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Monday, November 24, 2014
3:00 PM - 4:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Colloquium

The Higgs (Amplitude) Mode in Ferromagnetic Metals

Kevin Bedell
Boston College

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!

Host: Robert Ecke