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We study the electronic structure of the ferromagnetic spinel HgCr2Se4 by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments. We conclude that the theoretical prediction of a Weyl semimetal state in HgCr2Se4 underestimates the band gap, and this material is a ferromagnetic semiconductor.

We demonstrate a method to control the Fermi level around the Van Hove singularity (VHS) in Li-intercalated graphene on the SiC substrate. By angle-resolved photoemission spectroscopy, we observed a clear Lifshitz transition in the vicinity of the VHS when the thickness of graphene exceeds four layers. We calculated the band structure of a multilayer system with different stacking sequences of graphene and Li layer. The so-called stage 2 model reproduces the Lifshitz transition, where Li occupies every other interlayer of graphene. In addition, we found that a sizable Schottky barrier is formed between graphene and the substrate. These properties allow us to explore the electronic phase diagram around the VHS by controlling the thickness.

We studied the band structure of NdFeAs1-xPxO0.9F0.1 (x=0, 0.2, 0.4, and 0.6) using angle-resolved photoemission spectroscopy (ARPES) measurements. Two of the hole bands, a1 (dxz) and a3 (dz2), were observed at the Brillouin zone center in the P-polarized light configuration, while the other two hole bands, a2 (dyz) and ?(dxy), were observed in the S-polarized alternative. The observed ?band shifts downward as x increases, which is consistent with the theoretical prediction for the change in bond angle of As/P-Fe-As/P. Furthermore, a small amount of the dxy orbital component was observed at the same binding energy as that of the top of the a1 band, thus indicating the band reconstruction of the originally degenerate a1 and a2 (dxz/dyz) bands by the unoccupied dxy band. The change in the energy level of the a1 band top with dxy orbital character is accompanied by a Tc upturn at 0.2<x<0.4. The Tc continues to increase as the a1 band shifts downward, crossing the Fermi level. The incipient band with the dxy orbital character on its top could be an important ingredient for high Tc 1111-type iron-based superconductors.

We fabricated superlattice films composed of Bi₂ bilayers (BLs) and Bi₂Te₃ quintuple layers (QLs) by annealing pure Bi₂Te₃ films. It was found that Te desorbs from the QL to form the BL with an activation energy of 2.7 eV. Eventually, two distinct stoichiometric phases were formed, Bi₁Te₁ (QL–BL–QL) and Bi₄Te₃ (QL–BL), as evidenced by scanning transmission emission microscopy measurements. The surface-state dispersion was measured with angle-resolved photoemission spectroscopy, and the topological nature of each sample is discussed. Our method offers a convenient and simple way to fabricate superlattice films with different topological properties.

We study the mechanism of the exceptionally large anomalous Hall effect (AHE) in the noncentrosymmetric antiferromagnet CoNb3S6 by angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements. From ARPES measurements of CoNb3S6 and its family compounds (FeNb3S6 and NiNb3S6), we find a band dispersion unique to the Co intercalation existing near the Fermi level. We further demonstrate that a slight deficiency of sulfur in CoNb3S6 eliminates the ferromagnetism and the AHE simultaneously while hardly changing the band structure, indicating that the weak ferromagnetism is responsible for the emergence of the large AHE. Based on our results, we propose Weyl points near the Fermi level to cause the large AHE.

In order to study the possible superconductivity at the polar surfaces of 1111-type iron-based superconductors, which is doped with a large amount of holes in spite of the electron doping in bulk materials, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on superconducting PrFeAsO1-y crystals. We have indeed observed the opening of a superconducting gap on surface-derived hole pockets as well as on a bulk-derived hole pocket. The superconducting gap is found to open on the surface-derived hole pockets below the bulk Tc, which suggests that the surface superconductivity is possibly induced by proximity effect from the bulk. We have also observed the opening of a large pseudogap on the surface-derived hole pockets, which is similar to the pseudogap in 122-type bulk superconductors doped with a smaller amount of holes. This suggests that the opening of a large pseudogap is a characteristic property of hole-doped iron-based superconductors.

The nature of the pseudogap and its relationship with superconductivity are one of the central issues of cuprate superconductors. Recently, a possible scenario has been proposed that the pseudogap state is a distinct phase characterized by spontaneous rotational symmetry breaking called “nematicity” based on transport and magnetic susceptibility measurements, where the symmetry breaking was observed below the pseudogap temperature T∗. Here, we report a temperature-dependent ARPES study of nematicity in slightly overdoped Bi1.7Pb0.5Sr1.9CaCu2O8+δ triggered by a uniaxial strain applied along one of the Cu–O bond directions. While the nematicity was enhanced in the pseudogap state as in the previous studies, it was suppressed in the superconducting state. These results indicate that the pseudogap state is characterized by spontaneous rotational symmetry breaking and that the nematicity may compete with superconductivity. Relationship between the nematicity and charge-density waves, both of which are observed in the pseudogap state, is discussed.

Hybridization of Bogoliubov quasiparticles (BQPs) between the CuO2 layers in the triple-layer cuprate high-temperature superconductor Bi2Sr2Cu2Cu3O10+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 CuO2 layers and spin fluctuations represented by a four-well model are discussed.

We successfully fabricated a MnBi2Te4/Bi2Te3 heterostructure by incorporating Mn and Te inside the topmost quintuple layer of Bi2Te3, as unambiguously confirmed by low-energy electron diffraction I-V and scanning transmission electron microscopy measurements. The surface-state Dirac cone of the heterostructure showed little change compared to that of pristine Bi2Te3, and x-ray magnetic circular dichroism measurements showed that the system was paramagnetic down to 5.6 K. These results are in contrast to previous works on related materials that showed magnetic order around 10 K as well as theoretical predictions and suggest the intricacy of the magnetic properties of two-dimensional van der Waals magnets.

The electron-doped cuprates are usually characterized by a more robust antiferromagnetic phase and a much narrower superconducting (SC) dome than those of the hole-doped counterparts. Recently, bulk single crystals of Pr1.3-xLa0.7CexCuO4-δ (PLCCO) prepared by the protect annealing method have been studied extensively and revealed many intriguing properties that were different from those obtained from samples annealed by the conventional methods. Here we report on a systematic angle-resolved photoemission spectroscopy study of PLCCO single crystals after protect annealing. The results indicate that the actual electron concentration (nFS) estimated from the Fermi-surface area is significantly larger than the Ce concentration x and the new nFS-based SC dome of PLCCO is more extended towards the overdoped side than the x-based SC dome derived for samples prepared using the conventional annealing method.

<title>Abstract</title>Solids with competing interactions often undergo complex phase transitions with a variety of long-periodic modulations. Among such transition, devil’s staircase is the most complex phenomenon, and for it, CeSb is the most famous material, where a number of the distinct phases with long-periodic magnetostructures sequentially appear below the Néel temperature. An evolution of the low-energy electronic structure going through the devil’s staircase is of special interest, which has, however, been elusive so far despite 40 years of intense research. Here, we use bulk-sensitive angle-resolved photoemission spectroscopy and reveal the devil’s staircase transition of the electronic structures. The magnetic reconstruction dramatically alters the band dispersions at each transition. Moreover, we find that the well-defined band picture largely collapses around the Fermi energy under the long-periodic modulation of the transitional phase, while it recovers at the transition into the lowest-temperature ground state. Our data provide the first direct evidence for a significant reorganization of the electronic structures and spectral functions occurring during the devil’s staircase.

<title>Abstract</title> Materials that possess nontrivial topology and magnetism is known to exhibit exotic quantum phenomena such as the quantum anomalous Hall effect. Here, we fabricate a novel magnetic topological heterostructure Mn₄Bi₂Te₇/Bi₂Te₃ where multiple magnetic layers are inserted into the topmost quintuple layer of the original topological insulator Bi₂Te₃. A massive Dirac cone (DC) with a gap of 40–75 meV at 16 K is observed. By tracing the temperature evolution, this gap is shown to gradually decrease with increasing temperature and a blunt transition from a massive to a massless DC occurs around 200–250 K. Structural analysis shows that the samples also contain MnBi₂Te₄/Bi₂Te₃. Magnetic measurements show that there are two distinct Mn components in the system that corresponds to the two heterostructures; MnBi₂Te₄/Bi₂Te₃ is paramagnetic at 6 K while Mn₄Bi₂Te₇/Bi₂Te₃ is ferromagnetic with a negative hysteresis (critical temperature  ~20 K). This novel heterostructure is potentially important for future device applications.

We report a temperature-dependent study of Kondo resonance in the heavy-fermion compounds YbXCu4 with X = Mg, Cd, and Sn. A sharp peak of the Yb2+ 4f (7/2) state has been observed in photoemission spectra, and its energy position in the limit of zero temperature is in agreement with the energy scale of the Kondo temperature T-K. The peak develops in the form of a dispersionless peak with large spectral weight at T-coh well below T-K. The onset temperature of the robust Kondo state is comparable to the temperature at a local maximum in magnetic susceptibility. This nontrivial development of changes in the Kondo resonance at T-coh demonstrates the formation of the coherent heavy-fermion state.

Ba1-xKxFe2As2 has an exotic physical property, i.e., the C2 spin-density-wave phase suddenly changes to the C4 magnetic phase, and superconductivity is suppressed within a narrow composition range around x=0.25. We have investigated the doping dependence of the electronic structure of Ba1-xKxFe2As2 using angle-resolved photoemission spectroscopy (ARPES), and we found an anomaly in the superconducting (SC) gap structure only for x=0.25 in both the hole and electron Fermi surfaces. We also propose that the mechanism of a newly observed double-resonance peak by inelastic neutron scattering is understandable because of the SC gap obtained by ARPES. Our discovery reveals the important relationship between the C4-magnetic phase and superconductivity, and it provides an opportunity to survey the underlying relationship between the SC gap and the resonance mode in the hole-doped iron-based superconductor.

We fabricated spin-polarized surface electronic states with tunable Fermi levels from a semiconductor to low-dimensional metal in the Bi/GaSb(110)-(2 x 1) surface using angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES. The spin-polarized surface band of Bi/GaSb(110) exhibits quasi-onedimensional character with the Rashba parameter alpha(R) of 4.1 and 2.6 eV angstrom at the (Gamma) over bar and (Y) over bar points of the surface Brillouin zone, respectively. The Fermi level of the surface electronic state is tuned in situ by element-selective Ar-ion sputtering on the GaSb substrate. The giant Rashba-type spin splitting with switchable metallic/semiconducting character on a semiconductor substrate makes this system a promising candidate for future research in low-dimensional spintronic phenomena.

Methylammonium lead triiodide (CH3NH3PbI3) is an essential material in prototype perovskite solar cells, and its intrinsic electronic structures have been of great interest. In this study, the clean surface of single crystal samples of CH3NH3PbI3 was elucidated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and photoelectron yield spectroscopy (PYS), and was closely compared with as-prepared single crystal surfaces with considerable native impurities. Low-energy UPS and PYS successfully revealed a presence of electronic states exceeding the Fermi level, which are presumably attributed to electrons transferring from the surface impurities into the conduction band of CH3NH3PbI3. Methylammonium lead triiodide (CH3NH3PbI3) is an essential material in prototype perovskite solar cells, and its intrinsic electronic structures have been of great interest. In this study, the clean surface of single crystal samples of CH3NH3PbI3 was elucidated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and photoelectron yield spectroscopy (PYS), and was closely compared with as-prepared single crystal surfaces with considerable native impurities. Low-energy UPS and PYS successfully revealed a presence of electronic states exceeding the Fermi level, which are presumably attributed to electrons transferring from the surface impurities into the conduction band of CH3NH3PbI3.

We studied the electronic structure of YbCdCu4, which is known as a heavy fermion compound, by means of photoemission spectroscopy. An energy shift and an intensity enhancement of the Yb2+ 4f(7/2) peak are clearly observed with decreasing temperature. The characteristic energy scale of the Kondo temperature is estimated as 26.5 meV, which is in agreement with the Kondo temperature obtained by the magnetic susceptibility measurements. It suggests that the observed 4f7/2 state is associated with the heavy fermion behavior of YbCdCu4.

© 2018 American Physical Society. The evolution of Fermi surface (FS) states of NdFeAs1-xPxO0.9F0.1 single crystals with As/P substitution has been investigated. The critical temperature Tc and the power-law exponent (n) of temperature-dependent resistivity [ρ(T)=ρ0+ATn] show a clear correlation above x=0.2, suggesting that Tc is enhanced with increasing bosonic fluctuations in the same type of FS state. Around x=0.2, all the transport properties show anomalies, indicating that x∼0.2 is the critical composition of a drastic FS change. Angle-resolved photoemission spectroscopy has more directly revealed the distinct change of FS around x=0.2, that one hole FS disappears at the Brillouin zone center and the other FS with a propellerlike shape appears at the zone corner with decreasing x. These results are indicative of the existence of two types of FS states with different nesting conditions that are related with two Tc-raising mechanisms in this system.

Electronic states of the Bi/InAs(110)-(2 x 1) surface and its spin-polarized structure are revealed by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES, and density-functional-theory calculation. The surface electronic state showed quasi-one-dimensional (Q1D) dispersion curves and a nearly metallic character; the top of the holelike band is just below the Fermi level. The size of the Rashba parameter alpha(R) reached a much larger value (similar to 5.5 eV angstrom) than for previously reported 1D systems. The present result will provide a fertile playground for further studies of the exotic electronic phenomena in 1D or Q1D systems with the spin-split electronic states as well as for advanced spintronic devices.

We have deposited Sn on corrugated hexagonal boron nitride (h-BN) nanomeshs formed on Rh(111) and found that Sn atoms are intercalated between h-BN and Rh, flattening the h-BN. Our reflection high-energy electron diffraction (RHEED) analysis showed that the average in-plane lattice constant of h-BN increases due to the loss of the corrugation. Furthermore, electronic structure measurements based on angle-resolved photoemission spectroscopy (ARPES) showed that the h-BN π band width increases significantly while the σ band width does not change as much. These behaviors were partly different from previous reports on the intercalation of h-BN/Rh system. Our results offer a novel, simple method to control the electronic structure of h-BN.

The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe2 using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding d(xy) orbital. Bond length changes of 20% in IrTe2 are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner.

We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, Au64Ge22Yb14 (AGY-I), Au63.5Ge20.5Yb16 (AGY-II), and Zn85.4Yb14.6 (Zn-Yb), by means of photoemission spectroscopy. In the valence band hard x-ray photoemission spectra of AGY-II and Zn-Yb, we separately observe a fully occupied Yb 4f state and a valence fluctuation derived Kondo resonance peak, reflecting two inequivalent Yb sites, a single Yb atom in the cluster center and its surrounding Yb icosahedron, respectively. The fully occupied 4f signal is absent in AGY-I containing no Yb atom in the cluster center. The results provide direct evidence for a heterogeneous valence state in AGY-II and Zn-Yb.

One-dimensional (1D) electronic states were discovered on the 1D surface atomic structure of Bi fabricated on semiconductor InSb(001) substrates by angle-resolved photoelectron spectroscopy (ARPES). The 1D state showed steep, Dirac-cone-like dispersion along the 1D atomic structure with a finite direct band gap opening as large as 150 meV. Moreover, spin-resolved ARPES revealed the spin polarization of the 1D unoccupied states as well as that of the occupied states, the orientation of which inverted depending on the wave-vector direction parallel to the 1D array on the surface. These results reveal that a spin-polarized quasi-1D carrier was realized on the surface of 1D Bi with highly efficient backscattering suppression, showing promise for use in future spintronics and energy-saving devices.

We have investigated the superconducting (SC) gap on hole Fermi surfaces (FSs) of optimally substituted Ba(Fe0.65Ru0.35)(2)As-2 by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that, whereas the gap is isotropic in the kx -ky plane, the gap magnitudes of two resolved hole FSs show similar kz dependences and decrease as kz approaches similar to 2 pi/c (i.e., around the Z point), unlike the other Fe-based superconductors reported so far, where the SC gap of only one hole FS shows a strong kz dependence. This unique gap structure can be understood in the scenario that the d(z)2 orbital character is mixed into both hole FSs due to finite spin-orbit coupling (SOC) and is reproduced by calculation within the random phase approximation including the SOC.

We have investigated the superconducting (SC) gap on hole Fermi surfaces (FSs) of optimally substituted Ba(Fe0.65Ru0.35)(2)As-2 by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that, whereas the gap is isotropic in the kx -ky plane, the gap magnitudes of two resolved hole FSs show similar kz dependences and decrease as kz approaches similar to 2 pi/c (i.e., around the Z point), unlike the other Fe-based superconductors reported so far, where the SC gap of only one hole FS shows a strong kz dependence. This unique gap structure can be understood in the scenario that the d(z)2 orbital character is mixed into both hole FSs due to finite spin-orbit coupling (SOC) and is reproduced by calculation within the random phase approximation including the SOC.

We have grown tellurium (Te) thin films on Bi2Te3 and investigated the atomic structure. From low energy electron diffraction (LEED) measurements, we found that the Te films are [10 (1) over bar0]-oriented with six domains. A detailed analysis of the reflection high-energy electron diffraction (RHEED) pattern revealed that the films are strained with the in-plane lattice constant compressed by similar to 1.5% compared to the bulk value due to the epitaxy between Te and Bi2Te3. These films will be interesting systems to investigate the predicted topological phases that occur in strained Te. (C) 2016 Elsevier B.V. All rights reserved.

The electronic structures of the highest occupied molecular orbital (HOMO) or the HOMO-derived valence bands dominate the transport nature of positive charge carriers (holes) in organic semiconductors. In the present study, the valence-band structures of single-crystal pentacene and the temperature dependence of their energy momentum dispersion relations are successfully demonstrated using angle-resolved ultraviolet photoelectron spectroscopy(ARUPS). For the shallowest valence band, the intermolecular transfer integral and effective mass of the holes are evaluated as 43.1 meV and 3.43 times the electron rest mass, respectively, at room temperature along the crystallographic direction for which the widest energy dispersion is expected. The temperature dependence of the ARUPS results reveals that the transfer integral values (hole effective mass) are enhanced (reduced) by similar to 20% on cooling the sample to 110 K.

We present an angle-resolved photoemission study of the electronic structure of the three-dimensional pyrochlore iridate Nd2Ir2O7 through its magnetic metal-insulator transition. Our data reveal that metallic Nd2Ir2O7 has a quadratic band, touching the Fermi level at the G point, similar to that of Pr2Ir2O7. The Fermi node state is, therefore, a common feature of the metallic phase of the pyrochlore iridates. Upon cooling below the transition temperature, this compound exhibits a gap opening with an energy shift of quasiparticle peaks like a band gap insulator. The quasiparticle peaks are strongly suppressed, however, with further decrease of temperature, and eventually vanish at the lowest temperature, leaving a nondispersive flat band lacking long-lived electrons. We thereby identify a remarkable crossover from Slater to Mott insulators with decreasing temperature. These observations explain the puzzling absence of Weyl points in this material, despite its proximity to the zero temperature metal-insulator transition.

We present an angle-resolved photoemission study of the electronic structure of the three-dimensional pyrochlore iridate Nd2Ir2O7 through its magnetic metal-insulator transition. Our data reveal that metallic Nd2Ir2O7 has a quadratic band, touching the Fermi level at the G point, similar to that of Pr2Ir2O7. The Fermi node state is, therefore, a common feature of the metallic phase of the pyrochlore iridates. Upon cooling below the transition temperature, this compound exhibits a gap opening with an energy shift of quasiparticle peaks like a band gap insulator. The quasiparticle peaks are strongly suppressed, however, with further decrease of temperature, and eventually vanish at the lowest temperature, leaving a nondispersive flat band lacking long-lived electrons. We thereby identify a remarkable crossover from Slater to Mott insulators with decreasing temperature. These observations explain the puzzling absence of Weyl points in this material, despite its proximity to the zero temperature metal-insulator transition.

<p>近年、新たなトポロジカル物質としてワイル半金属が注目を集めている。昨年、高圧下におけるTe単結晶が歪みによってワイル半金属になると提案された。私たちはTe薄膜を格子定数の異なるBi2Te3基板上にエピタキシャル成長させ、この歪みの再現することを試みた。作成試料は反射高速電子回折と角度分解光電子分光で結晶構造とバンド分散を測定した。その結果、歪みを持ちバンド分散が変化するが、ワイル半金属ではないことがわかった。</p>

<p>極端紫外光研究施設(UVSOR) BL7Uにおいて、三層系銅酸化物高温超伝導体 Bi2223の低エネルギー高分解能角度分解光電子分光実験を行った。CuO2三重層間の電子のホッピングとCuO2面のキャリアの違いに由来する三重層分裂を明瞭に観測した。本講演では、その電子構造の詳細について報告する。</p>

高い電荷移動度を示すペンタセン単結晶は，有機固体内の電荷輸送メカニズムを解明するモデル物質の一つと注目されている。近年，有機半導体結晶の価電子バンド構造の測定が，角度分解型光電子分光法によって可能となった。しかし，これまでの研究では大気曝露を経た試料に対して測定が行われており，試料表面の酸化物の影響が懸念される。本研究では，ペンタセン単結晶を真空内で劈開し，得られた清浄表面のバンド分散を実測した。

© 2015 American Physical Society. We have performed an angle-resolved photoemission study of the nodal quasiparticle spectra of the high-Tc cuprate trilayer Bi2Sr2Ca2Cu3O10+δ (Tc∼110 K). The spectral weight Z of the nodal quasiparticle increases with decreasing temperature across Tc. Such a temperature dependence is qualitatively similar to that of the coherence peak intensity in the antinodal 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.

We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As1-xPx)(2) (x = 0.30, T-c = 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.

We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As1-xPx)(2) (x = 0.30, T-c = 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.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of BaNi2P2 that shows a superconducting transition at T-c similar to 2.5 K. We observed hole and electron Fermi surfaces (FSs) around the Brillouin zone center and corner, respectively, and the shapes of the hole FSs dramatically changed with photon energy, indicating strong three dimensionality. The observed FSs are consistent with band-structure calculations and de Haas-van Alphen measurements. The mass enhancement factors estimated in the normal state were m*/m(b) &lt;= 2, indicating weak electron correlation compared to typical iron-pnictide superconductors. An electronlike Fermi surface around the Z point was observed in contrast with BaNi2As2 and may be related to the higher T-c of BaNi2P2.

The isovalent-substituted iron-pnictide superconductor SrFe2(As1-xPx)(2) (x = 0.35) has a slightly higher optimum critical temperature than the similar system BaFe2(As1-xPx)(2), and its parent compound SrFe2As2 has a much higher Neel temperature than BaFe2As2. We have studied the band structure and the Fermi surfaces of optimally doped SrFe2(As1-xPx)(2) by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around (0,0) and two electronlike FSs around (pi,pi) have been observed as in the case of BaFe2(As1-xPx)(2). Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the k(z) direction than the corresponding one in BaFe(As1-xPx)(2), and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(As1-xPx)(2). A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the d(xy) and/or d(xz/yz) orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

The isovalent-substituted iron-pnictide superconductor SrFe2(As1-xPx)(2) (x = 0.35) has a slightly higher optimum critical temperature than the similar system BaFe2(As1-xPx)(2), and its parent compound SrFe2As2 has a much higher Neel temperature than BaFe2As2. We have studied the band structure and the Fermi surfaces of optimally doped SrFe2(As1-xPx)(2) by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around (0,0) and two electronlike FSs around (pi,pi) have been observed as in the case of BaFe2(As1-xPx)(2). Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the k(z) direction than the corresponding one in BaFe(As1-xPx)(2), and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(As1-xPx)(2). A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the d(xy) and/or d(xz/yz) orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of BaNi2P2 that shows a superconducting transition at T-c similar to 2.5 K. We observed hole and electron Fermi surfaces (FSs) around the Brillouin zone center and corner, respectively, and the shapes of the hole FSs dramatically changed with photon energy, indicating strong three dimensionality. The observed FSs are consistent with band-structure calculations and de Haas-van Alphen measurements. The mass enhancement factors estimated in the normal state were m*/m(b) &lt;= 2, indicating weak electron correlation compared to typical iron-pnictide superconductors. An electronlike Fermi surface around the Z point was observed in contrast with BaNi2As2 and may be related to the higher T-c of BaNi2P2.

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

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

We have studied the electronic structure of Ba(Fe1-xMn x)2As2 (x=0.08), which fails to become a superconductor in spite of the formal hole doping like Ba1-xK xFe2As2, with photoemission spectroscopy and x-ray absorption spectroscopy (XAS). With decreasing temperature, a transition from the paramagnetic phase to the antiferromagnetic phase was clearly observed by angle-resolved photoemission spectroscopy. XAS results indicated that the substituted Mn atoms form a strongly hybridized ground state. Resonance-photoemission spectra at the Mn L3 edge revealed that the Mn 3d partial density of states is distributed over a wide energy range of 2-13 eV below the Fermi level (EF), with little contribution around E F. This indicates that the dopant Mn 3d states are localized in spite of the strong Mn 3d-As 4p hybridization and split into the occupied and unoccupied parts due to the on-site Coulomb and exchange interaction. The absence of superconductivity in Ba(Fe1-xMnx) 2As2 can thus be ascribed both to the absence of carrier doping in the FeAs plane and to the stabilization of the competing G-type antiferromagnetic order by the Mn impurities. © 2013 American Physical Society.

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe1-xZnx)(2)As-2 (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around T similar to 135 K, indicating antiferromagnetic order below the Neel temperature of T-N similar to 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below T-N. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3d state similar to 10 eV below the Fermi level and do not participate in the formation of the FSs.

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe1-xZnx)(2)As-2 (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around T similar to 135 K, indicating antiferromagnetic order below the Neel temperature of T-N similar to 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below T-N. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3d state similar to 10 eV below the Fermi level and do not participate in the formation of the FSs.

In order to examine to what extent the rigid-band-like electron doping scenario is applicable to the transition metal-substituted Fe-based superconductors, we have performed angle-resolved photoemission spectroscopy studies of Ba(Fe1-xNix)(2)As-2 (Ni-122) and Ba(Fe1-xCux)(2)As-2 (Cu-122), and compared the results with Ba(Fe1-xCox)(2)As-2 (Co-122). We find that Ni 3d-derived features are formed below the Fe 3d band and that Cu 3d-derived ones further below it. The electron and hole Fermi surface (FS) volumes are found to increase and decrease with substitution, respectively, qualitatively consistent with the rigid-band model. However, the total extra electron number estimated from the FS volumes (the total electron FS volume minus the total hole FS volume) is found to decrease in going from Co-, Ni-, to Cu-122 for a fixed nominal extra electron number, that is, the number of electrons that participate in the formation of FS decreases with increasing impurity potential. We find that the Neel temperature T-N and the critical temperature T-c maximum are determined by the FS volumes rather than the nominal extra electron concentration or the substituted atom concentration. DOI

In order to examine to what extent the rigid-band-like electron doping scenario is applicable to the transition metal-substituted Fe-based superconductors, we have performed angle-resolved photoemission spectroscopy studies of Ba(Fe1-xNix)(2)As-2 (Ni-122) and Ba(Fe1-xCux)(2)As-2 (Cu-122), and compared the results with Ba(Fe1-xCox)(2)As-2 (Co-122). We find that Ni 3d-derived features are formed below the Fe 3d band and that Cu 3d-derived ones further below it. The electron and hole Fermi surface (FS) volumes are found to increase and decrease with substitution, respectively, qualitatively consistent with the rigid-band model. However, the total extra electron number estimated from the FS volumes (the total electron FS volume minus the total hole FS volume) is found to decrease in going from Co-, Ni-, to Cu-122 for a fixed nominal extra electron number, that is, the number of electrons that participate in the formation of FS decreases with increasing impurity potential. We find that the Neel temperature T-N and the critical temperature T-c maximum are determined by the FS volumes rather than the nominal extra electron concentration or the substituted atom concentration. DOI: 10.1103/PhysRevLett.110.107007

In the iron pnictide superconductors, two distinct unconventional mechanisms of superconductivity have been put forth

In the iron pnictide superconductors, two distinct unconventional mechanisms<br /> of superconductivity have been put forth: One is mediated by spin fluctuations<br /> leading to the s+- state with sign change of superconducting gap between the<br /> hole and electron bands, and the other is orbital fluctuations which favor the<br /> s++ state without sign reversal. Here we report direct observation of peculiar<br /> momentum-dependent anisotropy in the superconducting gap from angle-resolved<br /> photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large<br /> anisotropy found only in the electron Fermi surface (FS) and the nearly<br /> isotropic gap on the entire hole FSs are together consistent with modified s+-<br /> gap with nodal loops, which can be theoretically reproduced by considering both<br /> spin and orbital fluctuations whose competition generates the gap modulation.<br /> This indicates that these two fluctuations are nearly equally important to the<br /> high-Tc superconductivity in this system. In the iron pnictide superconductors, two distinct unconventional mechanisms<br /> of superconductivity have been put forth: One is mediated by spin fluctuations<br /> leading to the s+- state with sign change of superconducting gap between the<br /> hole and electron bands, and the other is orbital fluctuations which favor the<br /> s++ state without sign reversal. Here we report direct observation of peculiar<br /> momentum-dependent anisotropy in the superconducting gap from angle-resolved<br /> photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large<br /> anisotropy found only in the electron Fermi surface (FS) and the nearly<br /> isotropic gap on the entire hole FSs are together consistent with modified s+-<br /> gap with nodal loops, which can be theoretically reproduced by considering both<br /> spin and orbital fluctuations whose competition generates the gap modulation.<br /> This indicates that these two fluctuations are nearly equally important to the<br /> high-Tc superconductivity in this system.

The correlated electronic structure of SrVO3 has been investigated by angle-resolved photoemission spectroscopy using in situ prepared thin films. Pronounced features of band renormalization have been observed

The correlated electronic structure of SrVO3 has been investigated by angle-resolved photoemission spectroscopy using in situ prepared thin films. Pronounced features of band renormalization have been observed: a sharp kink similar to 60 meV below the Fermi level (E-F) and a broad so-called "high-energy kink" similar to 0.3 eV below E-F as in the high-T-c cuprates, although SrVO3 does not show magnetic fluctuations. We have deduced the self-energy in a wide energy range by applying the Kramers-Kronig relation to the observed spectra. The obtained self-energy clearly shows a large energy scale of similar to 0.7 eV, which is attributed to electron-electron interaction and gives rise to the similar to 0.3 eV kink in the band dispersion as well as the incoherent peak similar to 1.5 eV below E-F. The present analysis enables us to obtain a consistent picture for both the incoherent spectra and the band renormalization.

We have performed a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the trilayer high-T-c cuprate superconductor (HTSC) Bi2Sr2Ca2Cu3O10+delta (Bi2223), and have shown that the T-c (=110 K) and "effective" superconducting gap Delta(sc) defined at the end point of the Fermi arc follow the relationship 2 Delta(sc) approximate to 4k(B)T(c). Combining this result with previous ARPES results on single-and double-layer cuprates, we show that the relationship 2 Delta(sc) approximate to 4k(B)T(c) holds for various HTSCs.

We have performed a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the trilayer high-T-c cuprate superconductor (HTSC) Bi2Sr2Ca2Cu3O10+delta (Bi2223), and have shown that the T-c (=110 K) and "effective" superconducting gap Delta(sc) defined at the end point of the Fermi arc follow the relationship 2 Delta(sc) approximate to 4k(B)T(c). Combining this result with previous ARPES results on single-and double-layer cuprates, we show that the relationship 2 Delta(sc) approximate to 4k(B)T(c) holds for various HTSCs.

We investigate the kz dispersion in the hole Fermi surfaces (FSs) and the superconducting (SC) gaps of BaFe 2(As 0.65P 0.35) 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 kz∼π. The comparable magnitude of the SC gap is observed also at kz∼ 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. © 2012 Elsevier Ltd. All rights reserved.

We have performed a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the trilayer high-T c cuprate superconductor (HTSC) Bi 2Sr 2Ca 2Cu 3O 10+δ (Bi2223), and have shown that the T c (=110 K) and "effective" superconducting gap Δ sc defined at the end point of the Fermi arc follow the relationship 2Δ sc4k BT c. Combining this result with previous ARPES results on single- and double-layer cuprates, we show that the relationship 2Δ sc4k BT c holds for various HTSCs. © 2012 American Physical Society.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO0.7 and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d3z2-r2- and dyz,zx- derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation. © 2011 American Physical Society.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO0.7 and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d3z2-r2- and dyz,zx- derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation. © 2011 American Physical Society. We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO0.7 and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d3z2-r2- and dyz,zx- derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation. © 2011 American Physical Society.

We report the first observation of the multilayer band splitting in the optimally doped trilayer cuprate Bi2Sr2Ca2Cu3O10+delta (Bi2223) by angle-resolved photoemission spectroscopy. The observed energy bands and Fermi surfaces are originated from the outer and inner CuO2 planes (OP and IP). The OP band is overdoped with a large d-wave gap around the node of Delta(0) similar to 43 meV while the IP is underdoped with an even large gap of Delta(0) similar to 60 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

We report the first observation of the multilayer band splitting in the optimally doped trilayer cuprate Bi2Sr2Ca2Cu3O10+delta (Bi2223) by angle-resolved photoemission spectroscopy. The observed energy bands and Fermi surfaces are originated from the outer and inner CuO2 planes (OP and IP). The OP band is overdoped with a large d-wave gap around the node of Delta(0) similar to 43 meV while the IP is underdoped with an even large gap of Delta(0) similar to 60 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.

We have performed angle-resolved photoemission spectroscopy study of the electronic structure of the tri-layer high-Tc cuprate superconductor Bi 2Sr2Ca2Cu3O10+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. © 2009 Elsevier B.V. All rights reserved.

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 Bi2Sr2Ca2Cu3O10+δ (Bi2223). The kink energy of the OP band is ∼ 70 meV, as observed in various high-Tc 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 ∼35 meV buckling mode plus the large (∼ 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 ∼ 35 meV buckling-phonon mode plus the ∼ 40 meV SC gap of OP if interlayer scattering of QP is involved.