Evidence for spin current driven Bose-Einstein condensation of magnons

The quanta of magnetic excitations – magnons – are known for their unique ability to undergo Bose-Einstein condensation at room temperature. This fascinating phenomenon reveals itself as a spontaneous formation of a coherent state under the influence of incoherent stimuli. Spin currents have been predicted to offer electronic control of Bose-Einstein condensates, but this phenomenon has not been experimentally evidenced up to now. Here we show that current-driven Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic insulators with tailored nonlinearities and minimized magnon interactions. We demonstrate that, above a certain threshold, magnons injected by the spin current overpopulate the lowest-energy level forming a highly coherent spatially extended state. We quantify the chemical potential of the driven magnon gas and show that, at the critical current, it reaches the energy of the lowest magnon level. Our results pave the way for implementation of integrated microscopic quantum magnonic and spintronic devices.

Light-induced electron pairing in two-dimensional systems

The mechanism of electron pairing induced by a circularly polarized off-resonant electromagnetic field is suggested and examined theoretically for various two-dimensional (2D) nanostructures. Particularly, it is demonstrated that such a pairing can exist in 2D systems containing charge carriers with different effective masses. As a result of the pairing, the optically induced hybrid Bose-Fermy system appears. The elementary excitation in the system are analyzed and the possible Bose-Einstein condensation of the paired electrons and the related light-induced superconductivity are discussed.

Stimulated Axion-Like Bipolariton Generation in the Globular Photonic Crystal

Axion is the dark particle introduced to the quantum chromodynamics to solve the strong CP-problem. Because of its dark nature, there are many indirect evidences, but axion itself have not been registered till now. In the paper, we report the observation of dark axion-like particles formed by the polariton coupling in the resonant microcavity of a globular photonic crystal. To overcome the very small cross-section, we use the Bose-Einstein condensation of polaritons into the nearest-to-the-surface microcavity of an opal-like globular photonic crystal. This way, the synchronicity conditions are met and all polaritons have the same wavefunction to be coupled. Moreover, the giant density of states of a Bose-condensate makes polariton coupling not only allowed but stimulated. At the experiment, we observe “Light Shining through a Wall” Primakoff effect which proves dark particles. The additional spectral peak at the unitary polariton line of a maximal transparency of a crystal allows to differ bipolaritons from other particles. The results can be used not only to generate dark particles at a lab, but also to get a laboratory source of an optical-frequency gravitational waves.