Sales and Marketing Thesis
SCANNING HALL PROBE MICROSCOPY OF MAGNETIC VORTICES IN VERY UNDERDOPED YTTRIUM-BARIUM-COPPER-OXIDE
a dissertation submitted to the department of physics and the committee on graduate studies of stanford university in partial fulfillment of the requirements for the degree of doctor of philosophy
Janice Wynn Guikema March 2004
c Copyright by Janice Wynn Guikema 2004 All Rights Reserved
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iv
Abstract
Since their discovery by Bednorz and M¨ller (1986), high-temperature cuprate u superconductors have been the subject of intense experimental research and theoretical work. Despite this large-scale effort, agreement on the mechanism of high-Tc has not been reached. Many theories make their strongest predictions for underdoped superconductors with very low superfluid density ns /m∗ . For this dissertation I implemented a scanning Hall probe microscope and used it to study magnetic vortices in newly available single crystals of very underdoped YBa2 Cu3 O6+x (Liang et al. 1998, 2002). These studies have disproved a promising theory of spin-charge separation, measured the apparent vortex size (an upper bound on the penetration depth λab ), and revealed an intriguing phenomenon of “split” vortices. Scanning Hall probe microscopy is a non-invasive and direct method for magnetic field imaging. It is one of the few techniques capable of submicron spatial resolution coupled with sub-Φ0 (flux quantum) sensitivity, and it operates over a wide temperature range. Chapter 2 introduces the variable temperature scanning microscope and discusses the scanning Hall probe set-up and scanner characterizations. Chapter 3 details my fabrication of submicron GaAs/AlGaAs Hall probes and discusses noise studies for a range of probe sizes, which suggest that sub-100 nm probes could be made without compromising flux sensitivity. The subsequent chapters detail scanning Hall probe (and SQUID) microscopy studies of very underdoped YBa2 Cu3 O6+x crystals with Tc ≤ 15 K. Chapter 4
a dissertation submitted to the department of physics and the committee on graduate studies of stanford university in partial fulfillment of the requirements for the degree of doctor of philosophy
Janice Wynn Guikema March 2004
c Copyright by Janice Wynn Guikema 2004 All Rights Reserved
ii
iv
Abstract
Since their discovery by Bednorz and M¨ller (1986), high-temperature cuprate u superconductors have been the subject of intense experimental research and theoretical work. Despite this large-scale effort, agreement on the mechanism of high-Tc has not been reached. Many theories make their strongest predictions for underdoped superconductors with very low superfluid density ns /m∗ . For this dissertation I implemented a scanning Hall probe microscope and used it to study magnetic vortices in newly available single crystals of very underdoped YBa2 Cu3 O6+x (Liang et al. 1998, 2002). These studies have disproved a promising theory of spin-charge separation, measured the apparent vortex size (an upper bound on the penetration depth λab ), and revealed an intriguing phenomenon of “split” vortices. Scanning Hall probe microscopy is a non-invasive and direct method for magnetic field imaging. It is one of the few techniques capable of submicron spatial resolution coupled with sub-Φ0 (flux quantum) sensitivity, and it operates over a wide temperature range. Chapter 2 introduces the variable temperature scanning microscope and discusses the scanning Hall probe set-up and scanner characterizations. Chapter 3 details my fabrication of submicron GaAs/AlGaAs Hall probes and discusses noise studies for a range of probe sizes, which suggest that sub-100 nm probes could be made without compromising flux sensitivity. The subsequent chapters detail scanning Hall probe (and SQUID) microscopy studies of very underdoped YBa2 Cu3 O6+x crystals with Tc ≤ 15 K. Chapter 4