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 observe that the deviation from the standard behavior is concomitant with the fermion-fermion scattering time falling below the Planckian time, $\frac{\hbar}{k_{\rm B}T}$. We also observe that thermal diffusivity of this quantum liquid is bounded by a minimum set by fundamental physical constants, similarly to what was observed in classical liquids earlier. 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. Within a margin of 10\%, the experimental data from 0.007 K to 3 K can be accounted for if thermal conductivity is the sum of contributions from quasiparticles and sound: $\kappa=\kappa_{qp}+\kappa_s$; $\kappa_{qp}\propto T^{-1}$; $\kappa_s\propto T^{1/2}$. 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，我们观察到，标准行为的偏离与费米子-费米子散射时间低于普朗克时间$\frac{\hbar}{k_{\rm B}T}$相伴。 我们还观察到，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的限制，类似于早期在经典液体中观察到的情况。 这表明集体激发（声学模式）是热量的载体。 我们提出，这种模式的波矢为 2$k_F$，平均自由程等于德布罗意热长度。 这将提供一个具有$T^{1/2}$温度依赖性的额外导电通道，与实验中观察到的结果相匹配。 如果热导率是准粒子和声学模式贡献的总和，则在 10% 的范围内，从 0.007 K 到 3 K 的实验数据可以得到解释：$\kappa=\kappa_{qp}+\kappa_s$；$\kappa_{qp}\propto T^{-1}$；$\kappa_s\propto T^{1/2}$。