Local observations of orbital diamagnetism and excitation in three-dimensional dirac fermion systems Bi_1−xSb_x

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. (author)