Macroscopic magnonic quantum states

Burkard Hillebrands, Technische Universität Kaiserslautern

Finding new ways for fast and efficient processing and transfer of data is one the most challenging tasks nowadays. Elementary spin excitations ‒ magnons (spin wave quanta) ‒ open up very promising directions of high-speed and low-power information processing.
Magnons are bosons, and thus they can spontaneously form a spatially extended coherent ground state ‒ a Bose-Einstein condensate (BEC) – which can be established independently of the magnon excitation mechanism even at room temperature. An extraordinary challenge is the use of macroscopic quantum phenomena such as the magnon BEC for the information transfer and processing.
Very promising is the use of magnon supercurrents driven by a phase gradient in the magnon BEC. Imposing such a phase gradient onto the BEC’s wave function we have found using Brillouin light scattering spectroscopy that local heating by a probing laser beam leads to an excessive decay of the freely evolving magnon BEC. This is a fingerprint of the supercurrent efflux of condensed magnons. Moreover, we revealed that the condensed magnons being pushed out from the heated area form compact density humps, which propagate over long distances through the thermally homogeneous magnetic medium. We refer to them as a superposition of Bogoliubov waves with oscillations of both the amplitude and the phase of the magnon BEC’s wave function. In the long-wavelength limit, these waves have a linear dispersion law and can be considered as a magnon second sound potentially featuring viscosity-free propagation.
A further consequence of a magnon BEC is the prediction of the existence of a magnon ac Josephson effect. Recently, we discovered this effect in a room-temperature magnon BEC. The magnon condensate was prepared in a parametrically populated magnon gas around a potential trench created by a dc electric current. The appearance of the magnonic Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between magnon condensates from the left and right zones of the trench.

PDF file of the talk available here