Charge-density-waves in quasi-one and quasi-two-dimensional metallic crystal systems

Thesis

In this thesis I present experimental measurements on a number of different quasi-one and quasi-two-dimensional metallic crystal systems susceptible to density-wave formation.

I outline the discovery of a density-wave superstructure found via X-ray diffraction measurements in the quasi-two-dimensional Na₂Ti₂As₂O and Na₂Ti₂Sb₂O compounds. Na₂Ti₂Sb₂O and Na₂Ti₂As₂O are members of the Ti-based oxy-pnictides a group of compounds which exhibit complex phase diagrams and share structural similarities with the high temperature superconductors. Temperature-dependent X-ray diffraction measurements confirmed the superstructure in both materials to be concomitant with transitions seen in resistivity and magnetic data. The observation of the superstructure combined with results from other experimental techniques demonstrated the transition to be a charge-density-wave.

I also present results on a series of intercalated charge-density-wave compounds Ni_xZrTe₃. Ni_xZrTe₃ was measured using X-ray diffraction and ARPES to investigate the effects of chemical pressure on charge-density-wave formation. The transition temperature for density-wave formation in this series of compounds had been previously shown to vary as a result of Ni-content. X-ray diffraction measurements on the series revealed no changes in the wavevector of the associated superstructure modulation across the series. However ARPES measurements on Ni_xZrTe₃ showed subtle changes in the binding energy of the one-dimensional band associated with the charge-density-wave thought to be a result of the Ni-intercalation. Through a combination of XPS, EDX and ARPES measurements the Ni-content in these crystals was deduced to be much lower than growth parameters suggested.

Finally I describe the construction and testing of a straining device designed specifically for use with X-ray synchrotron type measurements. The straining device was successfully tested at the I16 beamline at the Diamond Light Source and shown to induce dynamic strain in a test sample of M₂Mo₆Se₆. Further testing at the ID28 beamline at the ESRF revealed that the strain induced in a M₂Mo₆Se₆ was significant and resulted in a change in the lattice dynamics of the material.

Authors: Gannon, L

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Keywords: Condensed Matter Physics; charge density waves
• Subjects: Condensed matter