Non-technical abstract Electrons behave differently in metals than they do in a vacuum: in a sense each metal has its own laws of physics that electrons must obey, turning each new material into its own universe where discoveries can be made. This work focuses on a new class of metals - Weyl semimetals - that have massless electrons similar to those in graphene but that are free to move in three dimensions. By applying extremely large magnetic fields this project seeks to force the Weyl electrons to interact with one another and form new states of matter. There is a large gap in our understanding of what happens to three dimensional metals in extreme magnetic fields, and this project fills that gap. The plan includes measurements under these extreme conditions, pushing the frontier for techniques such as ultrasound spectroscopy up to 100 tesla and eventually beyond. This broadens the range of parameters where discoveries can be made by all researchers investigating new materials. A resonant ultrasound spectroscopy lab module will be developed, bringing this modern experimental technique to the classroom. This module will be made available through the American Association of Physics Teachers Advanced Lab website, and the undergraduate will have the opportunity to present their work at the "Beyond First Year Labs" conference.
The quantum limit is the magnetic field beyond which all electrons in a metal are confined to the last Landau level. This state is highly degenerate and prone to the formation of density waves, excitonic phases, and possibly Wigner crystals. While the quantum limit has been studied extensively in gate-tunable two dimensional systems such as GaAs/AlGaAs heterostructures and graphene, very little is known about 3D metals in the quantum limit, and even less is known about 3D Dirac and Weyl semimetals in this limit. This project studies how Weyl fermions behave when magnetic fields provide the dominant energy scale in the system. Questions include: do Weyl fermions survive when the cyclotron and Zeeman energies are larger than the spin-orbit coupling; are Weyl fermions unstable to the formation of a new state of matter in high magnetic fields; and does the Weyl topology of the electronic structure play an important role in magnetic-field induced correlated states? This project develops experimental tools aligned along two complementary directions of ultrasound and electrical transport to answer these questions. This includes quantitative resistivity measurements using focused-ion-beam lithography for investigating phenomena such as the chiral anomaly. These transport measurements (including Hall effect) are used to develop a realistic tight-binding model for TaAs in the quantum limit, enabling theoretical calculations of possible high-field correlated states. The principle investigator's previous work on TaAs suggests the formation of a correlated electronic phase above 80 tesla. This project develops frequency-dependent pulse echo ultrasound, covering frequencies from the low MHz up to a few GHz, to probe quasiparticle dynamics near the field-induced phase transition, and to uncover which (if any) symmetry is broken at the transition. The synthesis of these two approaches will yield both a quantitative picture of how the electronic structure of a Weyl semimetal evolves with field into the quantum limit, and how that electronic structure becomes unstable to interactions and ultimately leads to a new field-induced state of matter.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
