Magnetic Torque and Transport Investigation of the Pseudogap and CDW in YBa2Cu3O6+x

Abstract

In this thesis, the results of torque magnetometry and electronic transport studies of the archetypal cuprate superconductor YBa₂Cu₂O_6+x (YBCO) are presented. By variation of temperature, doping or application of a magnetic field, it is possible to access several underlying electronic phases that occur on the backdrop of superconductivity. Of particular interest in this thesis are the pseudogap, an elusive phase possibly tied to a quantum critical point at p » 0.19 within the superconducting dome, and charge density wave (CDW) order, which seems ubiquitous in the cuprates and appears to be competing with superconductivity.
Firstly, magnetic torque measurements at 14 T in the underdoped region of the phase diagram (p= 0.08 - 0.15) are presented. Magnetic torque is used to investigate the nature of the pseudogap transition, believed to be a second order nematic phase transition, and determine whether the transition temperature T* marks the boundaries of a thermodynamic phase. The in-plane magnetic susceptibility anisotropy feature observed does not track the pseudogap onset, it is the result of a doping dependent linear contribution and a p-independent contribution, either coming from the experimental setup or the orthorhombicity of the crystal.
Secondly, resistivity and Hall effect measurements at 14 T in disordered YBCO (p »0.125) are presented. Point-like disorder, either by chemical substitution of Zn and Ni inside the material or by electron irradiation, is introduced in YBCO to gradually suppress superconductivity. The evolution of the superconducting transition temperature Tc and the temperature at which the Hall coefficient changes sign T₀, an indicator of a Fermi surface reconstruction due to the onset
of charge order, are tracked as a function of point-like disorder. Tc is used as a measure of the strength of superconductivity whilst T0 is used as a proxy for the strength of charge order. Both Tc and T₀ are shown to decrease as a function of disorder, in contrast with X-ray measurements where CDW seem to be weakly affected by disorder and in competition with superconductivity. The implications with regard to this discrepancy are discussed.
Thirdly, resistivity and Hall effect measurements at 14 T in YBCO (p »0.13) under uniaxial pressure are presented. Two uniaxial pressure devices, newly designed for high magnetic field, are used to apply strain to YBCO crystals. The evolution of Tc and T₀ are tracked as a function of strain. The devices proved to be able to effectively change the superconducting critical temperature and affect T₀ with the same success rate as a commercially available setup unsuited for high field measurements, making these results a promising proof of concept for future uniaxial pressure research at high magnetic fields.

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Antony Carrington (Supervisor)