If you have a symmetry in a classical system are you guaranteed that the symmetry will persist in the quantum system? An anomaly is the breakdown of a symmetry in the quantum system arising from the impossibility to assign a measure that preserves that symmetry when the action is summed over all paths. This profound question has been discussed in particle physics to understand the matter-antimatter asymmetry in the universe that may arise because baryon number is not conserved due to an anomaly. With this backdrop it is remarkable that in our experiment-theory collaboration reported in Nature Materials we observe the breaking of a chiral symmetry, or a left-right handedness, in a topological Weyl semimetal revealed by a new pathway of heat conduction. 

In a Weyl semimetal free electrons and holes (positively charged particles) merge in energy and behave like massless particles, called Weyl fermions, for two distinct values of momentum, called Weyl points. The complex phases of the wave functions of the Weyl fermions show textures of opposite chirality between the two Weyl points, looking like monopoles of magnetic field. We created such a Weyl semimetal by alloying bismuth and antimony at a specific concentration and applying an external magnetic field; we adjust the composition of the alloys so as to generate only Weyl fermions and eliminate all other electrons or holes that would normally conduct electricity or energy in the solid. Next, by applying a thermal gradient, a special highway is created that allows the Weyl fermions of one momentum and chirality to connect to those of the opposite momentum and chirality. The mathematical description of this phenomenon closely parallels the chiral anomaly in quantum field theories for elementary particles. No such pathways exist in ordinary metals. By activating this highway our group showed that it is possible to pump energy between the two Weyl points of opposite chirality arising from the breaking of chiral symmetry. The energy transfer between the Weyl points in the bismuth alloy results in a 300% increase in the thermal conductivity, a mechanism that can be used to create thermal switches, the heat equivalent of the transistor.

Prof. Joseph Heremans

Dung Vu¹, Wenjuan Zhang², Cüneyt Şahin^3,4, Michael E. Flatté^3,4, Nandini Trivedi², Joseph P. Heremans^1,2,5,*

1. Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, Ohio 43210

2. Department of Physics, The Ohio State University, Columbus, Ohio 43210

3. Optical Science and Technology Center and Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52242

4. Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637

5. Department of Material Science and Engineering, The Ohio State University, Columbus, Ohio 43210

To appear in Nature Materials [impact factor 38]