Tilman Enss

Publications (2)7.37 Total impact

• Article: Quantum Mechanical Limitations to Spin Diffusion in the Unitary Fermi Gas.

  Tilman Enss, Rudolf Haussmann
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  ABSTRACT: We compute spin transport in the unitary Fermi gas using the strong-coupling Luttinger-Ward theory. In the quantum degenerate regime the spin diffusivity attains a minimum value of D_{s}≃1.3ℏ/m approaching the quantum limit of diffusion for a particle of mass m. Conversely, the spin drag rate reaches a maximum value of Γ_{sd}≃1.2k_{B}T_{F}/ℏ in terms of the Fermi temperature T_{F}. The frequency-dependent spin conductivity σ_{s}(ω) exhibits a broad Drude peak, with spectral weight transferred to a universal high-frequency tail σ_{s}(ω→∞)=ℏ^{1/2}C/3π(mω)^{3/2} proportional to the Tan contact density C. For the spin susceptibility χ_{s}(T) we find no downturn in the normal phase.
  Physical Review Letters 11/2012; 109(19):195303. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Viscosity and scale invariance in the unitary Fermi gas

  Tilman Enss, Rudolf Haussmann, Wilhelm Zwerger
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  ABSTRACT: We compute the shear viscosity of the unitary Fermi gas above the superfluid transition temperature, using a diagrammatic technique that starts from the exact Kubo formula. The formalism obeys a Ward identity associated with scale invariance which guarantees that the bulk viscosity vanishes identically. For the shear viscosity, vertex corrections and the associated Aslamazov–Larkin contributions are shown to be crucial to reproduce the full Boltzmann equation result in the high-temperature, low fugacity limit. The frequency dependent shear viscosity η(ω) exhibits a Drude-like transport peak and a power-law tail at large frequencies which is proportional to the Tan contact. The weight in the transport peak is given by the equilibrium pressure, in agreement with a sum rule due to Taylor and Randeria. Near the superfluid transition the peak width is of the order of 0.5TF, thus invalidating a quasiparticle description. The ratio η/s between the static shear viscosity and the entropy density exhibits a minimum near the superfluid transition temperature whose value is larger than the string theory bound ℏ/(4πkB) by a factor of about seven.
  Annals of Physics.