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Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by 31 P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states D(E).The temperature and pressure dependences of nuclear spin lattice relaxation rate 1/T 1 in the semiconducting phase are well reproduced using the derived D(E), and the resultant pressure dependence of semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of 1/T 1 data with the complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of D(E) and chemical potential, as well as to decompose observed 1/T 1 into intrinsic and extrinsic contributions. An abrupt increase in 1/T 1 at 1.63 GPa indicates that the semiconducting gap closes, resulting in an enhancement of conductivity. * Electronic address: mito@sci.u-hyogo.ac.jp

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We report the results of 31 P-nuclear magnetic resonance measurements on two-dimensional layered semiconductor black phosphorus, a promising pressure induced Dirac system as predicted by band structure calculations. The reliable temperature dependence of nuclear spin lattice relaxation rate 1/T 1 was successfully obtained despite difficulties arising from very long T 1 especially at low temperatures. The analyses using free electron models with two-or three-dimensionality give the reasonable magnitude of band gap, 0.24 ∼ 0.25 eV, however the whole temperature dependence of 1/T 1 as well as its pressure dependence at high temperatures are not sufficiently reproduced. The results suggest that one needs to analyze the data based on an improved model with the realistic structure of density of states.

The intermediate-valence compound YbPd exhibit successive structural transitions at T a = 125 K and T b = 105 K that may involve valence order. However, little is known about the dynamical nature of the valence order as well as its relationship with the structural transition. In order to shed light on the nature of the structural transition, 105 Pd NMR measurements were performed. In the high-temperature cubic phase, a single-peak 105 Pd NMR spectrum was observed. By contrast, five peaks were observed at 4.2 K due to the appearance of the nuclear quadrupole interaction, which is clear evidence for crystal symmetry lowering. Unexpectedly, a decrease in the NMR intensity was observed from 160 K toward the structural transition at T a. Since there is no magnetic order at T a , the results indicate the development of low-energy fluctuations that may originate from Yb valence fluctuations toward the structural transition.