元素超导体铌中的热霍尔效应：一个双能隙超导体
Thermal Hall effect in elemental niobium, a two-gap superconductor
- Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院及武汉国家强磁场中心）
- Laboratoire de Physique et d’Étude de Matériaux (CNRS) ESPCI Paris, PSL Research University（巴黎高等物理化工大学校PSL研究大学材料物理与研究所（CNRS））
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
通过热霍尔效应测量，在元素超导体铌中明确观察到横向热输运的载流子类型交叉，证实了双能隙超导性，并利用BRT模型拟合得到第二能隙约为主导能隙的11%，解决了长期争议。
AI中文摘要：
铌在超导历史中占据着关键地位：它不仅是临界温度最高的元素超导体，而且长期以来一直被认为是多带配对的候选者，但其证据一直存在争议。早在1959年就提出了双能隙的设想，然而实验证明始终难以获得。在此，通过热霍尔效应测量，我们明确证明了在$T_{\rm c}$以下横向热输运中存在一个交叉现象，即随着冷却，主导载流子从空穴型转变为电子型。这一交叉是两种不同超导凝聚体的清晰标志。将我们的数据拟合到双能隙Bardeen-Rickayzen-Tewordt (BRT)模型，得到第二个能隙约为0.22 $k_{\rm B}T_{\rm c}$，仅为主导能隙的11%。这一小能隙尺寸解释了早期实验中难以分辨双能隙结构的原因。至关重要的是，纵向热导率中电子型贡献开始占主导的温度与热霍尔系数符号反转的开始温度相吻合，证实了两种方法之间的一致性，并为将超导能隙分别归属于各自的空穴型或电子型费米面提供了关键信息。这些发现不仅解决了一个长期存在的争议，而且确立了多能隙超导现象远比先前认为的更为普遍，并证明了热霍尔效应是解析超导体中能隙多重性的有力探针。
英文摘要：
Niobium holds a pivotal place in superconductivity history: it is not only the elemental superconductor with the highest critical temperature, but also a long-standing candidate for multiband pairing whose evidence has remained controversial. A two-gap scenario was proposed as early as 1959, yet experimental proof stayed elusive. Here, through thermal Hall effect measurements, we unambiguously demonstrate a crossover in transverse thermal transport below $T_{\rm c}$, where the dominant carrier switches from hole-like to electron-like upon cooling. This crossover is a clear hallmark of two distinct superconducting condensates. Fitting our data to a two-gap Bardeen-Rickayzen-Tewordt (BRT) model yields a second energy gap of approximately 0.22 $k_{\rm B}T_{\rm c}$, only 11\% of the dominant gap. This small gap size accounts for the difficulty in resolving the two-gap structure in earlier experiments. Crucially, the temperature at which the electron-like contribution to longitudinal thermal conductivity begins to dominate coincides with the onset of the sign reversal in the thermal Hall coefficient, confirming consistency between the two methods, and providing crucial information on the assignment of the superconducting gaps to their respective hole- or electron-like Fermi surface sheets. These findings not only resolve a longstanding controversy, but also establish that multigap superconductivity is far more common than previously assumed, and demonstrate the thermal Hall effect as a powerful probe for resolving gap multiplicities in superconductors.
