Angle-resolved Photoemission Spectra Research Articles

Hot electron cooling in graphite: Supercollision versus hot phonon decay

Disorder-assisted electron-phonon scattering processes (supercollision processes) have been reported to dominate the cooling of hot carriers in graphene. Here, we determine to what extent this type of relaxation mechanism governs the hot carrier dynamics in the parent compound graphite. Electron temperature transients derived from time- and angle-resolved extreme ultraviolet photoemission spectra are analyzed based on a three-temperature model which considers electron gas, optical phonons, and acoustic phonons as coupled subsystems. In the probed fluence regime of $0.035--1.4\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$, we find no indications for supercollision processes being involved in the cooling of the hot carriers. The data are, by contrast, compatible with a hot phonon assisted mechanism involving anharmonic coupling between optical phonons and acoustic phonons, a process which has previously been suggested for graphite. We attribute the striking difference to the reported findings for (substrate-supported) graphene to the low defect density of highly ordered pyrolitic graphite.

Angle resolved photoemission from Cu single crystals: Known facts and a few surprises about the photoemission process

We present angle resolved photoemission spectra for Cu(100) and Cu(111) singly crystals in normal emission geometry, taken at tightly spaced intervals for photon energies between 8eV and 150eV. This systematic collection of spectra gives unprecedented insight into the influence of the final states to the photoemission process as well as the band structure and lifetimes of highly excited electrons in Cu.

Quantized massive collective modes and massive spin fluctuations in high-Tc cuprates

We have analyzed angle-resolved photoemission spectra of the single- and double-layered Bi-family high-Tc superconductors by using quantized massive gauge fields, which might contain effects of spin fluctuations, charge fluctuations, and phonons. It is suggested strongly that the quantized massive gauge fields might be mediating Cooper pairing in high-Tc cuprates.

General theory of intraband relaxation processes in heavily doped graphene

The frequency and wave-vector-dependent memory function in the longitudinal conductivity tensor of weakly interacting electronic systems is calculated by using an approach based on quantum transport equations. In this paper, we show that there is a close relation between the single-electron self-energy, the electron-hole pair self-energy, and the memory function. It is also shown in which way singular long-range Coulomb interactions, together with other $\mathbf{q}\ensuremath{\approx}\mathbf{0}$ scattering processes, drop out of both the memory function and the related transport equations. The theory is illustrated on heavily doped graphene, which is the prototype of weakly interacting single-band electron-phonon systems. A steplike increase of the width of the quasiparticle peak in angle-resolved photoemission spectra at frequencies of the order of the frequency of in-plane optical phonons is shown to be consistent with the behavior of an intraband plasmon peak in the energy loss spectroscopy spectra. Both anomalies can be understood as a direct consequence of weak electron scattering from in-plane optical phonons.

Fermi arcs, pseudogap, and collective excitations in dopedSr2IrO4: A generalized fluctuation exchange study

Motivated by recent experimental measurements, we study the quasiparticle spectra and the collective excitations in doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$, in which the interesting interplay between the electronic correlations and strong spin-orbital coupling (SOC) exists. To include the SOC, we use the Hugenholtz diagrams to extend the fluctuation exchange (FLEX) approach to the case where the SU(2) symmetry can be broken. By using this generalized FLEX method, we find a weak pseudogap behavior near $(\ensuremath{\pi},0)$ in the slightly electron-doped system, with the corresponding Fermi arc formed by the partial destruction of Fermi surface. Similar features also appear in the hole-doped system; however, the position of the Fermi arc is rotated ${45}^{\ensuremath{\circ}}$ with respect to the former. These results are consistent with the recent angle-resolved photoemission spectra in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. We suggest that these anomalous phenomena are caused by the scatterings of quasiparticles off the isospin fluctuation derived from the effective ${J}_{\text{eff}}=1/2$ doublet.

Simulation of angle-resolved photoemission spectra by approximating the final state by a plane wave: From graphene to polycyclic aromatic hydrocarbon molecules

We present a computational study on the angular-resolved photoemission spectra (ARPES) from a number of polycyclic aromatic hydrocarbons and graphene. Our theoretical approach is based on ab-initio density functional theory and the one-step model where we greatly simplify the evaluation of the matrix element by assuming a plane wave for the final state. Before comparing our ARPES simulations with available experimental data, we discuss how typical approximations for the exchange-correlation energy affect orbital energies. In particular, we show that by employing a hybrid functional, considerable improvement can be obtained over semi-local functionals in terms of band widths and relative energies of π and σ states. Our ARPES simulations for graphene show that the plane wave final state approximation provides indeed an excellent description when compared to experimental band maps and constant binding energy maps. Furthermore, our ARPES simulations for a number of polycyclic aromatic molecules from the oligo-acene, oligo-phenylene, phen-anthrene families as well as for disc-shaped molecules nicely illustrate the evolution of the electronic structure from molecules with increasing size towards graphene.

Tight-binding parameters of graphite determined with angle-resolved photoemission spectra

An exfoliated Kish graphite sample on a heavily n-doped Si substrate covered with native oxide was prepared with a conventional micromechanical cleavage method. From angle-resolved photoemission spectra (ARPES), we measured the band structure of graphite over photon energies from 28eV to 116eV. The inner potential V0=17.25eV is determined with a period from the band dispersion in the KH direction. A set of parameters in the tight-binding method and the SWMcC model for graphite is extracted from the fitted results. A comparison of constant-energy mapping results at large binding energy indicates the reliability of the tight-binding parameters extracted from the ARPES results.

Epitaxial growth of Cu on Ag(1 1 1) studied with angle-resolved photoemission spectroscopy

The growth of Cu on Ag(111) under varied growth conditions was investigated with angle-resolved photoemission spectra. The evolution of Cu/Ag(111) during annealing was characterized on monitoring its surface states. The surface morphology of a Cu film on Ag(111) depends on the temperature and the Cu coverage. Islands or crystalline films develop when Cu is deposited at ∼300K. Defects in the Cu films penetrate deeply into the Ag(111) substrate and expose the Ag(111) surface. The deposition of Cu at a low temperature results in disordered films. On annealing, the films become ordered with defects. Our results show the segregation of Ag on the Cu surface, which occurs at 300K and becomes accelerated significantly at ∼380K. After being annealed above 430K, all islands and films of Cu are fully covered with Ag, showing a (9×9) reconstruction. Our results indicate also that the segregation of Ag on the Cu surface occurs only after the Ag(111) surface is exposed, indicating that Ag atoms migrate to the Cu(111) surface, not through bulk Cu, but along the walls of the islands and the defects in the films.

Adsorption and desorption of thermally generated hydrogen atoms on Au(111) and Ag/Au(111)

To explore the possibility of employing thermally generated H atoms to determine the chemical composition of a metallic surface, we investigated the adsorption and desorption of H atoms generated with a thermal cracker on surfaces Au(111) and Ag/Au(111). Angle-resolved photoemission spectra showed that the noble-metallic surfaces deteriorated upon exposure to a flux of H atoms at ~ 100 K. Upon subsequent annealing, the order of the surfaces was mostly recovered when H atoms on the surfaces desorbed, but the recovery was incomplete even with annealing at a temperature much higher than that at which H atoms desorb. X-ray photoemission spectra showed that O-containing contaminants existed on the surfaces after the H dosing. The evolution of O 1s during annealing indicated that the O-containing contaminants were H 2 O and its moieties generated during thermal cracking; the disturbances of the surfaces remaining above the desorption temperature of H atoms were likely caused by chemisorbed O. Our results show that it is possible to employ thermally generated H atoms to determine the chemical composition of a metallic surface, but a small proportion of H 2 O in a H 2 gas line might be unavoidable; precautions against possible O contamination are required when a thermal cracker is employed. • The noble-metallic surfaces dosed with thermally generated H atoms are studied. • The order of the H-dosed surfaces is not fully recovered after H atoms desorb. • O-containing contaminants are detected on the H-dosed surfaces. • The contaminants are water and its moieties generated with thermal cracking. • Precautions against O contamination are required when a thermal cracker is employed.

General analysis of the angle-resolved photoemission line shape for strongly correlated electron systems

In many cases the standard perturbation approach appears to be too simple to describe precisely the angle-resolved photoemission spectrum of a strongly correlated electron system. In particular, to describe the momentum asymmetry observed in the photoemission spectra of high-${T}_{c}$ cuprates, a phenomenological approach based on an extremely correlated Fermi-liquid model has been recently introduced. Here we analyze the general structure of the Green's function of quasiparticles in strongly correlated electron systems and stress that it is defined not only by the self-energy of Hubbard quasiparticles but also by a strength operator. The latter leads to an additional odd momentum contribution to the spectral function and alone can explain the observed asymmetry. So, the asymmetry of the angle-resolved photoemission spectra can be a measure of the strength of electron correlations in materials.

Superconductivity. Fermi arcs in a doped pseudospin-1/2 Heisenberg antiferromagnet.

High-temperature superconductivity in cuprates arises from an electronic state that remains poorly understood. We report the observation of a related electronic state in a noncuprate material, strontium iridate (Sr2IrO4), in which the distinct cuprate fermiology is largely reproduced. Upon surface electron doping through in situ deposition of alkali-metal atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments of zero-energy states, known as Fermi arcs, and a gap as large as 80 millielectron volts. Its evolution toward a normal metal phase with a closed Fermi surface as a function of doping and temperature parallels that in the cuprates. Our result suggests that Sr2IrO4 is a useful model system for comparison to the cuprates.

Nodal Landau Fermi-liquid quasiparticles in overdoped La1.77Sr0.23CuO4

Nodal angle-resolved photoemission spectra taken on overdoped La${}_{1.77}$Sr${}_{0.23}$CuO${}_{4}$ are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy $\ensuremath{\Sigma}$, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magnetoresistance, and high-field quantum oscillation experiments on overdoped Tl${}_{2}$Ba${}_{2}$CuO${}_{6+\ensuremath{\delta}}$.

Visualizing the influence of point defects on the electronic band structure of graphene

The supercell approach enables us to treat the electronic structure of defective crystals, but the calculated energy bands are too complicated to understand or compare with angle-resolved photoemission spectra because of inevitable zone folding. We discuss how to visualize supercell band structures more effectively by incorporating unfolded spectral weights and orbital decompositions into them. We then apply these ideas to gain a better understanding of the band structure of graphene containing various types of point defects, including nitrogen impurity, hydrogen adsorbate, vacancy defects and the Stone–Wales defect.

Attosecond evolution of energy- and angle-resolved photoemission spectra in two-color (XUV+IR) ionization of rare gases

We have analyzed the angular distributions of the photoelectrons emitted upon photoionization of rare gases by a comb of harmonics in the extreme ultraviolet range, in the presence of a “dressing” infrared (IR) field with controlled delay ?, stabilized down to about ±60 as. The measurements have been performed with the help of the coincidence momentum imaging technique. We evidence marked differences in the measured angular distributions of the photoelectrons, depending on the number of IR photons exchanged. Joined to a theoretical interpretation, these observations bring new insights into the dynamics of this class of two-color photoionization processes that are a key step towards studying photoionization in the time domain, with attosecond time resolution.

Theory of extreme correlations using canonical Fermions and path integrals

The t–J model is studied using a novel and rigorous mapping of the Gutzwiller projected electrons, in terms of canonical electrons. The mapping has considerable similarity to the Dyson–Maleev transformation relating spin operators to canonical Bosons. This representation gives rise to a non Hermitian quantum theory, characterized by minimal redundancies. A path integral representation of the canonical theory is given. Using it, the salient results of the extremely correlated Fermi liquid (ECFL) theory, including the previously found Schwinger equations of motion, are easily rederived. Further, a transparent physical interpretation of the previously introduced auxiliary Greens function and the ‘caparison factor’, is obtained.The low energy electron spectral function in this theory, with a strong intrinsic asymmetry, is summarized in terms of a few expansion coefficients. These include an important emergent energy scale Δ0 that shrinks to zero on approaching the insulating state, thereby making it difficult to access the underlying very low energy Fermi liquid behavior. The scaled low frequency ECFL spectral function, related simply to the Fano line shape, has a peculiar energy dependence unlike that of a Lorentzian. The resulting energy dispersion obtained by maximization is a hybrid of a massive and a massless Dirac spectrum EQ∗∼γQ−Γ02+Q2, where the vanishing of Q, a momentum type variable, locates the kink minimum. Therefore the quasiparticle velocity interpolates between (γ∓1) over a width Γ0 on the two sides of Q=0, implying a kink there that strongly resembles a prominent low energy feature seen in angle resolved photoemission spectra (ARPES) of cuprate materials. We also propose novel ways of analyzing the ARPES data to isolate the predicted asymmetry between particle and hole excitations.

Chemical-state-resolved depth profiles of Al/Pr0.7Ca0.3MnO3 stacked structures for application in resistive switching devices

We have investigated the interfacial chemical reaction between Al metal electrodes and Pr0.7Ca0.3MnO3 perovskite oxides to shed light on the underlying mechanism of resistive switching phenomenon that can be utilized in random access memory devices. Depth profiles of the Al/Pr0.7Ca0.3MnO3 interface show the formation of Al2O3 layers resulting from redox reactions between Al metal and perovskite oxide. Angle-resolved Mn 2p core-level photoemission spectra, which are independent of the Al metal electrode thickness, show that a Mn-Al alloy is formed at the interface between Al metal and Al2O3 layers during the initial growth of the Al metal electrode. This suggests that the Mn impurity level in the band gap of Al2O3 layers plays an important role in resistive switching characteristics.

Correlated band structure of superconducting NdFeAsO0.9F0.1: Dynamical mean-field study

We report the LDA + DMFT (method combining local density approximation with dynamical mean-field theory) results for spectral properties of the superconductor NdFeAsO0.9F0.1 in the paramagnetic phase. The calculated momentum-resolved spectral functions are in good agreement with angle-resolved photoemission spectra (ARPES). The obtained effective quasiparticle mass enhancement (m*/m = 1.4) is smaller than the one in isostructural parent compound LaFeAsO which critical temperature under the same fluorine doping (LaFeAsO0.9F0.1) is two times lower. Our results demonstrate that in quaternary FeAs-based superconductors of the same class, changes of the crystal structure caused by substitution of one rare-earth atom, implicitly result in reduction of the electronic correlation strength.

Doping dependence of low-energy quasiparticle excitations in superconducting Bi2212

The doping-dependent evolution of the d-wave superconducting state is studied from the perspective of the angle-resolved photoemission spectra of a high-Tc cuprate, Bi2Sr2CaCu2 O8+δ (Bi2212). The anisotropic evolution of the energy gap for Bogoliubov quasiparticles is parametrized by critical temperature and superfluid density. The renormalization of nodal quasiparticles is evaluated in terms of mass enhancement spectra. These quantities shed light on the strong coupling nature of electron pairing and the impact of forward elastic or inelastic scatterings. We suggest that the quasiparticle excitations in the superconducting cuprates are profoundly affected by doping-dependent screening.

Three-dimensional angle-resolved photoemission spectra of EuO thin film

We report the electronic structure of EuO thin films not only parallel to but also perpendicular to the (001) plane using synchrotron radiation angle-resolved photoemission spectroscopy. As a result, the electronic structure perpendicular to the (001) plane is consistent with in-plane electronic structure in spite of the thin films. This result implies that the electronic structure of EuO thin films is consistent with that of the bulk materials.

The ageing effect in topological insulators: evolution of the surface electronic structure of Bi2Se3 upon K adsorption

Topological insulators (TIs) have attracted a lot of interest in recent years due to their topologically protected surface states, as well as exotic proximity-induced phenomena and device applications for TI heterostructures. Since the first experimental studies of TIs, angle-resolved photoemission spectra (ARPES) showed that the electronic structure of the topological surface states significantly changes as a function of time after cleavage. The origin and underlying mechanism of this ageing effect are still under debate, despite its importance. Here we investigate the evolution of the surface Dirac cone for Bi2Se3 films upon asymmetric potassium (K) adsorption, using density-functional theory and a tight-binding model. We find that the K adatoms induce short-ranged downward band bending within 2–3 nm from the surface, due to charge transfer from the adatoms to the TI. These findings are in contrast to earlier proposals in the literature, that propose a long-ranged downward band bending up to 15 nm from the surface. Furthermore, as the charge transfer increases, we find that a new Dirac cone, localized slightly deeper into the TI than the original one, appears at the K-adsorbed surface, originating from strong Rashba-split conduction-band states. Our results suggest possible reinterpretations of experiments because the new Dirac cone might have been observed in ARPES measurements instead of the original one that appears immediately after cleavage. Our findings are consistent with ARPES data and provide insight into building TI-heterostructure devices by varying the band-bending potential or film thickness.