Repository logo
 

Interplay of charge density modulations and superconductivity

dc.contributor.advisorTanaka, Kaorien_US
dc.contributor.committeeMemberSowa, Arturen_US
dc.contributor.committeeMemberGhezelbash, Masouden_US
dc.contributor.committeeMemberTse, Johnen_US
dc.creatorSadowski, Jason Wayneen_US
dc.date.accessioned2011-04-13T15:27:52Zen_US
dc.date.accessioned2013-01-04T04:29:07Z
dc.date.available2012-04-15T08:00:00Zen_US
dc.date.available2013-01-04T04:29:07Z
dc.date.created2011-04en_US
dc.date.issued2011-04en_US
dc.date.submittedApril 2011en_US
dc.description.abstractRecent studies of the transition metal dichalcogenide niobium diselenide have led to debate in the scientific community regarding the mechanism of the charge density wave (CDW) instability in this material. Moreover, whether or not CDW boosts or competes with superconductivity (SC) is still unknown, as there are experimental measurements which supports both scenarios. Motivated by these measurements we study the interplay of charge density modulations and superconductivity in the context of the Bogoliubov de-Gennes (BdG) equations formulated on a tight-binding lattice. As the BdG equations require large numerical demand, software which utilizes parallel algorithms have been developed to solve these equations directly and numerically. Calculations were performed on a large-scale Beowulf-class PC cluster at the University of Saskatchewan. We first study the effects of inhomogeneity on nanoscale superconductors due to the presence of surfaces or a single impurity deposited in the sample. It is illustrated that CDW can coexist with SC in a finite-size s-wave superconductor. Our calculations show that a weak impurity potential can lead to significant suppression of the superconducting order parameter, more so than a strong impurity. In particular, in a nanoscale d-wave superconductor with strong electron-phonon coupling, the scattering by a weakly attractive impurity can nearly kill superconductivity over the entire sample. Calculations for periodic systems also show that CDW can coexist with s-wave superconductivity. In order to identify the cause of the CDW instability, the BdG equations have been generalized to include the next-nearest neighbour hopping integral. It is shown that the CDW state is strongly affected by the magnitude of the next-nearest neighbour hopping, while superconductivity is not. The difference between the CDW and SC states is a result of the anomalous, or off-diagonal, coupling between particle and hole components of quasiparticle excitations. The Fermi surface is changed as next-nearest neighbour hopping is varied; in particular, the perfect nesting and coincidence of the nesting vectors and the vectors connecting van Hove singularities (vHs) for zero next-nearest neighbor hopping is destroyed, and vHs move away from the Fermi energy. It is found that within our one-band tight-binding model with isotropic s-wave superconductivity, CDW and SC can coexist only for vanishing nearest neighbor hopping and for non-zero hopping, the homogeneous SC state always has the lowest ground-state energy. Furthermore, we find in our model that as the magnitude of the next-nearest neighbor hopping parameter increases, the main cause of the divergence in the dielectric response accompanying the CDW transition changes from nesting to the vHs mechanism proposed by Rice and Scott. It is still an open question as to the origin of CDW and its interplay with SC in multiple-band, anisotropic superconductors such as niobium diselenide, for which fundamental theory is lacking. The work presented in this thesis demonstrates the possible coexistence of charge density waves and superconductivity, and provides insight into the mechanism of electronic instability causing charge density waves.en_US
dc.identifier.urihttp://hdl.handle.net/10388/etd-04132011-152752en_US
dc.language.isoen_USen_US
dc.subjectvan Hove singularityen_US
dc.subjectcharge density waveen_US
dc.subjectsuperconductivityen_US
dc.subjectstrongly correlated electronsen_US
dc.subjectBCS theoryen_US
dc.titleInterplay of charge density modulations and superconductivityen_US
dc.type.genreThesisen_US
dc.type.materialtexten_US
thesis.degree.departmentPhysics and Engineering Physicsen_US
thesis.degree.disciplinePhysics and Engineering Physicsen_US
thesis.degree.grantorUniversity of Saskatchewanen_US
thesis.degree.levelMastersen_US
thesis.degree.nameMaster of Science (M.Sc.)en_US

Files

Original bundle
Now showing 1 - 1 of 1
Loading...
Thumbnail Image
Name:
sadowski_Thesis.pdf
Size:
9.36 MB
Format:
Adobe Portable Document Format
License bundle
Now showing 1 - 1 of 1
No Thumbnail Available
Name:
license.txt
Size:
905 B
Format:
Plain Text
Description: