We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of (40)K precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter γ=1.08(9) and a bare transverse spin diffusivity D(0)(⊥)=2.3(4)ℏ/m for a normal-state gas initialized with full polarization and at one-fifth of the Fermi temperature, where m is the atomic mass. One might expect γ=0 at unitarity, where two-body scattering is purely dissipative. We observe γ→0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a nonzero γ. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0<(k(F)a)(-1)≲1.3, where k(F) is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that the sign change in γ indicates a switching of branch between a repulsive and an attractive Fermi gas.We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of (40)K precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter γ=1.08(9) and a bare transverse spin diffusivity D(0)(⊥)=2.3(4)ℏ/m for a normal-state gas initialized with full polarization and at one-fifth of the Fermi temperature, where m is the atomic mass. One might expect γ=0 at unitarity, where two-body scattering is purely dissipative. We observe γ→0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a nonzero γ. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0<(k(F)a)(-1)≲1.3, where k(F) is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that the sign change in γ indicates a switching of branch between a repulsive and an attractive Fermi gas.

arXiv:1410.8517Physical Review LettersJan 7, 201557 citations

The unitary Fermi gas

Joaquín E. Drut

Jan 1, 20081 citation

Observation of field-induced spin-current relaxation in a Fermi liquid

L-J. Wei, Neal Kalechofsky, Donald Candela

We have studied NMR spin echoes in $^{3}\mathrm{He}$ liquid in a magnetic field ${\mathit{B}}_{0}$ =8 T for temperatures T\ensuremath{\ge}4.5 mK. The data are analyzed in terms of the Leggett-Rice effect to obtain the transverse spin-diffusion coefficient ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$(T). For T20 mK we find that ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$ is less than measured in earlier experiments at lower ${\mathit{B}}_{0}$ . This phenomenon has been predicted for degenerate Fermi systems, due to the phase space created for quasiparticle scattering by spin polarization. The effect we have measured is more pronounced than suggested by scaling a dilute-gas theory result by a Fermi-liquid factor.We have studied NMR spin echoes in $^{3}\mathrm{He}$ liquid in a magnetic field ${\mathit{B}}_{0}$=8 T for temperatures T\ensuremath{\ge}4.5 mK. The data are analyzed in terms of the Leggett-Rice effect to obtain the transverse spin-diffusion coefficient ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$(T). For T20 mK we find that ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$ is less than measured in earlier experiments at lower ${\mathit{B}}_{0}$. This phenomenon has been predicted for degenerate Fermi systems, due to the phase space created for quasiparticle scattering by spin polarization. The effect we have measured is more pronounced than suggested by scaling a dilute-gas theory result by a Fermi-liquid factor.

Physical Review LettersAug 9, 199391 citations

Mean-field and spin-rotation phenomena in fermi systems : the relation between the leggett-rice and lhuillier-laloë effects

K. Miyake, William J. Mullin, Philip C. E. Stamp

The term in the Boltzmann equation collision integral causing identical-particle spin rotation in the Lhuillier-Laloë (LL) theory of a non-degenerate quantum gas is shown to be equivalent in the dilute limit to the molecular field precessional term giving rise to the Leggett-Rice (LR) effect in a degenerate Fermi system. This equivalence is shown (a) by considering the spin-rotation term at low temperatures and (b) by deriving hydrodynamic equations valid for all temperatures from the Landau-Silin equation in the s-wave approximation. The spin-rotation factor μ resulting from (b) is found to agree with the LR value at low temperatures and with the LL value at high temperatures. The diffusion constant Do that results has the proper low temperature behaviour, including the standard mean-field correction factor ; at high temperatures Do has the LL form times a the mean-field correction factor. The importance of the missing mean-field term is illustrated by showing that it gives rise to second virial corrections to the pressure.The term in the Boltzmann equation collision integral causing identical-particle spin rotation in the Lhuillier-Laloë (LL) theory of a non-degenerate quantum gas is shown to be equivalent in the dilute limit to the molecular field precessional term giving rise to the Leggett-Rice (LR) effect in a degenerate Fermi system. This equivalence is shown (a) by considering the spin-rotation term at low temperatures and (b) by deriving hydrodynamic equations valid for all temperatures from the Landau-Silin equation in the s-wave approximation. The spin-rotation factor μ resulting from (b) is found to agree with the LR value at low temperatures and with the LL value at high temperatures. The diffusion constant Do that results has the proper low temperature behaviour, including the standard mean-field correction factor ; at high temperatures Do has the LL form times a the mean-field correction factor. The importance of the missing mean-field term is illustrated by showing that it gives rise to second virial corrections to the pressure.

Journal de physiqueJan 1, 198538 citations

We have studied NMR spin echoes in

^{3}\mathrm{He}

liquid in a magnetic field

{\mathit{B}}_{0}

=8 T for temperatures T\ensuremath{\ge}4.5 mK. The data are analyzed in terms of the Leggett-Rice effect to obtain the transverse spin-diffusion coefficient ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$(T). For T20 mK we find that ${\mathit{D}}_{\mathrm{\ensuremath{\perp}}}$ is less than measured in earlier experiments at lower

{\mathit{B}}_{0}

. This phenomenon has been predicted for degenerate Fermi systems, due to the phase space created for quasiparticle scattering by spin polarization. The effect we have measured is more pronounced than suggested by scaling a dilute-gas theory result by a Fermi-liquid factor.