Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevresearch.3.033222

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.3.033222/fulltext

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science (AAAS)  

DOI： 10.1126/science.abd6701

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Magnetic skyrmions are self-organized topological spin textures that behave like particles. Because of their fast creation and typically long lifetime, experimental verification of skyrmion's creation/annihilation processes has been challenging. Here, we successfully track skyrmion dynamics in defect-introduced Co9Zn9Mn2 by using pump-probe Lorentz transmission electron microscope. Following the nanosecond photothermal excitation, we resolve 160-nm skyrmion's proliferation at <1 ns, contraction at 5 ns, drift from 10 ns to 4 μs, and coalescence at ~5 μs. These motions relay the multiscale arrangement and relaxation of skyrmion clusters in a repeatable cycle of 20 kHz. Such repeatable dynamics of skyrmions, arising from the weakened but still persistent topological protection around defects, enables us to visualize the whole life of the skyrmions and demonstrates the possible high-frequency manipulations of topological charges brought by skyrmions.

DOI： 10.1126/sciadv.abg1322

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

In this paper, we investigate the femtosecond-optical-pulse-induced strain dynamics in relatively thin (100 nm) and thick (10 000 nm) silicon plates based on finite-element simulations. In the thin sample, almost spatially homogeneous excitation by the optical pulse predominantly generates a standing wave of the lowest-order acoustic resonance mode along the out-of-plane direction. At the same time, laterally propagating plate waves are emitted at the sample edge through the open edge deformation. Fourier transformation analysis reveals that the plate waves in the thin sample are mainly composed of two symmetric Lamb waves, reflecting the spatially uniform photoexcitation. In the thick sample, on the other hand, only the near surface region is photo-excited and thus a strain pulse that propagates along the out-of-plane direction is generated, accompanying the laterally propagating pulse-like strain dynamics through the edge deformation. These lateral strain pulses consist of multiple Lamb waves, including asymmetric and higher-order symmetric modes. Our simulations quantitatively demonstrate the out-of-plane and in-plane photoinduced strain dynamics in realistic silicon plates, ranging from the plate wave form to pulse trains, depending on material parameters such as sample thickness, optical penetration depth, and sound velocity.

DOI： 10.1063/4.0000059

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The control of acoustic phonons, which are the carriers of sound and heat, has become the focus of increasing attention because of a demand for manipulating the sonic and thermal properties of nanometric devices. In particular, the photoacoustic effect using ultrafast optical pulses has a promising potential for the optical manipulation of phonons in the picosecond time regime. So far, its mechanism has been mostly based on the commonplace thermoelastic expansion in isotropic media, which has limited applicability. In this study, we investigate a conceptually new mechanism of the photoacoustic effect involving a structural instability that utilizes a transition-metal dichalcogenide VTe2 with a ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging and diffraction measurements reveal the generation and propagation of unusual acoustic waves in a nanometric thin plate associated with optically induced instantaneous CDW dissolution. Our results highlight the capability of photoinduced structural instabilities as a source of coherent acoustic waves.

DOI： 10.1021/acs.nanolett.0c01006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe2, that hosts a charge density wave (CDW) coupled with the band inversion involving V3d and Te5p orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon the CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli. Manipulating topological states by coupled electronic orders is promising for future dissipation-less electronic devices. Here, Mitsuishi et al. report selective vanishing of Dirac-type topological surface states by the formation of coupled charge density wave in a transition-metal dichalcogenide VTe2.

DOI： 10.1038/s41467-020-16290-w

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Ultrahigh-repetition-rate (1.1 GHz), deep-ultraviolet coherent light at 208.8 nm is generated by applying an external Fabry-Perot cavity for repetition-rate multiplication to the fourth harmonics of a 10-ps, mode-locked Ti:sapphire laser. Its small pulse energy minimizes the unwanted space charge effect, while its high repetition rate drastically reduces the acquisition time in high-energy resolution angle-resolved photoemission spectroscopy using hemispherical electron analyzers. The absence of the space charge effect in the photoemission spectrum near the Fermi edge of polycrystalline Au at 8 K demonstrates this idea. Published under license by AIP Publishing.

DOI： 10.1063/1.5124342

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The electronic nematic phase is an unconventional state of matter that spontaneously breaks the rotational symmetry of electrons. In iron-pnictides/chalcogenides and cuprates, the nematic ordering and fluctuations have been suggested to have as-yet-unconfirmed roles in superconductivity. However, most studies have been conducted in thermal equilibrium, where the dynamical property and excitation can be masked by the coupling with the lattice. Here we use femtosecond optical pulse to perturb the electronic nematic order in FeSe. Through time-, energy-, momentum- and orbital-resolved photo-emission spectroscopy, we detect the ultrafast dynamics of electronic nematicity. In the strong-excitation regime, through the observation of Fermi surface anisotropy, we find a quick disappearance of the nematicity followed by a heavily-damped oscillation. This short-life nematicity oscillation is seemingly related to the imbalance of Fe 3d(xz) and d(yz) orbitals. These phenomena show critical behavior as a function of pump fluence. Our real-time observations reveal the nature of the electronic nematic excitation instantly decoupled from the underlying lattice.

DOI： 10.1038/s41467-019-09869-5

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigate photo-induced lattice strains of a monoclinic VTe2 thin flake using ultrafast electron diffraction. After photoexcitation by a 190 fs pulse, we observe diffraction intensity oscillations with periods of 35 and 75 ps, which indicate coherent acoustic phonons of two distinct branches. The oscillations of 9 (1) over bar 10 and (9) over bar 10 diffraction intensities have opposite signs, indicating the change of the diffraction angle due to the shear strain. By numerically simulating these diffraction intensities as a function of the monoclinic angle, we evaluate the amplitude of the photo-induced shear strain. (c) 2018 The Japan Society of Applied Physics

DOI： 10.7567/APEX.11.092601

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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.

DOI： 10.1126/sciadv.aar3867

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

High-temperature superconductivity in iron-pnictides/chalcogenides arises in balance with several electronic and lattice instabilities. Beside the antiferromagnetic order, the orbital anisotropy between Fe 3d(xz) and 3d(yz) occurs near the orthorhombic structural transition in several parent compounds. However, the extent of the survival of orbital anisotropy against the ion-substitution remains to be established. Here we report the composition (x) and temperature (T) dependences of the orbital anisotropy in the electronic structure of a BaFe2(As1-xPx)(2) system by using angle-resolved photoemission spectroscopy. In the low-x regime, the orbital anisotropy starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. By increasing x, it is gradually suppressed and survives in the optimally doped regime. We find that the in-plane orbital anisotropy persists in a large area of the nonmagnetic phase, including the superconducting dome. These results suggest that the rotational symmetry-broken electronic state acts as the stage for superconductivity in BaFe2(As1-xPx)(2).

DOI： 10.1038/s41598-018-20332-1

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Quantum states characterized by nontrivial topology produce interesting electrodynamics and versatile electronic functionalities. One source for such remarkable phenomena is emergent electromagnetic field, which is the outcome of interplay between topological spin structures with scalar spin chirality and conduction electrons. However, it has scarcely been exploited for emergent function related to heat-electricity conversion. Here we report an unusually enhanced thermopower by application of magnetic field in MnGe hosting topological spin textures. By considering all conceivable origins through quantitative investigations of electronic structures and properties, a possible origin of large magneto-thermopower is assigned to the strong energy dependence of charge-transport lifetime caused by unconventional carrier scattering via the dynamics of emergent magnetic field. Furthermore, high-magnetic-field measurements corroborate the presence of residual magnetic fluctuations even in the nominally ferromagnetic region, leading to a subsisting behavior of field-enhanced thermopower. The present finding may pave a way for thermoelectric function of topological magnets.

DOI： 10.1038/s41467-018-02857-1

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The structure of the superconducting gap in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions. In superconducting FeSe, there have been controversial results reporting nodal and nodeless gap structures, raising a fundamental issue of pairing mechanisms of iron-based superconductivity. Here, by utilizing polarization-dependent laser-excited angle-resolved photoemission spectroscopy, we report a detailed momentum dependence of the gap in single-and multi-domain regions of orthorhombic FeSe crystals. We confirm that the superconducting gap has a twofold in-plane anisotropy, associated with the nematicity due to orbital ordering. In twinned regions, we clearly find finite gap minima near the vertices of the major axis of the elliptical zone-centered Fermi surface, indicating a nodeless state. In contrast, the single-domain gap drops steeply to zero in a narrow angle range, evidencing for nascent nodes. Such unusual node lifting in multi-domain regions can be explained by the nematicity-induced time-reversal symmetry breaking near the twin boundaries.

DOI： 10.1038/s41467-017-02739-y

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

A major problem in the field of high-transition temperature (T-c) superconductivity is the identification of the electronic instabilities near superconductivity. It is known that the iron-based superconductors exhibit antiferromagnetic order, which competes with the superconductivity. However, in the nonmagnetic state, there are many aspects of the electronic instabilities that remain unclarified, as represented by the orbital instability and several in-plane anisotropic physical properties. We report a new aspect of the electronic state of the optimally doped iron-based superconductors by using high-energy resolution angle-resolved photoemission spectroscopy. We find spectral evidence for the folded electronic structure suggestive of an antiferroic electronic instability, coexisting with the superconductivity in the nonmagnetic state of Ba1-xKxFe2As2. We further establish a phase diagram showing that the antiferroic electronic structure persists in a large portion of the nonmagnetic phase covering the superconducting dome. These results motivate consideration of a key unknown electronic instability, which is necessary for the achievement of high-T-c superconductivity in the iron-based superconductors.

DOI： 10.1126/sciadv.1700466

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The low-energy electronic structure of LaFeAsO1-x H-x (0.0 <= x <= 0.60), a system which exhibits two superconducting domes and antiferromagnetic orders in its phase diagram, is investigated by utilizing laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find a suppression of the density of states with cooling, namely a pseudogap formation, for all doping range. The pseudogap gets suppressed on doping through the first superconducting dome regime to the higher-x region, whereas it tends to recover when further increasing x toward the second antiferromagnetic ordered phase. The systematic doping dependence indicates the different origins of pseudogaps in the low-and high-x regions, possibly related to the respective antiferromagnetic ground states residing at both ends of the phase diagram.

DOI： 10.1103/PhysRevB.95.064501

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We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods. For a single-crystalline Au thin film, we observe the suppression of the diffraction intensity occuring in 10 ps, which direcly reflects the lattice thermalization via the electron-phonon interaction. By using the two-temperature model, the electron-phonon coupling constant (g) and the electron and lattice temperatures (T-e, T-1) are evaluated from UED, with which we simulate the transient optical transmittance. The simulation well agrees with the experimentally obtained transmittance data, except for the slight deviations at the initial photoexcitation and the relaxed quasi-equilibrium state. We also present the results similarly obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths. (C) 2016 Author(s).

DOI： 10.1063/1.4971210

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the Gamma and M points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in d(xz)/d(yz) orbitals. The d(xz) band shifts upward at the Gamma point, while it moves downward at the M point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the Gamma and M points are rotated by 90 degrees with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.

DOI： 10.1103/PhysRevB.92.205117

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Language：English   Publisher：PHYSICAL SOC JAPAN  

We present a review on the developments in the photoemission spectrometer with a vacuum ultraviolet laser at Institute for Solid State Physics at the University of Tokyo. The advantages of high energy resolution, high cooling ability, and bulk sensitivity enable applications with a wide range of materials. We introduce some examples of fine electronic structures detected by laser photoemission spectroscopy and discuss the prospects of research on low-transition-temperature superconductors exhibiting unconventional superconductivity.

DOI： 10.7566/JPSJ.84.072001

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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.

DOI： 10.1038/srep07292

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on awide doping range of the Ba1-x K-x Fe2As2 (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure (kink1) in the dispersion which is doping dependent whereby its energy peaks at the optimally doped level (x similar to 0.4) and decreases towards the underdoped and overdoped sides. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound where a similar bosonic mode associated with spin excitations was observed. The relation between the mode energy (Omega) and the SC transition temperature (T-c) deduced from our laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE (kink2) showing less doping and temperature dependence compared to kink1 and which thus may be of different origin.

DOI： 10.1103/PhysRevB.90.195124

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A bulk material comprising stacked nanosheets of nickel bis(dithiolene) complexes is investigated. The average oxidation number is -3/4 for each complex unit in the as-prepared sample; oxidation or reduction respectively can change this to 0 or -1. Refined electrical conductivity measurement, involving a single microflake sample being subjected to the van der Pauw method under scanning electron microscopy control, reveals a conductivity of 1.6 x 10 (2) S cm(1), which is remarkably high for a coordination polymeric material. Conductivity is also noted to modulate with the change of oxidation state. Theoretical calculation and photoelectron emission spectroscopy reveal the stacked nanosheets to have a metallic nature. This work provides a foothold for the development of the first organic-based two-dimensional topological insulator, which will require the precise control of the oxidation state in the single-layer nickel bisdithiolene complex nanosheet (cf. Liu, F. et al. Nano Lett. 2013, 13, 2842).

DOI： 10.1021/ja507619d

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study superconducting FeSe (T-c = 9K) exhibiting the tetragonal-orthorhombic structural transition (T-s similar to 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the d(yz) orbital band at the Brillouin zone corner is 50 meV higher than that of d(xz), indicating the orbital order similar to the NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to T-s, thus suggesting that the electronic nematicity above T-s is considerably weaker in FeSe compared to BaFe2As2 family.

DOI： 10.1103/PhysRevB.90.121111

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We have investigated the superconducting (SC)-gap anisotropy for several Ba-doped KFe2As2 samples using laser-based angle-resolved photoemission spectroscopy. We show that the SC-gap anisotropy and node positions drastically change with a small amount of Ba doping. Our results totally exclude a possibility of d-wave symmetry and strongly suggest that both spin and orbital fluctuations are important for the pairing interaction in the Ba-doped K122. © 2014 American Physical Society.

DOI： 10.1103/PhysRevB.89.081103

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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 FeTe0.6Se0.4(T-c = 14.5 K similar to 1.2 meV) in an accessible range below and above the Fermi level (E-F) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV (similar to 8 K) above E-F at the Gamma-point, which shows a sharp superconducting coherence peak with gap formation below T-c. The estimated superconducting gap Delta and Fermi energy epsilon(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.

DOI： 10.1038/srep04109

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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.

DOI： 10.1103/PhysRevB.89.045101

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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.

DOI： 10.1103/PhysRevB.87.201110

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We investigate the two-dimensional highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X = I, Br, and Cl) by angular-resolved photoemission spectroscopy. Because of the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin splitting. The characteristic 2D confinement effect is clearly observed also in the valence bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX. DOI: 10.1103/PhysRevLett.110.107204

DOI： 10.1103/PhysRevLett.110.107204

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We study the superconducting-gap anisotropy of the Gamma-centered hole Fermi surface in optimally doped FeTe0.6Se0.4 (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(4 phi) 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.

DOI： 10.1103/PhysRevLett.109.237011

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We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting gaps in this compound. The gap size of the outer hole Fermi-surface (FS) sheet around the Brillouin zone (BZ) center shows an abrupt drop with overdoping (for x greater than or similar to 0.6) while the inner and middle FS gaps roughly scale with T-c. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results indicate the different contributions of X-2-Y-2 and XZ/YZ orbitals to superconductivity in BaK and can hardly be completely reproduced by the available theories on iron-based superconductors.

DOI： 10.1103/PhysRevB.86.165117

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In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe2As2 has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe2As2 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.

DOI： 10.1126/science.1222793

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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 omega(2) dependence of the imaginary part of the self energy, limited by a kink at similar to 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.

DOI： 10.1038/ncomms2079

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In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the three-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive h nu(-)dependent soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of similar to 300 meV at the conduction-band minimum and valence-band maximum located in the k(z) = pi/c plane. It provides direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound.

DOI： 10.1103/PhysRevB.86.085204

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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) similar to pi. The comparable magnitude of the SC gap is observed also at k(z) similar to 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. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2011.12.032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin- dependent electronic functions.

DOI： 10.1038/NMAT3051

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The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle-resolved photoemission spectroscopy on BaFe2(As0.65P0.35)(2) and Ba0.6K0.4Fe2As2 to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

DOI： 10.1126/science.1202150

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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 similar to 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) similar to 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.

DOI： 10.1103/PhysRevLett.104.057002

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have performed temperature (T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results we, find a sudden deviation from the BCS prediction just below T-N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.101

Web of Science

Language：English   Publisher：AMER PHYSICAL SOC  

A Reply to the Comment by J. D. Lee et al..

DOI： 10.1103/PhysRevLett.102.199702

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B are studied by high-resolution photoemission spectroscopy. The valence band spectra of Li2Pd3B and Li2Pt3B are well explained by first principles band calculations. The sub-meV photoemission result shows that the size of the superconducting gap of Li2Pd3B is about 1.1 meV at 5.4 K with a reduced gap value (2 Delta(0)/k(B)T(c)) of about 4.5, which is greater than the Bardeen-Cooper-Schrieffer (BCS) value of 3.52. The obtained superconducting gap can be explained more clearly with an s-wave superconducting gap than with p- and d-wave superconducting gaps. This result indicates that Li2Pd3B is a conventional strong-coupling superconductor.

DOI： 10.1143/JPSJ.78.034711

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical-transition temperatures (T-c's) of 26 and 5 K, respectively. We find that high-T-c LaFeAsO:F exhibits a temperature-dependent pseudogaplike feature extending over similar to 0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-T-c LaFePO:F. We also find similar to 20-meV pseudogaplike features and signatures of superconducting gaps in both LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogaplike features through comparison with the high-T-c cuprates.

DOI： 10.1103/PhysRevB.79.060503

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

The temperature (T) dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing T, the evolution of a small gap (_60meV) at the Fermi level is observed in the DOS, indicating a p-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at 5K in accordance with transport and optical experiments. The T dependence of the gap, which gets smeared out quickly at high T as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

DOI： 10.1103/PhysRevB.72.233202

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We present angle-integrated photoemission spectroscopy (PES) study of LaOFeAs: F (T-c = 26 K) and LaOFeP:F (T-c = 5 K) using He I line (h nu = 21.2 eV) and laser (h nu = 6.994 eV) as excitation light sources. We find quantitatively consistent results between the He I and the laser PES measurements: LaFeAsO:F exhibits a temperature-dependent pseudogap as large as similar to 0.1 eV at 300 K, whereas LaFePO:F exhibits a similar to 20-meV pseudogap. We discuss possible origin of the T-dependent pseudogap specifically observed in LaFeAsO: F through comparison with other materials that exhibit unusual T-dependent pseudogaps.

DOI： 10.1143/JPSJS.77SC.61

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (similar to 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

DOI： 10.1103/PhysRevLett.100.166402

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hv=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 mu eV 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 mu eV, 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, 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. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2839010

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the doping dependent low-energy, low temperature (T=5 K) properties of nodal quasiparticles in the d-wave superconductor Bi2.1Sr1.9CaCu2O8+delta (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near E-F, mean free paths, and scattering rates (Gamma) of quasiparticles. For optimally and overdoped samples, we obtain very sharp quasiparticle peaks of 8 and 6 meV full width at half maximum, respectively. The value of Gamma for optimally doped sample is in good accordance with terahertz conductivity. For all doping levels, we find the energy dependence of Gamma similar to vertical bar omega vertical bar, while Gamma(omega=0) shows a monotonic increase from overdoping to underdoping. The doping-dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K less than or similar to 0.07T(c)) pronounced in the underdoped region. The quasiparticle peak spectra match well with a single Lorentzian function, thus indicating that the nodal carriers can be described as well-defined quasiparticles, even in the underdoped region.

DOI： 10.1103/PhysRevB.77.064522

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi(2)B(2)C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T(N). This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

DOI： 10.1103/PhysRevLett.100.017003

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic structure near the Fermi level (E-F) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (T-c=9.6 K), with the strong electron-phonon coupling value of 2 Delta(0)/k(B)T(c)>= 4.56. A fitting analysis identifies clear anomalies at T-p=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.

DOI： 10.1103/PhysRevLett.99.117003

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (T-c = 9.0 K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (T-N = 6.0 K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.132

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We have performed a sub-meV photoemission study on Mg(B1-xCx)(2) for x = 0-0.075. The superconducting (SC) transition temperature (T-c) changes systematically from 38 K to 24 K with increasing the carbon concentration. Because of the extremely high-energy resolution of sub-meV, two features corresponding to the two SC gaps originated from pi and sigma bands are clearly observed even in raw spectra up to x = 0.075. From numerical analyses, we found that the gap size of the sigma band is proportional to T-c, while that of the pi band is almost constant. The temperature and carbon concentration dependences of the two gaps are explained within a simple two-band model based on a mean field theory. Using the theoretical model, we deduced interband and intraband coupling strength constants. The transition temperature is dominated by the intraband coupling of the sigma band, while the interband coupling slightly increases with increasing the carbon concentration. The two-band model shows that the interband coupling slightly pushes up T-c. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.071

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We investigate the temperature (T)-dependent low-energy electronic structure of a boron-doped diamond thin film using ultrahigh resolution laser-excited photoemission spectroscopy. We observe a clear shift of the leading edge below T=11 K, indicative of a superconducting gap opening (Delta similar to 0.78 meV at T=4.5 K). The gap feature is significantly broad and a well-defined quasiparticle peak is lacking even at the lowest temperature of measurement (=4.5 K). We discuss our results in terms of disorder effects on the normal state transport and superconductivity in this system.

DOI： 10.1103/PhysRevLett.98.047003

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The cobalt oxide superconductor NaxCoO2 center dot yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E-F) in the normal state of NaxCoO2 center dot yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to NaxCoO2. The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within similar to 30 meV below E-F, closer to E-F than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.

DOI： 10.1103/PhysRevLett.97.267003

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi2B2C and ErNi2B2C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi2B2C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNiAC. For YNi2B2C, s + g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi2B2C, the observed coherent peak is very broad and finite density of state exists at Fermi level. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.03.074

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We study the superconducting gap of the filled skutterudite superconductor LaRu4P12 (transition temperature T-c = 7.2 K) using a newly developed sub-meV resolution photoemission spectrometer with a vacuum-UV laser as a light source. The result at 4.8 K shows the opening of a superconducting gap, whose spectral shape is well explained with an isotropic s-wave Dynes function using a gap size of 1.3 meV. This corresponds to the reduced gap 2 Delta(0)/k(B)T(c) of 4.2. The temperature dependence of the superconducting gap shows fairly good agreement with the conventional BCS theory. The present results are consistent with those of a moderately strong-coupling and isotropic s-wave superconductor.

DOI： 10.1143/JPSJ.75.064711

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with bulk and surface sensitive high-resolution photoemission spectroscopy using low-energy photons (hv = 7-21 eV), including a VUV laser. A clear suppression of the density of states within similar to 0.2 eV of the Fermi level (E-F) is found. The coherent band in SrVO3 and CaVO3 is shown to consist of surface and bulk-derived features. The results indicate that the stronger distortion on the surface of CaVO3 compared to SrVO3 is directly reflected in the coherent DOS at EF, consistent with recent theory. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.120

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We have performed laser-excited ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPt2)(2)B2C (x = 0.0, 0.2) and numerical simulation using s, anisotropic s and even s + g symmetry to study the superconducting gap anisotropy in borocarbide supercodnuctors. We observed the reduction of gap anisotropy in the x = 0.2 sample in comparison to the x = 0.0 sample, confirming the anisotropic s-wave gap. We also found that the spectrum of x = 0.0 at 3.5 K cannot be fitted well with the s + g-wave symmetry, implying that the s + g symmetry is too simple to describe over all supercodncuting gap electronic Structure of borocarbides. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.204

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with bulk and surface-sensitive high-resolution photoemission spectroscopy, using a vacuum ultraviolet laser, synchrotron radiation, and a discharge lamp (h nu=7-21 eV). A systematic suppression of the density of states (DOS) within similar to 0.2 eV of the Fermi level (E-F) is found on decreasing photon energy, i.e., on increasing bulk sensitivity. The coherent band in SrVO3 and CaVO3 is shown to consist of surface and bulk-derived features, separated in energy. The stronger distortion on surface of CaVO3 compared to SrVO3 leads to a higher surface metallicity in the coherent DOS at E-F, consistent with recent theory.

DOI： 10.1103/PhysRevLett.96.076402

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We have performed a vacuum ultraviolet laser excited photoemission spectroscopy on a d-electron heavy-fermion-like material LiV2O4 single crystal. We observed a sharp peak structure in the density of states at similar to 4 meV above the Fermi level (E-F). The evolution of the peak height corresponds well with the crossover behavior to the heavy-fermion-like state as observed in the thermal and transport properties. The position, shape, and temperature (T) dependence of the peak structure is quite similar to the Kondo resonance observed in conventional f-electron heavy Fermion compounds.

DOI： 10.1103/PhysRevLett.96.026403

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na(0.35)CoO2 center dot 1.3H(2)0 using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of similar to 20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Delta of 0.5-0.7 meV. The energy scale of the SC gap is similar to 20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.10.124

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor-metal boundary, together with the characteristic structures at 150 x n meV possibly due to the strong electron-lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T-c indicative of a superconducting gap opening. We discuss the electron-lattice coupling and the superconductivity in doped diamond. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2006.05.008

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We study the effect of photocarrier injection on the electronic structure of VO2/TiO2:Nb thin films using photoemission spectroscopy. The results indicate that the valence band and core electronic states of VO2 shift systematically to lower binding energy upon photocarrier injection, consistent with doping in a rigid-band picture. The shift in binding energy, and its saturation, follows the known photovoltage behavior. In addition, the V 3d states near E-F exhibit a redistribution/transfer of spectral weight, similar to the temperature dependent insulator to metal transition in VO2. The study provides evidence for hole-doping induced electronic structure changes due to the photocarriers in VO2. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.2132078

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg (B1-x Cx) 2. Two features corresponding to σ and π gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the σ gap is proportional to Tc while the π gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.064527

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We study the bulk electronic structure of Na(x)CoO(2 center dot)yH(2)O using Hard X-ray (HX, hv = 5.95KeV) synchrotron photoelectron spectroscopy (PES). The Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy U-dd similar to 3-5.5 eV. Photon-dependent valence band PES identifies Co 3d and 0 2p derived states, in near agreement with band structure calculations. We discuss the importance of HX-PES for studying correlated transition metal oxides. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nima.2005.05.021

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

We have developed an ultrahigh-resolution photoemission system using a quasi continuous wave (quasi-CW) VUV laser. The photon energy of the VUV laser is 6.994 eV. It has a line width of 260 mu eV and a repetition rate of 80 MHz. The photon energy of this laser is the highest for a quasi-CW laser. Using a quasi-CW laser removes the space charge effect and hence does not degrade the energy resolution. For the electron analyser, we use the newly designed ultrahigh-resolution photoelectron analyser GAMMADATA-SCIENTA R4000. The analyser resolution is about 250 mu eV with a wide collecting angle of about 38 degrees. The total energy resolution of this system is measured to be 360 mu eV for the Fermi edge of gold. Using this system, we have measured several low transition temperature superconductors and succeeded to observe the bulk superconducting state of these materials. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.154

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We used low-temperature ultrahigh-resolution (360 mueV) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an f-electron superconductor CeRu2. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an f-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in CeRu2, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated d- and f-electron superconductors.

DOI： 10.1103/PhysRevLett.94.057001

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

We have studied the temperature (T)-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, Na 0.35CoO 2·1.3H 2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of ∼20 meV in Na 0.35CoO 2·1.3H 2O and Na 0.35CoO 2·0.7H 2O, which is clearly absent in Na 0.7CoO 2. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low T. We discuss implications of the PG in relation to available transport and magnetic susceptibility results. ©2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.020505

Scopus

Publishing type：Research paper (scientific journal)  

High-energy (hv = 5.95 keV) synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of Na0.35CoO2· 1.3H2O, the layered superconductor. In contrast to three-dimensional doped Co oxides, Co 2p core level spectra show well-separated Co3+and Co4+ions. Cluster calculations suggest low-spin Co3+and Co4+character, and a moderate on-site Coulomb correlation energy Udd∼3-5.5 eV. Photon dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band-structure calculations.

DOI： 10.1103/PhysRevB.69.180508

Scopus

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