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Title: Electron-phonon superconductivity in Y and La-based intermetallics and metallic magnetic frustration in PdCrO₂
Author: Billington , David
ISNI:       0000 0004 5924 3138
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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The electronic structure of several complex metallic systems of current interest, namely YIn3, YIr2Si2, LaIr2Si2 and PdCr02, has been investigated. The emphasis of these investigations has been on the role of the Fermi surface in the electronic ordering and phase behaviour of these materials. For the cubic intermetallic compound YIn3, first-principles calculations of the electron-phonon coupling were performed. The electron-phonon coupling constant was found to be /\ = 0.42. Using the Allen-Dynes formula with a Coulomb pseudopotential of JL* = 0.10, a Tc of approximately 0.77 K is obtained which is reasonably consistent with the experimentally observed temperature (between 0.8 K and 1.1 K). The results indicate that conventional electron-phonon coupling is capable of producing the superconductivity in this compound. For the superconducting high-temperature polymorphs of Ylr2Si2 and LaIr2Si2 , ab initio calculations of the electronic structure and lattice-vibrational properties were performed. The electron-phonon coupling constants were found to be 0.61 and 0.56 for the high-temperature polymorphs of YIr2Si2 and LaIr2Si2, respectively. In both compounds, the electron-phonon coupling is dominated by the low-energy phonon modes. The superconducting critical temperatures estimated from the Allen-Dynes formula agree well with the available experimental data and indicate that the superconductivity in these compounds can be explained by intermediate strength electron-phonon coupling. For t.he highly frustrated metallic magnet PdCr02, single-crystal neutron diffraction was used to determine the structure and symmetry of the short-range magnetic order above the ordering temperature. Further to this, high-resolution x-ray Compton scattering was used to determine the paramagnetic bulk Fermi surface. Through comparison with electronic structure calculations , a previously unseen sheet of Fermi surface was revealed . Calculations of the generalised susceptibility revealed that this sheet is nest ed at a wave-vector equal to the magnetic ordering vector. This is also where the diffuse magnetic scattering intensity is greatest. By calculating the k-dependence of the susceptibility, the electronic states that contribute to the electronic response were revealed. These results indicate that the itinerant electrons on the nested Fermi surface sheet compete with the local frustrated magnetic interactions, culminating in the observed low-temperature magnetic ordering.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available