Among topological materials, experimental study of topological semimetals that host Dirac or Weyl fermions has just begun, even though these topological concepts were proposed nearly a century ago by Dirac and Weyl. The Dirac/Weyl fermions should be massless with extremely high mobility and non-trivial Berry phase, characteristics that are highly desirable for applications. Our bulk studies show that the magnetic-semimetal BaMnSb2 exhibits nearly zero mass fermions with high mobility and a non-trivial Berry phase. The Shubnikov-de Hass (SdH) oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the non-trivial Berry phase. . I will discuss the procedure for determining the effective mass, where the standard procedure gives m=0.05me, but the correct procedure gives m=0.086me. What is unique is the magnetic order, ferromagnetic along the ab plane but antiferromagnetically coupled along the c direction where the system should be Weyl type due to time-reversal symmetry breaking. Theory shows that the spin order is very fragile, so it is expected that the application of magnetic field or uniaxial pressure could drive the material to be a type-II Weyl semimetal.
The surface properties were determined using LEED and STM/STS , revealing a persistent 2x1 reconstruction. STS measurments indicate that the surface is semiconducting not semimetallic.
* Funded by the National Science Foundation.
1) “Non-Trivial Berry Phase in Magnetic BaMnSb2 Semimetal,” Silu Huang, Jisun Kim, W. A. Shelton, E. W. Plummer, Rongying Jin, PNAS (2017).
2) Experiments conducted at CNMS (ORNL) by Zheng Gai, Kun Zhao, and Qiang Zhang.
Prof. Ward Plummer received a Bachelor of Arts degree from Lewis and Clark College in 1962 and his Ph. D. degree in Physics from Cornell University in 1968 under Prof. Thor Rhodin. He accepted a National Research Council Postdoctoral Fellowship at the National Bureau of Standards, before joining the University of Pennsylvania in 1973. In 1993, Prof. Plummer moved to Tennessee with a joint appointment at The University of Tennessee (Distinguished Professor), Knoxville, and Oak Ridge National Laboratory (Distinguished Scientist). He was the director of the Joint Institute for Advanced Materials (JIAM) until he came to Louisiana State University in 2009. He is a professor of Physics and Astronomy and special assistant to the Vice Chancellor for Research.
Prof. Plummer is author of >400 refereed papers and is included in the list of the 1,000 Most Cited Physicists. His main contributions are in surface physics of metals. He was elected to the National Academy of Sciences (USA) in 2006 and the American Academy of Arts and Sciences in 2014. He was awarded various prizes, including Wayne B. Nottingham Prize (1968), Davisson-Germer Prize in Atomic or Surface Physics (1983), and Medard W. Welch Award (2001).