The Higgs (Amplitude) Mode in Ferromagnetic Metals
Dr. Kevin Bedell
John H. Rourke Professor of Physics, Boston College
Refreshments will be served at 1:15 PM
The emergence of new modes in systems with spontaneously broken continuous symmetries plays an important role in our fundamental understanding of nature. Two of the fundamental particles that can be present in the field theory descriptions of these spontaneously broken symmetries are: massless Nambu-Goldstone bosons (phase modes) and massive Higgs bosons (amplitude modes). These two modes can be understood as following from fluctuations in the order parameter describing the spontaneously broken symmetry: φ(x) = ρ(x)eiθ(x). The massless mode, arises from fluctuations of the phase of the order parameter, θ(x), and the massive mode, comes from fluctuations of the amplitude of the order parameter, ρ(x). The Higgs amplitude mode can exist in a variety of condensed matter systems with broken continuous symmetry including, antiferromagnets, fermionic superfluids, and charge density waves, to name a few. In a ferromagnetic metal the existence of the phase mode (magnon /spin wave) is well know but the Higgs mode was not expected. In (2001) Bedell and Blagoev showed that the Ferromagnetic Fermi Liquid (FFL) theory predicted the existence of a gaped collective excitation in a ferromagnetic metal. Recently Yi, Farinas and Bedell (2013) identified this mode as the Higgs amplitude mode. Using existing measurements of some of the parameters needed in the FFL theory we predicted that there is a well-defined propagating Higgs amplitude mode in MnSi. From the calculation of the spin density response function we can estimate the relative intensity of the Higgs amplitude mode in neutron scattering experiments on MnSi and we expect that it should be observable. A recent experiment searching for the Higgs in MnSi was carried out at ORNL; currently the data is being analyzed and there is nothing to report now. If you want an update on the experiment you will have to come to my talk!
Dr. Kevin Bedell received his B.S. in Physics from Dowling College, Oakdale NY in 1971, M.S. in Applied Mathematics and Ph.D. in Physics from SUNY Stony Brook, NY in 1972 and 1979, respectively. Dr. Bedell was a research associate at University of Illinois at Urbana-Champagne from 1979 to 1982, a research fellow, at ITP, SUNY Stony Brook from 1982 to 1985, and a visiting professor at Kamerling Onnes Laboratory at Leiden from 1985 to 1986. Dr. Bedell worked at Los Alamos National Laboratory, T-11 from 1986-1996. He was the director of Program in Correlated Electron Theory from 1991 to 1995 and the co-director of Many-Body Theory Program from 1996 to 2002. Dr. Bedell was the chair of Physics Department, Boston College from 1996 to 2006 and the Vice Provost for Research at Boston College from 2006 to 2010. He became the Rourke Professor of Physics at Boston College in 1999. Dr. Bedell is an APS fellow and also the editor for Advances in Physics since 1995.