Ion-induced electron-emission study of high-Tc superconductors and phase transitions

Abstract

We have investigated the phenomenon of kinetic ion-induced electron emission from high-temperature copper oxide superconductors. The total yield γT of electrons induced by H+ and Ar+ (0.8 MeV) from polycrystalline YBa2Cu3O7−δ has been measured as a function of the probe temperature (70 K ≤T≤300 K). It shows a minimum close to the superconducting phase-transition temperature of TC≊93 K. Electron-energy distributions (0 eV ≤E≤40 eV, 25°≤θ≤90°) from H+- and C+- (2-MeV) bombardment of single crystals of YBa2Cu307−δ and EuBa2Cu3O7−δ high-TC superconductors show the low-energy peak of ‘‘true secondary electrons’’ at Emax≊5 eV and in the case of YBa2Cu3O7−δ a further weak structure at E≊14 eV, which can possibly be attributed to Auger electron emission from yttrium. The electron angular distributions N(θ) have been measured at sample temperatures above (T=300 K) and below (T=35 K) the superconducting transition temperature (TC≊90 K) as a function of the angle of incidence (δ=55°,65°,70°,75°) of the projectiles. The ratio R=N(C+)/N(H+) of electron intensities from heavy C+ ion impact to the electron intensities from proton impact shows broad peaks (Δθ≊25°) at low electron energies E<20 eV. The mean emission angle θem of the excess electrons belonging to this peak shifts to higher angles θ with increasing δ. This can be explained by the directed emission of so-called ‘‘shock electrons’’ perpendicular to the conical charge density fluctuations (‘‘wake’’) caused by the ion. The experimental results are in agreement with calculations within the framework of a simple model, which takes into account the refraction of low-energy electrons at surfaces, as well as the electronic structure of the copper oxide superconductors. They are characterized by two different collective excitation frequencies, i.e., the plasma frequency ωPFC of the free charge carriers (O 2p holes) (ħωPFC≊1.5 eV) and the volume plasma frequency ωPVP of the collective excitation of all the electrons in one unit cell of the crystals (ħωPVP≊25 eV). Finally, the influence of magnetic and structural phase transitions and, in particular, the superconducting phase transition at T=TC on ion-induced electron emission from solids is discussed.

Source-reported materials — not catalogue approval

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