ARPES Spectra Research Articles

Bi- and tri-layer splittings in a new formulation for the t–J model of cuprates

The t–J model of the cuprates is formulated in a novel fashion in terms of a single spin and charge particles along with flux tubes. The theory has been developed [1,2] to simulate ARPES spectra and this earlier formulation is here applied to compare the k={π,0} spectra for mono-, bi- and tri-layer Bi cuprates.

Theory of angle-resolved photoemission spectroscopy intensities from theCuO2plane

We present a theory for the photon energy and polarization dependence of ARPES intensities from the CuO$_2$ plane in the framework of strong correlation models. We show that for electric field vector in the CuO$_2$ plane the `radiation characteristics' of the $O$ $2p_\sigma$ and $Cu$ $3d_{x^2-y^2}$ orbitals are strongly peaked along the CuO$_2$ plane, i.e. most photoelectrons are emitted at grazing angles. This suggests that surface states play an important role in the observed ARPES spectra, consistent with recent data from Sr$_2$CuCl$_2$O$_2$. We show that a combination of surface state dispersion and Fano resonance between surface state and the continuum of LEED-states may produce a precipitous drop in the observed photoelectron current as a function of in-plane momentum, which may well mimic a Fermi-surface crossing. This effect may explain the simultaneous `observation' of a hole-like and an electron-like Fermi surfaces in Bi2212 at different photon energies. We show that by suitable choice of photon polarization one can on one hand `focus' the radiation characteristics of the in-plane orbitals towards the detector and on the other hand make the interference between partial waves from different orbitals `more constructive'.

Superconducting gap in the presence of bilayer splitting in underdoped(Pb,Bi)2Sr2CaCu2O8+δ

The clearly resolved bilayer splitting in ARPES spectra of the underdoped Pb-Bi2212 compound rises the question of how the bonding and antibonding sheets of the Fermi surface are gapped in the superconducting state. Here we compare the superconducting gaps for both split components and show that within the experimental uncertainties they are identical. By tuning the relative intensity of the bonding and antibonding bands using different excitation conditions we determine the precise {\bf k}-dependence of the leading edge gap. Significant deviations from the simple cos($k_{x}$)-cos($k_{y}$) gap function for the studied doping level are detected.

Polarized ARPES spectra of undoped cuprates

The spectral density (SD) in the ARPES spectra of antiferromagnetic (AFM) dielectrics Sr2CuO2Cl2 and Ca2CuO2Cl2 along the principal symmetry directions of the Brillouin zone was studied by the generalized tight binding method. At the valence band top of these undoped cuprates in the AFM state, there is a pseudogap of magnetic nature with Es(k)∼0–0.4 eV between a virtual level and the valence band proper. The observed similarity of dispersion along the Γ-M and X-Y directions can be explained by the proximity of the 3B1g triplet and the Zhang-Rice singlet levels. The value of parity of the polarized ARPES spectra at the Γ, M, and X points calculated for the AFM phase of undoped cuprates with an allowance for the partial contributions is even. The conditions favoring observation of the partial contributions in polarized ARPES spectra are indicated. Due to the spin fluctuations, the virtual level acquires dispersion and possesses a small spectral weight. Probably, this level cannot be resolved on the background of the main quasi-particle peak as a result of the damping effects.

Doping-induced inhomogeneity in high- Tc superconductors

Doping and disorder are inseparable in the superconducting cuprates. Assuming the simplest possible disordered doping, we construct a semiphenomenological model and analyze its experimental consequences. Among the affected experimental quantities are the ARPES spectra and thermodynamic properties. From our model we make a prediction for the width of the local superconducting gap distribution with the only experimentally unknown parameter being the superconducting correlation length. Thus, our model provides a direct way of determining the superconducting correlation length from a known experimental gap distribution.

Superconducting phase coherence in the presence of a pseudogap: Relation to specific heat, tunneling, and vortex core spectroscopies

In this paper we demonstrate how, using a natural generalization of BCS theory, superconducting phase coherence manifests itself in phase insensitive measurements, when there is a smooth evolution of the excitation gap \Delta from above to below Tc. In this context, we address the underdoped cuprates. Our premise is that just as Fermi liquid theory is failing above Tc, BCS theory is failing below. The order parameter \Delta_{sc} is different from the excitation gap \Delta. Equivalently there is a (pseudo)gap in the excitation spectrum above Tc which is also present in the underlying normal state of the superconducting phase, and can be directly inferred from specific heat and vortex core experiments. At the same time many features of BCS theory, e.g., fermionic quasiparticles below Tc, are clearly present. These observations can be reconciled by a natural extension of BCS theory, which includes finite center-of-mass momentum pair excitations, in addition to the usual fermionic quasiparticles. Applying this theory we find that the Bose condensation of Cooper pairs, which is reflected in \Delta_{sc}, leads to sharp peaks in the spectral function once $T \le T_c$. These are manifested in ARPES spectra as well as in specific heat jumps, which become more like the behavior in a \lambda transition as the pseudogap develops. We end with a discussion of tunneling experiments and condensation energy issues. Comparison between theoretical and experimental plots of C_v, and of tunneling and vortex core spectroscopy measurements is good.

COMPETING QUANTUM ORDERINGS IN CUPRATE SUPERCONDUCTORS: A MINIMAL MODEL

We present a minimal model for cuprate superconductors. At the unrestricted mean-field level, the model produces homogeneous superconductivity at large doping, striped superconductivity in the underdoped regime and various antiferromagnetic phases at low doping and for high temperatures. On the underdoped side, the superconductor is intrinsically inhomogeneous and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. The model is applied to calculate experimentally measurable ARPES spectra.

Two different types of pseudogaps in high- Tc superconductors

Ultrahigh-resolution ( ΔE=7 meV angle-resolved (ARPES) and angle-integrated (AIPES) photoemission spectroscopy have been performed on optimally doped Bi 2Sr 2CaCu 2O 8 (Bi2212) and La 1.85Sr 0.15CuO 4 (LSCO). The ARPES spectrum of Bi2212 at the (π, 0)–(π, π) crossing shows a pseudogap at E F above T c, which is smoothly connected from the superconducting gap across T c and closes slightly above T c (T ∗=110 K). On the other hand, the AIPES spectra both in Bi2212 and LSCO show a remarkable depletion of the density of states in a relatively wider energy (100–150 meV) region near E F. The temperature-induced filling of the depletion persists far above T c and T ∗. This suggests that there are two different pseudogaps in the high- T c superconductors; one is likely associated with pairing and the other is attributed to the development of antiferromagnetic correlation.

Angle-resolved photoemission in YBa 2Cu 3O 7 and YBa 2Cu 4O 8

We derive the coherent part of the ARPES spectra with the exact spectral function of a one-dimensional hole in a random potential, taking into account bipolaron formation due to the strong long-range electron-phonon interaction. Some unusual features of ARPES including the polarization dependence and spectral shape in YBa 2Cu 3O 7 and YBa 2Cu 4O 8 are described without any Fermi-surface, large or small. The theory is compatible with the doping dependence of kinetic and thermodynamic properties of cuprates as well as with the d-wave symmetry of the superconducting order parameter.

ARPES for real-space d-wave pairs

Motivated by recent experiments on high-temperature superconductors (HTSCs) which have revealed a normal-state pseudogap, we consider a simple model of real-space d-wave pairs in the infinitely low-density limit. We calculate the excitation spectrum and hence deduce the form of an ARPES spectrum. While we do not claim that this is an accurate model for the cuprates, we believe it provides some insight into possible approaches.

Collective modes in high-temperature superconductors

The role of collective modes in various experiments on the cuprates is investigated. We calculate the neutron scattering, photoemission (ARPES), and Raman scattering intensities below $T_c$ within the fluctuation-exchange (FLEX) approximation for the two-dimensional Hubbard model. It is shown that the large peak in the dynamical spin susceptibility arises from a weakly damped spin-density-wave collective mode. This gives rise to a dip between the sharp low energy peak and the higher binding energy hump in the ARPES spectrum. Furthermore, we show that the collective mode of the amplitude fluctuation of the d-wave gap yields a broad peak above the pair-breaking threshold in the $B_{1g}$ Raman spectrum.

Influence of the orthorhombic distortion on the van Hove singularities in the DOS and ARPES spectra of layered cuprates

Four atom states Cu3dx2− y2, Cu4s, Oa2pxare involved in a tight-binding model for the superconducting CuO2plane. The orthorhombic distortion is taken into account by the differences of Cu–O hopping amplitudes and single-site oxygen energies εaand εbof two oxygen positions in the elementary cell as well. An effective ‘oxygen’ Hamiltonian including only the electron amplitudes at the oxygen ions is derived. Simple expressions for the constant energy contours and the Fermi surface are obtained and they qualitatively describe the photoemission spectra. Extended saddle points nearp=(π,0) andp=(0,π) are observed in qualitative agreement with the ARPES data. The van Hove singularities of the density of states (DOS) related to the extended saddle points are calculated by a Monte Carlo method. It is found that the splitting of the singularity of the DOS at the bottom of the conduction band is created by the energy difference εa− εb=2δ.

Fermi liquids and non-Fermi liquids—The view from photoemission

This paper presents an improved Fermi liquid lineshape analysis of the ARPES spectra of 1TTiTe 2, and a brief overview of recent efforts to identify Luttinger liquid type behavior in photoemission spectra of quasi one-dimensional materials such as K 0.3MoO 3

An electron spectroscopy study of manganese on silver (100)

The growth mode and magnetic behaviour of Mn deposited on Ag(100) have been studied using the techniques of LEED, AES and PES. Overlayer growth occurs through Mn atoms occupying fourfold hollow sites in a c(2*2) arrangement; at higher coverages, a disordered simultaneous multi-layer growth is observed. Measurements of the Mn 3s multiplet splitting indicate a significantly lower magnetic moment than the bulk value for coverages up to 0.5 monolayers. ARPES spectra of the Mn 3d valence states, as a function of coverage, are also presented.

Mapping bulk Ge electronic energy bands along Δ using ARPES spectra of the (001) 2 × 1 surface

The E versus k dispersion relations of the second and third bulk valence bands as well as one conduction band of Ge along the Δ line of the Brillouin Zone have been experimentally determined from angle-resolved photoelectron spectra collected at the Stanford Synchrotron Radiation Laboratory. The experimentally observed bands show good agreement with those calculated using the empirical pseudopotential parameters of Chelikowsky and Cohen for bulk Ge, even though the Ge(001) surface is reconstructed and ARPES is generally considered to be a surface sensitive technique.

Angle-Resolved Photoemission and Band Structure of Copper

The triangulation method, the appearance angle method and for the first time the surface emission method have been used for the determination of the electronic band structure of copper along the [001], [110] and [111] direction from ARPES spectra taken on the (110) and (111) faces. The experimentally derived bands are compared with first-principles calculations. For the bands up to energies of 20 eV above the Fermi energies it is found that Burdick's band calculation is in good agreement with experiment. Together with published normal emission data an almost complete experimental band structure below the Fermi energy is derived which clearly shows the spin-orbit splitting of the d-bands.

On the temperature behavior of second sound and Poiseuille flow

We examined in detail the geometric and electronic structure of thin In chains on vicinal Si(111) surfaces by means of low energy electron diffraction and ultrahigh-resolution photoemission as a function of temperature. Our data reveal a transition around T c = 115 K from a high temperature ( 4×1)- to a low temperature ( 8×2)-phase being reversible with a small hysteresis of the order 10 K. ARPES spectra exhibit clearly important concomitant changes in the electronic band structure near the Fermi surfaces and at the border of the surface Brillouin zones. We derive the dispersive behavior of the bands involved in the transition in detail and demonstrate that at least two surface state bands m2 and m3 show the opening of a pseudo energy gap on the Fermi surface leaving small but finite spectral weight in the low-temperature state. We conclude that this transition is probably driven by a similar but more complex mechanism than in a conventional Peierls transition.