Atomic Hydrogen

The  Turku Atomic Hydrogen Group is a part of the  Wihuri Physical Laboratory of the Department of Physics and Astronomy, University of Turku, Finland. At the same time it is one of the five research groups of the Turku Centre for Quantum Physics. Research of the Turku Atomic Hydrogen Group is now focussed on three main projects:

Quantum properties of atomic hydrogen gas

Physics and chemistry of atomic hydrogen and deuterium stabilized in H2 and D2 crystals

Magnetic resonance and quantum computing with Phosphorus in Silicon

Research in these areas is performed at temperatures below 1 K and in strong magnetic fields of 5 T. As a common diagnostic tool we utilize methods of magnetic resonance : ESR, NMR and ENDOR.

History

Studies of spin-polarized atomic hydrogen at Turku were started by Matti Krusius and Simo Jaakkola more than 30 years ago. Research groups at a few universities (Amsterdam, MIT, Turku) compressed H gas hydraulically to high density at low temperatures. The goal was to achieve the Bose -Einstein Condensation (BEC), which occurs when the matter (de Broglie) wavelength of the atoms exceeds the mean interatomic distance.
In 1985  bubbles of atomic hydrogen gas were compressed at Turku to densities exceeding 5*1018 cm-3 [Phys. Rev. Lett. 56, 941 (1986)]. It was found that bubbles of such dense gas were unstable against thermal explosions triggered by three-body recombination. This process prevented  a further increase of the density and did not allow reaching BEC with this compression method.
The next generation of experiments was based on magnetic compression of hydrogen gas by specially shaped ferromagnetic needles. In this case strong magnetic field gradients play a role of the wall confining the gaseous sample, and still remain transparent for the recombination products - hydrogen molecules, thus allowing better cooling of the compressed gas.  Although BEC could not be reached in three-dimensional hydrogen gas in  these experiments either, analysis of the data showed that it could happen on the surface of the magnetic field intensifier, in the adsorbed two-dimensional gas.
The experiments were optimized for the  compression of the two-dimensional gas,  quantum degeneracy regime was reached and appearance of two-dimensional condensate was demonstrated for the first time in atomic hydrogen gas [Phys. Rev. Lett. 81, 4545 (1998)]. The transition to a new state of matter manifested itself by an abrupt decrease of the three-body recombination rate (click to read an article in Physics World).
Further studies of hydrogen at Turku were based on another compression method of two dimensional gas, which utilizes thermal compression of the adsorbed atoms on a small area of the sample cell wall, cooled somewhat below the rest of the sample cell. Such a "cold spot" method allows sensitive and direct detection of the adsorbed atoms by means of Electron Spin Resonance [Phys. Rev. Lett. 89, 153002 (2002),  Phys. Rev. A 69, 023610 (2004)].

Present activities

In our recent experiments we studied electron spin waves (e-magnons) in H gas at very high densities approaching to 5*1018 cm-3. In quantum gases this phenomenon occurs due to the Identical Spin Rotation Effect, and has been experimentally observed for the nuclear spins of H and 3He. Despite of many efforts electron spin waves in H gas were not found for many years due to severe technical difficulties. In our work we managed to solve these diffculties and the e-magnons were observed in H gas in 2008. In our further work we demostrated that e-magnons in H gas can be trapped in regions of strong magnetic fields, like real particles of cold gases. At high enough density of H gas we observed a strong ESR absorption peak originating from the e-magnons in the ground state in a magnetic trap. This macroscopic population of the ground state can be interpreted in terms of Bose-Einstein Condensation (BEC) of these quasiparticles. Similar effects have been observed also for magnons in liquid 3He and ferromagnets. BEC of quasiparticles and its differences from real particles formed an interesting research topic and is currently a subject of hot discussions in this field. In 2013 our department received a four year research grant from finnish academy: "Magnons in Spin-Polarized Atomic Hydrogen Gas" headed by Prof. K.-A. Suominen and Dr. S. Vasiliev. Two research positions: post doc (experiment) and PhD student (Lauri Lehtonen - theory) are opened within this project in 2013-2014.

Atomic hydrogen can be also stabilized inside solids, with the method known as matrix isolation. About 5 years ago we found that a system of atoms trapped inside solid H2 crystals can be created in our sample cell at temperatures below 100 mK, and remain stable for many days. At low enough temperatures the atoms will become delocalized and one expects that such "gas inside solid" would also undergo a BEC transition. Several exotic features were discovered in this system [PRL 2007]. Thus a new direction of research has been started at Turku in collaboration with Prof. David Lee  and Dr. Vladimir Khmelenko at Cornell University.  In 2011 their group has moved to Texas A&M University, and in 2012 our collaborative work was awarded with the joint NSF-Finnish Academy research grant:  ATOMIC HYDROGEN IN CRYOCRYSTALS: SUPERFLUID OR SUPERSOLID? In 2009 an Oxford 2000 dilution refrigerator insert has been donated to Tuku group by David Lee, and served as a starting point for construction of a new experimental setup fully devoted to the studies of atomic hydrogen in solids. In 2013 we started experiments with H and D atoms in soli H2 and D2 in the new setup and at the moment have already got several new and intersting observations. This work is pursued by academy researcher Janne Ahokas and PhD student Sergey Sheludyakov.

Magnetic resonance methods can be effectively utilized for studies of other hydrogen like atoms, like phosphorus. In 1998 Prof. B.E. Kane from the University of New South Wales, Australia suggested to use P impurities in crystalline silicon as qubits for quantum computing. His proposal created a real boom of magnetic resonance studies of the Si:P system. That is why our colleague Prof. Takao Mizusaki suggested to use our ESR equipment and expertise for studies of this system. Thus, a new collaborative project has been started in 2012 together with Prof. Mizusaki, Profs. Yutaka Fujii and Seitaro Mitsudo from the University of Fukui and Prof. SangGap Lee from the Korean Basic Science Institute. We have started with a basic sample of silicon with normal isotopic composition doped with P in a high field of 4.6T and temperatures below 1 K, the conditions never tried for these samples before. In our preliminary work we found several new and interesting phenomena, which encouraged us to continue working with this system. This research project is carried out by Dr. Jarno Järvinen and PhD student Denis Zvezdov (further reading).

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