Angle-resolved Photoemission Experiments Research Articles

Neutron scattering studies of spin excitations in hole-doped Ba(0.67)K(0.33)Fe(2)As(2) superconductor.

We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.

Communication: Momentum-resolved quantum interference in optically excited surface states

Surface states play essential roles in condensed matter physics, e.g., as model two-dimensional (2D) electron gases and as the basis for topological insulators. Here, we demonstrate quantum interference in the optical excitation of 2D surface states using the model system of C(60)/Au(111). These surface states are transiently populated and probed in a femtosecond time- and angle-resolved two-photon photoemission experiment. We observe quantum interference within the excited populations of these surface states as a function of parallel momentum vector. Such quantum interference in momentum space may allow one to control 2D transport properties by optical fields.

Antiferromagnetic spin excitations in single crystals of nonsuperconducting Li1−xFeAs

We use neutron scattering to determine spin excitations in single crystals of nonsuperconducting Li1-xFeAs throughout the Brillouin zone. Although angle resolved photoemission experiments and local density approximation calculations suggest poor Fermi surface nesting conditions for antiferromagnetic(AF) order, spin excitations in Li1-xFeAs occur at the AF wave vectors Q = (1, 0) at low energies, but move to wave vectors Q = (\pm 0.5, \pm0.5) near the zone boundary with a total magnetic bandwidth comparable to that of BaFe2As2. These results reveal that AF spin excitations still dominate the low-energy physics of these materials and suggest both itinerancy and strong electron-electron correlations are essential to understand the measured magnetic excitations.

A precise method for visualizing dispersive features in image plots

In order to improve the advantages and the reliability of the second derivative method in tracking the position of extrema from experimental curves, we develop a novel analysis method based on the mathematical concept of curvature. We derive the formulas for the curvature in one and two dimensions and demonstrate their applicability to simulated and experimental angle-resolved photoemission spectroscopy data. As compared to the second derivative, our new method improves the localization of the extrema and reduces the peak broadness for a better visualization on intensity image plots.

Interplay between structure and electronic states in step arrays explored with curved surfaces

Atomic staircases in noble-metal surfaces are model one-dimensional superlattices, where free-electron-like surface states transform into superlattice bands with sizable quantum size shifts and gaps. At critical step spacings $d=n\ifmmode\times\else\texttimes\fi{}({\ensuremath{\lambda}}_{F}/2)$, such superlattice gaps lie at the Fermi energy, affecting the electronic energy and hence the structural stability of the step lattice, which is held by weak elastic interactions. We use Cu, Ag, and Au curved crystals to smoothly tune the superlattice constant $d$ in angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) experiments. With ARPES we accurately quantify terrace-size effects and determine the superlattice potential, which increases from Ag to Cu and to Au. With STM we analyze the $d$-dependent terrace width distribution for Cu and Ag, and observe nonlinear variations in Cu. On the grounds of simple electronic and elastic models, we conclude that terrace width distribution instabilities and electronic energy variations at $d=n\ifmmode\times\else\texttimes\fi{}({\ensuremath{\lambda}}_{F}/2)$ have the same order of magnitude for Cu. In contrast, the weak superlattice potential in Ag, i.e., its smoother band-structure modulation, is not sufficient to alter the step lattice.

Existence, Character, and Origin of Surface-Related Bands in the High Temperature Iron Pnictide SuperconductorBaFe2−xCoxAs2

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.

LDA+DMFTstudy of Ru-based perovskiteSrRuO3andCaRuO3

We carried out a $\mathrm{LDA}+\mathrm{DMFT}$ study of perovskite ruthenates ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{CaRuO}}_{3}$, compounds which have been discussed in great detail in recent literature. Our results reproduced the observed mass enhancements and magnetic properties, and were in reasonable agreement with the measured photoemission spectra. Our calculations produced rather different coherence temperatures for ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{CaRuO}}_{3}$, providing a possible explanation for the observed differences between these compounds. We also present $\mathbf{k}$-resolved spectra for the sake of comparison with future angle-resolved photoemission experiments.

KDependence of the Crystal-Field Splittings of4fStates in Rare-Earth Systems

The occupation, energy separation, and order of the crystal-field-split 4f states are crucial for the understanding of the magnetic properties of rare-earth systems. We provide the experimental evidence that crystal-field-split 4f states exhibit energy dispersion in momentum space leading to variations of energy spacings between them and even of their energy sequence across the Brillouin zone. These observations were made by performing angle-resolved photoemission experiments on YbRh(2)Si(2) and properly simulated within a simple model based on results obtained by inelastic neutron scattering experiments and band structure calculations. Our findings should be generally applicable to rare-earth systems and have considerable impact on the understanding of magnetism and related phenomena.

Fermion correlators in non-abelian holographic superconductors

We consider fermion correlators in non-abelian holographic superconductors. The spectral function of the fermions exhibits several interesting features such as support in displaced Dirac cones and an asymmetric distribution of normal modes. These features are compared to similar ones observed in angle resolved photoemission experiments on high T_c superconductors. Along the way we elucidate some properties of p-wave superconductors in AdS_4 and discuss the construction of SO(4) superconductors.

Theoretical investigation of the superconducting gaps in the Fe-based superconductors

Based on an antiferromagnetic (AFM) spin fluctuation approximation, we study the superconducting gaps in Fe-based compound using two-band model. We find that our results are consistent with the previous work that concludes sign-reversal extended s-wave pairings between different Fermi surface sheets. The different superconducting gap magnitude around different Fermi surface sheets is probably due to the different density of states on them. This calculation can give insight to the recent angle-resolved photoemission (ARPES) experiments on these materials. To detect the phase variation of the superconducting gap over the Fermi surfaces, we propose a new method for measuring the particular wave vector phonon linewidth. In the case of the sign-reversal superconducting pairing, the linewidth shows continuities compared to the case of no phase variation.

A Fertile Majorana Habitat

Physics Most species of (near-) elementary particles (such as electrons and protons) have their own antiparticle partners with which they annihilate in a burst of gamma rays. Usually the antiparticle is distinct from the particle. An exotic exception to that rule is the spin-1/2 Majorana fermion, which is its own antiparticle. Although predicted more than 70 years ago, Majorana fermions are yet to be observed, with neutrinos and dark matter WIMPs being the prime candidates. Recently, another possible setting for Majoranas emerged in solid-state materials such as certain topological insulators (TIs) that have a linear electron dispersion but are also superconducting. On doping the TI Bi2Se3 with copper, a superconducting state was observed; however, calculations indicated that such doping would destroy the conditions necessary to support the Majorana state. Now, Wray et al. find in their angle-resolved photoemission experiments that this is not the case, raising a realistic possibility of a first observation of a Majorana fermion. Nat. Phys. 6 , 10.1038/nphys1762 (2010).

Dynamical charge and spin density wave scattering in cuprate superconductors

We show that a variety of spectral features of high-Tc cuprates can be understood on the basis of the coupling of charge carriers to some kind of dynamical order that we exemplify in terms of fluctuating charge and spin density waves. Two theoretical models that capture different aspects of such a dynamical scattering are investigated. The first approach leaves the ground state in the disordered phase but couples the electrons to bosonic degrees of freedom, corresponding to the quasi-singular scattering associated with closeness to an ordered phase. The second, more phenomological approach starts from the construction of a frequency-dependent order parameter that vanishes for small energies. Both theories capture scanning tunneling microscopy and angle-resolved photoemission experiments that suggest the protection of quasi-particles close to the Fermi energy but the manifestation of long-range order at higher frequencies.

Neutron and ARPES constraints on the couplings of the multiorbital Hubbard model for the iron pnictides

The results of neutron scattering and angle-resolved photoemission experiments for the Fe-pnictide parent compounds, and their metallic nature, are shown to impose severe constraints on the range of values that can be considered "realistic" for the intraorbital Hubbard repulsion U and Hund coupling J in multiorbital Hubbard models treated in the mean-field approximation. Phase diagrams for three- and five-orbital models are here provided, and the physically realistic regime of couplings is highlighted, to guide future theoretical work into the proper region of parameters of Hubbard models. In addition, using the random phase approximation, the pairing tendencies in these realistic coupling regions are investigated. It is shown that the dominant spin-singlet pairing channels in these coupling regimes correspond to nodal superconductivity, with strong competition between several states that belong to different irreducible representations. This is compatible with experimental bulk measurements that have reported the existence of nodes in several Fe-pnictide compounds.

Bulk vs. surface effects in ARPES experiment from La 2/3Sr 1/3MnO 3 thin films

Experiments directly probing the electronic states using angle-resolved photoemission (ARPES) were carried out on La 2/3Sr 1/3MnO 3 in order to elucidate its electronic properties. ARPES is a surface sensitive technique where bulk and surface states are usually both present. We present high-resolution ARPES studies in the (1 0 0) and (1 1 0) mirror planes and compare them with simulated ARPES based on GGA + U band structure calculations. In the (1 1 0) mirror plane we identify surface umklapps accounted by ( 2 × 2 ) R 45 ° surface reconstruction which couple to bulk electronic states. As predicted by the simulated spectra there is additional spectral intensity at the Fermi level detected in ARPES data due to k ⊥-broadening effects in the photoemission final states. We demonstrate that this additional spectral intensity is a convenient spectral marker for determination of the k F Fermi momenta.

Magnetic degeneracy and hidden metallicity of the spin-density-wave state in ferropnictides

We analyze spin-density-wave (SDW) order in iron-based superconductors and electronic structure in the SDW phase. We consider an itinerant model for Fe pnictides with two hole bands centered at (0,0) and two electron bands centered at $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi},0)$ in the unfolded Brillouin zone. A SDW order in such a model is generally a combination of two components with momenta $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi},0)$, both yield $(\ensuremath{\pi},\ensuremath{\pi})$ order in the folded zone. Neutron experiments, however, indicate that only one component is present. We show that $(0,\ensuremath{\pi})$ or $(\ensuremath{\pi},0)$ order is selected if we assume that only one hole band is involved in the SDW mixing with electron bands. A SDW order in such three-band model is highly degenerate for a perfect nesting and hole-electron interaction only but we show that ellipticity of electron pockets and interactions between electron bands break the degeneracy and favor the desired $(0,\ensuremath{\pi})$ or $(\ensuremath{\pi},0)$ order. We further show that stripe-ordered system remains a metal for arbitrary coupling. We analyze electronic structure for parameters relevant to the pnictides and argue that the resulting electronic structure is in good agreement with angle-resolved photoemission experiments. We discuss the differences between our model and ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model of localized spins.

Effect of electronic correlations on the quasiparticle dispersion of USb2

Angle resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band which LSDA predicts to osculate the Fermi-energy. The observed band is found to be depressed by 17 meV below the Fermi-energy, furthermore, the inferred quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi-energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band which does not cross the Fermi-energy is unprecedented. The origin of the observed depression of the band, its quasi-particle mass enhancement, and the characteristic energy are discussed on the basis of a theoretical model.

Theory of angle-resolved photoemission experiments on a two-band model

Considering the electron states inside and outside the solid, we derive a formula of photoemission intensity. A general theoretical way to determine electronic structures of solids from ARPES experiments is outlined. It is shown that the spectral function inside the solids cannot be measured directly by ARPES, the effects of free electron states on the electronic structure observed by ARPES measurements must be considered, and the results from ARPES experiments cannot be understood until these results have been made consistent with a theoretical calculation.

Electron–phonon coupling at surfaces and interfaces

Over the recent years, electronic surface states have been used for a detailed spectroscopic study of the electron–phonon (e–ph) interaction, both experimentally and theoretically. This review discusses the basic physics of e–ph coupling and how information can be obtained from angle-resolved photoemission experiments and first principles calculations. Several recent results for clean and adsorbate-covered surfaces, quantum wells and free-standing monolayers are also discussed.

First Direct Observation of a Nearly Ideal Graphene Band Structure

Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.

Evidence for Two Competing Order Parameters in Underdoped Cuprate Superconductors from a Model Analysis of Fermi-Arc Effects

Preformed pairs above T{c} and the two-gap scenarios are two main proposals for describing the low-doping pseudogap phase of high-T{c} cuprates. Very recent angle-resolved photoemission experiments have shown features which were interpreted as evidence for preformed pairs. Here it is shown that those results can be explained also in the context of the two-gap scenario if self-energy effects are considered. The discussion is based on the d charge-density wave theory or the flux phase of the t-J model.