Transcript "Universal Spin Transport in Strongly Interacting Fermi Gases"

Universal Spin Transport in
Strongly Interacting Fermi Gases
Ariel Sommer
Mark Ku, Giacomo Roati, Martin Zwierlein
MIT
INT Experimental Symposium
May 19, 2011
Spin Transport
• Net relative motion of atoms with different
spin
Spin
current
• Damped due to collisions
Spin transport parameters
• Spin drag coefficient GSD
FSD = ½ M GSD vrel
• Spin diffusivity Ds
Motivation
• Learn about the unitary Fermi gas through
transport properties
• Understand transport properties of strongly
interacting systems
Expectations
• Known power laws at high temperature
• For T≈TF, expect Ds ≈ ħ/m
• For T<<TF, correlations may influence
transport
See:
Riedl et al., PRA 78, 053609 (2008)
Bruun and Smith PRA 72 043605 (2005)
Experiment 1
• Use 6Li in the two lowest hyperfine states
• Prepare a 50/50 spin mixture at 300G
• Ramp to 50G, separate the spin states via
magnetic gradient pulses
• Jump to the final field near the Feshbach
resonance
• Observe the evolution
Collision of Two Fermi Gases
Position (a.u.)
Collision of Two Fermi Gases
Center of Mass Difference
Time (ms)
• Bouncing at early times
• Exponential relaxation at late times
Varying Interactions
Experiment 2
•
•
•
•
•
Prepare a 50/50 spin mixture at 300G
Cool or heat at 300G
Ramp to 50G, separate the spin states
Jump to the final field
Cool again, or heat and wait, until separation
is small
• Observe the evolution to equilibrium
Measuring Spin Drag
Overdamped, use an exponential fit to get GSD
Results for Spin Drag: Unitarity
• Maximum drag near TF
Nature 472, 201 (2011)
Spin Drag vs Interaction Strength
T/TF = 0.32,
0.16
• Maximum drag on resonance
Nature 472, 201 (2011)
Comparison
• No enhanced spin drag at low T
Riedl et al., PRA 78, 053609 (2008)
Sommer et al, Nature 472, 201 (2011)
Measuring Spin Diffusivity
• Ds from spin density gradient and equilibration time
Gradient decays at the same rate as d
Spin Diffusion at Unitarity
Minimum Ds = 6.3(3) ħ/m
for T < 0.5 TF
Nature 472, 201 (2011)
Spin Diffusion vs Interaction Strength
• Minimum diffusivity on resonance
Nature 472, 201 (2011)
Spin Susceptibility
• Spin conductivity:
• Einstein relation:
• Derivation – Magnetization in a spin-dependent
potential:
spin-diffusion ↔ spin-conduction
Spin Susceptibility
Susceptibility < Compressibility
for T< TF
Nature 472, 201 (2011)
Relation to Homogeneous Values
• Non-uniform density and average velocity
• Trap average:
• Measured values:
Highly Polarized Gas
• Same procedure, but with a 90/10 mixture
arXiv: 1103.2337 (NJP, in production)
Solid line: Bruun et al., PRL 100, 240406 (2008)
• More Pauli blocking in the polaron case
Polaron damping
50/50 Spin drag
Drag reduction from peak down to 0.15 TF:
Polaron :
51(6)% reduction
Balanced Gas:
26(4) % reduction
Conclusion
• Spin drag is strong at unitarity
• Maximum spin drag on resonance, near TF
– Contrast to viscosity calculation at low T
• Minimum spin diffusion on res., below TF
• Non-uniform density and average velocity affects the
measurement