Bose–Einstein condensation of quasiparticles by rapid cooling

Abstract

The fundamental phenomenon of Bose-Einstein Condensation (BEC) has been observed in different systems of real and quasi-particles. The condensation of real particles is achieved through a major reduction in temperature while for quasi-particles a mechanism of external injection of bosons by irradiation is required. Here, we present a novel and universal approach to enable BEC of quasi-particles and to corroborate it experimentally by using magnons as the Bose-particle model system. The critical point to this approach is the introduction of a disequilibrium of magnons with the phonon bath. After heating to an elevated temperature, a sudden decrease in the temperature of the phonons, which is approximately instant on the time scales of the magnon system, results in a large excess of incoherent magnons. The consequent spectral redistribution of these magnons triggers the Bose-Einstein condensation.Bosons are particles of integer spin that allow for the fundamental quantum effect of Bose-Einstein Condensation (BEC), which manifests itself in the formation of a macroscopic coherent state in an otherwise incoherent, thermalized many-particle system. The phenomenon of BEC was originally predicted for an ideal gas by Albert Einstein in 1924 based on the theory developed by Satyendra Nath Bose. Nowadays, Bose-Einstein condensates are investigated experimentally in a variety of different systems which includes real particles such as ultra-cold gases (1, 2) as well as quasi-particles with the likes of exciton-polaritons (3, 4), photons (5, 6) or magnons (7-9). The phenomenon can be reached by a major decrease in the system temperature or by an increase in the particle density. In order to condensate atomic gases, extremely low temperatures on the order of mK are required since the density of such gases must be very low to prevent their cohesion. In contrast, the quasi-stationary cooling of a quasi-particle system is accompanied by a decrease in its population and prevents BEC. Thus, an artificial injection of bosons is required to reach the threshold for BEC. Since quasi-particle systems allow for high

Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy

“…The significant thickness reduction achieved today allows reducing the circuit sizes from classical millimeter dimensions [1] down to 50 nm [10][11][12]. Another important field is (ii) spintronics: By increasing the YIG surfaceto-volume ratio as much as possible (while keeping its magnetic properties), physical phenomena, such as the inverse spin Hall effect [13], spin-transfer torque [14], and the spin Seebeck effect [15] (generated by a spin angular momentum transfer at the interfaces between YIG and a nonmagnetic metallic conductor layer) become much more efficient [7,[16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Also (iii) the field of terahertz physics, which uses ultrafast spin dynamics to control ultrafast magnetism, for example for potential terahertz spintronic devices [30,31,32], and (iv) the field of low-temperature physics, which deals with magnetization dynamics at cryogenic temperatures [33] for prospective quantum computer systems, are possible fields of applications for nanometer-thin iron garnet films.…”

Enhancement of the Spin Pumping Effect by Magnon Confluence Process in YIG/Pt Bilayers

“…The large variety of the effects in magnon systems is observed due to the manifold of multi‐magnon scattering processes in combination with the inherent nonlinearity in a magnetic medium. Furthermore, it has recently been demonstrated that microscopic‐scale systems exhibit novel linear and nonlinear phenomena having no analogue or exactly opposite to those found on macroscopic scale …”

Recent Trends in Microwave Magnetism and Superconductivity

“…After switching-off the heater, the system rapidly cools down, and the pronounced peak of magnon density is formed at the lowest frequency in the spectrum. Fitting the observed magnon populations with the Bose-Einstein distribution confirmed the formation of a magnon BEC [141].…”

“…In macroscopic samples as used in the previous experiments, it is very hard to create preconditions for the rapid cooling due to the limited thermal conductivity of YIG and its substrate. However, in nanostructures with a reduced volume of the sample, the characteristic time scales for the cooling can be made as short as a few nanoseconds [141]. The magnon population around the low-frequency part of the spectrum in a nanostructured waveguide was monitored by means of timeresolved microfocused BLS.…”