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A comparative study of the gaps measured in two techniques, angle-resolved photoelectron spectroscopy and spectroscopic imaging scanning tunneling microscopy, is presented. In particular the study focuses on the more overdoped region of the cuprate phase diagram in the superconducting state. While the total densities of states measured in the two techniques agree very well, it is shown that the peak in the density of states, ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{DOS}}$, is consistently displaced to higher energies relative to the maximal superconducting gap, ${\mathrm{\ensuremath{\Delta}}}_{0}$, determined in photoemission. The difference between the two gaps is more evident for the less doped samples reflecting increased normalization of bands. This observation will clearly influence the boundaries in the phase diagrams of the cuprates defined by these two techniques.

We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the SC transition at $T_c\simeq$10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the dramatic decrease in the low-frequency optical conductivity signals the formation of a SC energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron- and hole-like bands at the corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below $T_r$ the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ~20 meV. The magnetic states, with resulting SDW and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the SC in this material.

The high-${T}_{c}$ superconductor, $\mathrm{FeT}{\mathrm{e}}_{0.55}\mathrm{S}{\mathrm{e}}_{0.45}$, has recently been shown to support a surface state with topological character. Here we use low-energy laser-based angle-resolved photoemission spectroscopy with variable light polarization, including both linear and circular polarizations, to reexamine the same material and the related $\mathrm{FeT}{\mathrm{e}}_{0.7}\mathrm{S}{\mathrm{e}}_{0.3}$, with a larger Te concentration. In both cases, we observe the presence of a surface state displaying linear dispersion in a cone-like configuration. The use of circular polarization confirms the presence of a helical spin structure. These experimental studies are compared with theoretical studies that account for the local magnetic effects related to the paramagnetism observed in this system in the normal state. In contrast to previous studies, we find that including the magnetic contributions is necessary to bring the chemical potential of the calculated electronic band structure naturally into alignment with the experimental observations.

Here, we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi-liquid phase (LiFeAs) and an incoherent bad-metal phase (${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum-superconducting-gap and ${T}_{c}$ ratio $2{\mathrm{\ensuremath{\Delta}}}_{max}/{k}_{B}{T}_{c}$, which shows a universal scaling that is valid for all iron-based superconductors (FeSCs). Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong-coupling pairing mechanism for the FeSCs.

The dressing of quasiparticles in solids is investigated by changes in the electronic structure $E$($k$) driven by femtosecond laser pulses. Employing time- and angle-resolved photoemission on an optimally doped cuprate above ${T}_{\mathrm{c}}$, we observe two effects with different characteristic temporal evolutions and, therefore, different microscopic origins. First, a marked change in the effective mass due to the 70-meV kink in $E$($k$) is found to occur during the experiment's 100-fs temporal resolution and is assigned to laser-driven perturbation of an electronic interaction dressing the bare band. Second, a change in ${k}_{\mathrm{F}}$ is explained by effective photodoping due to particle-hole asymmetry and offers opportunities for ultrafast optoelectronic switches based on an optically driven insulator-superconductor transition.

Using low-temperature scanning tunneling microscopy, we demonstrate a one-dimensional system that undergoes a charge-density-wave (CDW) instability on a metallic substrate. For our measurements we utilize a self-assembled monatomic chain of Co atoms aligned by the steps on a vicinal Cu(111) surface. We assign the measured CDW instability to ferromagnetic interactions along the chain. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.

Spin-polarized angle-resolved sulfur ${\mathit{L}}_{2,3}$VV Auger-electron spectra have been recorded for the c(2\ifmmode\times\else\texttimes\fi{}2)S/Fe(001) system. The data show the modulation of the sulfur Auger spin polarization as a function of emission angle, which represents an observation of spin-polarized Auger-electron diffraction (SPAED), a potentially powerful tool for the study of local magnetic structure at surfaces, interfaces, and thin films. Theoretical modeling of the SPAED data indicates a large decrease in the magnetization of the top iron layer, suggesting a magnetic poisoning induced by the sulfur overlayer. These findings are independently supported by the observation of a large decrease of secondary electron spin polarization upon sulfur adsorption.

Quantum confinement is studied with angle-resolved photoemission for ultrathin Cu separated from the Cu(100) substrate by an epitaxial wedge of Co (0--11 A) that serves as a finite barrier. For 2 ML of Cu, the photoemission intensities of the Cu [ital sp] quantum-well (QW) state at 1.6 eV binding energy and the corresponding [ital d]-band-derived QW state at 2.4 eV increase rapidly with the thickness of the Co barrier and indicate stronger confinement of the [ital d] QW state than of the [ital sp] state. Implications for the magnetic coupling and magnetoresistance are discussed.

The results of a spin-polarized photoemission study of the Fe 3s level clearly reveal the multiplet structure characterizing the final states of the excitation process. The energy separation between the high-spin and low-spin ionic states of 3.5 eV is smaller than has previously been measured, to our knowledge. The two spin components of the high-spin ionic state are clearly resolved and shown to be separated in binding energy by 0.9 eV. Line-shape fitting provides evidence that the individual peak widths reflect the total spin of the ionic final state rather than the spin of the core hole.

We present the results of a tight-binding calculation of the soft-x-ray emission spectra obtained in an earlier study of resonant inelastic scattering of x rays at the carbon k edge of diamond. The close agreement between the calculated spectra and the experimental observations lends support to a picture of momentum conservation restricting the emission process to the same point in the Brillouin zone sampled by the initial excitation. We thus demonstrate qualitatively the feasibility of band-structure determinations with this technique.

A microscopic scaling relation linking the normal and superconducting states of the cuprates in the presence of a pseudogap is presented using angle-resolved photoemission spectroscopy. This scaling relation, complementary to the bulk universal scaling relation embodied by Homes' law, explicitly connects the momentum-dependent amplitude of the $d$-wave superconducting order parameter at $T\ensuremath{\sim}0$ to quasiparticle scattering mechanisms operative at $T\ensuremath{\gtrsim}{T}_{c}$. The form of the scaling is proposed to be a consequence of the marginal Fermi-liquid phenomenology and the inherently strong dissipation of the normal pseudogap state of the cuprates.

We report angle-resolved photoemission spectroscopy (ARPES) and a set of in situ scanning tunneling microscopy (STM) measurements on a narrow-terrace-width vicinal Cu(111) crystal surface, Cu(775), whose vicinal cut lies close to the transition between terrace and step modulation. These measurements show sharp zone-folding (or umklapp) features with a periodicity in ${k}_{\ensuremath{\parallel}}$, indicating that the predominant reference plane is that of Cu(775), i.e., that the surface is predominately step-modulated. Our measurements also show variation in umklapp intensity with photon energy, which is consistent with prior ARPES experiments on other vicinal Cu(111) surfaces and in agreement with our designation of the state as being step-modulated. The measurements also show a weak terrace-modulated state, which, based on several characteristics, we attribute to the presence of terrace widths larger than the ideal terrace width. By measuring the intensity ratio of the two distinct surface-state modulations from photoemission and the terrace-width distribution from STM, we derive a value for the terrace width at which the surface state switches between the two modulations.

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe{sub 0.5}Te{sub 0.5}. The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0) whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

Superconducting thin films of ${\text{Fe}}_{1.08}\text{Te}:{\text{O}}_{x}$ have been epitaxially grown on ${\text{SrTiO}}_{3}$ substrates by pulsed-laser deposition in controlled oxygen atmosphere. Although the bulk ${\text{Fe}}_{1.08}\text{Te}$ is not superconducting, thin films with oxygen are superconducting with an onset and a zero resistance transition temperature around 12 and 8 K, respectively. Oxygen was found to be crucial to the superconducting properties of these films, suggesting that the oxygen incorporation can induce superconductivity in FeTe thin films. A metal-insulator transition is found at about 60 K, lower than that of bulk $(\ensuremath{\sim}70\text{ }\text{K})$. From magnetoresistive measurements, we obtained the irreversibility line and the upper critical field.

Inverse photoemission is used to examine the adsorbate-induced unoccupied band structure of the c(2\ifmmode\times\else\texttimes\fi{}2)CO/Cu(001) adsorption system. As in an earlier study we observe two adsorbate-induced peaks above the Fermi level. By comparing with a simple tight-binding model we associate one unoccupied adsorbate band with the molecular 2\ensuremath{\pi} level and another band with a substrate surface band interacting with the molecular 5\ensuremath{\sigma} level.

Laser-based photoemission with photons of energy 6 eV is used to examine the fine details of the very low-energy electron dispersion and associated dynamics in the nodal region of optimally doped Bi2212. A ``kink'' in the dispersion in the immediate vicinity of the Fermi energy is associated with scattering from an optical phonon previously identified in Raman studies. The identification of this phonon as the appropriate mode is confirmed by comparing the scattering rates observed experimentally with the results of calculated scattering rates based on the properties of the phonon mode.

We measure the electronic structure of low-coverage Co on a Cu(775) stepped substrate using high-resolution photoemission spectroscopy with the particular goal of relating the electronic dispersion to the coverage-dependent surface structure. In particular, we follow the evolution of the electronic dispersion of the $sp$-like Cu surface state and the position of the band minimum as a function of Co coverage. On the bare Cu(775) surface, we observe band folding of this state due to the stepped surface-superlattice array. In addition, we determine that the reference plane, as measured by the position of the band minimum of this state, changes dramatically after addition of just 0.03 ML Co. At 0.06 ML, we observe the formation of a second surface state at a binding energy of $0.68\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This feature is attributed to a quantum-well state hybridized with the substrate.

Angle-resolved photoemission and optical spectroscopy are used to probe the electronic structure of the one-dimensional Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$, both at half-filling and with small concentrations of excess oxygen doping. Spin-charge separation can be seen as evidenced by the existence of spinon and holon branches in the photoemission spectra. Optical studies reveal no significant doping dependence, while photoemission studies show a large energy shift in the spinon and holon states with respect to the main valence band states which remain nearly unaffected. The results suggest the excess dopant carriers are incorporated solely within the one-dimensional Hubbard band which is electronically isolated from the rest of the states in the system.

We present an angle-resolved photoemission spectroscopy (ARPES) study of a Mott-Hubbard-type bismuth cobaltate system across a metal-insulator transition. By varying the amount of Pb substitution, and by doping with Sr or Ba cation, a range of insulating to metallic properties is obtained. We observe a systematic change in the spectral weight of the coherent and incoherent parts, accompanied by an energy shift of the incoherent part. The band dispersion also shows the emergence of a weakly dispersing state at the Fermi energy with increasing conductivity. These changes correspond with the changes in the temperature-dependent resistivity behavior. We address the nature of the coherent-incoherent parts in relation to the peak-dip-hump feature seen in cuprates superconductors.

We investigate the nature of oxygen dopant-induced electronic states in the high-temperature Bi2Sr2CaCu2O8+x superconductor by measuring photoemission spectra of underdoped, optimally doped, and overdoped samples using a wide photon energy range (15-100 e

Very high energy resolution photoemission experiments on high quality samples of optimally doped Bi2Sr2CaCu2O8+delta show new features in the low-energy electronic excitations. A marked change in the binding energy and temperature dependence of the near-n

The element-specific valence- and conduction-band densities of states for the high-dielectric-material $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ have been measured using soft x-ray emission and absorption spectroscopies. Ti ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, Cu ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, and O $K$ soft x-ray emission spectra of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ were measured with monochromatic photon excitation on selected energies above the Ti and Cu ${L}_{2,3}$ and O $K$ absorption edges, respectively. X-ray absorption spectra were recorded at the same edges. The electronic structure was also calculated using density functional theory employing the full-potential linearized augmented plane-wave method. Excellent agreement is seen between the results of these calculations and the measured x-ray emission and absorption spectra. This agreement is particularly good at the O $K$ edge where the resonant behavior of the x-ray emission spectrum can be attributed directly to $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-state emission from valence-band O $2p$ states when in resonance with ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ conduction-band O $2p$ states. Resonant inelastic x-ray scattering is observed at the Ti ${L}_{2,3}$ absorption edge and is compared to previous studies of Ti containing perovskite compounds. The role of Cu $3d$ states in determining the band gap of this material is discussed.

We present a method for extracting the electron-boson spectral function a 2 Fsvd from infrared and photoemission data. This procedure is based on inverse theory and will be shown to be superior to previous techniques. Numerical implementation of the algorithm is presented in detail and then used to accurately determine the doping and temperature dependence of the spectral function in several families of high-Tc superconductors. Principal limitations of extracting a 2 Fsvd from experimental data will be pointed out. We directly compare the IR and angular-resolved photoemission spectroscopy a 2 Fsvd and discuss the resonance structure in the spectra in terms of existing theoretical models.

We report an angle-resolved photoemission study of d-derived quantum well (QW) states in ultrathin silver films grown on V(100). Distinct d-derived QW states were observed for film thicknesses between 1 and 5 ML. Using a line shape analysis we conclude that for thicker films the d electrons are almost completely confined within the silver overlayer, suggesting a high reflectivity at the vanadium-silver interface. The energies of the QW states were found to be in good agreement with tight-binding calculations as well as with a phase accumulation model analysis. The calculations indicate a strong influence of the vanadium substrate on the energy of d-band QW states for the single-monolayer film.

Angle-resolved-photoemission studies of the dispersion of the quantum-well states in copper thin films deposited on a Co(001) substrate reveal that hybridization in the interface leads to a large increase in the effective mass of the electrons. These observations have implications for theories of the oscillatory exchange coupling in the related magnetic multilayers, particularly where Fermi-surface spanning vectors away from the center of the zone are invoked as in the case of the short-period oscillation in the Co/Cu(001) multilayers.

We present O K-edge absorption and emission spectra for \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ obtained using a high-resolution-undulator soft-x-ray radiation source. The occupied and the unoccupied O p densities of states are obtained and compared with the valence-band photoemission results. While these results confirm that \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ can be classified as a charge transfer or an intermediate-type insulator, they suggest that ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ is more likely of the Mott-Hubbard type. The excitation-energy dependence of the emission spectra is also discussed.

We present k-resolved inverse photoemission spectroscopy (KRIPES) results for Ag(111) in the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} K\ifmmode\bar\else\textasciimacron\fi{} direction of the surface Brillouin zone. The spectra were taken as a function of electron energy and angle of incidence at a fixed photon energy (9.7 eV) of the emitted radiation. The crystal-induced surface state and the image-potential-induced surface state are observed with final-state electron energies at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} in good agreement with a recently proposed multiple reflection model and with dispersion characterized by effective masses ${m}^{\mathrm{*}}$=0.7m and ${m}^{\mathrm{*}}$\ensuremath{\approxeq}m, respectively. The measured intensity of the surface state roughly follows a ${\mathrm{cos}}^{2}$\ensuremath{\alpha} dependence, where \ensuremath{\alpha} is the angle between the sample normal and the polarization vector of the detected photons. Transitions into bulk-derived final states are observed, with final-state electron energies ranging from ${E}_{F}$ to ${E}_{F}$+20 eV. The dispersions of these bulk-derived final states agree with very simple ``empty-lattice isochromats'' derived from free-electron bands of proper symmetry, as well as with results of a band-structure calculation based upon a combined interpolation scheme.

We compare new high-resolution inverse-photoemission spectra of the unoccupied surface states on Cu(001) with a first-principles calculation of the same surface. This theoretical calculation includes the asymtotic long-range image potential. The commonly observed image state is found to correspond to the n=1 member of the Rydberg series of states derived from the image potential, i.e., the wave function has a single extremum beyond the crystal edge characteristic of the hydrogenic n=1 state. Agreement between theory and experiment for all states is excellent. The experimentally determined binding energies and effective masses are 0.62 eV and 0.98${m}_{e}$ for the n=1 image state and 0.8 eV (with respect to ${E}_{F}$) and 0.4${m}_{e}$ for the n=0 surface resonance. The results indicate that surface corrugation effects are negligible.

Momentum-resolved inverse photoemission spectra have been taken on Pd(111) at h..omega.. = 9.7 eV. In addition to the unoccupied part of the d band, we observe an s,p peak which disperses, but lies somewhat higher in energy (by approx.0.5 eV) than a bulk direct-transition peak predicted by band-structure calculations. The s,p peak does not fall in the fundamental s,p gap, and is therefore not attributable to a surface state. A step structure close to the vacuum level is attributed to band-gap emission and image-potential states.

Single crystals of lanthanum cuprate have been studied using k-resolved inverse photoemission. The predicted dispersion of the Cu 3d--O2p-derived band through the Fermi level is not observed and this may influence models for high-temperature superconductivity based on band-structure calculations. In addition, resonant inverse photoemission of the lanthanum-derived features is also observed at the plasmon frequencies.

Inverse photoemission spectra on Pd(110) reveal bulk band-structure features, and surface-state features in the projected gaps centered at points Y-bar and X-bar. The energy dispersion of the bulk band-structure features is in agreement with direct-transition predictions. Surface-state energies and their dispersion are reproduced with a nearly-free-electron model and a surface-barrier model which places the image plane 2.3 a.u. beyond the outermost atomic layer. The experimentally determined energies of surface states are as follows: at point X-bar, 6.6 eV above E/sub F/, designated S/sub 0//sup +/; at point Y-bar, 3.4 and 6.6 eV above E/sub F/, designated S/sub 0//sup +/ and S/sub 1//sup -/, respectively.

We have identified a new unoccupied surface resonance approximately 1.15 eV above the Fermi level on Cu(001) with the use of inverse photoemission. The resonance has to be separated from a nearby intense peak due to a bulk direct transition. This has been done in two ways: (a) varying the photon energy, which causes a shift in the bulk peak, and (2) varying the work function, wich causes a shift in the resonance. The occurrence of the resonance is consistent with a recently proposed phase analysis.

We present the first observation to our knowledge of the molecular shape resonance in adsorbed CO using the technique of inverse photoemission. By comparison with earlier photoionization measurements we are able to examine the role of the final-state potential in determining the binding energy of the shape resonance. We find that screening of the photohole is more effective for valence-band photoionization than for the more localized core-level photoionization.

Valence-band photoemission measurements have been made both above and below the ${T}_{c}$ of ${\mathrm{Ba}}_{2}$${\mathrm{YaCu}}_{3}$${\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ where \ensuremath{\delta}=2.1. We find similarities to recent calculations of the electronic properties. However, the experimental binding energies are between 1.0 and 1.4 eV further from the Fermi level than the calculation. We suggest that this difference may affect attempts to predict ${T}_{c}$ from tight-binding modeling of the electron-phonon mechanism.

An inverse-photoemission study (at $\ensuremath{\hbar}\ensuremath{\omega}=9.7$ eV) of the adsorption of carbon monoxide on a Pd(111) surface is compared with a first-principles calculation of the cross section for the emission of photons as a function of electron angle of incidence. It is found that the calculation is able to explain the experimental results in terms of the relative strengths of the $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\pi}$ and $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\pi}$ channels in the optical transitions.