Superstructures of Graphite Intercalation Compound Formed by Sodium-Hydroxide Solution Studied by Scanning Tunneling Microscopy

Highly oriented pyrolytic graphite (HOPG) treated with sodium hydroxide solution was found to show an X-ray diffraction pattern characteristic of a stage-8 intercalation compound. Superstructures such as 2 X 2, √3 X √3 and noble orthorhombic lattices were observed on the NaOH-treated graphite surfaces by scanning tunneling microscopy, as was previously observed on stage-1 alkali-metal graphite intercalation compound (GIC) surfaces formed by Li, K, Rb and Cs intercalates. The orthorhombic phase has a periodic structure formed by four bright and two dark rows, which were explained by the intercalant arrangement occupying two different sites, in the GIC

Exploring spin dynamics visualization and its application to new functional devices

科学研究費助成事業 研究成果報告書：基盤研究(S)2010-2014課題番号：2222600

Surface Charge-Density Wave on the One-Dimensional Organic Conductor β-(BEDT-TTF)2PF6

We performed STM and theoretical studies of the surface of β-(BEDT-TTF)2PF6 which is known to show a metal-insulator transition at 297 K with the nesting vector of Qbulk = a*+0.5c* as a bulk phase. Two typical charge-density wave (CDW) phases with QS1 = 0.13a*+0.26c* and QS2 = 0.19a*+0.20c* were observed at 285 K on the surface. The observed structures were correlated well with the analysis of the susceptibility χ(Q), given the incomplete surface-charge transfer in the polar surface of this material. This is the first observation of surface CDW phases that reflect the characteristics of the electronic structure. These results suggest the possible discovery of other surface properties, such as surface superconductivity, in organic materials with a polar surface

Phasons Formed on Si(100) Dimer Rows Observed by Scanning Tunneling Microscopy(STM-Si(100))

At～6K, dimers on Si(100) surface are buckled, and structural change occurs between c(4x2) and p(2x2) arrangements due to dimer flip-flop motion at phase boundaries on dimer rows. The phase defect appearing at boundaries has a structure similar to that of the type-C defect, two adjacent dimers are buckled in the same orientation. In consideration of the dimer arrangement around the phase boundary, there exist structures with two different conformations for the phase defect, however, according to the Ising spin model, both of which have the same energy higher compared to other buckled dimers with 2x anticorrelation along a dimer row. Therefore, dimer flip-flop motion at a phase boundary results in the migration of a solitary phase defect with higher energy, as a phason

Time-resolved force microscopy using delay-time modulation method

We developed a time-resolved force microscopy technique by integrating atomic force microscopy using a tuning-fork-type cantilever with the delay time modulation method for optical pump-probe light. We successfully measured the dynamics of surface recombination and diffusion of photoexcited carriers in bulk WSe2, which is challenging owing to the effect of tunneling current in time-resolved scanning tunneling microscopy. The obtained results were comprehensively explained with the model based on the dipole-dipole interaction induced by photo illumination.Comment: 13 pages, 4 figure

Giant nonlinear optical effects induced by nitrogen-vacancy centers in diamond crystals

We investigate the effect of nitrogen-vacancy (NV) centers in single crystal diamond on nonlinear optical effects using 40 fs femtosecond laser pulses. The near-infrared femtosecond pulses allow us to study purely nonlinear optical effects, such as optical Kerr effect (OKE) and two-photon absorption (TPA), related to unique optical transitions by electronic structures with NV centers. It is found that both nonlinear optical effects are enhanced by the introduction of NV centers in the N+ dose levels of 2.0×1011 and 1.0×1012 N+/cm2. In particular, our data demonstrate that the OKE signal is strongly enhanced for the heavily implanted type-IIa diamond. We suggest that the strong enhancement of the OKE is possibly originated from cascading OKE, where the high-density NV centers effectively break the inversion symmetry near the surface region of diamond