Exploring the Fermi surfaces of novel quantum materials using high magnetic fields

Thesis

This thesis presents the results of torque magnetometry and resistivity measurements of the electronic structure of novel quantum materials, specifically using the techniques of quantum oscillations and angle-dependent magnetoresistance oscillations. Measurements of the Fermi surfaces of these materials, alongside comparisons to the electronic structure predicted by density functional theory calculations, can elucidate much about the novel physical properties they exhibit and the competing interactions which govern their phase diagrams.

The first system studied is the Iron-based superconductor FeSe_1-xS_x, an isoelectronically doped version of a system of much current interest, FeSe. Doping up to x = 0.2 is found to suppress the structural transition in this system entirely, with superconductivity continually present at low temperatures. Shubnikov-de Haas measurements across this range find a small quasi-two dimensional Fermi surface that increases in size and warping continuously with doping, with orbital dependent effective masses that do not change significantly within the orthorhombic phase.

The second material studied is the antiferromagnetic intermetallic CeZn₁₁ which, featuring an unpaired 4f electron, is considered a possible candidate for heavy fermion behaviour. De Haas-van Alphen oscillations are seen once the antiferromagnetic phase is suppressed, and comparable frequencies of oscillation are measured in the non-magnetic analogue LaZn11, although with relatively smaller effective masses. GGA+U calculations, once magnetic breakdown is considered, match well the measured frequencies, confirming CeZn₁₁ to be a localised moment system with the 4f electron well below the Fermi level.

The final material studied is the transition metal dichalcogenide IrTe₂, which undergoes dimerisation upon cooling into a number of possible charge modulated structures. Low temperature de Haas-van Alphen measurements find multiple domains of a quasi-two dimensional Fermi surface, no longer perpendicular to the lattice planes. Angular-dependent magnetoresistance oscillations observe a similarly tilted quasi-one dimensional Fermi surface, again with many domains present. Together these measurements confirm the unusual dimensionality of the dimerised Fermi surface of IrTe₂.

Authors: Blake, S

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Quantum oscillations; Density functional theory
• Subjects: Condensed matter