High-resolution Angle-resolved Photoemission Study Research Articles

Lifetime of quasiparticles in the nearly free electron metal sodium

We report a high-resolution angle-resolved photoemission (ARPES) study of the prototypical nearly-free-electron metal sodium. The observed mass enhancement is slightly smaller than that derived in previous studies. The new results on the lifetime broadening increase the demand for theories beyond the random phase approximation. Our results do not support the proposed strong enhancement of the scattering rates of the charge carriers due to a coupling to spin fluctuations. Moreover, a comparison with earlier electron energy-loss data on sodium yields a strong reduction of the mass enhancement of dipolar electron-hole excitations compared to that of monopole hole excitations, measured by ARPES.

High-resolution angle-resolved photoemission study of large magnetoresistance topological semimetal CaAl4 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774421, 12074425, 11874422, 11574391, and 11774424), the National Key Research and Development Program of China (Grant No. 2019YFA0308602), and the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China

Extremely large magnetoresistance (XMR) has been explored in many nonmagnetic topologically nontrivial/trivial semimetals, while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures. In this paper, the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated, exhibiting unsaturated magnetoresistance of ∼ 3000% at 2.5 K and 14 T as the fingerprints of XMR materials. By the combination of ARPES and the first-principles calculations, we elaborate multiband features and anisotropic Fermi surfaces, which can explain the mismatch of isotropic two-band model. Although the structural phase transition from I4/mmm to C2/m has been recognized, the subtle impact on electronic structure is revealed by our ARPES measurements. Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4, our findings report CaAl4 as a new reference material for exploring the XMR phenomena.

High-resolution angle-resolved photoemission study of oxygen adsorbed Fe/MgO(001)**Present address: Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Science and Technology, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan

We have investigated the electronic states of clean Fe(001) and oxygen adsorbed Fe(001)–p(1 × 1)-O films epitaxially grown on MgO(001) substrates by means of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and extensive density-functional theory (DFT) calculations. The observed Fermi surfaces and band dispersions of pure Fe near the Fermi level were modified upon oxygen adsorption. By the detailed comparison of ARPES and DFT results of the oxygen adsorbed Fe surface, we have clarified the orbital-dependent p–d hybridization in the topmost and second Fe layers. Furthermore, the observed energy levels and Fermi wave numbers for the oxygen adsorbed Fe surface were deviated from the DFT calculations depending on the orbital characters and momentum directions, indicating an anisotropic interplay of the electron correlation and p–d hybridization effects in the surface region.

Temperature-Dependent Electron-Electron Interaction in Graphene on SrTiO3.

The electron band structure of graphene on SrTiO3 substrate has been investigated as a function of temperature. The high-resolution angle-resolved photoemission study reveals that the spectral width at Fermi energy and the Fermi velocity of graphene on SrTiO3 are comparable to those of graphene on a BN substrate. Near the charge neutrality, the energy-momentum dispersion of graphene exhibits a strong deviation from the well-known linearity, which is magnified as temperature decreases. Such modification resembles the characteristics of enhanced electron-electron interaction. Our results not only suggest that SrTiO3 can be a plausible candidate as a substrate material for applications in graphene-based electronics but also provide a possible route toward the realization of a new type of strongly correlated electron phases in the prototypical two-dimensional system via the manipulation of temperature and a proper choice of dielectric substrates.

Spectroscopic studies ofCdTe(111)bulk and surface electronic structure

Cadmium telluride (CdTe) is a direct band-gap semiconducting material with broad applications in optoelectronic devices. Here we report on a high-resolution angle-resolved photoemission (ARPES) study of $\mathrm{CdTe}(111)$ surfaces prepared by sputtering and annealing that show a $(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction as observed by electron diffraction. The ARPES maps along high-symmetry directions show prominent features with their intensities modulated by varying the incident photon energy, thus suggesting important matrix element effects associated with photoemission. The results are in excellent agreement with first-principles calculations of the bulk band structure and one-dimensional density of states. A prominent surface state is observed that exhibits a $(2\ifmmode\times\else\texttimes\fi{}2)$ periodicity in agreement with the symmetry of the surface reconstruction.

Electronic structure of transition metal dichalcogenides PdTe2 and Cu0.05PdTe2 superconductors obtained by angle-resolved photoemission spectroscopy**Project supported by the National Natural Science Foundation of China (Grant No. 11190022), the National Basic Research Program of China (Grant Nos. 2011CB921703 and 2011CBA00110), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020300).

The layered transition metal chalcogenides have been a fertile land in solid state physics for many decades. Various MX2-type transition metal dichalcogenides, such as WTe2, IrTe2, and MoS2, have triggered great attention recently, either for the discovery of novel phenomena or some extreme or exotic physical properties, or for their potential applications. PdTe2 is a superconductor in the class of transition metal dichalcogenides, and superconductivity is enhanced in its Cu-intercalated form, Cu0.05PdTe2. It is important to study the electronic structures of PdTe2 and its intercalated form in order to explore for new phenomena and physical properties and understand the related superconductivity enhancement mechanism. Here we report systematic high resolution angle-resolved photoemission (ARPES) studies on PdTe2 and Cu0.05PdTe2 single crystals, combined with the band structure calculations. We present in detail for the first time the complex multi-band Fermi surface topology and densely-arranged band structure of these compounds. By carefully examining the electronic structures of the two systems, we find that Cu-intercalation in PdTe2 results in electron-doping, which causes the band structure to shift downwards by nearly 16 meV in Cu0.05PdTe2. Our results lay a foundation for further exploration and investigation on PdTe2 and related superconductors.

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).

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.

Coherent Heavy Quasiparticles in aCePt5Surface Alloy

We report on the results of a high-resolution angle-resolved photoemission study on the ordered surface alloy CePt(5). The temperature dependence of the spectra show the formation of the coherent low-energy heavy-fermion band near the Fermi level. These experimental data are supported by a multiband model calculation in the framework of the dynamical mean-field theory.

Unconventional electronic reconstruction in undoped(Ba,Sr)Fe2As2across the spin density wave transition

Through a systematic high-resolution angle-resolved photoemission study of the iron pnictide compounds $(\text{Ba},\text{Sr}){\text{Fe}}_{2}{\text{As}}_{2}$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of $0.5{\ensuremath{\mu}}_{\text{B}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

High-resolution angle-resolved photoemission studies of quasiparticle dynamics in graphite

We obtained the spectral function of the graphite $H$ point using high-resolution angle-resolved photoelectron spectroscopy (ARPES). The extracted width of the spectral function (inverse of the photohole lifetime) near the $H$ point is approximately proportional to the energy as expected from the linearly increasing density of states (DOS) near the Fermi energy. This is well accounted for by our electron-phonon coupling theory considering the peculiar electronic DOS near the Fermi level. We also investigated the temperature dependence of the peak widths both experimentally and theoretically. The upper bound for the electron-phonon coupling parameter is 0.23, nearly the same value as previously reported at the $K$ point. Our analysis of temperature-dependent ARPES data at $K$ shows that the energy of a phonon mode of graphite has a much higher energy scale than 125 K, which is dominant in electron-phonon coupling.

The angle-resolved photoemission study of the quasi-one-dimensional chain cuprate LiCu2O2

We have performed a high resolution angle-resolved photoemission study on edge-sharing one-dimensional (1D) chain cuprate LiCu2O2 single crystals at room temperature. The crystals were grown by the floating zone technique in air and cleaved in-situ at a pressure better than 9x10-11 Torr. The low energy electron diffraction (LEED) of cleaved (001) surfaces show a well ordered (2x1) pattern with twin domains. The normal emission spectra were collected at various photon energies. While bands of deeper binding energies display a clear dispersion with photon energies disclosing a coupling along the c-axis, the bands nearest the Fermi energy shows no such dependence revealing a localized character within the ab-plannar layers. Off-normal emission measurements were carried out along the high symmetry directions. Several dispersive bands near the Fermi energy are observed. The highest energy band can be identified as copper 3dxy states strongly hybridized with oxygen 2p states.

High-resolution angle-resolved photoemission study of bulk electronic states in [formula omitted

We have studied low-energy electronic structure of YBa 2 Cu 3 O 7 - δ using high-resolution angle-resolved photoemission spectroscopy (ARPES). By carefully tuning photon energy and polarization of incident light, we identified five different bands in the vicinity of the Fermi level, those are surface-bilayer-derived bonding and antibonding bands, their bulk counterparts, and CuO-chain-derived band, respectively. The surface bands show metallic property reflecting highly overdoped character, while bulk bands show signature of a d x 2 - y 2 -wave superconducting gap opening. The present ARPES result suggests the universality of the bulk electronic states in bilayer high- T c superconductors.

The angle-resolved photoemission study for ultrathin NiO and CoO thin films on Ag(1 0 0) surfaces

We have performed a high resolution angle-resolved photoemission study on ultrathin NiO and CoO films on Ag(1 0 0) surfaces. The NiO and CoO thin films were grown on Ag(1 0 0) at room temperature and 180 ∘ C , respectively, for an O 2 pressure of 7.5 × 10 - 7 Torr . The thickness of thin film was determined by a quartz monitor and RHEED. A clearest ( 2 × 1 ) low-energy electron diffraction (LEED) pattern was observed at a NiO coverage of 0.9 monolayer(ML), consistent with literature. LEED shows a clear ( 1 × 1 ) pattern indicating well-ordered domains for the 2 ML NiO and CoO thin films. Angler-integrated photoemission spectra (AIPES) show a finite density of states at the Fermi level for the 2 ML NiO thin film, suggesting a metallic character, while the 2 ML CoO thin film was insulating. Angle-resolved photoemission spectroscopy (ARPES) was used to detect the band dispersion of 2 ML NiO and CoO thin films. Very different band dispersions of the valence bands were observed. The insulating nature of bulk NiO and CoO is related to their antiferromagnetic order, but the unusual metallic behavior of ultra NiO thin film may affect its magnetic order. The detail comparison of electronic structure for 2 ML NiO and CoO thin films will be discussed.

Evidence for an Excitonic Insulator Phase in1T−TiSe2

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2.

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the Γ ¯ point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9–27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron–electron interaction in the SS.

High-resolution photoemission study of electron–electron interaction in the Ni(1 1 1) surface state

High-resolution angle-resolved photoemission study of the surface state in ferromagnetic Ni(1 1 1) has been performed to evaluate the strength of many-body interactions. We could not observe a kink structure in the energy band dispersion derived from the electron–phonon interaction. The imaginary part of the self-energy was evaluated; it increases significantly with increasing energy, indicating a strong electron–electron interaction.

The electronic structure of ultrathin NiO films on Ag(1 0 0) surfaces studied by high resolution angle-resolved photoemission spectroscopy

We have performed a high resolution angle-resolved photoemission study on ultrathin NiO films on Ag(1 0 0) surfaces. The NiO thin films were prepared on Ag(1 0 0) at room temperature by Ni deposition in O 2 atmosphere. The thickness of NiO thin films were determined by a quartz monitor and reflection high energy electron diffraction (RHEED). The low-energy electron diffraction (LEED) result shows a two-domain ( 2 × 1 ) pattern for the 0.9 monolayer (ML) NiO thin film at the optimum O 2 pressure of 7.5 × 10 - 7 Torr at room temperature with subsequent annealing to 450 K. The ( 2 × 1 ) pattern evolves to a ( 1 × 1 ) pattern upon further annealing to 620 K in the presence of O 2 or upon increasing film thickness. Photoemission spectra show finite density of states at Fermi level for 0.9 ML ( 2 × 1 ) NiO and 2 ML ( 1 × 1 ) NiO on Ag(1 0 0). Angle-resolved photoemission spectroscopy was used to measure the band dispersion of the 0.9 and 2 ML NiO thin films on Ag(1 0 0), as well as of a 10 ML film. The band dispersion of the 10 ML thick film was similar to that of the bulk crystals. Weak dispersion of valence bands was observed on the ( 2 × 1 ) NiO thin film, and it was very different from that on the ( 1 × 1 ) for 0.9 ML and 2 ML NiO thin films. The interpretation of the electronic structure of these thin films is presented.

High-resolution angle-resolved photoemission study of the spin-polarized Fermi surface of Ni(1 0 0)

A high-resolution angle-resolved photoemission spectroscopy study of Ni(1 0 0) has been performed to elucidate the Fermi surfaces (FSs) in the ΓXWK plane. We observed three bulk-derived FSs around the Γ point, and two closed FSs around the X point. The observed bulk-derived FSs were explained reasonably well by a first-principles band-structure calculation with the local-density approximation.

High-resolution photoemission study of electron–phonon interaction in Pd(1 1 0)

High-resolution angle-resolved photoemission study of Pd(1 1 0) single crystal has been done to evaluate the strength of electron–phonon interaction. We found a clear kink structure in the energy-band dispersion at E ∼ - 30 meV , which corresponds well to the energy scale of the Debye temperature. The obtained real part of the self-energy has been explained well by the self-energy due to the electron–phonon interaction. We have evaluated the electron–phonon coupling constant λ = 0.41 ± 0.03 for the s-like Fermi surface around the Γ point.