Magnons in H gas

Atomic hydrogen is the simplest, smallest and lightest of atoms. It is the only element or substance, which remains a gas and does not condense into liquid or solid even at absolute zero of temperature. Furthermore, hydrogen atoms interact with each other on a very short range, when they approach to the distance less than 1 Å. This allows observing quantum properties in the H gas in rather wide range of temperatures. The so-called quantum gas regime is reached when the thermal de-Broglie wavelength becomes larger than the characteristic range of elastic interaction. For atomic hydrogen this condition is well justified when the gas is cooled below 1K, relatively high temperature on the standards of cold gases. Having a 1/2 electron and nuclear spins atomic hydrogen represents a unique possibility for performing magnetic resonance experiments. Thus the methods of Electron Spin (ESR) and Nuclear Magnetic (NMR) Resonance methods and their combinations are the main diagnostic tools to study these gaseous samples.

At present we are the only lab in the world, capable of creating samples of H gas at temperatures below 1 K and still having very high densities aproaching to 5*1018 cm-3. Atoms are stabilized against recombination into H2 molecules due to high degree of electron spin polarization which is achieved in strong magnetic fields of 4.6 T and low temperatures. We utilize hydraulic compressiion technique, when small bubbles of H gas are pressurized inside superfluid helium. High frequency 130 GHz ESR is used to study the properties of the gaseous samples.

Our present efforts in the H gas research are directed on the investigation of magnetic excitations in this system called spin waves. This phenomenon has been predicted for quantum gases in the beginning of 1980s by the group of Frank Lalöe in Paris, and is caused by the identical particle efects during atomic collisions when the atoms approach over the distance shorter than their thermal de-Broglie wavelength. The exchange interaction leads to the rotation of their spins around their sum, which has got the name of an Identical Spin Rotation (ISR) effect. Accumulating in numerous quantum collisions ISR leads to the wave-like propagation of the spin perturbations: spin waves, or magnons. Soon after their prediction the spin waves associated with the nuclear spins were detected experimentally for the nuclear spins of H and 3He gases.

Electron spin waves (e-magnons) were much harder to realize in experiments, and only in 2007 in our group we managed to excite and detect them.

It turned out that the electron spin wave modes and functions obey an equation, which has exactly the same form as the Schrödinger equation. There the magnetic field deviations play a role of potential energy in exactly same wave as the Zeeman energy for free atoms. This allows conducting trapping experiments with the e-magnons silmilar to cold atoms trapping experiments.

 

 

References:

1. H. Fröhlich, Bose Condensation of Strongly Excited Longitudinal Electric Modes, Physics Letters 26A, p.402 (1968)

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