补偿半金属中的热输运：一个谜团的解释
Thermal transport in compensated semimetals: a mystery explained
摘要：
众所周知，清洁补偿半金属的电子热导率可以通过双极传导机制的可用性而大大增强于电导率，由此电子和空穴在相同方向上流动，经历可忽略的库仑散射以及可忽略的杂质散射。最近在电荷中性点附近的二维单层和双层石墨烯中观察到了这种增强-导致Wiedemann-Franz定律的崩溃，洛伦兹比非常大。与此相反，通常发现三维补偿半金属如WP $_2$和Sb显示出降低的洛伦兹比。这种行为上的巨大差异通常归因于两种情况下费米统计的不同机制：补偿半金属中的简并电子-空穴液体与石墨烯中的非简并电子-空穴液体。我们表明，这种差异是不足以解释的补偿半金属的洛伦兹比的减少。我们认为，这个难题的解决方案在于补偿半金属的能力，以维持相当大的区域附近的接触，这反过来又是一个结果的电子和空穴口袋的动量空间中的大分离的电子-空穴积累。这些积累抑制双极导电机制，并有效地分裂成两个独立的电子和空穴导体的系统。我们提出了一个定量理论的交叉从双极到单极传导的电子-空穴积累区域的大小的函数，并表明，它自然会导致样品尺寸依赖的热导率。
英文摘要：
It is well known that the electronic thermal conductivity of clean compensated semimetals can be greatly enhanced over the electric conductivity by the availability of an ambipolar mechanism of conduction, whereby electrons and holes flow in the same direction experiencing negligible Coulomb scattering as well as negligible impurity scattering. This enhancement -- resulting in a breakdown of the Wiedemann-Franz law with an anomalously large Lorenz ratio -- has been recently observed in two-dimensional monolayer and bilayer graphene near the charge neutrality point. In contrast to this, three-dimensional compensated semimetals such as WP$_2$ and Sb are typically found to show a reduced Lorenz ratio. This dramatic difference in behavior is generally attributed to different regimes of Fermi statistics in the two cases: degenerate electron-hole liquid in compensated semimetals versus non-degenerate electron-hole liquid in graphene. We show that this difference is not sufficient to explain the reduction of the Lorenz ratio in compensated semimetals. We argue that the solution of the puzzle lies in the ability of compensated semimetals to sustain sizeable regions of electron-hole accumulation near the contacts, which in turn is a consequence of the large separation of electron and hole pockets in momentum space. These accumulations suppress the ambipolar conduction mechanism and effectively split the system into two independent electron and hole conductors. We present a quantitative theory of the crossover from ambipolar to unipolar conduction as a function of the size of the electron-hole accumulation regions, and show that it naturally leads to a sample-size-dependent thermal conductivity.
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