Hole Spectral Function Research Articles

SELF-CONSISTENT TREATMENT OF THE SELF-ENERGY IN NUCLEAR MATTER

The influence of hole–hole propagation in addition to the conventional particle–particle propagation on the energy per nucleon and the momentum distribution is investigated. The results are compared to the Brueckner–Hartree–Fock (BHF) calculations with a continuous choice and a conventional choice for the single-particle spectrum. Also, the structure of nucleon self-energy in nuclear matter is evaluated. All the off-shell self-energies are calculated self-consistently. Using the self-consistent self-energy, the hole and particle self-consistent spectral functions including the particle–particle and hole–hole ladder contributions in nuclear matter are calculated using realistic NN interactions. We found that the hole–hole ladder brought about non-negligible contributions to the nuclear matter binding energy per nucleon.

Quasiparticle vanishing driven by geometrical frustration

We investigate the single hole dynamics in the triangular t-J model. We study the structure of the hole spectral function, assuming the existence of a 120 magnetic Neel order. Within the self-consistent Born approximation (SCBA) there is a strong momentum and t sign dependence of the spectra, related to the underlying magnetic structure and the particle-hole asymmetry of the model. For positive t, and in the strong coupling regime, we find that the low energy quasiparticle excitations vanish outside the neighbourhood of the magnetic Goldstone modes; while for negative t the quasiparticle excitations are always well defined. In the latter, we also find resonances of magnetic origin whose energies scale as (J/t)^2/3 and can be identified with string excitations. We argue that this complex structure of the spectra is due to the subtle interplay between magnon-assisted and free hopping mechanisms. Our predictions are supported by an excellent agreement between the SCBA and the exact results on finite size clusters. We conclude that the conventional quasiparticle picture can be broken by the effect of geometrical magnetic frustration.

Spin dependent many-body effects in the photoemission of Co

Many-body effects influence the energy versus momentum relation that is measured in angle-resolved photoemission experiments and the quasi-particle band structure may be significantly different from what is deduced within the independent particle model. In the case of cobalt many body effects are even more drastic than an energy renormalization giving rise to a quenching of quasi-particle peaks. Augmenting ab-initio band structure with many-body e–e interactions, we have obtained spin- and k-dependent self-energies, hole spectral functions and quasi-particle energies to be compared with photoemission spectra; our results show that e–e correlations are responsible for strong spin-dependent energy renormalizations.

Spherical Hartree-Fock calculations with linear-momentum projection before the variation

The hole spectral functions and from these the spectroscopic factors have been calculated in a Galilei-invariant way for the ground-state wave functions resulting from spherical Hartree-Fock calculations with projection onto zero total linear momentum before the variation for the nuclei 4He, 12C, 16O, 28Si, 32S and 40Ca. The results are compared to those of the conventional approach which uses the ground states resulting from usual spherical Hartree-Fock calculations subtracting the kinetic energy of the center-of-mass motion before the variation and to the results obtained analytically with oscillator occupations.

Resonance peak in underdoped cuprates

The magnetic susceptibility measured in neutron-scattering experiments in underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ is interpreted based on the self-consistent solution of the $t\ensuremath{-}J$ model of a Cu-O plane. The calculations reproduce correctly the frequency and momentum dependencies of the susceptibility and its variation with doping and temperature in the normal and superconducting states. This allows us to interpret the maximum in the frequency dependence---the resonance peak---as a manifestation of the excitation branch of localized Cu spins and to relate the frequency of the maximum to the size of the spin gap. The low-frequency shoulder well resolved in the susceptibility of superconducting crystals is connected with a pronounced maximum in the damping of the spin excitations. This maximum is caused by intense quasiparticle peaks in the hole spectral function for momenta near the Fermi surface and by the nesting.

Nuclear matter spectral function in the Bethe-Brueckner-Goldstone approach

The microscopic many-body theory of the Nuclear Equation of State is discussed in the framework of the Bethe-Brueckner-Goldstone method. The expansion is extended up to the three hole-line diagrams contribution. Within the same scheme, the hole spectral function is calculated in nuclear matter to assess the relevance of nucleon-nucleon short-range correlations. The calculation is carried out by using several nucleon-nucleon realistic interactions. Results are compared with other approaches based on variational methods and transport theory. Discrepancies appear in the high-energy region, which is sensitive to short-range correlations, and are due to the different many-body treatment more than to the specific NN interaction used. Both nuclear matter Equation of State and spectral function appear to be dominated by two-body correlations.

Two-dimensional t-J model at moderate doping

Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 <= x <= 0.16 and temperatures 0.01t <= T <= 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes and other types of phase separation are not connected with the strong electron correlations described by the model. A series of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has a pseudogap in the vicinity of (0,\pi) and (\pi,0). For x \approx 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (\pm\pi/2,\pm\pi/2) to two large rhombuses centered at (0,0) and (\pi,\pi). The calculated temperature and concentration dependencies of the spin correlation length and the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum of the model.

Coordinate representation of the one-spinon one-holon wave function and spinon-holon interaction

By deriving and studying the coordinate representation for the one-spinon one-holon wavefunction we show that spinons and holons in the supersymmetric $t - J$ model with $1/r^2$ interaction attract each other. The interaction causes a probability enhancement in the one-spinon one-holon wavefunction at short separation between the particles. We express the hole spectral function for a finite lattice in terms of the probability enhancement, given by the one-spinon one-holon wavefunction at zero separation. In the thermodynamic limit, the spinon-holon attraction turns into the square-root divergence in the hole spectral function.

The hole spectral function in the finite temperature Green's function scheme

Using the finite temperature (Matsubara) Green's function method, the hole spectral function is calculated in a representation where holes are described as spinless fermions (holons) and spins as normal bosons. The holon self-energy term for multiple spin-wave processes is analytically determined with the spin polaron Hamiltonian as the interaction term in the S-matrix. The usual prescription of analytic continuation then enables us to obtain the retarded expression from the Matsubara self-energy term yielding the holon spectral function.

Nuclear matter hole spectral function in the Bethe–Brueckner–Goldstone approach

The hole spectral function is calculated in nuclear matter to assess the relevance of nucleon-nucleon short range correlations. The calculation is carried out within the Brueckner scheme of many-body theory by using several nucleon-nucleon realistic interactions. Results are compared with other approaches based on variational methods and transport theory. Discrepancies appear in the high energy region, which is sensitive to short range correlations, and are due to the different many-body treatment more than to the specific N-N interaction used. Another conclusion is that the momentum dependence of the G-matrix should be taken into account in any self consistent approach.

Spinon-holon attraction in the supersymmetric t-J model with 1/r(2) interaction.

We derive the coordinate representation of the one-spinon one-holon wave function for the supersymmetric t-J model with 1/r(2) interaction. This result allows us to show that a spinon and a holon attract each other at short distance. The attraction gets stronger as the size of the system is increased and, in the thermodynamic limit, it is responsible for the square-root singularity in the hole spectral function [Y. Kato, Phys. Rev. Lett. 81, 5402 (1998)].

Quantum decoherence in the spectral functions of undoped lamno3.

We study the one hole spectral function in a model for LaMnO3 including both the effects of orbital ordering and the quantum decoherence due to the antiferromagnetic coupling between the ferromagnetic layers. We find that the classical picture of a ferromagnetic polaron does not apply and free dispersion is replaced by rigid quasiparticles on the scale of magnetic excitations, while the spectra are dominated by the incoherent spectral weight at high energies. These results have important implications on the in-plane transport and angular resolved photoemission in the manganites.

Single-hole spectral function and spin-charge separation in thet−Jmodel

Worm algorithm quantum Monte Carlo simulations of the hole Green function with subsequent spectral analysis were performed for J/t 0.1, 0.2, 0.4 on lattices with up to LxL=32x32 sites at temperatures as low as T=J/40, and present, apparently, the hole spectral function in the thermodynamic limit. Spectral analysis reveals a delta-function-sharp quasiparticle peak at the lower edge of the spectrum which is incompatible with the power-law singularity and thus rules out the possibility of spin-charge separation in this parameter range. Spectral continuum features two peaks separated by a gap ~4t.

Evidence of electron fractionalization from photoemission spectra in the high temperature superconductors.

In the normal state of the high temperature superconductors Bi(2)Sr(2)CaCu(2)O(8+delta) and La2(-x)Sr(x)CuO4, and in the related "stripe ordered" material, La(1.25)Nd(0.6)Sr(0.15)CuO4, there is sharp structure in the measured single hole spectral function, A<(k-->,omega), considered as a function of k--> at fixed small binding energy omega. At the same time, as a function of omega at fixed k--> on much of the putative Fermi surface, any structure in A<(k-->,omega), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one-dimensional electron gas.

Spectral functions for the Tomonaga-Luttinger and Luther-Emery liquids

We calculate the finite temperature single hole spectral function and the spin dynamic structure factor of spinfull one-dimensional Tomonaga-Luttinger liquid. Analytical expressions are obtained for a number of special cases. We also calculate the single hole spectral function of a spin gapped Luther-Emery liquid and obtain exact results at the free fermion point K_s=1/2. These results may be applied to the analysis of angle resolved photoemission and neutron scattering experiments on quasi-one-dimensional materials.

Angle-resolved photoemission in doped charge-transfer Mott insulators

A theory of angle-resolved photoemission (ARPES) in doped cuprates and other charge-transfer Mott insulators is developed taking into account the realistic (LDA+U) band structure, (bi)polaron formation due to the strong electron-phonon interaction, and a random field potential. In most of these materials the first band to be doped is the oxygen band inside the Mott-Hubbard gap. We derive the coherent part of the ARPES spectra with the oxygen hole spectral function calculated in the non-crossing (ladder) approximation and with the exact spectral function of a one-dimensional hole in a random potential. Some unusual features of ARPES including the polarisation dependence and spectral shape in YBa2Cu3O7 and YBa2Cu4O8 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.

Theoretical study of the4plevel photoemission spectra of Ba metal and related systems

The $4p$ hole spectral function of Ba metal is calculated by an ab initio atomic Green's-function method. The atom-metal energy shifts of the interacting hole (particle) states are taken into account. It is shown that for a consistent description of the spectral line positions, intensities, and linewidths, it is necessary to take into account explicitly such energy shifts. The $4p$ x-ray photoelectron spectroscopy spectra of Ba metal and compounds are analyzed in detail. The $2s\ensuremath{-}4p$ x-ray emission spectroscopy spectrum of La metal is also discussed. For the $4p$ level ionization of Ba metal and compounds, the one-electron approximation breaks down. This is due to the Coster-Kronig (CK) ${4p\ensuremath{-}4d}^{\ensuremath{-}2}4f$ process by which an empty $4f$ level is filled by a $4d$ electron rather than by a ligand valence electron [charge-transfer (CT) screening] or a conduction-band electron. A possibility of suppression of both CK and CT core-hole screening in a final state by decay of a resonantly excited core-hole state and a fully relaxed core-hole state where an $4f$ empty level is already occupied in an initial core-hole state is discussed.

Spin-polaron damping in the spin-fermion model for cuprate superconductors

A self-consistent, spin rotational invariant Green's function procedure has been developed to calculate the spectral function of carrier excitations in the spin-fermion model for the CuO2 plane. We start from the mean field description of a spin polaron in the Mori-Zwanzig projection method. In order to determine the spin polaron lifetime in the self-consistent Born approximation, the self-energy is expressed by an irreducible Green's function. Both, spin polaron and bare hole spectral functions are calculated. The numerical results show a well pronounced quasiparticle peak near the bottom of the dispersion at (pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather large damping away from the minimum and an asymmetry of the spectral function with respect to the antiferromagnetic Brillouin zone. These findings are in qualitative agreement with photoemission data for undoped cuprates. The direct oxygen-oxygen hopping is responsible for a more isotropic minimum at (pi/2,pi/2).

Dynamics of spin ladders

We derive an approximate theory for Heisenberg spin ladders with two legs by mapping the spin dynamics onto the problem of hard-core `bond-Bosons'. The parameters of the Bosonic Hamiltonian are obtained by matching anomalous Green's functions to Lanczos results and we find evidence for a strong renormalization due to quantum fluctuations. Various dynamical spin correlation functions are calculated and found to be in good agreement with Lanczos results. We then enlarge the effective Hamiltonian to describe the coupling of the bond-Bosons to a single hole injected into the system and treat the hole-dynamics within the `rainbow-diagram' approximation by Schmidt-Rink et. al. Theoretical predictions for the single hole spectral function are obtained and found to be in good agreement with Lanczos results.

Theoretical study of the valence level photoemission spectrum of C 6 adsorbed on Ni, Pd and Pt metal surfaces

The valence hole spectral functions of C 6, NiC 6, PdC 6 and PtC 6 are calculated by the ab initio third-order algebraic-diagrammatic-construction (ADC(3)) Green function method using an extended basis set. The calculations for XC 6, (X = Ni, Pd, Pt) were performed, assuming the top site adsorption of the two ring forms of C 6. These are the benzene structure ( 1A 1g) possessing D 6h symmetry and a distorted cyclic form ( 1A' t) of D 3h symmetry. The changes in spectral features (spectral intensity and energy level separations) from the free molecule to the adsorbate are predicted. The possibility of using the adsorbate data to determine the geometric structure of the adsorbed C 6 molecule is discussed.