藤井 拓斗

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0 T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0 T reaches more than 20 times larger than that at 2.0 T. The increase in 1/T_{1}T above 6.5 T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100 K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.

<p>黒リンは層状構造のナローギャップ半導体であるが、圧力印加によってギャップが減少し、グラファイトやビスマスのような半金属物質に転移する。本研究では黒リンの半導体特性を微視的実験手法である核磁気共鳴法を用い調べた。常圧下の核スピン格子緩和率は、半導体特性を反映して高温に伴い減少を示した。講演では、半金属状態への転移とそこでの電子状態を明らかにするため、高圧下での測定結果も報告する予定である。</p>