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
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 describes two experimental tests for visons, essential excitations of a spin-charge separation theory proposed by Senthil and Fisher (2000, 2001b). We searched for predicted hc/e vortices (Wynn et al. 2001) and a vortex memory eﬀect (Bonn et al. 2001) with
null results, placing upper bounds on the vison energy inconsistent with the theory. Chapter 5 discusses imaging of isolated vortices as a function of Tc . Vortex images were ﬁt with theoretical magnetic ﬁeld proﬁles in order to extract the apparent vortex size. The data for the lowest Tc ’s (5 and 6.5 K) show some inhomogeneity and suggest that λab might be larger than predicted by the Tc ∝ ns (0)/m∗ relation ﬁrst suggested by results of Uemura et al. (1989) for underdoped cuprates. Finally, Chapter 6 examines observations of apparent “partial vortices” in the crystals. My studies of these features indicate that they are likely split pancake vortex stacks. Qualitatively, these split stacks reveal information about pinning and anisotropy in the samples. Collectively these magnetic imaging studies deepen our knowledge of cuprate superconductivity, especially in the important regime of low superﬂuid density.
First and foremost I want to thank my advisor Kathryn (Kam) Moler. It has been an honor to be her ﬁrst Ph.D. student. She has taught me, both consciously and unconsciously, how good experimental physics is done. I appreciate all her contributions of time, ideas, and funding to make my Ph.D. experience productive and stimulating. The joy and enthusiasm she has for her research was contagious and motivational for me, even during tough times in the Ph.D. pursuit. I am also thankful for the excellent example she has provided as a successful woman physicist and professor. The members of the Moler group have contributed immensely to my personal and professional time at Stanford. The group has been a source of friendships as well as good advice and collaboration. I am especially...