Magnetic resonance in Dense Atomic Hydrogen Gas

Transcription

1 Magnetic resonance in Dense Atomic Hydrogen Gas S. Vasiliev University of Turku, Finland Turku

2 Magnetic resonance in Dense Atomic Hydrogen Gas Sergey Vasiliev University of Turku H group at Turku: Janne Ahokas Otto Vainio Ville Virtanen Sergey Sheludyakov Denis Zvezdov* *also at Kazan Federal University, Kazan, Russia

3 Why to study H gas? - The only gas at T=0 - Bose-Einstein Condensation - H-H interactions, cold collisions - Magnetic resonance - Spin waves, BEC of magnons

4 n as 0.72Å Basic parameters = cm T = mk λ = 0 2 4π an 0.1 mm 2π 17.4Å Λ th = mkbt T Λ 36Å at T =0.3 K th B 4.6 T Quantum gas limit Λ th a Onset of quantum degeneracy Λ th nλ 3 th 1 Cold collisions, Spin waves

5 Spin waves in quantum gases V. P. Silin, JETP 6,945 (1958) A.J. Legett and M.J. Rice, PRL, 20, 586 (1968) E.P. Bashkin, JETP Lett. 33, 11 (1981) C. Lhuillier and F. Laloe, J. Phys (Paris) 43, 197 (1982) liquid 3 He spin-polarized gases μ Λ >> 1 th a 1 2 Ψ = A + B Ψ = Ae iϕ1 + Be iϕ Identical Spin Rotation Effect (ISR)

6 Nuclear spin waves in H gas Pulsed NMR, spectra after FT D.M. Lee at Cornell University B. R. Johnson et al., PRL 52, 1508 (1984)

7 M + = M x + im y Basic equations of ISR spin waves C. Lhuillier and F. Laloe, J. Phys (Paris) 43, 197 (1982) M+ 1 iεμm = + z t 1+ μ M 2 iγδ B0M+ D0 M ε =± 1 μ Λ th a for bosons/fermions - spin wave quality factor i+ με Mz T ω( k) = D0 k k μ M n D 0 spin diffusion coefficient in unpolarized gas 2 2

8 μ Λ >> 1 th a M M M + z Schrödinger type equation: M t i = M+ + γδ B0M+ Particle ε γ μ D ε μ kinetic potential 3 He -1 negative neg/pos H proton H electron +1 positive negative +1 negative, negative large High field seekers μ m = μn ε D 0 Low field seekers

9 Calculations of μ for H and 3 He C. Lhuillier, J. Physique 44, 1-12 (1983)

10 Experimental setup 100 μm B 4.6 T T= K n~10 15 cm -3 n cm -3 Sample lifetime ~3-100 min

11 80 μm 3 μm polyimide wall (tube) Compression region H gas 300 nm gold layer 12 μm polyimide (Kapton) film Epoxy 4 He coolant

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13 m S, m I ½, ½ ½, -½ F=1 F=0 Doubly polarized H gas -½, -½ -½, ½ hyperfine state selective recombination: a + a + M = H 2 + M a + b + M = H 2 + M b + b + b = H 2 + b

14 Sweep, V [0.39 G/V] CW spectra in static field of optimized homogeneity 4 75 mg 2 ESR absorption

15 B(z) Large positive gradient ~40G/cm z H gas G

16 Magnetic field sweep, V (@0.4G/V) B(z) Large negative gradient ~40G/cm 3 2 z H gas G

17 Sweep, V [0.39 G/V] CW spectra in static field of optimized homogeneity ω = D μ 0 2 π k = = λ k 2 π L n 4 H gas 2 75 mg π D Δω μ L 0.5 mg For L=0.5 mm n=10 17 cm khz 2 ESR absorption

18 U(z) Large positive gradient ~40G/cm, CW z H gas ( ) z 2 2 i t M J0 κr Ai λ kz e ω + = + λ 2 κ = μ ω D 0 λ D 0 = γ eg zμ 1/3

19 U(z) Large negative gradient ~40G/cm 3 2 z H gas 1 0 ( ) z 2 2 i t M J0 κr Ai λ kz e ω + = + λ 2 κ = μ ω D 0 λ 0 = γ eg zμ D 1/ G Magnetic field sweep, V (@0.4G/V)

20 Fit result: μ 8 T=360 mk

21 ESR spectra with no gradients applied main ESR peak Is the observed lineshape caused by inhomogeneity of RF field plus the static field inhomogeneity? magnon peak Sweep, G

22 Spectra in various gradients I = -3A I = -2A I = -1A 0.00 I = 0A Peak position, V (3.8G/V) I = +1A I = +2A SR coil current, Amps I = +3A Sweep [V ] (0.78 G/V)

23 Static magnetic field profile, no gradients applied Epoxy 4π M 0.8 G

24 Moving trap position with applied gradients

25 Particle in a box + parabolic potential M t D ε = + μ i M+ γδ H0M+ Δω 1 Δω μn Magnetic field, mg n = = cm, μ Sweep, G

26 CW spectra, no grad applied 10 mg density increases 0.1G Magnetic field or frequency

27 n c increases at higher T and shallower trap n c

28 Free Induction Decay FID signal Precession frequency does not depend on time f 130 GHz time, ns Q τ~5 s at 1 MHz

29 Conclusions Spin-polarized H gas is a rich system where a large number of magnetic excitations (magnons) can be studied by means of magnetic resonance. We identified four basic types of magnon modes, distinguished by their behavior in magnetic field gradient, at different densities, sample shapes and gas temperatures. Applying negative axial magnetic field gradients we observed magnon modes of exchange origin, caused by the ISR effect. These modes do not depend on the sample shape, and are not pure standing waves. We found a spectrum of magnon modes trapped in the magnetic field maximum. At high enough gas densities we observed excessive occupation of the ground state in this trap, with long coherent precession of magnetization, similar to the HPD in experiments with liquid 3 He.