arXiv subjects

Katsuya Iwaya

Publications and source records attributed to Katsuya Iwaya.

12 recordsLinked to original sources

arXiv2026

Concerted Carrier-Barrier Dynamics in van der Waals Schottky Junctions Revealed by Time-Resolved Atomic Force Microscopy

Schottky junctions based on transition-metal dichalcogenides (TMDCs) have emerged as key building blocks for next-generation optoelectronic devices that demand ultrafast response and high sensitivity. However, the ultrafast, nanoscale carrier dynamics at these interfaces, crucial for device performance, have remained experimentally elusive. Here, we introduce optical pump-probe time-resolved atomic force microscopy to directly visualize, in real space, the nanosecond-scale modulation of the Schottky barrier potential at a van der Waals junction formed by point contact between WSe2 and a PtIr tip. Complementary analyses using transient absorption spectroscopy and light-modulated current-voltage characteristics together with model simulations reveal that time-resolved currents originate from the concerted temporal evolution of photoexcited carriers and the subsequent barrier response, processes that also define the rate-limiting steps of the photocurrent. Our results uncover the essential interfacial dynamics that underpin TMDC-based photodetectors and photovoltaic elements, while establishing a new measurement paradigm that complements and extends existing spectroscopic techniques. This approach provides direct access to nonequilibrium processes hidden at nanoscale interfaces, offering a powerful route to rational design of high-performance optoelectronic devices.

cond-mat.mes-hall↗

arXiv2024

Next-Generation Time-Resolved Scanning Probe Microscopy

Understanding the nanoscale carrier dynamics induced by light excitation is the key to unlocking futuristic devices and innovative functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) has opened a window to these phenomena. However, mastering the combination of ultrafast pulsed lasers with STM requires high expertise and effort. We have shattered this barrier and developed a compact OPP-STM system accessible to all. This system precisely controls laser pulse timing electrically and enables stable laser irradiation on sample surfaces. Furthermore, by applying this technique to atomic force microscopy (AFM), we have captured time-resolved force signals with an exceptionally high signal-to-noise ratio. Originating from the dipole-dipole interactions, these signals provide insights into the carrier dynamics on sample surfaces, which are activated by photo-illumination. These technologies are promising as powerful tools for exploring a wide range of photoinduced phenomena in conductive and insulating materials.

cond-mat.mtrl-sci↗

arXiv2024

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.

physics.app-ph↗

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↗

arXiv2012

STM Studies of Isolated Mn12-Ph Single Molecule Magnets

We study Mn12O12(C6H5COO)16(H2O)4 (Mn12-Ph) single-molecule magnets on highly ordered pyrolytic graphite (HOPG) using low temperature scanning tunneling microscopy (LT-STM) experiments. We report Mn12-Ph in isolation, resembling single molecules with metallic core atoms and organic outer ligands. The local tunneling current observed within the molecular structure shows a strong bias voltage dependency, which is distinct from that of the HOPG surface. Further, evidence of internal inhomogeneity in the local density of states has been observed with high spatial resolution, and this inhomogeneity appears to be due to localized metallic behavior. These results facilitate magneto-metric studies of single molecule magnets in isolation. As compared to bulk crystal studies, our experiments allow the specific investigation of atomic sites in the molecule.

cond-mat.mes-hall↗

arXiv2011

Atomic-scale visualization of initial growth of homoepitaxial SrTiO3 thin film on an atomically ordered substrate

The initial homoepitaxial growth of SrTiO3 on a (\surd13\times\surd13) - R33.7°SrTiO3(001) substrate surface, which can be prepared under oxide growth conditions, is atomically resolved by scanning tunneling microscopy. The identical (\surd13\times\surd13) atomic structure is clearly visualized on the deposited SrTiO3 film surface as well as on the substrate. This result indicates the transfer of the topmost Ti-rich (\surd13\times\surd13) structure to the film surface and atomic-scale coherent epitaxy at the film/substrate interface. Such atomically ordered SrTiO3 substrates can be applied to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures.

cond-mat.mtrl-sci↗

arXiv2011

Stripe charge ordering in SrO-terminated SrTiO3(001) surfaces

The local electronic structure of the SrO-terminated SrTiO3(001) surface was explored using scanning tunneling microscopy. At low bias voltages in the empty states, a unidirectional structure with a periodicity of 3 unit cells, superimposed on a c(2 x 2) reconstructed structure, was found to develop along the crystallographic a axis. This structure indicates a charge-ordered stripe induced by carrier doping from oxygen vacancies in the SrO and the subsurface TiO2 planes. In the filled states, localized deep in-gap states were observed in addition to large energy gaps in the tunneling spectra. This result represents inelastic tunneling due to significant electron-lattice interaction associated with unidirectional lattice distortion in the SrO-terminated surface.

cond-mat.str-el↗

arXiv2010

Thickness-dependent Local Surface Electronic Structures of Homoepitaxial SrTiO3 Thin Films

We have investigated the atomically-resolved substrate and homoepitaxial thin film surfaces of SrTiO3(001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) combined with pulsed laser deposition (PLD). It was found that annealing at 1000 oC in an oxygen partial pressure of 1*10-6 Torr, which is a typical annealing treatment for the preparation of SrTiO3 substrates, unexpectedly resulted in a disordered surface on an atomic scale. In contrast, homoepitaxial SrTiO3 thin films grown on this disordered substrate exhibited (2*2) surface reconstructions. The differential conductance spectra, dI/dV in STS measurements, revealed a number of surface defects in a 10-unit-cell-thick SrTiO3 film, but much fewer in a 50-unit-cell-thick film. These results indicate that the defect density in the film strongly depends on the film thickness, suggesting non-uniform stoichiometry along the growth direction.

cond-mat.mtrl-sci↗

arXiv2010

Atomically Resolved Surface Structure of SrTiO3(001) Thin Films Grown in Step-Flow Mode by Pulsed Laser Deposition

The surface structure of SrTiO3(001) thin films homoepitaxially grown by PLD in step-flow mode was characterized using low temperature STM. It was found that one-dimensional (1D) TiOx-based nanostructures were formed on the thin film surface and their density increased with increasing thin film thickness. Most of the 1D nanostructures disappeared after a post-deposition annealing, indicating that this structure is metastable due to the nonequilibrium growth mode. The resulting surface after annealing exhibited similar features to that of a thinner film, having a domain structure with (2x1) and (1x2) reconstructions, but with fewer oxygen-vacancy-type defects. These results imply that the step-flow growth is likely to produce TiOx-rich surface and Ti deficiencies in the film. By the post-deposition annealing, the rich TiOx would diffuse from the surface into the film to compensate defects associated with Ti vacancies and oxygen vacancies, resulting in the stable surface structure with fewer oxygen vacancies. Thus, STM measurements can provide us with a microscopic picture of surface stoichiometry of thin films originating in the dynamics of the growth process, and can present a new approach for designing functional oxide films.

cond-mat.mtrl-sci↗

arXiv2001

The electronic state of vortices in YBa2Cu3Oy investigated by complex surface impedance measurement

The electromagnetic response to microwaves in the mixed state of YBa2Cu3Oy(YBCO) was measured in order to investigate the electronic state inside and outside the vortex core. The magnetic-field dependence of the complex surface impedance at low temperatures was in good agreement with a general vortex dynamics description assuming that the field-independent viscous damping force and the linear restoring force were acting on the vortices. In other words, both real and imaginary parts of the complex resistivity, ρ_1, and ρ_2, were linear in B. This is explained by theories for d-wave superconductors. Using analysis based on the Coffey-Clem description of the complex penetration depth, we estimated that the vortex viscosity ηat 10 K was (4 \sim 5) \times 10^{-7} Ns/m^2. This value corresponds to ω_0 τ\sim 0.3 - 0.5, where ω_0 and τare the minimal gap frequency and the quasiparticle lifetime in the vortex core, respectively. These results suggest that the vortex core in YBCO is in the moderately clean regime. Investigation of the moderately clean vortex core in high-temperature superconductors is significant because physically new effects may be expected due to d-wave characteristics and to the quantum nature of cuprate superconductors. The behavior of Z_s as a function of B across the first order transition (FOT) of the vortex lattice was also investigated. Unlike Bi2Sr2CaCu2Oy (BSCCO), no distinct anomaly was observed around the FOT in YBCO. Our results suggest that the rapid increase of X_s due to the change of superfluid density at the FOT would be observed only in highly anisotropic two-dimensional vortex systems like BSCCO. We discuss these results in terms of the difference of the interlayer coupling and the energy scale between the two materials.

cond-mat.supr-con↗

arXiv2000

Direct observation of the washboard noise of a driven vortex lattice in a high-temperature superconductor, Bi2Sr2CaCu2Oy

We studied the conduction noise spectrum in the vortex state of a high-temperature superconductor, Bi2Sr2CaCu2Oy, subject to a uniform driving force. Two characteristic features, a broadband noise (BBN) and a narrow-band noise (NBN), were observed in the vortex-solid phase. The origin of the large BBN was determined to be plastic motion of the vortices, whereas the NBN was found to originate from the washboard modulation of the translational velocity of the driven vortices. We believe this to be the first observation of

cond-mat.supr-con↗