arXiv subjects

Atsushi Yamada

Publications and source records attributed to Atsushi Yamada.

At least 19 recordsLinked to original sources

arXiv2026

Microscopic Origin of Dephasing in Solids from First-Principles Electron-Phonon Interactions

Electron-phonon interactions provide a microscopic origin of effective dephasing in solids within real -time TDDFT via a time-domain Williams-Lax framework. In metals, Drude -like damping emerges from a single disordered configuration; in dielectrics, the same mechanism yields clean high-harmonic spectra without introducing ultrashort phenomenological dephasing times. Mapping supercell dynamics onto a primitive-cell density matrix reveals that dephasing is governed primarily by population dynamics (diagonal elements) rather than by the decay of off-diagonal coherence.

cond-mat.other↗

arXiv2025

Record negative photoconductivity in N-polar AlGaN/GaN quantum-well heterostructures

The AlGaN/GaN quantum-well heterostructures typically exhibit a positive photoconductivity (PPC) during the light illumination. Surprisingly, we found that introducing the GaN/AlN superlattice (SL) back barrier into N-polar AlGaN/GaN quantum-well heterostructures induces a transition in these heterostructures from PPC to negativie photoconductivity (NPC) as the SL period number increased at room temperature. This transition occurred under an infrared light illumination and can be well explained in terms of the excitation of hot electrons from the two-dimensional electron gas and subsequent trapping them in a SL structure. The NPC effect observed in N-polar AlGaN/GaN heterostructures with SL back barrier exhibits photoconductivity yield exceeding 85 % and thus is the largest ones reported so far for semiconductors. In addition, NPC signal remains relatively stable at high temperatures up to 400 K. The obtained results can be interesting for the development of NPC related devices such as photoelectric logic gates, photoelectronic memory and infrared photodetectors.

cond-mat.mtrl-sci↗

arXiv2025

A simple method for detection and quantitative estimation of deep levels in a barrier layer of AlGaN/GaN HEMT structures by analysis of light induced threshold voltage shift

The characterization of deep levels in AlGaN/GaN heterostructures is one of the most important problems in GaN high electron mobility transistors (HEMTs) technology. This work reports on a technique for determination of deep level concentration in AlGaN/GaN HEMT structures. The proposed method is relatively simple, and it is based on the detection of free holes created by optically induced transitions of electrons from the deep levels to the conduction band. The developed method can detect and provide quantitative estimation of deep level traps in a barrier layer of AlGaN/GaN HEMT structures. Furthermore, it provides a framework for analysis of light induced threshold voltage shift, which includes an important experimental criterion of determination whether the holes are generated or not in the AlGaN/GaN HEMT structures by sub-band gap illumination. The method was verified by applications it to a study of the deep levels in GaN HEMTs grown on various substrates, i.e. SiC and GaN.

cond-mat.mtrl-sci↗

arXiv2025

Unusually high-density 2D electron gases in N-polar AlGaN/GaN heterostructures with GaN/AlN superlattice back barriers grown on sapphire substrates

We reported on the observation of extremely high-density ($>10^{14}$cm$^{-2}$) 2D electron gas in N-polar AlGaN/GaN heterostructures grown on sapphire substrates. Due to introducing the GaN/AlN superlattice (SL) back barrier between the GaN buffer layer and AlGaN barrier layer, we observed a giant enhancement of the 2D electron gas density at the GaN/AlGaN interface from $3\times10^{13}$cm$^{-2}$ (without SL) to $1.4\times10^{14}$cm$^{-2}$ (with SL back barrier) that is only one order of magnitude below the intrinsic crystal limit of $\approx10^{15}$cm$^{-2}$. We found that the changes of 2D electron gas density with SL correlated well with the changes of the wafer warp parameter which suggests that the strains are responsible for the 2D electron gas density enhancement (reduction of the piezoelectric polarization in the GaN channel). Nevertheless, this finding is probably insufficient to fully explain the observed high 2D electron gas density. Simultaneously, the room temperature electron mobility was 169 cm$^2$/Vs, which with the electron density of $1.4\times10^{14}$cm$^{-2}$ gives a low sheet resistance of 264 $Ω$/sq (one of the lowest reported so far for the N-polar 2D electron gas channel). Finally, the possibility of application of such high-density 2D electron gas with low sheet resistance to transistors, emitters and detectors was discussed.

cond-mat.mes-hall↗

arXiv2024

Interaction of intense ultrashort laser pulses with solid targets: A systematic analysis using first-principles calculations

Intense ultrashort laser pulse irradiation of solid targets was systematically investigated at the first-principles level, both theoretically and computationally. In the method, the propagation of a pulsed light through a thin film is described by a one-dimensional Maxwell's equation, and the microscopic electronic motion at different positions in the film is described by employing first-principles time-dependent density functional theory (TDDFT). The method uses a coarse-graining approximation to couple light propagation and electronic motion, and is termed the multiscale Maxwell-TDDFT method. The reflectance, transmittance, and absorbance of pulsed light incident normally on thin films of 50-200 nm thickness were calculated for materials with different optical properties, such as aluminum (simple metal), graphite (semi-metal), silicon (small-gap dielectric), and quartz (wide-gap dielectric). Optical response transitions were explored as the light intensity shifted from the linear regime, represented by the dielectric function for weak light, to the extremely nonlinear regime, represented by plasma reflection under intense light conditions. Numerous mechanisms that depend on the laser pulse intensity and material type were found to contribute to these changes. These include multiphoton absorption, saturable absorption, sign change of the effective dielectric constant, and transition from quantum occupation to classical Boltzmann distribution. Thus, the calculations provide a unified understanding of the interaction of intense pulsed light with solids, occurring on an extremely short time scale.

physics.optics↗

arXiv2024

SAR-RARP50: Segmentation of surgical instrumentation and Action Recognition on Robot-Assisted Radical Prostatectomy Challenge

Surgical tool segmentation and action recognition are fundamental building blocks in many computer-assisted intervention applications, ranging from surgical skills assessment to decision support systems. Nowadays, learning-based action recognition and segmentation approaches outperform classical methods, relying, however, on large, annotated datasets. Furthermore, action recognition and tool segmentation algorithms are often trained and make predictions in isolation from each other, without exploiting potential cross-task relationships. With the EndoVis 2022 SAR-RARP50 challenge, we release the first multimodal, publicly available, in-vivo, dataset for surgical action recognition and semantic instrumentation segmentation, containing 50 suturing video segments of Robotic Assisted Radical Prostatectomy (RARP). The aim of the challenge is twofold. First, to enable researchers to leverage the scale of the provided dataset and develop robust and highly accurate single-task action recognition and tool segmentation approaches in the surgical domain. Second, to further explore the potential of multitask-based learning approaches and determine their comparative advantage against their single-task counterparts. A total of 12 teams participated in the challenge, contributing 7 action recognition methods, 9 instrument segmentation techniques, and 4 multitask approaches that integrated both action recognition and instrument segmentation. The complete SAR-RARP50 dataset is available at: https://rdr.ucl.ac.uk/projects/SARRARP50_Segmentation_of_surgical_instrumentation_and_Action_Recognition_on_Robot-Assisted_Radical_Prostatectomy_Challenge/191091

cs.CV↗

arXiv2022

High order harmonic generation in semiconductors driven at near- and mid-IR wavelengths

We study high order harmonics generation (HHG) in crystalline silicon and diamond subjected to near and mid-infrared laser pulses. We employ time-dependent density functional theory and solve the time-dependent Kohn-Sham equation in the single-cell geometry. We demonstrate that clear and clean HHG spectra can be generated with careful selection of the pulse duration. In addition, we simulate dephasing effects in a large silicon super-cell through displacement of atomic positions prepared by a molecular dynamics simulation. We compare our results with the previous calculations by Floss et al. [arXiv:1705.10707] [Phys. Rev. A 97, 011401(R) (2018)] on Diamond at 800 nm and by Tancogne-Dejean et al. [arXiv:1609.09298] [Phys. Rev. Lett. 118, 087403 (2017)] on Si at 3000 nm.

cond-mat.mtrl-sci↗

arXiv2021

Nonlinear dynamics of electromagnetic field and valley polarization in WSe$_{2}$ monolayer

Linear and nonlinear optical response of WSe$_{2}$ monolayer is investigated by two-dimensional Maxwell plus time-dependent density functional theory with spin-orbit interaction. By applying the chiral resonant pulses, the electron dynamics along with high harmonic generation are examined at weak and strong laser fields. WSe$_{2}$ monolayer shows the linear optical response at the intensity I = 10$^{10}$~W/cm$^{2}$ while a complex nonlinear behavior is observed at I = 10$^{12}$~W/cm$^{2}$. The nonlinear response of WSe$_{2}$ monolayer in terms of saturable absorption is observed at strong laser field. By changing the chirality of the resonant light, a strong circular dichroic effect is observed in the excited state population. A relatively weak laser field shows effective valley polarization while strong field induces spin-polarized carrier peak between $K$($K'$) and $\mathitΓ$-point via nonlinear process. On the other hand, the strong laser field shows high harmonics up to the 11th order. Our results demonstrate that circularly polarized resonant pulse generate high harmonics in WSe$_{2}$ monolayer of order 3n$\pm1$.

cond-mat.mtrl-sci↗

arXiv2021

Valley polarization control in WSe2 monolayer by a single-cycle laser pulse

Abstract The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronics devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here, we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K'-point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-Stückelberg interference of wave packets of the electron wave function.

physics.optics↗

arXiv2020

Modulation of probe signal in coherent phonon detection revisited: Analytical and first-principles computational analyses

Modulation of probe signal in pump-probe measurements of coherent phonons in dielectrics, with and without spectral resolution, are investigated theoretically taking diamond as an example. Analytical investigation as well as first-principles calculations based on time-dependent density functional theory is utilized to clarify the mechanism of the modulation of the probe signals. Boundary and bulk effects are investigated systematically, putting emphasis on the phase relation between the modulation and the atomic motion of the coherent phonon. They are summarized as follows: Modulation by the boundary effect is in phase with the coherent phonon amplitude, while that by the bulk effect shows $π/2$ phase difference. Strong frequency dependence appears in the modulation by the bulk effect, while no frequency dependence by the boundary effect. First-principles calculations support the reliability of the analytical result.

physics.optics↗

arXiv2018

Time-dependent density functional theory for a unified description of ultrafast dynamics: pulsed light, electrons, and atoms in crystalline solids

We have developed a novel multiscale computational scheme to describe coupled dynamics of light electromagnetic field with electrons and atoms in crystalline solids, where first-principles molecular dynamics based on time-dependent density functional theory is used to describe the microscopic dynamics. The method is applicable to wide phenomena in nonlinear and ultrafast optics. To show usefulness of the method, we apply it to a pump-probe measurement of coherent phonon in diamond where a Raman amplification takes place during the propagation of the probe pulse.

physics.comp-ph↗

arXiv2018

Energy transfer from intense laser pulse to dielectrics in time-dependent density functional theory

Energy transfer processes from a high-intensity ultrashort laser pulse to electrons in simple dielectrics, silicon, diamond, and $α$-quartz are theoretically investigated by first-principles calculations based on time-dependent density functional theory (TDDFT). Dependences on frequency as well as intensity of the laser pulse are examined in detail, making a comparison with the Keldysh theory. Although the Keldysh theory reliably reproduces the main features of the TDDFT calculation, we find some deviations between results by the two theories. The origin of the differences is examined in detail.

physics.optics↗

arXiv2018

SALMON: Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience

SALMON (Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience, http://salmon-tddft.jp) is a software package for the simulation of electron dynamics and optical properties of molecules, nanostructures, and crystalline solids based on first-principles time-dependent density functional theory. The core part of the software is the real-time, real-space calculation of the electron dynamics induced in molecules and solids by an external electric field solving the time-dependent Kohn-Sham equation. Using a weak instantaneous perturbing field, linear response properties such as polarizabilities and photoabsorptions in isolated systems and dielectric functions in periodic systems are determined. Using an optical laser pulse, the ultrafast electronic response that may be highly nonlinear in the field strength is investigated in time domain. The propagation of the laser pulse in bulk solids and thin films can also be included in the simulation via coupling the electron dynamics in many microscopic unit cells using Maxwell's equations describing the time evolution of the electromagnetic fields. The code is efficiently parallelized so that it may describe the electron dynamics in large systems including up to a few thousand atoms. The present paper provides an overview of the capabilities of the software package showing several sample calculations.

physics.comp-ph↗

arXiv2015

Absence of Spin Liquid in the Hubbard model on the Honeycomb Lattice

The possible emergence of a spin liquid phase in the half-filled Hubbard model on the honeycomb lattice; a simple model of graphene, is studied using the variational cluster approximation. We found that the critical interaction strength of a magnetic transition is slightly lower than that of the non-magnetic metal-to-insulator transition. Thus a semi-metallic state becomes a magnetic insulator as the interaction strength increases and a non-magnetic insulating state does not occur. Therefore a spin liquid state characterized by an insulating state without long range order, is not realized in this system.

cond-mat.str-el↗

arXiv2014

Magnetic phase diagram and Mott transition of the half-filled 1/5-depleted Hubbard model with frustration

The magnetic properties and Mott transition of the half-filled Hubbard model on the 1/5 depleted square lattice with frustration is studied at zero temperature by the variational cluster approximation. The $(π,π)$ Néel ordering (AF) is stable in a wide region of the phase diagram and almost completely veils the non-magnetic Mott transition for the unfrustrated case. However, AF is severely suppressed by the frustration and even with moderate frustrations the non-magnetic Mott transition takes place in the range where the intra-dimer hoppings are larger than the intra-plaquette hoppings.

cond-mat.str-el↗

arXiv2014

Magnetic properties and Mott transition of the Hubbard model for weakly coupled chains on the anisotropic triangular lattice

We investigate the magnetic properties and Mott transition in the Hubbard model for weakly coupled chains on the anisotropic triangular lattice. Taking into account 120$^\circ$ Néel, and collinear orderings, the magnetic phase diagram is studied at zero temperature and half-filling by the variational cluster approximation. We found that when the on-site Coulomb repulsion $U$ is relatively large, nonmagnetic insulator, which is a candidate of the spin liquid state, is realized for wide range of the interchain hopping from quasi two-dimensional to almost one-dimensional regime. When the interchain hopping is relatively large, this nonmagnetic insulator becomes magnetic states as $U$ decreases. For rather small interchain hopping, it changes to the paramagnetic metal, thus purely paramagnetic metal-insulator transition (Mott transition) takes place. Implications of our results for the Cs$_2$CuBr$_4$ and Cs$_2$CuCl$_4$ are discussed.

cond-mat.str-el↗

arXiv2009

Space efficient opposed-anvil high-pressure cell and its application to optical and NMR measurements up to 9 GPa

We have developed a new type of opposed-anvil high pressure cell with substantially improved space efficiency. The clamp cell and the gasket are made of non-magnetic Ni-Cr-Al alloy. Non-magnetic tungsten carbide (NMWC) is used for the anvils. The assembled cell with the dimension ϕ29mm \times 41mm is capable of generating pressure up to 9 GPa over a relatively large volume of 7 mm3. Our cell is particularly suitable for those experiments which require large sample space to achieve good signal-to-noise ratio, such as the nuclear magnetic resonance (NMR) experiment. Argon is used as the pressure transmitting medium to obtain good hydrostaticity. The pressure was calibrated in situ by measuring the fluorescence from ruby through a transparent moissanite (6H-SiC) window. We have measured the pressure and temperature dependences of the 63Cu nuclear-quadrupole-resonance (NQR) frequency of Cu2O, the in-plane Knight shift of metallic tin, and the Knight shift of platinum. These quantities can be used as reliable manometers to determine the pressure values in situ during the NMR/NQR experiments up to 9 GPa.

cond-mat.mtrl-sci↗

arXiv2009

Intrinsic Properties of AFe2As2 (A = Ba, Sr) Single Crystal under Highly Hydrostatic Pressure Conditions

We measured the electrical resistivity and ac magnetic susceptibility of BaFe2As2 and SrFe2As2 single crystals under pressure using a cubic anvil apparatus. For BaFe2As2, the antiferromagnetic (AF) and structural transitions are suppressed with increasing pressure. Unexpectedly, these transitions persist up to 8 GPa, and no signature of a superconducting transition was observed in the pressure range investigated here. On the other hand, the AF and structural transitions of SrFe2As2 collapse at around the critical pressure Pc ~ 5 GPa, resulting in the appearance of bulk superconductivity. The superconducting volume fraction abruptly increases above Pc, and shows a dome centered at approximately 6 GPa. Our results suggest that the bulk superconducting phase competes with the AF/orthorhombic phase and only appears in the narrow pressure region of the tetragonal phase.

cond-mat.supr-con↗