We present the first magnetic resonance study of atomic hydrogen embedded in solid H2 films for temperatures 150-900 mK. We found that at T approximately 150 mK average concentrations of H atoms of order 10(18 cm(-3) are very stable against recombination during two weeks of observations. The distribution of the population of the two lowest hyperfine states is found to be non-Boltzmann, with a very large occupation of the ground state. We consider the possibility of formation in solid H2 of regions with high local concentrations of H atoms, where collective quantum phenomena might occur.

We report a measurement of the cold collision frequency shift in atomic hydrogen gas adsorbed on the surface of superfluid (4)He at T approximately < 90 mK. Using two-photon electron and nuclear magnetic resonance in 4.6 T field we separate the resonance line shifts due to the dipolar and exchange interactions, both proportional to surface density sigma. We find the clock shift Delta nu(c) = -1.0(1) x 10(-7) Hz cm(-2) x sigma, which is about 100 times smaller than the value predicted by the mean field theory and known scattering lengths in the three-dimensional case.

We present an experimental study of H atoms embedded in thin films of solid H 2 at temperatures below 1 K. H 2 films up to 50 nm thick were first grown as a result of slow recombination of atomic hydrogen gas on the sample cell walls. If the recombination occurred in three-body atomic collisions in the gas phase, small concentrations of atoms could be captured inside the films during the film deposition. As a second method of generating atomic populations inside the H 2 films, we used a direct dissociation by a low power rf discharge in the sample cell. With this latter method, we achieved record high atomic concentrations exceeding 2 ϫ 10 19 cm −3 . The samples were characterized by means of magnetic resonance: electron spin resonance ͑ESR͒ and electron-nuclear double resonance ͑ENDOR͒ in a magnetic field of 4.6 T. We observed density-dependent broadening and shifts of the ESR lines due to the dipolar interactions, and resolved these effects for like and unlike atoms. Relaxation of the relative hyperfine populations was measured as a function of temperature for H in H 2 films grown on different substrates. For H 2 films on Mylar substrates, the relative equilibrium populations of the two lowest hyperfine states of H were found to deviate substantially from the prediction of Boltzmann statistics. We also found two narrow lines in the ENDOR spectra of H in H 2 films shifted to the red from the position for free atoms. This indicates two possible substitutional positions of the atoms in H 2 matrices, both characterized by very homogeneous crystalline fields.

We report on experimental observation of BEC-like behaviour of quantized electron spin waves (magnons) in a dense gas of spin polarized atomic hydrogen. The magnons are trapped and controlled with inhomogeneous magnetic fields, and described by a Schrödinger-like wave equation, in analogy to the BEC experiments with neutral atoms. We have observed the appearance of a sharp feature in the ESR spectrum displaced from the normal spin wave spectrum. We believe that this observation corresponds to a sudden growth of the ground state population of the magnons and emergence of their spontaneous coherence for hydrogen gas densities exceeding a critical value, dependent on the trapping potential. We interpret the results as a BEC of non-equilibrium magnons which were formed by applying the rf power.In contrast to the Bose-Einstein condensation (BEC) experiments with real particles, e.g. with alkali atoms [1], a condensation of quasiparticles cannot be achieved by lowering the temperature of a thermalized system, but instead a non-equilibrium state is necessary [2]. This overpopulation of quantum states compared to the thermal equilibrium population given by the Planck distribution is achieved by injecting additional quasiparticles to the system externally, i.e. pumping the system. BEC-like behavior, or spontaneous coherence, in systems of coupled oscillators has been predicted by H. Fröhlich [3] and is often referred to as the Fröhlich coherence. In recent years a Bose-Einstein-like condensation of quasiparticles has been reported in several distinct systems. These include exciton polaritons [4], triplet states in magnetic insulators [5], magnons in ferromagnets [6] and liquid 3 He [7], and photons in a microcavity [8]. Understanding the properties of quasiparticles is increasingly important due to advancing technologies pushing ever further into the quantum realm.Quantized electron spin waves (magnons) form a quasiparticle system in a dense quantum gas of ultracold atomic hydrogen [9]. The definition of a quantum gas is that the thermal de Broglie wavelength Λ th substantially exceeds the scattering length of elastic collisions a s . By tuning the magnetic field profile in our experiment we can modify the magnon-trapping potential. Furthermore, by changing the atomic hydrogen gas density we are able to modify the spin-exchange interaction strength and thereby the dynamics of the magnons. These two tools together provide unique control of the quasiparticle dynamics and allow the formation of a BEC of magnons similar to that observed in the BEC of neutral atoms [1].In the present study of dense H gas by electron spin resonance (ESR), we observed a sudden change in the spectrum of the trapped magnons: a sharp and intense peak corresponding to their ground state in the trap grows rapidly after H gas density exceeds a critical value. We believe that this is associated with the emergence of a spontaneous coherence in the system. Based on these two observations we conclude that the magnons undergo a transition to a BEC when th...