The longitudinal relaxation time T $\\sb1$ is calculated for a dilute Fermi gas. We include the possibility of arbitrary degrees of polarization P, and uniform external magnetic fields. At low temperatures T $\\sb1$ is proportional to T $\\sp{-2}$ for the unpolarized gas in agreement with earlier calculations. For a polarized system in zero applied field the separation between up and down Fermi spheres provides an open channel in phase space for spin flipping processes. As temperature is reduced to zero T $\\sb1$ levels off and approaches a constant value which depends sensitively on P, leading to much smaller values of T $\\sb1$ than in the unpolarized case. For relaxation in a uniform external field H, Zeeman energy conservation during spin flipping interactions essentially shifts the location of the T $\\sp{-2}$ behavior from zero polarization to the polarization P(H) which would be produced in equilibrium with the external field H. When the initial polarization of the system is different from P(H), T $\\sb1$ again approaches a constant value which is equally sensitive to the initial polarization as temperature is reduced to zero. We apply our calculations to the case of dilute $\\sp3$ He-HeII solutions. We also consider the calculation of density dependent corrections to T $\\sb1$ for unpolarized, or weakly polarized $\\sp3$ He-HeII using the long wavelength limit of the Bardeen-Baym-Pines interaction as a form for the spin independent part of the $\\sp3$ He- $\\sp3$ He interaction. Owing to the positive sign of the antisymmetric Landau parameter $\\rm F\\sbsp{o}{a}$ in dilute $\\sp3$ He-HeII Fermi Liquid interactions act to screen the magnetic dipole interactions which cause relaxation. To lowest order in density we find that the relaxation rate 1/T $\\sb1$ is reduced by factor $\\rm (1 - 3F\\sbsp{o}{a})$ which compares favorably with recent calculations of T $\\sb1$ for pure $\\sp3$ He.The longitudinal relaxation time T$\\sb1$ is calculated for a dilute Fermi gas. We include the possibility of arbitrary degrees of polarization P, and uniform external magnetic fields. At low temperatures T$\\sb1$ is proportional to T$\\sp{-2}$ for the unpolarized gas in agreement with earlier calculations. For a polarized system in zero applied field the separation between up and down Fermi spheres provides an open channel in phase space for spin flipping processes. As temperature is reduced to zero T$\\sb1$ levels off and approaches a constant value which depends sensitively on P, leading to much smaller values of T$\\sb1$ than in the unpolarized case. For relaxation in a uniform external field H, Zeeman energy conservation during spin flipping interactions essentially shifts the location of the T$\\sp{-2}$ behavior from zero polarization to the polarization P(H) which would be produced in equilibrium with the external field H. When the initial polarization of the system is different from P(H), T$\\sb1$ again approaches a constant value which is equally sensitive to the initial polarization as temperature is reduced to zero. We apply our calculations to the case of dilute $\\sp3$He-HeII solutions. We also consider the calculation of density dependent corrections to T$\\sb1$ for unpolarized, or weakly polarized $\\sp3$He-HeII using the long wavelength limit of the Bardeen-Baym-Pines interaction as a form for the spin independent part of the $\\sp3$He-$\\sp3$He interaction. Owing to the positive sign of the antisymmetric Landau parameter $\\rm F\\sbsp{o}{a}$ in dilute $\\sp3$He-HeII Fermi Liquid interactions act to screen the magnetic dipole interactions which cause relaxation. To lowest order in density we find that the relaxation rate 1/T$\\sb1$ is reduced by factor $\\rm (1 - 3F\\sbsp{o}{a})$ which compares favorably with recent calculations of T$\\sb1$ for pure $\\sp3$He.

Jan 1, 19930 citations

Transverse spin diffusion in strongly interacting Fermi gases

Tilman Enss

We compute spin diffusion in a dilute Fermi gas at arbitrary temperature, polarization, and strong interaction in the normal phase using kinetic theory. While the longitudinal spin diffusivity ${D}_{\ensuremath{\parallel}}$ depends weakly on polarization and diverges for small temperatures, the transverse spin diffusivity ${D}_{\ensuremath{\perp}}$ has a strong polarization dependence and approaches a finite value for $T\ensuremath{\rightarrow}0$ in the Fermi liquid phase. For a 3D unitary Fermi gas at infinite scattering length, the diffusivities reach a minimum near the quantum limit of diffusion $\ensuremath{\hbar}/m$ in the quantum degenerate regime and are strongly suppressed by medium scattering, and we discuss the importance of the spin-rotation effect. In two dimensions, ${D}_{\ensuremath{\perp}}$ attains a minimum at strong coupling $\ensuremath{-}1\ensuremath{\lesssim}\mathrm{ln}({k}_{F}{a}_{2D})\ensuremath{\lesssim}1$ and reaches ${D}_{\ensuremath{\perp}}\ensuremath{\sim}0.2...\phantom{\rule{0.16em}{0ex}}0.3\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar}/m$ at large polarization. These values are consistent with recent measurements of two-dimensional ultracold atomic gases in the strong coupling regime.We compute spin diffusion in a dilute Fermi gas at arbitrary temperature, polarization, and strong interaction in the normal phase using kinetic theory. While the longitudinal spin diffusivity ${D}_{\ensuremath{\parallel}}$ depends weakly on polarization and diverges for small temperatures, the transverse spin diffusivity ${D}_{\ensuremath{\perp}}$ has a strong polarization dependence and approaches a finite value for $T\ensuremath{\rightarrow}0$ in the Fermi liquid phase. For a 3D unitary Fermi gas at infinite scattering length, the diffusivities reach a minimum near the quantum limit of diffusion $\ensuremath{\hbar}/m$ in the quantum degenerate regime and are strongly suppressed by medium scattering, and we discuss the importance of the spin-rotation effect. In two dimensions, ${D}_{\ensuremath{\perp}}$ attains a minimum at strong coupling $\ensuremath{-}1\ensuremath{\lesssim}\mathrm{ln}({k}_{F}{a}_{2D})\ensuremath{\lesssim}1$ and reaches ${D}_{\ensuremath{\perp}}\ensuremath{\sim}0.2...\phantom{\rule{0.16em}{0ex}}0.3\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar}/m$ at large polarization. These values are consistent with recent measurements of two-dimensional ultracold atomic gases in the strong coupling regime.

arXiv:1307.5175Physical Review ASep 27, 201319 citations

Degeneracy effects in the spin dynamics of spin-polarized Fermi gases

A. E. Meyerovich

Physics Letters AJan 1, 198563 citations

The longitudinal relaxation time T

\\sb1

is calculated for a dilute Fermi gas. We include the possibility of arbitrary degrees of polarization P, and uniform external magnetic fields. At low temperatures T

\\sb1

is proportional to T

\\sp{-2}

for the unpolarized gas in agreement with earlier calculations. For a polarized system in zero applied field the separation between up and down Fermi spheres provides an open channel in phase space for spin flipping processes. As temperature is reduced to zero T

\\sb1

levels off and approaches a constant value which depends sensitively on P, leading to much smaller values of T

\\sb1

than in the unpolarized case. For relaxation in a uniform external field H, Zeeman energy conservation during spin flipping interactions essentially shifts the location of the T

\\sp{-2}

behavior from zero polarization to the polarization P(H) which would be produced in equilibrium with the external field H. When the initial polarization of the system is different from P(H), T

\\sb1

again approaches a constant value which is equally sensitive to the initial polarization as temperature is reduced to zero. We apply our calculations to the case of dilute

\\sp3

He-HeII solutions. We also consider the calculation of density dependent corrections to T

\\sb1

for unpolarized, or weakly polarized

\\sp3

He-HeII using the long wavelength limit of the Bardeen-Baym-Pines interaction as a form for the spin independent part of the

\\sp3

He-

\\sp3

He interaction. Owing to the positive sign of the antisymmetric Landau parameter

\\rm F\\sbsp{o}{a}

in dilute

\\sp3

He-HeII Fermi Liquid interactions act to screen the magnetic dipole interactions which cause relaxation. To lowest order in density we find that the relaxation rate 1/T

\\sb1

is reduced by factor

\\rm (1 - 3F\\sbsp{o}{a})

which compares favorably with recent calculations of T

\\sb1

for pure

\\sp3

He.