Research Area 6: Physics: Transport and Tunneling Experiments on Dirac and Weyl Semimetals
Abstract
The purpose of the proposed research is to expand the family of Dirac and Weyl semimetals, directly measure Berry curvature, induce superconductivity in Dirac and Weyl semimetals and search for evidence of anyons in these systems. New Dirac and Weyl semimetals will be sought among the full Heusler alloys, Pb-based and III-V semiconductors, and layered transition metal chalcogenides. The chiral anomaly will be used to evidence Weyl states. The anomalous Nernst effect will be employed to develop a measurement technique for studying Berry curvature in these and related materials. Superconductivity will be sought in Dirac and Weyl materials by the proximity effect with normal metal superconductors and NbN. Radio-frequency superconducting quantum interference devices will be employed to search for the induced superconductivity. The Nernst effect will also be employed in these studies as a technique to detect fractional (anyonic) particles. The proposed research maps out 6 directions for discovering new Weyl systems, some using high pressure. Some of these directions have begun to bear fruit. The Fermi surface of Weyl fermions acts like a monopole source or sink of a field called the Berry curvature which behaves like an intense internal magnetic field confined to k (momentum) space. The Berry curvature has profound effects on the resistivity, Hall conductivity, the thermopower and the Nernst effect. The research will explore these effects quantitatively. For example, with a piezoelectric goniometer, the angular variation of the Hall effect can yield a 3D map of the Berry curvature. Supercurrents induced in the Weyl materials by proximity to a BCS superconductor effect will be investigated in two approaches. One uses a microtunneling probe to detect subgap states arising from the induced pairing. This will allow direct tests of theories for how Weyl fermions form Cooper pairs. A second approach is to inject a Josephson supercurrent in the material and to measure its current-phase relation or CPR directly. The high-resolution technique detects shift of the resonance frequency of a tank circuit caused by screening of the supercurrent in the SQUID as the external flux is slowly varied. This will detect how subgap states influence the CPR. The technique, already refined to operate at 10 millikelvin, will also be used to search for the fractional Josephson effect Ð a key hallmark of topological superconductivity. Applied to devices displaying the fractional Quantum Hall Effect, the RF technique may be used to hunt down the parafermion, an exotic excitation that has been proposed for topological quantum computing.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 12, 2017
- Source ID
- W911NF1610116
Entities
People
- Nai Phuan Ong
Organizations
- Army Contracting Command
- Princeton University
- United States Army