Abstract
We present an experimental study of electron spin waves in atomic hydrogen gas compressed to high densities of ∼5 × 1018 cm−3 at temperatures ranging from 0.26 to 0.6 K in the strong magnetic field of 4.6 T. Hydrogen gas is in a quantum regime when the thermal de-Broglie wavelength is much larger than the s-wave scattering length. In this regime the identical particle effects play a major role in atomic collisions and lead to the identical spin rotation effect (ISR). We observed a variety of spin wave modes caused by this effect with strong dependence on the magnetic potential caused by variations of the polarizing magnetic field. We demonstrate confinement of the ISR modes in the magnetic potential and manipulate their properties by changing the spatial profile of the magnetic field. We have found that at a high enough density of H gas the magnons accumulate in their ground state in the magnetic trap and exhibit long coherence, which has a profound effect on the electron spin resonance spectra. Such macroscopic accumulation of the ground state occurs at a certain critical density of hydrogen gas, where the chemical potential of the magnons becomes equal to the energy of their ground state in the trapping potential.
Highlights
The concept of spin is one of the main features which distinguish between quantum and classical behaviour of matter
We observed a variety of spin wave modes caused by this effect with strong dependence on the magnetic potential caused by variations of the polarizing magnetic field
We demonstrate confinement of the Identical Spin Rotation effect (ISR) modes in the magnetic potential and manipulate their properties by changing the spatial profile of magnetic field
Summary
The concept of spin is one of the main features which distinguish between quantum and classical behaviour of matter. Quantization of the spin-wave oscillations leads to a description of the system as a gas of quasi-particles, or magnons. The quantum gas condition is not the same as that for the degenerate quantum gas The latter situation occurs when Λth exceeds the mean interatomic spacing r, and it is associated with drastic changes in the macroscopic properties of the gas; the phenomenon of BEC of atoms. In the quantum gas regime there appears a special type of collisions when identical particles exchange (rotate) their spins without changing their momentum. Such Identical Spin Rotation effect was predicted by Bashkin[16] and Lhuillier and Laloe[17]. The efficiency of ISR depends on the ratio of the frequencies of spin changing collisions to the elastic collisions and is given by the so-called ISR quality factor μ∗ ∼ Λth/as, which is large in the quantum gas regime
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