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  1. Abstract

    In Landau’s Fermi liquid picture, transport is governed by scattering between quasi-particles. The normal liquid3He conforms to this picture but only at very low temperature. Here, we show that the deviation from the standard behavior is concomitant with the fermion-fermion scattering time falling below the Planckian time,$$\frac{\hslash }{{k}_{{{{{{{{\rm{B}}}}}}}}}T}$$kBTand the thermal diffusivity of this quantum liquid is bounded by a minimum set by fundamental physical constants and observed in classical liquids. This points to collective excitations (a sound mode) as carriers of heat. We propose that this mode has a wavevector of 2kFand a mean free path equal to the de Broglie thermal length. This would provide an additional conducting channel with aT1/2temperature dependence, matching what is observed by experiments. The experimental data from 0.007 K to 3 K can be accounted for, with a margin of 10%, if thermal conductivity is the sum of two contributions: one by quasi-particles (varying as the inverse of temperature) and another by sound (following the square root of temperature).

     
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  3. Abstract The Wiedemann–Franz law establishes a link between heat and charge transport due to electrons in solids. The extent of its validity in the presence of inelastic scattering is a question raised in different contexts. Here we report on a study of the electrical, σ , and thermal, κ , conductivities in WP 2 single crystals. The Wiedemann-Franz law holds at 2 K, but a downward deviation rapidly emerges upon warming. At 13 K, there is an exceptionally large mismatch between the Lorenz number and the Sommerfeld value. We show that this is driven by a fivefold discrepancy between the T -square prefactors of electrical and thermal resistivities, both caused by electron–electron scattering. This implies the existence of abundant small-scattering-angle collisions between electrons, due to strong screening. By quantifying the relative frequency of collisions conserving momentum flux, but degrading heat flux, we identify a narrow temperature window where the hierarchy of scattering times may correspond to the hydrodynamic regime. 
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