PhD Defenses


Date Range
Wed August 9th 2023, 9:30 - 10:30am
PAB 232
Stanford overview

Photo by Linda Cicero

Ph.D. Candidate: Linsey Rodenbach

Research Advisor: 
David Goldhaber-Gordon

Date: August 9, 2023
Time: 9:30AM

Location: PAB 232

Zoom Link:

Zoom Password: email nickswan [at] (nickswan[at]stanford[dot]edu) for password.

The quantum anomalous Hall effect: uses in electrical metrology, and understanding residual dissipation

When a company manufactures and sells a resistor, they typically aim to traceably link its resistance value to a quantum Hall (QH) effect measurement. When a thin semiconductor is cooled to a low enough temperature and exposed to a large enough magnetic field, it exhibits zero longitudinal resistance and quantized Hall (transverse) resistance. This quantized resistance is a topological phenomenon, insensitive to sample details, and since 1990 has been used to define the ohm. Magnetic topological insulators exhibit the same resistance properties, but at zero magnetic field — this is the quantum anomalous Hall (QAH) effect. QAH materials offer significant advantages as possible replacements for QH resistance standards. Eliminating the need for large magnetic fields not only allows simplifying measurement infrastructure, but also allows a quantum resistance standard to be directly integrated with a Josephson voltage standard to produce a quantum current standard. However, all measurements of the QAH effect to date have observed non-zero longitudinal resistance despite quantization of the Hall resistance to h/e2 (where h is Planck’s constant and e the electron charge) being confirmed to within one part per billion[1]. This residual dissipation is strongly temperature-dependent, lowering the temperature at which the QAH effect is well-quantized and thus limiting industrial applications.

This talk will focus on our efforts to understand residual dissipation in the QAH state of the canonical magnetic topological insulator — Cr-doped (BiSb)2Te3 — by decoupling edge and bulk dissipation using an annular geometry known as the Corbino disk[2]. I will show that despite their limitations, these materials are already useful within the field of electrical metrology. I will discuss how metrological measurements of resistance are made[3] and highlight the construction of a novel quantum current sensor based on a single-cryostat combination of a QAH resistor with a programmable Josephson voltage standard[4].