C) charge density wave (CDW) phases. In the ICCDW phase at 370 K, Ta 5d states are observed as a single branch along the Γ–M direction over the binding energy range EB=0–1.2 eV. In the CCDW phase at 17 K, on the other hand, Ta 5d states are divided into three sub-band manifolds centered at EB=0.2, 0.5 and 1.0 eV. The former sub-band at EB=0.2 eV is less dispersive, while latter two show clear dispersive nature with the bandwidths of ∼0.5 and 0.3 eV. Furthermore, the spectral intensities are concentrated around the original Ta 5d band.

Angle-resolved photoemission spectroscopy using tunable low energy photons allows us to study the quasi-particle (QP) dispersions of the inner and outer CuO2 planes (IP and OP) separately in the tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223). The kink energy of the OP band is $\sim$ 70 meV, as observed in various high-$T_c$ cuprates, while that of the IP band is as large as 100 meV in the superconducting (SC) state. This large kink energy is attributed to the $\sim$ 35 meV buckling mode plus the large ($\sim$ 60 meV) SC gap of IP. The IP band also shows a weak kink feature at 70 meV in the SC state. The latter feature can be explained either by the 70 meV half-breathing mode or by the $\sim$ 35 meV buckling-phonon mode plus the $\sim$ 40 meV SC gap of OP if interlayer scattering of QP is involved., Comment: 5 pages, 2 figures

We have characterized the electronic structure of FeSe1−xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

We have performed a temperature dependent angle-resolved photoemission spectroscopy (ARPES) study of the tri-layer high-$T_c$ cuprate superconductor (HTSC) Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223), and have shown that the \textquotedblleft effective\textquotedblright superconducting (SC) gap $\Delta_{\rm{sc}}$ defined at the end point of the Fermi arc and the $T_c$ (= 110 K) approximately satisfies the weak-coupling BCS-relationship 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$. Combining this result with previous ARPES results on single- and double-layer cuprates, we show that the relationship between 2$\Delta_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$ holds for various HTSCs. Furthermore, at $T$ $\sim$ $T_c$, the quasi-patricle width at the end point of the Fermi arc is found to coincide with $\Delta_{\rm{sc}}$, consistent with the context of Planckian dissipation.

The electronic structures of Yb 2 Co 3 X 9 ( X =Ga, Al), which show properties of heavy-fermion antiferromagnet for X =Al and mixed-valent behavior for X =Ga, are studied by means of high-resolution photoemission spectroscopy. A sharp Yb 4f related peak, which shows characteristics consistent with the prediction by the single impurity Anderson model, was observed just below E F only for X =Ga. The origin of the difference between the Ga-based and Al-based compounds is attributed to the difference in the 4f energy level and the 4f-ligand hybridization strength.

We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of a saddle-point singularity at the Z point, as well as the Gamma point. With antiferromagnetic ordering, both of these two van Hove singularities come down below the Fermi energy, leading to a topological change in the innermost Fermi surface around the kz axis from cylindrical to tear-shaped, as expected from first-principles calculation. These singularities may provide an additional instability for the Fermi surface of the superconductors derived from BaFe2As2., Comment: 14 pages, 4 figures, 1 table

We report the first observation of the multilayer band splitting in the optimally doped trilayer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\ensuremath{\delta}}$ (Bi2223) by angle-resolved photoemission spectroscopy. The observed energy bands and Fermi surfaces are originated from the outer and inner ${\mathrm{CuO}}_{2}$ planes (OP and IP). The OP band is overdoped with a large $d$-wave gap around the node of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}43\text{ }\text{ }\mathrm{meV}$ while the IP is underdoped with an even large gap of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}60\text{ }\text{ }\mathrm{meV}$. These energy gaps are much larger than those for the same doping level of the double-layer cuprates, which leads to the large ${T}_{c}$ in Bi2223. We propose possible origins of the large superconducting gaps for the OP and IP: (1) minimal influence of out-of-plane disorder and a proximity effect and (2) interlayer tunneling of Cooper pairs between the OP and IP.

The disorder dependence of the oxygen isotope effect in optimally doped ${\text{Bi}}_{2}{\text{Sr}}_{1.6}{\text{Ln}}_{0.4}{\text{CuO}}_{6+\ensuremath{\delta}}$ superconductors which is introduced into cation sites in the planes adjacent to ${\text{CuO}}_{2}$ planes is studied. The degree of out-of-plane disorder is controlled by substituting Ln ions with different ionic radii from La to Gd. This type of disorder does not explicitly alter the nodal electronic structure or the underlying Fermi surface but markedly enhances the oxygen isotope exponent at the superconducting transition temperature $({\ensuremath{\alpha}}_{{T}_{c}})$. The enhancement in ${\ensuremath{\alpha}}_{{T}_{c}}$ is well explained by the pair-breaking model, suggesting the growth of a pseudogap state around the antinodal region due to the out-of-plane disorder.

Low energy ultrahigh momentum resolution angle resolved photoemission spectroscopy study on four-layer self-doped high Tc superconductor Ba2Ca3Cu4O8F2 (F0234) revealed fine structure in the band dispersion, identifying the unconventional association of hole and electron doping with the inner and outer CuO2 layers, respectively. For the states originating from two inequivalent CuO2 layers, different energy scales are observed in dispersion kinks associated with the collective mode coupling, with the larger energy scale found in the electron (n-) doped state which also has stronger coupling strength. Given the earlier finding that the superconducting gap is substantially larger along the n-type Fermi surface, our observations connect the mode coupling energy and strength with magnitude of the pairing gap.

Low energy ultrahigh momentum resolution angle resolved photoemission spectroscopy study on four-layer self-doped high Tc superconductor Ba2Ca3Cu4O8F2 (F0234) revealed fine structure in the band dispersion, identifying the unconventional association of hole and electron doping with the inner and outer CuO2 layers, respectively. For the states originating from two inequivalent CuO2 layers, different energy scales are observed in dispersion kinks associated with the collective mode coupling, with the larger energy scale found in the electron (n-) doped state which also has stronger coupling strength. Given the earlier finding that the superconducting gap is substantially larger along the n-type Fermi surface, our observations connect the mode coupling energy and strength with magnitude of the pairing gap.

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point., 8 pages, 3 figures

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi2Sr2CaCu2O8+delta reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, which are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches., 5 pages, 3 figures

The possibility that a pairing boson might act as the 'glue' to bind electrons into a Cooper pair in superconductors with a high critical temperature (T(c)) is being actively pursued in condensed-matter physics. Gweon et al. claim that there is a large and unusual oxygen-isotope effect on the electronic structure, indicating that phonons have a special importance in high-temperature superconductors. However, we are unable to detect this unusual oxygen-isotope effect in new data collected under almost identical material and experimental conditions. Our findings point towards a more conventional influence of phonons in these materials.