Spatially non-uniform ground state and quantized vortices in a two-component Bose-Einstein condensate of magnons

• Published in: Scientific reports Vol. 2; no. 1; p. 482
• Main Authors: Nowik-Boltyk, P, Dzyapko, O, Demidov, V E, Berloff, N G, Demokritov, S O
• Format: Journal Article
• Language: English
• Published: England Nature Publishing Group 29.06.2012
• Subjects: Helium; Spatial discrimination; Vortices
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep00482

A gas of magnons in magnetic films differs from all other known systems demonstrating Bose-Einstein condensation (BEC), since it possesses two energetically degenerate lowest-energy quantum states with non-zero wave vectors ±k(BEC). Therefore, BEC in this system results in a spontaneously formed two-component Bose-Einstein condensate described by a linear combination of two spatially non-uniform wave-functions ∝exp(±ik(BEC)z), while condensates found in other physical systems are characterized by spatially uniform wave-functions. Here we report a study of BEC of magnons with sub-micrometer spatial resolution. We experimentally confirm the existence of the two wave-functions and show that their interference results in a non-uniform ground state of the condensate with the density oscillating in space. Additionally, we observe stable topological defects in the condensate. By comparing the experimental results with predictions of a theoretical model based on the Ginzburg-Landau equation, we identify these defects as quantized vortices.