arXiv subjects

Munenori Yokota

Publications and source records attributed to Munenori Yokota.

3 recordsLinked to original sources

arXiv2026

Intertwined bulk photocarrier and interfacial barrier dynamics in van der Waals point-contact Schottky junctions

Schottky junctions based on transition-metal dichalcogenides underpin next-generation optoelectronic devices, yet the nanoscale carrier dynamics that ultimately govern their performance remain largely inaccessible. Here, we employ optical pump-probe time-resolved atomic force microscopy (OPP-TR-AFM) to investigate the spatiotemporal response of a WSe$_2$/PtIr van der Waals Schottky junction. Under backside illumination, photocarriers generated in bulk WSe$_2$ produce a local junction response through coupled bulk carrier transport and interfacial barrier modulation. We show that the time-resolved current is consistently explained by the competition between long-lived hole diffusion governed by the occupancy of bulk trap states and thermionic emission current modulated by the surface photovoltage. This mechanism is supported by transient absorption spectroscopy, light-modulated current-voltage measurements, and numerical simulations. Furthermore, spatial mapping reveals pronounced nanoscale variations in the coupled dynamics of carrier transport and interfacial barrier modulation. These results demonstrate that OPP-TR-AFM provides a new framework for resolving the coupled behavior of bulk carriers and interfacial barrier dynamics in van der Waals Schottky junctions with simultaneous spatial and temporal resolution, establishing it as a powerful approach for investigating the microscopic processes that play a crucial role in the development of next-generation optoelectronic devices.

cond-mat.mes-hall↗

arXiv2023

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of $\sim170$ ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

physics.optics↗

arXiv2022

Development of a near-5-Kelvin, cryogen-free, pulse-tube refrigerator-based scanning probe microscope

We report the design and performance of a cryogen-free, pulse-tube refrigerator (PTR)-based scanning probe microscopy (SPM) system capable of operating at the base temperature of near 5K. We achieve this by combining a home-made interface design between the PTR cold head and the SPM head, with an automatic gas-handling system. The interface design isolates the PTR vibrations by a combination of polytetrafluoroethylene and stainless-steel bellows, and by placing the SPM head on a passive vibration isolation table via two cold stages that are connected to thermal radiation shields using copper heat links. The gas-handling system regulates the helium heat-exchange gas pressures, facilitating both the cool down to and the maintenance of the base temperature. We discuss the effects of each component using measured vibration, current-noise, temperature, and pressure data. We demonstrate that our SPM system performance is comparable to known liquid-helium-based systems with the measurements of the superconducting gap spectrum of Pb, atomic-resolution scanning tunneling microscopy image and quasiparticle interference pattern of Au(111) surface, and non-contact atomic force microscopy image of NaCl(100) surface. Without the need for cryogen refills, the current SPM system enables uninterrupted low-temperature measurements.

cond-mat.mtrl-sci↗