Electron-Spin-Resonance Instability in Two-Dimensional Atomic Hydrogen Gas

Vasilyev, S. A.; Järvinen, J.; Safonov, A. I.; Kharitonov, A. P.; Lukashevich, I. I.; Jaakkola, S.

doi:10.1103/physrevlett.89.153002

Cited by 20 publications

(21 citation statements)

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“…While the experiments with ultracold alkali vapors [1, 2] demonstrate an excellent agreement with the spectroscopic data on scattering lengths [6], the measured shift of a certain transition in atomic hydrogen appeared to be two orders of magnitude lower than the expected value in both 2D and 3D cases [3,4]. Let us recall that the virtual absence of the collision frequency shift of ESR in two-dimensional hydrogen has been already observed in experiments by Shinkoda and Hardy [7] and, later on, by Vasiliev et al [8,9]. As an explanation, Shlyapnikov and also Prokofiev and Svistunov [10] pointed out that triplet-singlet transitions in spin-polarized atomic hydrogen accompanied by the change in the scattering length are impossible, because the absorption of microwave quanta by the spin system is coherent.…”

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“…Thus, the average interaction energy for the |bd〉 pair is equal to (7) Comparing Eqs. (5) and (7) and averaging over all of the atoms of the gas, we finally obtain the collision frequency shift of the a d transition in the gas of b atoms: (8) The frequency shift turns out to be negative, corresponding to an increase in the resonance value of the magnetic field when the microwave transition is detected at a fixed frequency. The resulting value of the shift agrees well with the experiment [4].…”

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10.1007/s11448-008-1006-8

2010

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23Collisions of particles in a gas lead to a shift of hyperfine transition frequencies with respect to isolated atoms [1][2][3][4]. Since the collision frequency shift is one of the major factors that affect the stability of the present frequency standards [5], its firm understanding is important for both fundamental physics and applications. This work was stimulated by the experiments to measure the collision frequency shift of hyperfine transitions in two-dimensional (2D), adsorbed on the surface of superfluid helium [3], and three-dimensional (3D) [4] spin-polarized atomic hydrogen at T ~ 0.1-0.3 K in a magnetic field of 4.6 T by means of ESR and ENDOR (electron-nuclear double resonance) techniques. While the experiments with ultracold alkali vapors [1, 2] demonstrate an excellent agreement with the spectroscopic data on scattering lengths [6], the measured shift of a certain transition in atomic hydrogen appeared to be two orders of magnitude lower than the expected value in both 2D and 3D cases [3,4]. Let us recall that the virtual absence of the collision frequency shift of ESR in two-dimensional hydrogen has been already observed in experiments by Shinkoda and Hardy [7] and, later on, by Vasiliev et al. [8,9]. As an explanation, Shlyapnikov and also Prokofiev and Svistunov [10] pointed out that triplet-singlet transitions in spin-polarized atomic hydrogen accompanied by the change in the scattering length are impossible, because the absorption of microwave quanta by the spin system is coherent. However, shortly after that, the experiments of Harber et al. with 87 Rb [1] and the accurate analyses of Zwierlein et al. [2] proved that the coherence of the interaction of spins with the microwave field is not directly related to the magnitude of the contact shift.The sharp contradiction between theory and experiment in the simplest system of atomic hydrogen, where the majority of the results may be obtained ab initio analytically with a high accuracy, seemingly questions the foundations of quantum mechanics. In this work, we show that the apparent contradiction may be resolved by taking into account the relation between the symmetry of the state of two atoms and their total electron spin S , such that triplet-singlet transitions are actually forbidden in some cases.To exclude the coherence-related effects, we assume the gas to be far from quantum degeneracy. The ground state of a separate hydrogen atom in a magnetic field B is known to split into four hyperfine sublevels [11] (1) where tan2 θ ≈ A /2 µ B B , the hyperfine constant for hydrogen is A / h ≈ 1420 MHz, and the arrows ⇓⇑ ( ↓↑ ) denote the projections of the electron (nuclear) spin on the direction of the magnetic field. In a high field, θ Ӷ 1, the hyperfine interaction is much smaller than the Zeeman interaction; therefore, the electron and nuclear spin almost independently interact with the magnetic field. This appears in the smallness of the impurity of inversed spin orientations in eigenstates a and c . At a low temperature, only the two lowest hy...

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10.1007/s11448-008-1006-8

2010

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“…This is 4...10 times smaller than the earlier values measured indirectly [3,9,10,11].Our experimental setup is shown in Fig.1. The sample cell has been described elsewhere [8]. The lowtemperature part of the ESR spectrometer, operating now as a mm-wave bridge, has been modified so that we may use three times smaller excitation powers and thus avoid ESR instability effects [8] in a wider temperatures range.…”

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Thermal compression of two-dimensional atomic hydrogen gas

et al. 2004

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We describe experiments where 2D atomic hydrogen gas is compressed thermally at a small "cold spot" on the surface of superfluid helium and detected directly with electron-spin resonance. We reach surface densities up to 5×10 12 cm −2 at temperatures ≈ 100 mK corresponding to the maximum 2D phase-space density ≈ 1.5. By independent measurements of the surface density and its decay rate we make the first direct determination of the three-body recombination rate constant and get the upper bound L 3b 2 × 10 −25 cm 4 /s which is an order of magnitude smaller than previously reported experimental results.PACS numbers: 05.30.Jp, 67.65.+z, 82.20.Pm When adsorbed on the surface of superfluid helium spin-polarized atomic hydrogen (H↓) is an ideal realization of a two-dimensional (2D) boson gas [1]. Helium provides a translationally invariant substrate and its surfacenormal potential supports only one bound state for hydrogen with the binding energy E a =1.14(1) K [2]. Even for such a weak interaction, lowering the surface temperature T s well below 1 K leads to a large adsorbate density σ. At high H↓ coverages three-body recombination is expected to be the dominant density decay mechanism setting the main obstacle to the achievement of the quantum degeneracy regime, where the thermal de Broglie wavelength Λ is larger than the average interatomic spacing. Degenerate 2D H↓ is expected to exhibit collective phenomena such as the Kosterlitz-Thouless superfluidity transition and the formation of a quasicondensate.Two methods of local compression of adsorbed H↓ have been employed to overcome limitations caused by recombination and its heat. Magnetic compression has been successfully used to achieve quantum degeneracy [3,4]. In this method the recombination heat is removed from the compressed H↓ by ripplons of the helium surface and the cooling efficiency depends on the length of the heat transfer path. By decreasing the size of the compressed region to 20 µm we were able to achieve σΛ 2 ≈ 9 [3]. However, the small size of the sample together with large magnetic field gradients did not allow to implement direct diagnostics of adsorbed H↓. In the thermal compression method [5,6] cooling a small part of the sample cell wall well below the temperature of the rest of the wall leads to an exponential increase of σ on such a "cold spot". In this method the recombination heat is transferred from the ripplons to the phonons of helium [7] and then to the substrate beneath the spot. Therefore a larger spot is preferable as long as the total recombination rate on the spot becomes a limitation. The larger sample size and the homogeneity of the magnetic field make thermal compression better suited for direct studies of adsorbed H↓.In the present work we use sensitive electron-spin resonance [8] to diagnose 2D H↓ gas thermally compressed to σΛ 2 ≈ 1.5 and discuss the limitations and possible improvements of the "cold spot" method to reach and detect the Kosterlitz-Thouless transition. By independent measurements of the recombination rate a...

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“…[24][25][26] For 2D systems, magnetization bistability was observed in spinpolarized atomic hydrogen gas absorbed on the superfluid helium film. 27 . For semiconductor heterostructures, the nonlinearity of inter-subband absorption was predicted and studied since late 80s [28][29][30][31][32][33] and absorption bistability has been long sought.…”

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Bistability and hysteresis of intersubband absorption in strongly interacting electrons on liquid helium

Konstantinov

Dykman²,

Lea

et al. 2012

Phys. Rev. B

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We study nonlinear inter-subband microwave absorption of electrons bound to the liquid helium surface. Already for a comparatively low radiation intensity, resonant absorption due to transitions between the two lowest subbands is accompanied by electron overheating. The overheating results in a significant population of higher subbands. The Coulomb interaction between electrons causes a shift of the resonant frequency, which depends on the population of the excited states and thus on the electron temperature Te. The latter is determined experimentally from the electron photoconductivity. The experimentally established relationship between the frequency shift and Te is in reasonable agreement with the theory. The dependence of the shift on the radiation intensity introduces nonlinearity into the rate of the inter-subband absorption resulting in bistability and hysteresis of the resonant response. The hysteresis of the response explains the behavior in the regime of frequency modulation, which we observe for electrons on liquid 3 He and which was previously seen for electrons on liquid 4 He.

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Electron-Spin-Resonance Instability in Two-Dimensional Atomic Hydrogen Gas

Cited by 20 publications

References 13 publications

10.1007/s11448-008-1006-8

10.1007/s11448-008-1006-8

Thermal compression of two-dimensional atomic hydrogen gas

Bistability and hysteresis of intersubband absorption in strongly interacting electrons on liquid helium

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