Monday, September 25, 2017 at 4:00pm to 5:00pm
Rockefeller Hall, Schwartz Auditorium
General Physics Colloquium, Professor Brad Ramshaw, Physics Department, Cornell University.
Refreshments from 3:30-3:50 pm
Title: Weyl Fermions: New Ingredients for Correlated States of Matter
Abstract: Interactions between electrons in a metal lead to some of the most interesting and bizarre states of quantum matter, including superconductors, magnets, quantum Hall states, and spin liquids. These states are `correlated' - the behaviour of the system cannot be understood by studying any single electron, as the fate of each electron is entangled with the others. A newly discovered family of materials - Weyl semimetals - contains electrons with very unusual properties: their relationship between energy and momentum is linear, similar to electrons in graphene, and instead of the usual `spin up` and `spin down` they posses chiral `left` and `right`-handed states. Electrons with these properties are called `Weyl fermions', and the pressing question is whether these new ingredients can form correlated states of matter that have not existed previously. We have used extreme magnetic fields - up to 95 tesla - to confine Weyl fermions in tantalum monoarsenide (TaAs) into a highly degenerate state know as the `quantum limit'. In this state Weyl fermions are forced to interact strongly with one another, and as the field is increase we observe dramatic changes in the resistance and ultrasonic attenuation, suggesting the formation of new states of matter. I will describe how we perform measurements under these extreme conditions, and discuss what the future holds for using high magnetic fields to uncover new correlated states.