Title: How heat propagates in liquid $^3$He Show Chinese title

Title: 热量如何在液态$^3$He 中传播

Abstract: In Landau's Fermi liquid picture, transport is governed by scattering between quasi-particles. The normal liquid $^3$He 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, $\frac{\hbar}{k_{\rm B}T}$ and 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 2$k_F$ and a mean free path equal to the de Broglie thermal length. This would provide an additional conducting channel with a $T^{1/2}$ temperature 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 and another by sound (following the square root of temperature). Show Chinese abstract

Abstract: 在朗道的费米液体图像中，输运由准粒子之间的散射所支配。正常的液态$^3$氦符合这一图像，但仅在非常低的温度下。在这里，我们表明，标准行为的偏离伴随着费米子-费米子散射时间低于普朗克时间，$\frac{\hbar}{k_{\rm B}T}$，并且这种量子液体的热扩散率受到由基本物理常数设定的最小值限制，并在经典液体中被观察到。这指向集体激发（声子模式）作为热量的载体。我们提出，这种模式具有波矢为 2$k_F$和平均自由程等于德布罗意热长度。这将提供一个额外的导电通道，具有$T^{1/2}$温度依赖性，与实验中观察到的一致。如果热导率是两个贡献之和：一个是准粒子（随温度倒数变化）和另一个是声子（随温度平方根变化），则可以从 0.007 K 到 3 K 的实验数据可以得到解释，误差范围为 10%。