Angle-resolved Photoelectron Spectroscopy Spectra Research Articles

Band structure of a nonparabolic two-dimensional electron gas system

Using angle-resolved photoelectron spectroscopy (ARPES), we study the band structures of two-dimensional electron gases (2DEGs) formed at Te-doped surfaces of InAs(110), as a result of band bending. The selected surface system is very clean and stable; therefore, ARPES spectra may be registered in short times, with minimal adsorption of residual gases and no photon beam-induced damage. We record a set of data that allows for a detailed analysis of the 2DEG band shapes, their thermal dependencies, and for an assessment of the many-body interactions. We find the electron-phonon interaction extremally weak and the electron-electron interactions hidden in dominant electron-donor interactions. While 2DEG band shapes are observed to be variable in correlation with the 2DEG density, this is tracked down to the variability of the band-bending potential and nonparabolicity of the system, and not to the many-body interactions. Thus, many-body renormalization of the 2DEG bands is not required, and the bands are described well by using the one-electron theoretical frame.

Surface valence transition in SmS by alkali metal adsorption

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm$^{2+}$ to Sm$^{3+}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons are trapped by isolated excitons, which is inconsistent with the phase transition from the black insulator with Sm$^{2+}$ to the gold metal with Sm$^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Electronic structure of ThPd2Al3 : Impact of the U 5f states on the electronic structure of UPd2Al3

The electronic structure of ThPd$_2$Al$_3$, which is isostructural to the heavy fermion superconductor UPd$_2$Al$_3$, was investigated by photoelectron spectroscopy. The band structure and Fermi surfaces of ThPd$_2$Al$_3$ were obtained by angle-resolved photoelectron spectroscopy (ARPES), and the results were well-explained by the band-structure calculation based on the local density approximation. The comparison between the ARPES spectra and the band-structure calculation suggests that the Fermi surface of ThPd$_2$Al$_3$ mainly consists of the Al $3p$ and Th $6d$ states with a minor contribution from the Pd $4d$ states. The comparison of the band structures between ThPd$_2$Al$_3$ and UPd$_2$Al$_3$ argues that the U $5f$ states form Fermi surfaces in UPd$_2$Al$_3$ through hybridization with the Al $3p$ state in the Al layer, suggesting that the Fermi surface of UPd$_2$Al$_3$ has a strong three-dimensional nature.

Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures

Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures.

Electronic phase separation at LaAlO3/SrTiO3 interfaces tunable by oxygen deficiency

Electronic phase separation is crucial for the fascinating macroscopic properties of the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics -- momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface -- of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{t}_{2\mathrm{g}}$ electrons coexist with localized strongly correlated $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized $\mathrm{Ti}\phantom{\rule{0.28em}{0ex}}{e}_{\mathrm{g}}$ electrons, and repelling of the mobile ${t}_{2\mathrm{g}}$ electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

Electronic band dispersion determination in azimuthally disordered transition-metal dichalcogenide monolayers

Generally, the lack of long-range order in materials prevents from experimentally addressing their electronic band dispersion by angle-resolved photoelectron spectroscopy (ARPES), limiting such assessment to single crystalline samples. Here we demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide (TMDC) monolayers with 2 H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. We exemplify this by analyzing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. A robust base for investigating TMDC monolayers significantly beyond single crystal samples is thus established.

Electronic structures of UX3 ( X=Al , Ga, and In) studied by photoelectron spectroscopy

The electronic structures of UX$_3$ (X=Al, Ga, and In) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl$_3$ and UGa$_3$ were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl$_3$ and UGa$_3$ were explained reasonably well by the calculation, although bands near the Fermi level ($E_\mathrm{F}$) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl$_3$ and UGa$_3$ are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa$_3$ in the paramagnetic and antiferromagnetic phases, suggesting that UGa$_3$ is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of UX$_3$ compounds was also studied by utilizing the smaller lattice constants of UAl$_3$ and UGa$_3$ than that of UIn$_3$. The valence band spectrum of UIn$_3$ is accompanied by a satellite-like structure on the high-binding-energy side. The core-level spectrum of UIn$_3$ is also qualitatively different from those of UAl$_3$ and UGa$_3$. These findings suggest that the U~$5f$ states in UIn$_3$ are more localized than those in UAl$_3$ and UGa$_3$.

Electronic structure ofThRu2Si2studied by angle-resolved photoelectron spectroscopy: Elucidating the contribution of U5fstates inURu2Si2

The electronic structure of ${\mathrm{ThRu}}_{2}{\mathrm{Si}}_{2}$ was studied using angle-resolved photoelectron spectroscopy (ARPES) with incident photon energies of $h\ensuremath{\nu}=655$--745 eV. Detailed band structure and the three-dimensional shapes of Fermi surfaces were derived experimentally, and their characteristic features were mostly explained by means of band-structure calculations based on density-functional theory. Comparison of the experimental ARPES spectra of ${\mathrm{ThRu}}_{2}{\mathrm{Si}}_{2}$ with those of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ shows that they have considerably different spectral profiles, particularly in the energy range of $1\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ from the Fermi level, suggesting that $\mathrm{U}\phantom{\rule{4pt}{0ex}}5f$ states are substantially hybridized in these bands. The relationship between the ARPES spectra of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ and ${\mathrm{ThRu}}_{2}{\mathrm{Si}}_{2}$ is very different from the one between the ARPES spectra of ${\mathrm{CeRu}}_{2}{\mathrm{Si}}_{2}$ and ${\mathrm{LaRu}}_{2}{\mathrm{Si}}_{2}$, where the intrinsic difference in their spectra is limited only in the very vicinity of the Fermi energy. The present result suggests that the $\mathrm{U}\phantom{\rule{4pt}{0ex}}5f$ electrons in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ have strong hybridization with ligand states and have an essentially itinerant character.

Electronic Structure of EuAl4 Studied by Photoelectron Spectroscopy

The electronic structure of the divalent Eu compound EuAl4, which shows a charge density wave transition at TCDW = 140 K, was studied by hard X-ray angle-integrated photoelectron spectroscopy (HAXPES) and soft X-ray angle-resolved photoelectron spectroscopy (ARPES). The valence band and core-level spectra obtained by HAXPES are consistent with the divalent nature of Eu atoms in EuAl4. From the ARPES results, the Fermi surface as well as band structure in the vicinity of the Fermi energy (EF) of EuAl4 are very similar to those of its isostructural divalent Sr compound SrAl4, which has no 4f electrons. This suggests that the Eu atoms are divalent in EuAl4, and the 4f electrons are localized below 1.8 eV with the Eu 4f7 electronic configuration in the ground state. The ARPES spectra measured along the Γ–(Σ)–Z high-symmetry line did not show significant temperature dependences above and below TCDW within the energy resolution of 80–90 meV. Moreover, the Fermi surface mapping along the kz direction showed that...

Electronic structure of reconstructed InAs(001) surfaces — identification of bulk and surface bands based on their symmetries

Using angle-resolved photoelectron spectroscopy (ARPES) band structures of indium- and arsenic-terminated InAs(001) surfaces are investigated. These surfaces are highly reconstructed, elementary cells of their lattices contain many atoms in different chemical configurations, and moreover, they are composed of domains having related but different reconstructions. These domain-type surface reconstructions result in the reciprocal spaces containing regions with well-defined k→∥-vector and regions with not-well-defined one. In the ARPES spectra most of the surface related features appear as straight lines in the indeterminate k→∥-vector space. It is shown that, thanks to differences in crystal and surface symmetries, the single photon energy ARPES may be successfully used for classification of surface and bulk bands of electronic states on complex, highly reconstructed surfaces instead of the most often used variable photon energy studies.

Itinerant nature of U5fstates in uranium mononitride revealed by angle-resolved photoelectron spectroscopy

The electronic structure of the antiferromagnet uranium nitride (UN) has been studied by angle-resolved photoelectron spectroscopy (ARPES) using soft x-rays ($h\ensuremath{\nu}=420$--520 eV). Strongly dispersive bands with large contributions from the U $5f$ states were observed in ARPES spectra and form Fermi surfaces. The band structure as well as the Fermi surfaces in the paramagnetic phase are well explained by the band-structure calculation treating all the U $5f$ electrons as being itinerant, suggesting that an itinerant description of the U $5f$ states is appropriate for this compound. On the other hand, changes in the spectral function due to the antiferromagnetic transition were very small. The shapes of the Fermi surfaces in a paramagnetic phase are highly three-dimensional, and the nesting of Fermi surfaces is unlikely as the origin of the magnetic ordering.

Surface atomic and electronic structure of Mn5Ge3on Ge(111)

The atomic and electronic structure of the Mn${}_{5}$Ge${}_{3}$(001) surface grown on Ge(111) $c$($2\ifmmode\times\else\texttimes\fi{}8$) has been studied in detail by angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy. ARPES spectra recorded from the $\overline{\ensuremath{\Gamma}}$-$\overline{K}$-$\overline{M}$ and $\overline{\ensuremath{\Gamma}}$-$\overline{M}$-$\overline{\ensuremath{\Gamma}}$ directions of the surface Brillouin zone show six surface-related features. The STM images recorded at biases higher/lower than $\ifmmode\pm\else\textpm\fi{}$0.4 V always show a honeycomb pattern with two bright protrusions in each unit cell. At lower biases, a hexagonal, intermediate transition, and a honeycomb pattern are observed. These can be explained as arising from Mn and Ge atoms in the sublayer arranged in triangular structures and Mn atoms in the top layer arranged in a honeycomb structure, respectively. The photoemission and STM data from the germanide surface are discussed and compared to earlier published theoretical, photoelectron spectroscopy, and scanning tunneling microscopy studies.

Time and angle resolved photoemission spectroscopy using femtosecond visible and high-harmonic light

The angle resolved photoelectron spectroscopy (ARPES) has emerged as a leading technique in identifying static key properties of complex systems such as the electronic band structure of adsorbed molecules, ultrathin quantum-well films or high temperature superconductors. We efficiently combined ARPES by using a two-dimensional analyzer for parallel energy (E) and momentum (k||) detection with femtosecond time-resolved spectroscopies. Using time and angle resolved two photon photoemission (2PPE) with visible light pulses, the hot electron dynamics in complex electronic structures are directly accessible by means of angle resolved hot electron lifetime mapping. Furthermore, femtosecond ARPES spectra recorded with high harmonic generation (HHG) light pulses are presented, showing the potential of this technique for future investigations of surface dynamics and photo-induced phase transition processes.

Fermi surface nesting in several transition metal dichalcogenides

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the CuxNbS2 family of materials.

Pb chains on ordered Si(3 3 5) surface

Abstract The electronic band structure of the Si(3 3 5)–Au surface decorated with Pb atoms was studied with angle resolved photoelectron spectroscopy (ARPES) in ultra high vacuum (UHV) conditions. The photoemission spectra were measured in two perpendicular directions, along and across the steps. In the direction parallel to the step edges the ARPES spectra show strongly dispersive electron energy band while in the perpendicular direction there is no electronic dispersion at all. This confirms one-dimensional character of the system. The theoretical band dispersion calculated within a tight-binding model was fitted to that obtained from the experiment.

Angle-resolved photoemission study of electronic structure development during Ag growth on the Si(001) surface

Angle-resolved photoelectron spectroscopy (ARPES) study of the initial Ag growth on the Si(001)2×1 surface has been performed using synchrotron radiation for Ag coverage up to 7.5 monolayer (ML) at room temperature. The evolution of Ag 4d and near-Fermi level valence band structures is investigated, which is interpreted consistently with the Stranski–Krastanov growth of Ag with an intermediate layer of roughly 1 ML. The ARPES spectra of the two-dimensional intermediate layer, the Si(001)2×1–Ag surface, are measured in more detail. This reveals three intrinsic surface-state bands and indicates the semiconducting nature of the intermediate layer. Two of these surface states at binding energies of ∼0.9 and ∼1.8 eV exhibit a close resemblance to the surface states of alkali-covered Si(001)2×1 surfaces. Based on these results the nature of bonding between Ag and Si for the initial adsorption is discussed.

Laser-induced band-bending variation on room-temperature CdTe(110)1 x 1 surfaces observed in photoemission and through the Franz-Keldish effect in surface differential reflectivity.

A laser-induced band-bending variation on ultrahigh vacuum cleaved CdTe(110)1\ifmmode\times\else\texttimes\fi{}1 surfaces kept at room temperature has been observed both in angle-resolved photoelectron spectroscopy (ARPES) and, through the Franz-Keldish (FK) effect, in surface-differential reflectivity. Through ARPES spectra, the shift of the Fermi level, between the sample obscured and illuminated with an ${\mathrm{Ar}}^{+}$ laser power of 500 mW has been determined to be 0.2 eV. The FK effect has been observed at the ${\mathit{E}}_{0}$ bulk critical point and studied as a function of laser power, showing a linear increase in its oscillation amplitude up to a power of 400 mW. Upon switching off the laser, the FK effect disappears exponentially with a time constant of about 1300 s. A surface photovoltage induced by the helium lamp has also been observed.

Angle-resolved photoemission study of a single-domain Si(001)2 × 1-Na surface with synchrotron radiation

The electronic structures of a single-domain Si(001)2 × 1-Na surface at a saturation coverage have been studied by angle-resolved photoelectron spectroscopy (ARPES) with linearly polarized synchrotron radiation (SR). To determine the dispersion of surface states in detail, the ARPES spectra have been measured along several symmetry axes of the 2 × 1 surface Brillouin zone. The result revealed a new assignment of surface-state dispersions. Symmetry properties of upper surface-state bands are determined using a linearly polarized character of SR. It has been found that the upper two surface states have the same symmetries as the dangling bond of the clean Si(001)2 × 1 dimer surface. The electronic structure of the Si(001)2 × 1-Na surface determined here is very similar to that of the Si(001)2 × 1-K surface [Surf. Sci. 261 (1992) 217] and is consistent with that expected for an alkali double-layer model.

Angle-resolved photoemission study of a single-domain Si(001)2 × 1-K surface with synchrotron radiation: symmetry and dispersion of surface-states

The electronic structures of a single-domain Si(001)2 × 1-K surface at saturation coverage have been studied by synchrotron radiation (SR) angle resolved photoelectron spectroscopy (ARPES). The ARPES spectra have been measured along several symmetry axes of the 2 × 1 surface Brillouin zone and surface state dispersions are depicted. Symmetry properties of upper surface-state bands are determined using the linearly polarized character of SR. The symmetries and dispersions of the upper surface states determined here are consistent with those expected for a K double-layer model.

Surface dependence of the Cr(001) 3s photoemission line shape.

An angle-resolved photoelectron spectroscopy (ARPES) study of the Cr(001) 3s core level is reported. The relative intensities of the exchange-split photoelectron components change as the ARPES spectra become more surface sensitive. Surface contamination suppresses these intensity variations, and decreases the energy width of the 3s spectral profile. Almost all of the spectral observations are consistent with a surface-sensitive enhancement of the atomic 3d spin at the Cr(001) surface.