High-resolution Angle-resolved Photoemission Research Articles

ARPES study of the effect of Cu substitution on the electronic structure of NaFeAs

Using high-resolution angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe${}_{1\ensuremath{-}x}$Cu${}_{x}$As ($x=0.019,0.045,0.14$). With increasing the doping concentration, we found that the Cu dopant introduces extra charge carriers. The overall band dispersions barely change with doping, suggesting that the Cu substitution does not affect local correlations. Similar to the case of NaFe${}_{1\ensuremath{-}x}$Co${}_{x}$As, one electron pocket emerges at the Brillouin zone center at high doping levels. Moreover, the near-E${}_{F}$ spectral weight decreases with increasing the Cu dopant, which explains why the NaFe${}_{1\ensuremath{-}x}$Cu${}_{x}$As shows a poor electronical conductivity at high doping levels.

Fermi Surface and Band Structure of (Ca,La)FeAs2 Superconductor from Angle-Resolved Photoemission Spectroscopy

The (Ca,R)FeAs2 (R=La, Pr, etc.) superconductors with a signature of superconductivity transition above 40 K possess a new kind of block layers that consist of zig-zag As chains. We report the electronic structure of the new (Ca,La)FeAs2 superconductor investigated by both band structure calculations and high resolution angle-resolved photoemission spectroscopy measurements. Band structure calculations indicate that there are four hole-like bands around the zone center Γ(0,0) and two electron-like bands near the zone corner M(π, π) in CaFeAs2. In our angle-resolved photoemission measurements on (Ca0.9La0.1)FeAs2, we have observed three hole-like bands around the Γ point and one electron-like Fermi surface near the M(π, π) point. These results provide important information to compare and contrast with the electronic structure of other iron-based compounds in understanding the superconductivity mechanism in the iron-based superconductors.

Superconductivity and bandwidth-controlled Mott metal-insulator transition in 1T-TaS2−xSex

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of layered chalcogenide 1$T$-TaS${}_{2\ensuremath{-}x}$Se${}_{x}$ to elucidate the electronic states especially relevant to the occurrence of superconductivity. We found a direct evidence for a Ta-5$d$-derived electron pocket associated with the superconductivity, which is fragile against a Mott-gap opening observed in the insulating ground state for S-rich samples. In particular, a strong electron-electron interaction-induced Mott gap driven by a Ta 5$d$ orbital also exists in the metallic ground state for Se-rich samples, while finite ARPES intensity near the Fermi level likely originating from a Se 4$p$ orbital survives, indicative of the orbital-selective nature of the Mott transition. Present results suggest that effective electron correlation and $p$-$d$ hybridization play a crucial role to tune the superconductivity and Mott metal-insulator transition.

Electronic structure and Fermi surface of Ru(0 0 0 1) and Ru(1 0 [formula omitted] 0) measured with high-resolution angle-resolved photoemission

We study the Fermi surface and the band structure of Ru(0001) and, for the first time, Ru(101¯0) by high-resolution angle-resolved photoemission spectroscopy using He Iα and He IIα radiation for excitation. The experimental results, though consistent with published data, show several features that were not observed previously. Theoretical calculations done in the framework of the density functional theory and the experimental data allows to derive detailed information about the momentum-resolved electronic structure. Our results can be used as a reference base to understand the chemical and catalytic properties of Ru surfaces.

Disappearance of nodal gap across the insulator–superconductor transition in a copper-oxide superconductor

The parent compound of the copper-oxide high-temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high-temperature superconductors and the superconductivity mechanism. Here we report high-resolution angle-resolved photoemission measurement on heavily underdoped Bi₂Sr₂-xLaxCuO(₆+δ) system. The electronic structure of the lightly doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.

Surface electronic structure for various surface preparations of Nb-doped SrTiO3 (001)

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to study the surface electronic structure of Nb-doped SrTiO3 (STO) single crystals prepared using a variety of surface preparations. ARPES measurements show that simple degreasing with subsequent anneal in vacuum is not an adequate surface preparation of STO, rather, preparations consisting of etching with buffered HF or HCl, and to a lesser extent, simple water leaching resulted in surfaces with much less disorder. A non-dispersing, mid-gap state was found ∼800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present for vacuum-annealed and water-leached samples, as well as for STO thin films grown using molecular beam epitaxy. Theoretical modeling using density functional theory suggests that this mid-gap state is not related to the SrO- and TiO2-terminated surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching.

High-resolution angle-resolved photoemission spectroscopy study of monolayer and bilayer graphene on the C-face of SiC

High-energy and $k$-space resolution angle-resolved photoemission spectroscopy experiments were achieved on nominally single and bilayer graphene grown by Si-flux assisted molecular beam epitaxy (MBE) on the C-face of SiC. This material shows the same structure as the graphene grown by standard high-temperature annealing of SiC, noticeably the rotational disorder and the very weak electronic coupling between stacked layers. The SiC substrate induces a strong doping by charge transfer, with a Dirac point located 320 meV below the Fermi level for monolayer graphene. The efficient screening by the successive graphene layers results in a reduction of this value to 190 meV for bilayer graphene. The opening of an energy band gap, whose width is inversely dependent on the thickness, is also reported. These measurements emphasize the potentialities of the Si-flux assisted MBE technique, more particularly for homogeneous low thickness graphene growth on the C-face of SiC.

Anisotropic Eliashberg function and electron-phonon coupling in doped graphene

We investigate, with high-resolution angle-resolved photoemission spectroscopy, the spectral function of potassium-doped quasi-free-standing graphene on Au. Angle-dependent x-ray photoemission and density functional theory calculations demonstrate that potassium intercalates into the graphene/Au interface, leading to an upshift of the K-derived electronic band above the Fermi level. This empty band is what makes this system perfectly suited to disentangle the contributions to electron-phonon coupling coming from the $\ensuremath{\pi}$ band and K-derived bands. From a self-energy analysis we find an anisotropic electron-phonon coupling strength $\ensuremath{\lambda}$ of 0.1 (0.2) for the $K\ensuremath{\Gamma}$ ($KM$) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in $KM$ does an additional low-energy part appear.

Formation of the Coherent Heavy Fermion Liquid at the Hidden Order Transition inURu2Si2

We present high-resolution angle-resolved photoemission spectra of the heavy-fermion superconductor URu2Si2. Detailed measurements as a function of both photon energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low-energy electronic structure across the "hidden order" transition. Above the transition, our measurements reveal the existence of weakly dispersive states that exhibit a large scattering rate and do not appear to shift from above to below the Fermi level, as previously reported. Upon entering the hidden order phase, these states rapidly hybridize with light conduction band states and transform into a coherent heavy fermion liquid, coincident with a dramatic drop in the scattering rate. This evolution is in stark contrast with the gradual crossover expected in Kondo lattice systems, which we attribute to the coupling of the heavy fermion states to the hidden order parameter.

Direct evidence for a metallic interlayer band in Rb-intercalated bilayer graphene

We have fabricated Rb-intercalated bilayer graphene on 6H-SiC(0001) and performed low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). In LEED experiments, we observed a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 pattern identical to that of the bulk graphite intercalation compound C${}_{8}$Rb. The existence of a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 Rb layer is also confirmed by ARPES, where band folding due to the periodic potential of intercalated atoms was observed. All these results indicate that Rb atoms are intercalated between the two graphene monolayers in a regular manner similar to that of bulk C${}_{8}$Rb. Further, we observed a parabolic metallic band at the Brillouin-zone center, analogous to the so-called interlayer band, which is thought to play an important role for the superconductivity in graphite intercalation compounds.

Momentum-resolved evolution of the Kondo lattice into "hidden order" in URu2Si2.

We study, using high-resolution angle-resolved photoemission spectroscopy, the evolution of the electronic structure in URu2Si2 at the Γ, Z, and X high-symmetry points from the high-temperature Kondo-screened regime to the low-temperature hidden-order (HO) state. At all temperatures and symmetry points, we find structures resulting from the interaction between heavy and light bands related to the Kondo-lattice formation. At the X point, we directly measure a hybridization gap of 11 meV already open at temperatures above the ordered phase. Strikingly, we find that while the HO induces pronounced changes at Γ and Z, the hybridization gap at X does not change, indicating that the hidden-order parameter is anisotropic. Furthermore, at the Γ and Z points, we observe the opening of a gap in momentum in the HO state, and show that the associated electronic structure results from the hybridization of a light electron band with the Kondo-lattice bands characterizing the paramagnetic state.

Anisotropic but Nodeless Superconducting Gap in the Presence of Spin-Density Wave in Iron-Pnictide SuperconductorNaFe1−xCoxAs

The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts strong constraints on theory.

Quasiparticle Mass Enhancement and Temperature Dependence of the Electronic Structure of FerromagneticSrRuO3Thin Films

We report high-resolution angle-resolved photoemission studies of epitaxial thin films of the correlated 4d transition metal oxide ferromagnet SrRuO(3). The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO(3).

Electronic and spin structures of solids investigated by means of synchrotron radiation photoemission

Recent progress in research on electronic and spin structures of solids and instrumentation on spin-resolved photoemission at Hiroshima Synchrotron Radiation Center are reported. The fine details of electron dynamics of a typical multiband superconductor Sr2RuO4 were uncovered by high-resolution angle-resolved photoemission spectroscopy (ARPES) with tunable polarizations, and the surface of W(110) was found to have a Dirac-corn-like state of d character with nearly massless energy dispersion by high-resolution ARPES and spin-resolved ARPES (SARPES). The SARPES system with very low energy electron diffraction spin detector and modified VG-SCIENTA R4000 electron analyzer brought a breakthrough in spin detection efficiency as well as energy and angular resolution, and enables precise SARPES measurements for materials that require high energy and angular resolution.

Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)

We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼170 meV centered ∼65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of the time-reversal symmetry protected topological surface states in Sb2Te3 in the presence of impurities and defects.

High-resolution angle-resolved photoemission study of electronic structure and electron self-energy in palladium

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\ensuremath{\Sigma}}_{1}$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\ensuremath{\sim}\ensuremath{-}20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\ensuremath{\lambda}}_{ep}=0.39\ifmmode\pm\else\textpm\fi{}0.05$ at 8 K for the ${\ensuremath{\Sigma}}_{1}$ band, which is consistent with the theoretical values of ${\ensuremath{\lambda}}_{ep}=0.35--0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}150$ meV. The evaluated electron-paramagnon coupling parameter is ${\ensuremath{\lambda}}_{em}\ensuremath{\sim}0.06$ for the ${\ensuremath{\Sigma}}_{1}$ band. We found that the magnitudes of ${\ensuremath{\lambda}}_{ep}$ and ${\ensuremath{\lambda}}_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\ensuremath{\lambda}}_{ep}+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{em}\ensuremath{\sim}1.5$ for the ${\ensuremath{\Sigma}}_{1}$ band, which is close to the values ${m}^{*}/{m}_{b}\ensuremath{\sim}1.5--1.7$ given by the de Haas--van Alphen and electron specific-heat measurements.

Angle-resolved photoemission and quasiparticle calculation of ZnO: The need fordband shift in oxide semiconductors

ZnO is a prototypical semiconductor with occupied ${d}^{10}$ bands that interact with the anion $p$ states and is thus challenging for electronic structure theories. Within the context of these theories, incomplete cancellation of the self-interaction energy results in a Zn $d$ band that is too high in energy, resulting in upwards repulsion of the valence band maximum (VBM) states, and an unphysical reduction of the band gap. Methods such as GW should significantly reduce the self-interaction error, and in order to evaluate such calculations, we measured high-resolution and resonant angle-resolved photoemission spectroscopy (ARPES) and compared these to several electronic structure calculations. We find that, in a standard GW calculation, the $d$ bands remain too high in energy by more than 1 eV irrespective of the Hamiltonian used for generating the input wave functions, causing a slight underestimation of the band gap due to the $p\ensuremath{-}d$ repulsion. We show that a good agreement with the ARPES data over the full valence band spectrum is obtained, when the Zn-$d$ band energy is shifted down by applying an on-site potential ${V}_{d}$ for Zn-$d$ states during the GW calculations to match the measured $d$ band position. The magnitude of the GW quasiparticle energy shift relative to the initial density functional calculation is of importance for the prediction of charged defect formation energies, band-offsets, and ionization potentials.

Electron-phonon coupling on strained Ge/Si(111)-(5×5) surfaces

We investigate the structural and electronic properties of strained Ge/Si(111)-($5\ifmmode\times\else\texttimes\fi{}5$) surfaces by means of scanning tunneling microscopy and high-resolution angle-resolved photoemission spectroscopy. The homogeneous ($5\ifmmode\times\else\texttimes\fi{}5$) reconstructed overlayers are characterized by three electronic surface states, similar to the Si(111)-($7\ifmmode\times\else\texttimes\fi{}7$) surface. The dispersion of the dangling bond related surface state exhibits the same periodicity as that of the ($5\ifmmode\times\else\texttimes\fi{}5$) reconstruction. Moreover, a careful analysis of the shape and width of this surface state provides striking evidence of electron-phonon coupling at low temperatures. By considering the spectral function within a simple Debye model, we determine both the Debye energy and the electron-phonon coupling strength. The latter value is further confirmed by analyzing the temperature-dependent phonon broadening of the dangling bond related surface state linewidth.

A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy

We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ~7 eV, delivering under typical conditions >10(12) ph/s within a 10 meV spectral bandwidth, which is comparable to helium plasma lamps and many synchrotron beamlines. We first describe the lamp and monochromator system and then characterize its output, with attention to those parameters which are of interest for photoemission experiments. Finally, we present angle-resolved photoemission spectroscopy data using the light source and compare its performance to a conventional helium plasma lamp.

Real-Space Coexistence of the Melted Mott State and Superconductivity in Fe-Substituted1T−TaS2

We have performed high-resolution angle-resolved photoemission spectroscopy of layered chalcogenide 1T-Fe(x)Ta(1-x)S(2) which undergoes a superconducting transition in the nearly commensurate charge-density-wave phase (melted Mott phase). We found a single electron pocket at the Brillouin-zone center in the melted Mott phase, which is created by the backfolding of bands due to the superlattice potential of charge-density-wave. This electron pocket appears in the x region where the samples show superconductivity, and is destroyed by the Mott- and Anderson-gap opening. Present results suggest that the melted Mott state and the superconductivity coexist in real space, providing a new insight into the interplay between electron correlation, charge order, and superconductivity.