Dirac fermions display a singular response against magnetic and electric fields. A distinct manifestation is the large diamagnetism originating in the interband effect of Bloch bands, as observed in bismuth alloys. Through ²⁰⁹Bi NMR spectroscopy, we extract diamagnetic orbital susceptibility inherent to Dirac fermions in the semiconducting bismuth alloys Bi_1−xSb_x (x = 0.08–0.16). The ²⁰⁹Bi hyperfine coupling constant provides an estimate of the effective orbital radius. In addition to the interband diamagnetism, the NMR shift includes an anomalous temperature-independent term originating in the enhanced intraband diamagnetism under strong spin–orbit coupling. The nuclear spin–lattice relaxation rate 1/T₁ is dominated by orbital excitation and follows cubic temperature dependence in an extensive temperature range. The result demonstrates the robust diamagnetism and low-lying orbital excitation against the small gap opening, whereas x-dependent spin excitation appears at low temperatures.

中文翻译：

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三维狄拉克费米子系统Bi1-xSbx的轨道磁和激发的局部观测

狄拉克费米子对磁场和电场表现出奇异的响应。一个明显的表现是大的反磁性，这是由于在铋合金中观察到的布洛赫带的带间效应引起的。通过²⁰⁹ Bi NMR光谱，我们提取了Bi铋半导体Bi _{1− x} Sb _x（x = 0.08–0.16）中狄拉克费米子固有的抗磁轨道磁化率。在²⁰⁹双超精细耦合常数提供了有效轨道半径的估计。除了带间反磁性外，NMR位移还包括一个与温度无关的异常项，其起源于强自旋轨道耦合下增强的带内反磁性。核自旋-晶格弛豫速率1 / T ₁受轨道激发支配，并在宽温度范围内遵循立方温度依赖性。结果证明了针对小间隙开口的强抗磁性和低地轨道激发，而在低温下出现x依赖性自旋激发。