Dresden 2014 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 47: Spintronics (organized by MA)

HL 47.1: Talk

Tuesday, April 1, 2014, 13:45–14:00, HSZ 401

Sound Waves in a Magnon Bose Einstein Condensate — •Patryk Nowik-Boltyk¹, Oleksandr Dzyapko¹, Vladislav E. Demidov¹, Sergej O. Demokritov¹, Vasyl Tyberkevych², and Andrej N. Slavin² — ¹Institute of Applied Physics, University of Muenster, Muenster, Germany — ²Department of Physics, Oakland University, Rochester, USA

Magnon Bose-Einstein condensation (mBEC) in Yttrium-Iron-Garnet films is a spectacular room-temperature macroscopic quantum phenomenon, which is under investigation since recently [1]. Although the basic properties of mBEC such as temporal [2] and spatial [3] coherence have extensively been studied during the last 5 years, the perturbed dynamics of the condensate have not been addressed so far. Here we report an experimental study of sound waves in a magnon gas, above and below the threshold for mBEC, performed using a space- and time-resolved Brillouin light scattering technique. The magnon gas was prepared using microwave pumping of magnons, while the sound waves were excited by, an additional, localized, oscillating, RF magnetic field. We show that at small wave vectors sound waves exhibit a linear dispersion law with a density independent group velocity, while at large wave vectors the dispersion changes from a linear dependence into a quadratic one at the threshold for mBEC. We demonstrate that this sudden change is due to an additional scattering mechanism that arises when an mBEC is formed. [1] S.O. Demokritov et al. Nature 443, 430 (2006) [2] V.E. Demidov et al. Phys. Rev. Lett. 100, 047205 (2008) [3] P. Nowik-Boltyk et al. Nature Sci. Rep. 2, 482 (2012)