A surprising “planar” thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators; this effect has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here, we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity 𝜅xy (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar 𝜅xy from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar 𝜅xy in those samples where the thermal Hall response is due to phonons, even though it should, in principle, be forbidden by the high crystal symmetry. Our findings call for a careful reexamination of the mechanisms responsible for the phonon thermal Hall effect in insulators.

中文翻译：

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铜酸盐中声子的平面热霍尔效应


最近在一些磁绝缘体中观察到一种令人惊讶的“平面”热霍尔效应，即磁场与电流平行;这种效应归因于奇异的激发，如马约拉纳费米子或手性磁振子。在这里，我们研究了三种不同铜酸盐材料中平面热霍尔效应的可能性，其中传统的热霍尔电导率 κxy（具有垂直于电流的平面外场）由电子或声子主导。我们的测量表明，铜酸盐中电子的平面 κxy 为零，这与平面构型中没有洛伦兹力所预期的那样。相比之下，我们在那些热霍尔响应由声子引起的样品中观察到相当大的平面 κxy，尽管原则上它应该被高晶体对称性所禁止。我们的研究结果要求仔细重新检查导致绝缘体中声子热霍尔效应的机制。