A dielectric resonator with the resonance frequency of ω mw/2π = f mw = 9.0 GHz is used to inject magnons of a frequency ω p/2π = f p = f mw/2 = 4.5 GHz into a 5.1 μm thick in-plane magnetized YIG film by means of parametric pumping (see experimental details in 15, 18). Thus, the magnonic second sound wave represents a novel type of a spin current, that is neither purely ballistic (carried by regular magnons) 28, 29, nor diffusive 29, and, also, rather different from the magnonic supercurrents manifesting themselves in a room temperature BEC of magnons 21, 22, 23, 24.įinally, an additional advantage of an externally pumped quasi-equilibrium magnon gas for the second sound experiments is the fact that by varying the pumping power one can change the magnon gas density, forcing it to undergo BEC, which allows one to study the influence of the BEC formation on the properties of the magnonic second sound waves.Ī scheme of the experimental setup for the investigation of the second sound waves in a quasi-equilibrium magnon gas is shown in Fig. This allows one to excite the magnonic second sound coherently, by an alternating magnetic field of a sufficiently low frequency lying inside the spectral gap, which makes this signal incapable of excitation of regular magnons (magnonic “first” sound).īesides, from a less general point of view of modern spintronics, the magnonic second sound would be rather interesting, because the propagating magnonic second sound transfers not only the energy (heat) but, also, the spin angular momentum. The gap is proportional to the magnitude of the bias magnetic field, and, typically, is of the order of a few GHz. The other peculiarity of magnons is the presence of a gap in the quasi-particle spectrum. The dense gas of magnons is also an excellent model system for which many interesting novel phenomena, such as formation of quantized BEC vortices 21, supercurrent flows in a magnonic BEC 21, 22, 23, 24, formation of hybrid magnetoelastic bosons 25, and spin superfluidity 26, 27 have been either experimentally observed or theoretically studied recently. This energy exchange leads to the formation of the room-temperature magnon Bose-Einstein condensate (BEC) 15, 16, 17, and is responsible for the unusual dynamics of the magnon gas 18. The low-energy magnons exchange energy not only with the pumping photons, but also with both the lattice and higher-energy magnon branches of the YIG film spin system. Therefore, it is important to clarify how the heat exchange between the quasi-particle gas and other subsystems influences the properties of the second sound.Ī particular example of such a “non-isolated” quasi-particle system is a dense quasi-equilibrium gas of parametrically pumped low-energy (microwave-frequency) magnons existing in high-quality yttrium-iron garnet (YIG) films 15, 16, 17, 18, 19, 20. The same, however, is not true for almost any other type of quasi-particles. This assumption works well for phonons, which are responsible for the dominant part of the heat capacity of a solid or liquid, and for which heat (or energy) exchange with other subsystems can be ignored. In all the previous theoretical studies of the second sound phenomenon it was implicitly assumed that the system of quasi-particles carrying the sound waves is perfectly thermally isolated from the rest of the world. It should be, also, noticed, that in most experiments the second sound was excited by heat pulses, received using a resonant cavity 3, 13, and additional measures were necessary to prevent simultaneous excitation of both the first and the second sound 14 in a phonon gas having a gapless spectrum. The condition for the observation of this interesting phenomenon is the requirement that gas of phonons exists in a state of thermal quasi-equilibrium, and the intensity of the so-called “umklapp” phonon-phonon scattering processes, that are responsible for the relaxation of the phonon momentum into a crystal lattice, is small 9, 10, 11, 12. Second sound has been observed in a superfluid liquid He 4 3, 4, 5, in solid He 4 6, and in some dielectric solids, such as Bi 7 or NaF 8 at low temperatures. Usually, second sound is discussed as a phenomenon in which energy transfer occurs in a wave-like fashion (in contrast with the usual diffusion-like heat transfer) in an equilibrium gas of phonon quasi-particles 2. If normal (or “first”) sound is a wave of density of molecules in a substance, the “second” sound is a wave of density of collective quasiparticle excitations that can be excited in the same substance 1.
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