Angle-resolved Photoemission Spectra Research Articles

Robust surface electronic properties of topological insulators: Bi2Te3 films grown by molecular beam epitaxy

The surface electronic properties of the important topological insulator Bi2Te3 are shown to be robust under an extended surface preparation procedure which includes exposure to atmosphere and subsequent cleaning and recrystallization by an optimized in-situ sputter-anneal procedure under ultra high vacuum conditions. Clear Dirac-cone features are displayed in high-resolution angle-resolved photoemission spectra from the resulting samples, indicating remarkable insensitivity of the topological surface state to cleaning-induced surface roughness.

Peculiar Linear Dispersive Bands Observed in Angle-Resolved Photoemission Spectra of Tl-Based Ternary Chalcogenide TlGaTe2

Peculiar Linear Dispersive Bands Observed in Angle-Resolved Photoemission Spectra of Tl-Based Ternary Chalcogenide TlGaTe₂

Two-Photon Photoemission Study of the Coverage-Dependent Electronic Structure of Chemisorbed Alkali Atoms on a Ag(111) Surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].

Real-space multiple scattering method for angle-resolved photoemission and valence-band photoelectron diffraction and its application to Cu(111)

A computational method is presented for angle-resolved photoemission spectra (ARPES) and photoelectron diffraction (PED) in the ultraviolet regime. The one-step model is employed and both initial valence and final continuum states are calculated using the finite-cluster, real-space multiple scattering method. Thereby the approach is versatile and provides a natural link to core-level PED. The method is applied to the Cu(111) valence band and good agreement with experiment is found for both ARPES spectra and PED patterns. When the PED patterns are integrated over a filled band of a single-orbital symmetry, such as Cu-3$d$, we show, both numerically and analytically, that the exact theory with delocalized initial states can be replaced by the much simpler, core-level-type theory where the initial states are taken as localized.

The pairing mechanism of high-temperature superconductivity: experimental constraints

Developing a theory of high-temperature superconductivity in copper oxides is one of the outstanding problems in physics. It is a challenge that has defeated theoretical physicists for more than 20 years. Attempts to understand this problem are hindered by the subtle interplay among a few mechanisms and the presence of several nearly degenerate and competing phases in these systems. Here, we present some crucial experiments that place essential constraints on the pairing mechanism of high-temperature superconductivity. The observed unconventional oxygen-isotope effects in cuprates have clearly shown strong electron–phonon interactions and the existence of polarons and/or bipolarons. Angle-resolved photoemission and tunneling spectra have provided direct evidence for strong coupling to multiple-phonon modes. In contrast, these spectra do not show strong coupling features expected for magnetic resonance modes. Angle-resolved photoemission spectra and the oxygen-isotope effect on the antiferromagnetic exchange energy J in undoped parent compounds consistently show that the polaron binding energy is about 2 eV, which is over one order of magnitude larger than J = 0.14 eV. The normal-state spin-susceptibility data of hole-doped cuprates indicate that intersite bipolarons are the dominant charge carriers in the underdoped region, while the component of Fermi-liquid-like polarons is dominant in the overdoped region. All the experiments for testing the gap or order-parameter symmetry consistently demonstrate that the intrinsic gap (pairing) symmetry for the Fermi-liquid-like component is anisotropic s-wave and the order-parameter symmetry of the Bose–Einstein condensation of bipolarons is d-wave.

High-temperature superconductivity: the explanation

Soon after the discovery of the first high-temperature superconductor by Georg Bednorz and Alex Müller in 1986, the late Sir Nevill Mott in answering his own question ‘Is there an explanation?’ (1987 Nature327 185) expressed the view that the Bose–Einstein condensation (BEC) of small bipolarons, predicted by us in 1981, could be the one. Several authors then contemplated BEC of real-space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with an electron–phonon interaction (EPI). However, a number of other researchers criticized the bipolaron (or any real-space pairing) scenario as incompatible with some angle-resolved photoemission spectra, with experimentally determined effective masses of carriers and unconventional symmetry of the superconducting order parameter in cuprates. Since then, the controversial issue of whether EPI is crucial for high-temperature superconductivity or is weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here I outline some developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron–electron correlations with a significant finite-range (Fröhlich) EPI, and that the theory is fully compatible with key experiments.

Calculation of angle-resolved photo emission spectra within the one-step model of photo emission—Recent developments

Various technical developments enlarged the potential of angle-resolved photo emission spectroscopy (ARPES) tremendously during the last one or two decades. In particular improved momentum and energy resolution as well as the use of photon energies from few eV up to several keV makes ARPES a rather unique tool to investigate the electronic properties of solids and surfaces. Obviously, this rises the need for a corresponding theoretical formalism that allows to accompany experimental ARPES studies in an adequate way. As will be demonstrated by several examples this goal could be achieved by various recent developments on the basis of the one-step model of photo emission: the spin–orbit induced Rashba-splitting of Shockley-type surface states is discussed using a fully relativistic description. The impact of chemical disorder within surface layers can be handled by means of the coherent potential approximation (CPA) alloy theory. Calculating phonon properties together with the corresponding electron–phonon self-energy allows a direct comparison with features in the ARPES spectra caused by electron–phonon interaction. The same holds for the influence of electronic correlation effects. These are accounted for by means of the dynamical mean field theory (DMFT) that removes the most serious short comings of standard calculations based on the standard local density approximation (LDA). The combination of this approach with the CPA allows the investigation of correlated transition metal alloys. Finally, accounting for the photon momentum and going beyond the single scatter approximation for the final state allows to deal quantitatively with ARPES in the high-energy regime (HAXPES) that reduces the influence of the surface on the spectra and probing primarily the bulk electronic structure this way. Corresponding calculations of ARPES spectra, however, have to deal with thermal vibrations in an adequate way. For this, a new scheme is suggested that makes use of the CPA.

Electronic Dispersion Anomalies in Iron Pnictide Superconductors

Recently, experimental studies of the spin excitation spectrum revealed a strong temperature dependence in the normal state and a resonance feature in the superconducting state of several Fe-based superconductors. Based on these findings, we develop a model of electrons interacting with a temperature dependent magnetic excitation spectrum and apply it to angle resolved photoemission in Ba(1-x)K(x)Fe(2)As(2). We reproduce in quantitative agreement with experiment a renormalization of the quasiparticle dispersion both in the normal and the superconducting state, and the dependence of the quasiparticle linewidth on binding energy. We estimate the strength of the coupling between electronic and spin excitations. Our findings support a dominantly magnetic pairing mechanism.

Electronic structure of URu2Si2 in paramagnetic phase studied by soft x-ray photoemission spectroscopy

We have performed soft x-ray photoemission spectroscopy study on the heavy-fermion compound URu2Si2. We have found that the U 5f states are distributed in the binding energy range between 0.6 eV and the Fermi level. In angle-resolved photoemission spectra, we have observed the U 5f-Ru 4d hybridized band. Our results strongly suggest that the U 5f states form itinerant energy bands in this system.

Energy-Dependent Enhancement of the Electron-Coupling Spectrum of the UnderdopedBi2Sr2CaCu2O8+δSuperconductor

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O(8+δ) from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies ≲15 meV shows a twofold and nearly band-independent enhancement, while that around ∼65 meV increases moderately, and that in ≳130 meV decreases leading to a crossover of dominant coupling excitation between them. Our results suggest the competition among multiple screening effects, and provide important clues to the source of sufficiently strong low-energy coupling, λ(LE)≈1, in an underdoped system.

Band Calculations for Ce Compounds with AuCu3-type Crystal Structure on the basis of Dynamical Mean Field Theory: I. CePd3 and CeRh3

Band calculations for Ce compounds with the AuCu 3 -type crystal structure were carried out on the basis of dynamical mean field theory (DMFT). The auxiliary impurity problem was solved by a method named NCA f 2 vc (noncrossing approximation including the f 2 state as a vertex correction). The calculations take into account the crystal-field splitting, the spin–orbit interaction, and the correct exchange process of the f 1 → f 0 , f 2 virtual excitation. These are necessary features in the quantitative band theory for Ce compounds and in the calculation of their excitation spectra. The results of applying the calculation to CePd 3 and CeRh 3 are presented as the first in a series of papers. The experimental results of the photoemission spectrum (PES), the inverse PES, the angle-resolved PES, and the magnetic excitation spectra were reasonably reproduced by the first-principles DMFT band calculation. At low temperatures, the Fermi surface (FS) structure of CePd 3 is similar to that of the band obtained by the local density approximation. It gradually changes into a form that is similar to the FS of LaPd 3 as the temperature increases, since the 4 f band shifts to the high-energy side and the lifetime broadening becomes large.

Comment on “Circular Dichroism in the Angle-Resolved Photoemission Spectrum of the High-TemperatureBi2Sr2CaCu2O8+δSuperconductor: Can These Measurements Be Interpreted as Evidence for Time-Reversal Symmetry Breaking?”

We report first-principles computations of the angle-resolved photoemission response with circularly polarized light in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ for the purpose of delineating contributions to the circular dichroism resulting from distortions and modulations of the crystal lattice. Comparison with available experimental results shows that the measured circular dichroism from antinodal mirror planes is reproduced in quantitative detail in calculations employing the average orthorhombic crystal structure. We thus conclude that the existing angle-resolved photoemission measurements can be understood essentially within the framework of the conventional picture, without the need to invoke unconventional mechanisms.

Electronic and Magnetic Structures in O/Cr(001) Surface from Angle-Resolved Photoemission Spectroscopy

We measured the angle-resolved photoemission spectra of Cr(001) surface with and without oxygen adsorption to study the surface and interfacial electronic structures in relation to its magnetism. The experimental results show that the surface states and Cr 3 d –O 2 p interfacial state are found near the Fermi level, and they are responsible to the surface ferromagnetism of Cr(001). The energy band dispersion of this interfacial state indicates the odd symmetry with respect to the mirror plane (010), and originates from the 3 d x y –2 p y antibonding state. It was found that the fully occupied antibonding state leading to the negative surface relaxation results in the enhancement of magnetic moments in O/Cr(001). This magneto-volume effect also realized that the energy band width of O/Cr(001) is at most 50% less than that of clean Cr(001) surface.

Interplay of matrix element, self-energy and geometric effects in various spectroscopies of the cuprates

We discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cuprates where effects of matrix element, crystal structure, strong electron correlations, and superconductivity are included realistically in material-specific detail. A number of illustrative examples drawn from our recent work are presented. Specific issues in the cuprate physics considered are: (i) Origin of high-energy kink or the waterfall effect; (ii) Dichroic effects in angle-resolved photoemission spectrum; (iii) Asymmetry of the scanning tunneling spectrum between the processes of electron extraction and injection; (iv) Persistence of ‘Mott’ like high-energy features with doping in optical spectra; (v) Magnetic excitations in electron and hole doped cuprates.

Effective Hamiltonian for transition-metal compounds: Application toNaxCoO2

We describe a simple scheme to construct a low-energy effective Hamiltonian H_eff for highly correlated systems containing non-metals like O, P or As (O in what follows) and a transition-metal (M) as the active part in the electronic structure, eliminating the O degrees of freedom from a starting Hamiltonian that contains all M d orbitals and all non-metal p orbitals. We calculate all interaction terms between d electrons originating from Coulomb repulsion, as a function of three parameters (F_0, F_2 and F_4) and write them in a basis of orbitals appropriate for cubic, tetragonal, tetrahedral or hexagonal symmetry around M. The approach is based on solving exactly (numerically if necessary) a MO_n cluster containing the transition-metal atom and its n nearest O atoms (for example a CoO_6 cluster in the case of the cobaltates, or a CuO_n cluster in the case of the cuprates, in which n depends on the number of apical O atoms), and mapping them into many-body states of the same symmetry containing d holes only. We illustrate the procedure for the case of Na_xCoO_2. The resulting H_eff, including a trigonal distortion D, has been studied recently and its electronic structure agrees well with angle-resolved photoemission spectra [A. Bourgeois, A. A. Aligia, and M. J. Rozenberg, Phys. Rev. Lett. 102, 066402 (2009)]. Although H_eff contains only 3d t_2g holes, the highly correlated states that they represent contain an important amount not only of O 2p holes but also of 3d e_g holes. When more holes are added, a significant redistribution of charge takes place. As a consequence of these facts, the resulting values of the effective interactions between t_2g states are smaller than previously assumed, rendering more important the effect of D in obtaining only one sheet around the center of the Brillouin zone for the Fermi surface (without additional pockets).

Magnetic circular dichroism study of ultrathin Ni films by threshold photoemission and angle resolved photoemission spectroscopy

Threshold photoemission magnetic circular dichroism (MCD) for 12 monolayer (ML) Ni thin films grown on Cu(0 0 1) was measured with a total electron yield method. The obtained MCD asymmetry using the total electron yield method reaches as much as 10% near the photoemission threshold. On the other hand, the MCD asymmetry in angle resolved photoemission spectra with the photon energy of 5.28 eV was also investigated for Cs/Ni(12 ML)/Cu(0 0 1) and was found to give a similar MCD asymmetry. The high MCD asymmetry in the total electron yield can be interpreted by the argument that the angle and energy selection is spontaneously achieved in the total electron yield near the photoemission threshold for low photon energy excitation.

Effective continuous model for surface states and thin films of three-dimensional topological insulators

Two-dimensional (2D) effective continuous models are derived for the surface states and thin films of a three-dimensional topological insulator (3DTI). Starting from an effective model for 3DTI based on first-principles calculations (Zhang et al 2009 Nat. Phys.5 438), we present solutions for both the surface states in a semi-infinite boundary condition and those in a thin film with finite thickness. The coupling between opposite topological surfaces and structure inversion asymmetry (SIA) gives rise to gapped Dirac hyperbolas with Rashba-like splittings in the energy spectrum. In addition, SIA leads to asymmetric distributions of wavefunctions for the surface states along the film growth direction, making some branches in the energy spectra much harder than others to probe by light. These features agree well with the recent angle-resolved photoemission spectra of Bi2Se3 films grown on SiC substrate (Zhang et al 2009 arXiv:0911.3706). More importantly, using the parameters fitted by experimental data, the result indicates that the thin film Bi2Se3 lies in the quantum spin Hall (QSH) region based on the calculation of the Chern number and Z2 invariant. In addition, strong SIA always tends to destroy the QSH state.

Preparation and photoemission investigation of bulklikeα-Mnfilms on W(110)

We report the successful stabilization of a thick bulk-like, distorted $\alpha$-Mn film with (110) orientation on a W(110) substrate. The observed $(3\times3)$ overstructure for the Mn film with respect to the original W(110) low-energy electron diffraction pattern is consistent with the presented structure model. The possibility to stabilize such a pseudomorphic Mn film is supported by density functional total energy calculations. Angle-resolved photoemission spectra of the stabilized $\alpha$-Mn(110) film show weak dispersions of the valence band electronic states in accordance with the large unit cell.

Material and Doping Dependence of the Nodal and Antinodal Dispersion Renormalizations in Single- and Multilayer Cuprates

We present a review of bosonic renormalization effects on electronic carriers observed from angle-resolved photoemission spectra in the cuprates. Specifically, we discuss the viewpoint that these renormalizations represent coupling of the electrons to the lattice and review how materials dependence, such as the number of CuO2layers, and doping dependence can be understood straightforwardly in terms of several aspects of electron-phonon coupling in layered correlated materials.

Anomalous angle-resolved photoemission spectra in the colossal magnetoresistive bilayer manganites

We have discussed anomalous angle-resolved photoemission spactra in the colossal magnetoresistive bilayer manganites by using quantized massive gauge fields, which are introduced theoretically in the path-integral method, around the photohole.