Scientists Discover New Magnet with Nearly Massless Charge Carriers

January 19, 2018 by Paul Shepard

Advances in modern electronics has demanded the requisite hardware, transistors, to be smaller in each new iteration. Recent progress in nanotechnology has reduced the size of silicon transistors down to the order of 10 nanometers. However, for such small transistors, other physical effects set in, which limit their functionality.

For example, the power consumption and heat production in these devices is creating significant problems for device design. Therefore, novel quantum materials and device concepts are required to develop a new generation of energy-saving information technology. The recent discoveries of topological materials — a new class of relativistic quantum materials — hold great promise for use in energy saving electronics.

Researchers in the Louisiana Consortium for Neutron Scattering, or LaCNS, led by Tulane University Professor Zhiqiang Mao and LSU Department of Physics & Astronomy Chair and Professor John F. DiTusa, with collaborators at Oak Ridge National Lab, the National High Magnetic Field Laboratory, Florida State University, and the University of New Orleans, recently reported the first observation of this topological behavior in a magnet, Sr1-yMn1-zSb2 (y, z < 0.1). These results were published July 2017 in Nature Materials.

“This first observation is a significant milestone in the advancement of novel quantum materials and this discovery opens the opportunity to explore its consequences. The nearly massless behavior of the charge carriers offers possibilities for novel device concepts taking advantage of the extremely low power dissipation,” DiTusa said.


The artwork depicts the magnetic and electronic states of the newly discovered magnetic topological semimetal (Sr1-yMn1-zSb2); its unusual and robust electronic behavior enables the flow of electrical current with little resistance. The violet spheres are strontium (Sr), blue is antimony (Sb), and purple is manganese (Mn). Arrows represent the alignment of magnetic moments. The two gold cones touching at a point (called Dirac point) represent the energy and momentum states of conducting electrons. (click on image to enlarge)

Neutron scattering experiments revealed an unusual combination of ferromagnetic and antiferromagnetic states in the material that vary as a function of temperature. In general, materials exhibit either ferromagnetism or antiferromagnetism, not both in the same material at a given condition.

The observed persistence of a ferromagnetic component in the antiferromagnetic state at low temperatures is the signature for the breaking of the time-reversal symmetry (that is, running backward does not return to its starting conditions).

The combination of unusual electronic behaviors in this new material offers a rare opportunity to investigate the interplay between relativistic quantum states and spontaneous breaking of symmetry associated with time. These materials offer great promise for highly efficient quantum devices and quantum computers.

The phrase “topological materials” refers to materials where the current carrying electrons act as if they have no mass similar to the properties of photons, the particles that make up light. Amazingly, these electronic states are robust and immune to defects and disorder because they are protected from scattering by symmetry.

This symmetry protection results in exceedingly high charge carrier mobility, creating little to no resistance to current flow. The result is expected to be a substantial reduction in heat production and energy saving efficiencies in electronic devices.

This new magnet displays electronic charge carriers that have almost no mass. The magnetism brings with it an important symmetry breaking property – time reversal symmetry, or TRS, breaking where the ability to run time backward would no longer return the system back to its starting conditions.

The combination of relativistic electron behavior, which is the cause of much reduced charge carrier mass, and TRS breaking has been predicted to cause even more unusual behavior, the much sought after magnetic Weyl semimetal phase. The material discovered by this collaboration is thought to be an excellent one to investigate for evidence of the Weyl phase and to uncover its consequences.

The researchers involved include J.Y. Liu, J. Hu, Y.L. Zhu, G.F. Cheng, X. Liu, J. Wei, and Z.Q. Mao (Tulane University, New Orleans); Q. Zhang, W. A. Phelan, and J. F. DiTusa (Louisiana State University, Baton Rouge); D. Graf, (National High Magnetic Field Lab, Tallahassee); Q. Zhang, H.B. Cao and D. A. Tennant (Oak Ridge National Laboratory); S.M.A. Radmanesh, D.J. Adams, and L. Spinu (University of New Orleans); Marcelo Jaime and Fedor Balakirev (Condensed Matter and Magnet Science, MPA-CMMS); and I. Chiorescu (NHMFL and Florida State University, Tallahassee).

Support for this research was provided by the U.S. Department of Energy Office of Science, the National Science Foundation and the Louisiana Board of Regents.