The recent development of optical technology has enabled the practical use of a carrier-envelope phase (CEP)-controlled monocycle electric field in the terahertz (THz) regime. By combining this technique with metal nanostructures such as nanotips, which induce near-field enhancement, the development of novel applications is anticipated. In particular, THz scanning tunneling microscopy (THz-STM) is a promising technique for probing ultrafast dynamics with the spatial resolution of STM. However, the modulation of the THz waveform generally accompanies the enhancement of the electric field, which is unknown in actual measurement environments. Here, we present a method capable of directly evaluating the enhanced near field in the tunnel junction of THz-STM in the femtosecond range, which is essential for the use of THz near field. In the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluated using the THz-STM, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.

The dynamics of the flip-flop motion of single buckled dimers of Si(100) was elucidated by locating the tip of a scanning tunneling microscope over a single flip-flopping dimer and measuring the tunneling current (time trace). Based on a statistical analysis of the time trace, we succeeded in estimating the activation energy and the energy splitting between the two stable configurations of buckling. Strong dependence of the dynamics of the flip-flop motion on the local environment was found: Activation energy differs significantly (directly measured 32 meV, estimated $\sim 110\mathrm{meV}$) for dimers in different domains.

We have investigated the basic surface reconstruction of Si(100) on well defined surfaces fabricated on various substrates at low temperatures ($-80\mathrm{K}$) by scanning tunneling microscopy. Below 40 K, the single $p(2\times 2)$ phase, a phase never observed before, was observed exclusively on $n$-type substrates doped in the range of 0.002 to $0.017\Omega \mathrm{c}\mathrm{m}$. We also exclude the possibility of the ($2\times 1$) symmetric dimer commonly observed at low temperature ($-10\mathrm{K}$) being the basic surface reconstruction by showing that a buckled dimer can be flip-flopped by the tunneling tip.

We revisit and refine the interpretation of the scanning tunneling microscopy (STM) images of the Si(100) dimers, based on results from an extensive set of STM observations carried out at low temperature (80 K) and total-energy calculations of Si(100) surfaces. The interpretation addresses some unresolved questions and brings much experimental and theoretical research into unanimous agreement. We show that tunneling from surface resonances and bulk states seriously contributes to the STM images within usual tunneling conditions. In the empty state, tunneling from these states overwhelms tunneling from the localized ${\pi }^{*}$ surface state, which STM is generally believed to observe.

Time-resolved tunneling current measurement in the subpicosecond range was realized byultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pairmethod. A low-temperature-grown GaNxAs1−x (x=0.36%) sample exhibited two ultrafast transientprocesses in the time-resolved tunnel current signal, whose lifetimes were determined to be0.653±0.025 and 55.1±5.0 ps. These values are of the same order of magnitude as those measuredin the conventional pump–probe reflectivity measurement