Phonon-Roton Interaction: from superfluid He-4 to quantum magnets

Date

May 22, 2013

Time

2:00 PM - 3:00 PM

Speaker

Mike Zhitomirsky

Language

en

Main Topic

Physik

Other Topics

Physik

Host

Paul McClarty

Description

High-energy gapped quasiparticles (rotons) interacting with low-energy acoustic excitations (phonons) are ubiquitous in Condensed matter physics. Here we consider two experimentally relevant examples. The high-precision neutron-spin-echo measurements of rotons in the superfluid helium reveal a non-monotonous temperature dependence of the roton gap [1]. We explain this phenomenon by competition between the standard roton-roton scattering, which is effective at T>1K, and the roton-phonon three-particle processes, which originate from the Bose-Einstein condensate in the ground state. In the second example, an optical magnon in an easy-plane collinear antiferromagnet interacts with acoustic spin waves. We consider the effect of disorder in such a system and demonstrate that it has a profound effect on the temperature dependence of the relaxation rate of optical magnons. The usual random-potential scattering yields a T-independent contribution while the impurity-assisted magnon scattering gives the leading temperature correction, which exceeds greatly the effect of magnon-magnon interaction in the bulk. Our theoretical prediction finds confirmation in high-resolution neutron measurements on BaNi2(PO4)2 [2]. [1] B. Fak, T. Keller, M. E. Zhitomirsky, and A. L. Chernyshev "Roton-Phonon Interactions in Superfluid 4He," Phys. Rev. Lett. 109, 155305 (2012) [2] A. L. Chernyshev, M. E. Zhitomirsky, N. Martin, and L.-P. Regnault "Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet," Phys. Rev. Lett. 109, 097201 (2012).

Last modified: May 22, 2013, 9:55:26 AM