4 Reports on Experimental Results 4.2 Full electric detection of a Bose-Einstein condensate via the spin-pumping

It is well known, that magnons excited in an insulating magnetic medium can transfer their angular momenta to free electrons in an adjacent non-magnetic metal layer, and thus generate a spin current via the spin-pumping (SP) effect [1]. Due to the inverse spin Hall effect (ISHE) [2, 3], caused by spin-dependent scattering, this current is transformed into a charge current and is measured as an electric voltage. It is commonly believed that the magnitude of the ISHE voltage increases with the total number of magnons in a spin-wave system [4] as well as with the energy of these magnons [5]. The electric signal, which corresponds to the formation of a magnon Bose-Einstein condensate (BEC) by a freely evolving magnon gas after the termination of an external pumping [6], breaks with this rule: The transition of the thermalized gaseous magnons to the lowest energy states, which develops alongside with the process of the magnon decay, leads to a pronounced upward jump of the voltage magnitude. This unusual behavior can be understood as a result of a rectiÞcation of a coherent microwave current induced by the magnon BEC in the non-magnetic metal layer. Thus, the voltage signal contains a Þngerprint of the coherency of the condensate. The experiment is performed using an in-plane magnetized 6.7μm thick single-crystal yttrium iron garnet Þlm (YIG) covered with a 10nm thick platinum layer (see Fig. 1) and placed on top of a 50μm microstrip resonator. The resonator is fabricated on top of an alumina substrate, which was used due to its low dielectric losses and good thermal conductivity to avoid spurious heating effects which could be potentially caused by high microwave powers. A pumping circuit consisting Microwave pumping- up to 100 WP