Positron annihilation in copper: Ak-space analysis

Electron momentum density studies in high-Tcmaterials by positron annihilation spectroscopy: theory and experiment

“…The 2~ EMD is nearly constant, a result close to that expected from filled bands [40,43]. Small residual variations of this order (-2%) have been observed in the LCW analysis of ID ACPAR data for the semiconductor (narrow band gap insulator) germanium [42]. Thus, it is more likely that the residual variation originates from the positronic wavefunction differently overlapping different electronic states [41], and/or from electron-electron correlation effects.…”

Fermiology of Cr and Mo

“…Since the integration direction [110] is common to the Cr and Mo projections in figures 2 and 3, an interpretation in terms of directional behaviour (as in [4]) would require the positron wavefunction 'cosines' to have much larger amplitudes in Cr. Another possibility is the existence of disordered local moments in Cr above T N , and the effect that these might have on the electronic structure.…”

“…Although the LCW theorem is only valid in the case of a constant (k-independent) positron wavefunction, these experiments have shown that the perturbations introduced are generally small enough not to mask the FS-related effects. Projection-dependent perturbations have been considered by Lock and West [4], where the LCW distributions from 1D-ACAR experiments on Cu along two different directions were considered. In one direction, [110], the distribution closely reflected the FS topology, but in the other ([100]), the distribution was heavily affected by the positron wavefunction.…”

Fermi surface determination from momentum density projections

“…The simplest option is to transform the density from pto k-space, i.e., subject it to the so-called LCW folding (Lock et al 1973), and define the Fermi surface as an iso-density contour, e.g., according to a maximum gradient criterion (Biasini et al 2002). The problem with such an approach in positron annihilation experiments is that due to the inequivalence between the measured two-photon momentum density and the electron momentum density, filled bands give rise to a non-constant background (Lock & West 1975), so that for finite resolution the Fermi surface does not strictly correspond to any iso-density contour. Edge-detection or enhancement methods (e.g., Dugdale et al 1994, O'Brien et al 1995 should be able to obviate this issue, but also in this case finite experimental resolution will tend to smooth regions of the Fermi surface with high curvature.…”