Photoemission Line Shape Research Articles

Carbon 1s X-ray Photoemission Line Shape Analysis of Highly Oriented Pyrolytic Graphite: The Influence of Structural Damage on Peak Asymmetry

C 1s XPS spectra of various highly oriented pyrolytic graphite (HOPG) surfaces, untreated, as well as those treated by keV Ar+ beam bombardment and low-energy O2, N2, Ar, and H2O plasmas, have been systematically studied by comparing two XPS peak-fitting procedures. These procedures treat the spectrum as either (1) the overlap of several symmetric component peaks or (2) a single asymmetric peak. The results indicate that, in the case of HOPG, the asymmetry parameter defining the single peak is directly related to the extent of damage to the alternant hydrocarbon structure of the HOPG surface, as manifested by its correlation with the symmetric peak component due to the damaged HOPG structure.

Normal state spectral lineshapes of nodal quasiparticles in single layer Bi2201 superconductor

Abstract A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi 2 Sr 2− x La x CuO 6+ δ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to 10 times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.

Luttinger liquid angle-resolved photoemission line shapes from samples ofLi0.9Mo6O17grown by the temperature-gradient-flux technique

Angle resolved photoemission spectroscopy line shapes measured for quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$ samples grown by a temperature gradient flux technique are found to show Luttinger liquid behavior, consistent with all previous data by us and other workers obtained from samples grown by the electrolyte reduction technique. This result eliminates the sample growth method as a possible origin of considerable differences in photoemission data reported in previous studies of Li$_{0.9}$Mo$_6$O$_{17}$.

Step-lattice-induced band-gap opening at the fermi level.

The interaction of the Shockley surface state with the step lattice of vicinal Cu(111) leads to the formation of an electronic superlattice state. On Cu(443), where the average terrace length forms a "shape resonance" with the Fermi wavelength, we find a step-lattice-induced band-gap opening at the Fermi level. A gap magnitude >200 meV is inferred from high resolution photoemission experiments and line shape analysis. The corresponding energy gain with respect to a gapless case is approximately 11 meV/unit cell, and is a substantial contribution to the stabilization of the step lattice.

High-resolution photoemission spectroscopy study of clean and Au-nanowire-decorated Si(5 5 12) surfaces

We have examined the Si $2p$ photoemission line shape of the clean $\mathrm{Si}(5512)2\ifmmode\times\else\texttimes\fi{}1$ surface in detail and have investigated the evolution of the Au-induced nanowires on this surface at different temperatures with low-energy-electron diffraction and photoemission spectroscopy. The Si $2p$ spectra from the clean $2\ifmmode\times\else\texttimes\fi{}1$ surface exhibit very complex line shapes composed at least of five different surface-related components. Possible origins of the most distinct surface component with a surface core-level shift of $\ensuremath{-}0.62\mathrm{eV}$ are discussed within the three different structure models proposed so far. Upon Au deposition, the intensity of that particular surface component was largely reduced indicating that the Au adsorbates interact dominantly with the specific Si surface atoms related. With an annealing above $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ after Au deposition of 0.2 ML, the surface drastically changes into a metallic phase, as shown clearly in the valence band photoemission spectra, along with the gradual formation of the $(337)\ifmmode\times\else\texttimes\fi{}2$ facets observed in LEED. In this transition, a new Au $4f$ component appears while the Si $2p$ core-level spectra keep alike. This indicates that the surface metallization involves the rearrangement of Au adsorbates, possibly the formation of new Au chains on the surface.

Photoemission line-shape study ofLa0.7Sr0.3Mn1−δAlδO3(δ=0,0.03, 0.06)

We have studied the line shapes of ultraviolet photoemission spectra of La$_{0.7}$Sr$_{0.3}$Mn$_{1-\delta}$Al$_\delta$O$_3$ ($\delta$ = 0, 0.03, 0.06) systems to test the extrinsic image charge screening effect on photoemission spectra recently suggested by R. Joynt (Science {\bf 284}, 777 (1999)), who argued that the photoemission spectrum near the Fermi energy, specially for poorly conducting system, can be very different from the intrinsic density of states because the outgoing electron has probability of losing its kinetic energy due to the image force. We tested this argument in real materials experimentally by measuring the photoemission line shapes of La$_{0.7}$Sr$_{0.3}$Mn$_{1-\delta}$Al$_\delta$O$_3$ systems, for which all the requirements of this theory are satisfied and sample resistivities change systematically. We found that experimental photoemission spectra do not show the change of line shapes expected from the extrinsic image force effect and we conclude that the influence of this long range interaction is not so large as suggested in that paper.

Anomalous temperature dependence in the photoemission spectral function of cuprates

Temperature-dependent angle-resolved photoemission experiments were performed on overdoped single layer ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6}$ ${(T}_{c}\ensuremath{\sim}4 \mathrm{K})$ and on the parent compounds ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}.$ Contrary to the more conventional results obtained on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6},$ the parent compounds show strong temperature dependence over a wide energy range that is two orders of magnitude larger than the temperature scale involved. The doping dependence strongly suggests that the anomalous temperature dependence has its origin in the novel many-body physics in the Mott insulators. We consider the observed temperature dependence using a heuristic model with multiple initial and final states in the photoemission process. The nature of the multiple states and broad photoemission line shape in parent compounds is also given.

Fermi surfaces of the two-dimensional surface states on vicinal Cu(111)

The Shockley $(L$-gap) surface states on Cu(111) and the vicinal (332) and (221) surfaces have been mapped by angle-scanned ultraviolet photoelectron spectroscopy. We find two-dimensional (2D) surface states on both vicinal surfaces. An analysis of the photoemission line shape that includes the effects of the terrace width distribution indicates an isotropic reduction of the photohole lifetime on the vicinal surfaces and allows us to quantify the intrinsic initial-state dispersion. For the larger step-step separation of $12 \AA{}$ on Cu(332), the Fermi contour shows the characteristic elliptical shape of a 2D Bloch state in a 1D lattice. On Cu(221) with $7.7 \AA{}$ terraces we find an isotropic dispersion within the accuracy of the experiment. These findings are interpreted as a continuous surface state to surface resonance transition with decreasing terrace length. The effective step potential is estimated using a simple perturbation theory ansatz.

Strong spin-fluctuation effects on the photoemission line shape in the normal state of high-T(c) cuprates.

We have studied the quasiboson model for photoemission from cuprates in the normal state, where quasiparticles are coupled to collective spin excitations strongly enhanced near momentum (pi,pi). The present model has a benefit to describe the observed crossover from the overdoped to the underdoped cases because it is in principle an infinite-order theory in the limit of narrow bandwidth. The anomalous dependences of the photoemission line shape on the momentum and hole concentration are consistently understood within the model with no adjustable parameter except for a doping-independent coupling constant.

Information from photoemission spectral weights and shapes

Abstract Photoemission line shapes can be used to extract important information about ground-state and low-energy properties. This is illustrated for the electron–phonon interaction in C 60 molecules, the occupancy and hybridization of the 4 f -level in Ce compounds, the effect of the Cu–O network on core-level photoemission in cuprates and the properties of one-dimensional conductors.

Final-state effects on photoemission line shapes at finite temperature

We have measured angle-resolved photoemission spectra from Al(001) over a large range of temperatures and photon energies. These data were analyzed using a model that allows one to calculate the photoemission intensity for transitions with the simultaneous excitation/absorption of 0, 1, 2, etc., phonons. By making a simple simulation of the line shape, we show that the so-called direct transition (or quasiparticle) peaks always contain a significant contribution from photoemission events with a simultaneous excitation and/or absorption of 1 and 2 phonons, i.e., from transitions that are actually indirect. At low photon energies and/or low temperatures these contributions are small; but as the photon energy or the temperature is raised they increase relative to the elastic or zero-phonon contribution and eventually become the dominant contribution to the so-called direct transition peak. The effect of these phonon-assisted transitions is a significant change of the photoemission line shape. Our model gives a good description of the temperature dependence in the experimental data but only if the phonon-assisted contributions to the photoemission peak are taken into account.

Influence of incommensurate dynamic charge-density-wave scattering on the photoemission line shape of superconducting high-Tccuprates

We show that the spectral line shape of superconducting ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) can be well described by the coupling of the charge carriers to collective incommensurate charge-density-wave (CDW) excitations. Our results imply that besides antiferromagnetic fluctuations also low-energy CDW modes can contribute to the observed dip-hump structure in the Bi2212 photoemission spectra. In case of underdoped LSCO we propose a possible interpretation of angle resolved photoemission spectroscopy data in terms of a grid pattern of fluctuating stripes where the charge and spin scattering directions deviate by $\ensuremath{\alpha}=\ensuremath{\pi}/4.$ Within this scenario we find that the spectral intensity along $(0,0)\ensuremath{\rightarrow}(\ensuremath{\pi},\ensuremath{\pi})$ is strongly suppressed, consistent with recent photoemission experiments. In addition, the incommensurate charge-density-wave scattering leads to a significant broadening of the quasiparticle peak around $(\ensuremath{\pi},0).$

Fermi surface and photoemission lineshape of incommensurate CDW systems

We study the single-particle spectra of a model which shows an incommensurate charge-density wave (CDW) instability arising from the competition between phase separation and long-range Coulomb interactions. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (1,0)- and (or) (0,1)-direction which (besides a purely one-dimensional structure) allows for a two-dimensional "eggbox type" charge modulation. This leads to a substantial enhancement of the van Hove singularities and the spectral weight of Fermi surface states near the M-points tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. Extending this approach to the inclusion of dynamic incommensurate CDW fluctuations in the superconducting state we calculate the photoemission lineshape which agrees well with experimental data of underdoped Bi2212 compounds.

Non-fermi liquid angle resolved photoemission line shapes of li0. 9Mo6O17

A recent letter by Xue et al. (PRL v.83, 1235 ('99)) reports a Fermi-Liquid (FL) angle resolved photoemission (ARPES) lineshape for quasi one-dimensional Li0.9Mo6O17, contradicting our report (PRL v.82, 2540 ('99)) of a non-FL lineshape in this material. Xue et al. attributed the difference to the improved angle resolution. In this comment, we point out that this reasoning is flawed. Rather, we find that their data have fundamental differences from other ARPES results and also band theory.

BCS-like density of states in superconducting A3C60 surfaces

We report on an ultrahigh resolution photoemission study on the topmost molecular layer of K3C60 and Rb3C60 films below and above the superconducting transition temperature T(C). We observed not only clear evidence for the opening of the superconducting gap, but also a modification in the photoemission line shape consistent with a change from a normal-metallic to a BCS-like density of states, including the formation of a condensation peak. The data can be accurately modeled by a BCS-type function with a gap Delta derived from T(C) in the weak-coupling limit ( 2Delta/k(B)T(C) = 3.53).

Condensation energy and spectral functions in high-temperature superconductors

If high temperature cuprate superconductivity is due to electronic correlations, then the energy difference between the normal and superconducting states can be expressed in terms of the occupied part of the single particle spectral function. The latter can, in principle, be determined from angle resolved photoemission (ARPES) data. As a consequence, the energy gain driving the development of the superconducting state is intimately related to the dramatic changes in the photoemission lineshape when going below Tc. These points are illustrated in the context of the "mode" model used to fit ARPES data in the normal and superconducting states, where the question of kinetic energy versus potential energy driven superconductivity is explored in detail. We use our findings to comment on the relation of ARPES data to the condensation energy, and to various other experimental data. In particular, our results suggest that the nature of the superconducting transition is strongly related to how anomalous (non Fermi liquid like) the normal state spectral function is, and as such, is dependent upon the doping level.

Electronic and optical investigation of hydrogenated amorphous carbon (a-C:H) by X-ray photoemission spectroscopy and spectroscopic ellipsometry

Hydrogenated amorphous carbon (a-C:H) films were prepared by low-pressure PECVD of CH 4 in a dual electron cyclotron resonance (ECR)–radio frequency (RF) discharge by applying an independently controlled RF substrate bias voltage from 0 to −200 V. The films were analyzed with core-level and valence band (VB) X-ray photoemission spectroscopy (XPS) as well as spectroscopic ellipsometry. A progressive change from soft transparent polymer-like to hard absorbing diamond-like was observed with increasing bias voltage in this range. The C 1s core-level spectra were fitted by using a Doniach–Sunjic photoemission line-shape. The C 1s spectra showed the enhanced asymmetric behavior with increasing bias voltage. In addition, the VB spectra exhibited distinct π density of states according to the bias. Such XPS features were analyzed and interpreted in terms of the π→π * electron transition model on amorphous carbon through spectroscopic ellipsometry.

Electronic Spectra and Their Relation to the(π,π)Collective Mode in High-TcSuperconductors

The photoemission line shape near $(\ensuremath{\pi},0)$ in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ below ${T}_{c}$ is characterized by a sharp peak, followed at higher energy by a dip and hump. We study the evolution of this line shape as a function of momentum, temperature, and doping. We find the hump scales with the peak and persists above ${T}_{c}$ in the pseudogap state. We present strong evidence that the peak-dip-hump structure arises from the interaction of electrons with a collective mode of wave vector $(\ensuremath{\pi},\ensuremath{\pi})$. The inferred mode energy and its doping dependence agree well with a magnetic resonance observed by neutron scattering.

Evidence for Quantum Critical Behavior in the Optimally Doped Cuprate Bi 2 Sr 2 CaCu 2 O 8+δ

The photoemission line shapes of the optimally doped cuprate Bi(2)Sr(2)CaCu(2)O(8+delta) were studied in the direction of a node in the superconducting order parameter by means of very high resolution photoemission spectroscopy. The peak width or inverse lifetime of the excitation displays a linear temperature dependence, independent of binding energy, for small energies, and a linear energy dependence, independent of temperature, for large binding energies. This behavior is unaffected by the superconducting transition, which is an indication that the nodal states play no role in the superconductivity. Temperature-dependent scaling suggests that the system displays quantum critical behavior.

Drastic Photoemission Line Shape Changes in Li due to Surface-Bulk Interference and Plasmon Excitations

Band mapping of Li by angle-resolved photoemission shows anomalous peak dispersion for near-uv photons, giving the false impression that an energy gap of about 1 eV is present just below the Fermi level. This is explained in terms of drastic line shape changes caused by interference between surface and bulk emission together with enhancements of these contributions for photon energies near the multipole surface plasmon and bulk plasmon energies, respectively.