We have performed magnetic compression experiments with the two-dimensional gas of hydrogen atoms on liquid 4 He at T 120 200 mK. For 2D phase-space densities higher than 3, the probability of three-body dipole recombination is observed to decrease gradually by an order of magnitude in comparison to its low-density value K bbb 8.4͑3.5͒ 3 10 225 cm 4 s 21 . This is attributed to local coherence developing in a 2D quasicondensate. We have also determined the value U͞k 5.1͑5͒ 3 10 215 K cm 2 for the mean-field energy of the gas. [S0031-9007(98)07787-4] PACS numbers: 03.75.Fi, 05.30.Jp, 67.65. + z, 68.35.Rh In addition to the occupation of a single quantum state, a Bose-Einstein condensate (BEC) is characterized by long-range order or, equivalently, global coherence. The concept of quasicondensate (QC) has been introduced [1] for the lesser known situation where a boson system is coherent only locally. Having similar particle interactions, BEC and QC would be hardly distinguishable by virtue of properties related to inelastic [2,3] or elastic [4] collisions. However, the difference is transparent in spatially uniform two-dimensional (2D) system where a BEC cannot exist at finite temperature T but a QC can.In fact, a QC may appear due to interactions which suppress density fluctuations and therefore allow the 2D system to be described by a single wave function. Although the phase of the function fluctuates spatially and thus thwarts a long-range order [1], the phase coherence persists within a finite length L~exp͑sl 2 ͒ [2]. Here, l p 2ph 2 ͞mkT is the de Broglie wavelength, s is the 2D gas density, and m is the particle mass. For T ! 0 the "local BEC" regions of size L tend to extend throughout the sample and the QC turns into a "true" condensate.The appearance of a QC is to be expected when the interaction energy becomes comparable with the kinetic energy, sU ϳ kT . Here, U 4ph 2 j͞m is the mean-field parameter for elastic interactions and j is a dimensionless interaction strength. It is known that at T 0 the ratio of densities of above-condensate and condensate particles is s 0 ͞s 0 ഠ j [5]. Therefore a QC may manifest itself preferably in weakly interacting (low-j) systems.This Letter reports on the first experimental evidence for a quasicondensate in 2D atomic hydrogen gas, where we have observed a significant reduction in three-body recombination probability [2,6]. Hydrogen atoms on liquid 4 He surface occupy a single bound state with low adsorption energy, E a ͞k Ӎ 1.0 K [7]. The H adatoms are localized quite far, 8 Å, from the liquid and their out-of-plane delocalization length, l h͞ p 2E a m Ӎ 5 Å, is large compared with, e.g., the 3D scattering length a 0 0.72 Å. Thus adsorbed hydrogen may be also regarded as quasi-2D in the sense that it obeys 2D statistics while particle interactions are three dimensional [2]. It may be shown [2] that j ഠ a 0 ͞l and consequently a QC in hydrogen on 4 He may be observed at sl 2 * l͞2a 0 Ӎ 3.5.It is not easy to achieve a high degree of quantum degeneracy, i.e., high s at ...

We describe a 128 GHz electron-spin-resonance spectrometer based on heterodyne detection with double frequency conversion utilizing cryogenic Schottky-diode mixers. Together with other mm-wave components installed into a dilution refrigerator cryostat, the mixers comprise a bridge operating at 1.5 K. A miniature Fabry-Perot resonator is used to detect samples of bulk and surface-adsorbed atomic hydrogen gas at temperatures below 100 mK. The sensitivity is 2 ϫ10 9 spins/Gauss at the excitation power of 20 pW.

We study by electron-spin-resonance spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures T(S) from 50 to 110 mK. The average dipolar field in this 2D system shifts the electron-spin-resonance peak of the adsorbed atoms relative to that of bulk atoms. The shift is directly proportional to surface density. The role of longitudinal magnetization relaxation is played by particle exchange between the 2D and the 3D phases, which diminishes exponentially with decreasing T(S). Therefore at T(S) less, similar 80 mK an excitation field of 0.1 mG disturbs the equilibrium surface density and leads to a magnetization instability observed as sawtooth shaped resonance lines.

No abstract

We present the first systematic measurement of the binding energy E(a) of hydrogen atoms to the surface of saturated 3He- 4He mixture films at temperatures 70-400 mK. E(a) is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the population of the ground surface state of 3He grows from zero to 6x10(14) cm(-2), yielding the value 1.2(1)x10(-15) K cm(2) for the mean-field parameter of H- 3He interaction in 2D. Measuring by ESR the rate constants K(aa) and K(ab) for second-order recombination of hydrogen atoms in hyperfine states a and b, we find the ratio K(ab)/K(aa) to be independent of the 3He content and to grow with temperature.