arXiv subjects

Katsunori Wakabayashi

Publications and source records attributed to Katsunori Wakabayashi.

At least 19 recordsLinked to original sources

arXiv2026

Direct Observation of the Zigzag Edge States of a Supramolecular Diatomic Kagome Lattice

Lattice geometry plays a fundamental role in the behavior of Bloch electrons in a crystal. The diatomic Kagome lattice, an extension of the honeycomb and Kagome lattices, is predicted to give rise to emergent and topological phenomena, but its experimental investigation has been limited thus far. Here, we fabricate a diatomic Kagome lattice through self-assembly of a triptycene derivative with phenazine moieties (Trip-Phz)---a $\mathrm{C_3}$-symmetric, non-planar $π$-conjugated molecule. Our scanning tunneling microscopy (STM) observations show that Trip-Phz forms a highly ordered diatomic Kagome lattice terminated by zigzag-type edges on the Pb(111) surface. Combined STM measurements and tight-binding calculations provide direct evidence for the existence of the edge states that correspond to those of graphene. These states are topological edge states dictated by the quantization of the Zak phase and the bulk-edge correspondence.This work reveals an ideal platform for exploring quantum materials with unique lattice geometries using supramolecular technology.

cond-mat.mes-hall↗

arXiv2026

Hot-Carrier Distribution Spectroscopy by Transconductance in Two-Dimensional Field-Effect Transistors

The transconductance $g_m = dI_D/dV_G$ of a field-effect transistor (FET) is conventionally read as a proxy for carrier density. We show that it is instead a spectroscopic probe of the carrier distribution: because $g_m$ weights the spectral current $j(E)$ by the gate-voltage derivative $\partial f(E)/\partial V_G$ and integrates over energy, it is sensitive to the \emph{shape} of $f(E)$, not merely its integrated weight $n$. We develop an energy-resolved transport framework for two-dimensional (2D) FETs and, within a gate-independent spectral-kernel approximation, derive the decomposition $g_m = g_m^{(n)} + g_m^{(α)}$ into the conventional density-modulation term $g_m^{(n)}$ and a distribution-shape-driven term $g_m^{(α)}$. The latter, obtained as the residual after subtracting the smooth density-modulation background from the measured $g_m$, exhibits a characteristic anomalous peak at a gate voltage $V_G^{\rm pk}$. This peak has no counterpart in equilibrium transport and \emph{cannot be explained by carrier density modulation alone}. With the spectral kernel calibrated, the peak position and height -- extracted from standard DC/lock-in $g_m$ sweeps -- constrain the hot-carrier energy $E_0$, spectral width $σ$, and generation threshold $n_c$, realizing a steady-state, all-electrical spectroscopy of the carrier distribution. An optional time-resolved extension further recovers the carrier relaxation time $τ$ from the transient response following a pump excitation, establishing the 2D FET as a distribution-function spectrometer that requires no optical readout.

cond-mat.mes-hall↗

arXiv2026

Valley-Landscape Engineering in Bilayer WSe$_2$ Gate-All-Around Transistors

The K-$Γ$ valence-band splitting $Δ_{KΓ}$ that governs hole transport in few-layer WSe$_2$ is not a fixed material constant. It is tuned by interlayer stacking, strain, pressure, and the dielectric and displacement-field environment. Its computed value also depends on the level of electronic-structure theory. Bilayer WSe$_2$ therefore realizes a tunable multivalley landscape rather than a single operating point. We combine first-principles inputs with an analytical two-valley device model for gate-all-around (GAA) field-effect transistors, and obtain three results. (i) The minimum subthreshold swing stays near the thermionic limit of $60$~mV~dec$^{-1}$ independently of layer number, because well below threshold the quantum capacitance remains far below the oxide capacitance. (ii) The effective mobility is set by the K-to-$Γ$ occupation ratio: valley redistribution is strong when $Δ_{KΓ}$ is of order $k_BT$ and fades toward single-valley $K$ transport as the splitting grows. (iii) In this small-splitting regime, biaxial strain tunes the effective mobility - and hence the on-current - through the valley population while the subthreshold swing stays at the thermionic limit, decoupling mobility control from electrostatic switching in a way distinct from scattering-based strategies. A symmetric GAA gate controls carrier density at essentially fixed splitting, and because an out-of-plane field increases $Δ_{KΓ}$, its midplane-symmetric potential avoids driving the channel out of the small-splitting regime. The design principle has two facets: small $Δ_{KΓ}$ - set by layer number, stacking, strain, and dielectric engineering - maximizes the valley tunability of $μ_\mathrm{eff}$, whereas larger $Δ_{KΓ}$, for example through compressive strain, suppresses the heavy $Γ$ valley and maximizes the on-state mobility.

cond-mat.mes-hall↗

arXiv2026

Graphene as a Tunable Nonradiative Bath for Moiré Excitons

A minimal theory of nonradiative energy transfer from a two-dimensional (2D) moiré exciton to a nearby graphene layer is presented. From Fermi's golden rule the transfer rate is the overlap of the exciton near-field spectrum with the dissipative density response of graphene, weighted by an exciton form factor, and it reproduces the established $\GET\propto z^{-4}$ law in the point-dipole limit. A finite exciton size filters out the high-momentum near field once the spacer thickness approaches the transition-polarization radius $R_X$, so the distance dependence of the rate---and of the photoluminescence (PL) quenching---probes the exciton size. A low-momentum expansion shows that, relative to the calibrated point-dipole response of the same bath, this leading correction is set by $R_X$ alone. In the ideal coherent-envelope limit the accompanying giant oscillator strength makes the rate non-monotonic in the exciton size, with a peak near $\lX\approx z$. Treating graphene as a gate-tunable bath, Pauli blocking suppresses the interband channel once $2|\muF|$ approaches $\hbarω$, partially restoring PL, and a full random-phase-approximation benchmark confirms the normalized interband distance dependence to within a few percent away from the threshold. Mapping the PL observables across the transition-metal dichalcogenide/hexagonal boron nitride/graphene parameter space, we find that a graphene gate acts not as a passive electrostatic element but as a tunable 2D electronic reservoir probed through exciton PL quenching.

cond-mat.mes-hall↗

arXiv2026

Peak-Decomposition-Free Inverse Metrology of Hyperspectral Moiré Photoluminescence

Hyperspectral photoluminescence (PL) of moiré transition-metal dichalcogenide heterobilayers encodes spatially varying exciton landscapes, but extracting that information is hampered by the ambiguity of multi-peak spectral decomposition. Here we develop a peak-decomposition-free inverse framework for quantitative optical metrology of effective disorder coordinates. From the raw cube $I(x,y,E)$ we construct physically motivated descriptor maps -- centroid energy, dominant emission energy, spectral width, low/high spectral-weight ratio, and dominant--centroid offset. Their spatial autocorrelation hierarchy and covariance structure form a robust descriptor fingerprint of multi-scale disorder-sensitive spectral statistics. By matching these descriptor summary statistics to a minimal smooth-plus-trap generative model through a grid-Bayesian inverse, we infer effective disorder coordinates $\Thetaeff=\{\Ws,\xis,\Wt,\nt\}$ with explicit uncertainties. Using synthetic PL cubes generated from controlled multi-scale landscapes, we recover the well-identified smooth-disorder coordinates and constrain the trap sector up to a strength--density degeneracy. We report this degeneracy explicitly as an intrinsic identifiability limit rather than a deficiency of the method, and map four canonical disorder regimes onto a disorder-coordinate diagram. The descriptor statistics are stable against shot noise and pixel pitch, and behave predictably under optical-resolution and energy-window changes. The same pipeline ingests experimental cubes without modification, making it directly applicable to two-dimensional and moiré materials. Our results establish descriptor-based hyperspectral PL as a practical, minimal-assumption route to optical disorder diagnostics and provide the validated core of a reusable analysis workflow (\texttt{HyperPL-Diag}) for moiré exciton systems.

physics.optics↗

arXiv2026

Electrical Spectroscopy of Intervalley Relaxation in WSe$_2$ Transistors

We show that the transconductance of multilayer WSe$_2$ field-effect transistors serves as a direct electrical spectrometer of the intervalley relaxation time $τ_{\rm iv}$, previously accessible only by ultrafast optical techniques. Extending an equilibrium valley-thermodynamics framework with a single relaxation equation for the $Γ$-valley carrier fraction $f_Γ(t)$, we predict three signatures: (i)~a Lorentzian transconductance $g_m(ω)=g_{m,0}+g_{m,v}^0/(1+iωτ_{\rm iv})$, whose imaginary part peaks at $ω_c=τ_{\rm iv}^{-1}$ with opposite signs for bilayer and trilayer; (ii)~a two-stage current transient after a gate step, exhibiting bilayer overshoot or trilayer undershoot; and (iii)~sweep-rate-proportional hysteresis whose gate-voltage profile and the layer-dependent transconductance sign reversal distinguish valley from trap-induced dynamics. All three signatures provide quantitative electrical access to $τ_{\rm iv}$ with standard radio-frequency (rf) and direct-current (dc) instrumentation.

cond-mat.mes-hall↗

arXiv2026

Hierarchical Disorder in Moiré Exciton Photoluminescence Probed by Spectral-Descriptor Correlations

Hyperspectral photoluminescence (PL) maps of moiré transition-metal dichalcogenide heterobilayers encode rich information about the underlying disorder, but extracting it is hampered by the ambiguity of assigning individual spectral peaks. We show that the spatial correlations among a set of simple, peak-decomposition-free spectral descriptors -- such as the centroid energy, the dominant-peak energy, and a sharp-line fraction -- provide a peak-decomposition-free route to infer features of that disorder. Different descriptors act as filters that select different components of a multi-scale disorder landscape -- a smooth, micron-correlated background and a dense set of localized traps -- and therefore acquire different spatial correlation lengths. The central prediction is a correlation-length hierarchy, $ξ(E_{\rm cent}) \ge ξ(E_{\rm dom})$, which we derive by splitting the dominant-peak energy into a smooth background part and a short-range trap-switching fluctuation. The same picture explains the measured inter-descriptor correlations, including the near-perfect anticorrelation $ρ_{\rm S}(ΔE_{\rm cd}, R_{\rm HL}) \approx -0.978$, which we show to be a robust geometric trend for spectra dominated by a common emission-envelope asymmetry. Benchmarked against phenomenological simulations, Hamiltonian diagonalization, and the measured MoSe$_2$/WSe$_2$ descriptor correlations, the framework turns descriptor maps into a quantitative, peak-decomposition-free probe of slow disorder and local traps in moiré excitons and, more broadly, in disordered semiconductor emitters.

cond-mat.mes-hall↗

arXiv2026

Spin-1 Dirac dispersion and Chern insulating phases in 2D honeycomb Sierpiński fractal

Graphene-based Sierpiński fractals host a zero-energy chiral mode and spin-1 Dirac dispersions within the nearest-neighbor tight-binding model. However, the presence of complex next-nearest neighbor hopping arising from the local flux and the staggered Semenoff mass terms, modeled within the Haldane Hamiltonian, breaks the time-reversal and spatial inversion symmetries, respectively, and makes these flat bands dispersive. Moreover, they introduce rich topological phases in this class of systems that can be characterized by Chern numbers up to $\pm 3$, i.e., beyond the conventional honeycomb lattice. These observations pave the way for the exploration of 2D periodic fractals beyond graphene, where topological phase transitions can be realized through externally applied fields.

cond-mat.mes-hall↗

arXiv2026

Transconductance as a Probe of Valley Thermodynamics in Multilayer WSe$_2$

Transconductance is a central figure of merit in field-effect transistors, typically governed by charge accumulation and carrier mobility. In multilayer WSe$_2$ transistors, however, it is shown to carry a nonlinear transport signature of inter-valley carrier redistribution between the $K$ and $Γ$ valleys. This valley-crossover contribution suppresses transconductance in bilayer WSe$_2$ and reverses sign in trilayer, while remaining absent in single-valley systems. Unlike extrinsic mechanisms such as trap-state filling or contact resistance, the anomaly leaves the subthreshold swing unchanged and cannot be reproduced within conventional single-valley transport models. Introducing the valley susceptibility $χ_v \equiv \partial f_Γ/\partial V_{\rm GS}$, bounded by an intrinsic thermodynamic limit $(4k_BT)^{-1}$, we quantify this response and show that it reaches ${\sim}0.20\,\mathrm{V}^{-1}$ in bilayer WSe$_2$ near threshold at room temperature. The sign, magnitude, and temperature dependence of the anomaly provide directly measurable fingerprints of valley thermodynamics, establishing transconductance as an electrical probe of internal electronic degrees of freedom and revealing a previously hidden nonlinear response in standard transistor measurements.

cond-mat.mes-hall↗

arXiv2026

Shift current conductivity in monolayer SnS: a tight-binding analysis

We investigate the bulk photovoltaic effect in monolayer SnS using an effective tight-binding model derived from first-principles calculations. By comparing short-range and long-range hopping models, we show that the essential features of the shift current conductivity are captured by a minimal model. The shift current is decomposed into transition intensity and shift vector, enabling identification of dominant interband transitions. The comparison reveals that long-range hopping processes quantitatively modify the peak positions and magnitudes, while the short-range model retains the characteristic low-energy structure of the nonlinear response. Our findings provide a transparent framework for understanding and designing bulk photovoltaic effects in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2026

Dilute Magnetism and Edge-State Engineering in Monolayer SnO

Tin monoxide (SnO) is a p-type oxide semiconductor whose electronic properties can be widely modified via atomic-scale engineering. Using density functional theory, we investigate the electronic and magnetic properties of transition-metal (TM = Mn, Fe, Co and W) doped SnO monolayer within a large supercell. We find that all dopants induce finite localized magnetic moments, primarily originating from $d$-orbitals of the impurity atoms. We show that these localized magnetic states give rise to nearly dispersionless bands in the vicinity of the Fermi energy (taking Co doped SnO as an example). In addition, we investigate dimensional effects by constructing nanoribbon geometries of SnO monolayer. The ribbons exhibit intrinsic edge-localized states that are largely independent of ribbon width. For chiral nanoribbons oriented along a low-symmetry direction of the square lattice, we find that oxygen-rich edges are thermodynamically most stable and remain semiconducting, whereas Sn-terminated edges host metallic one-dimensional conduction channels. Our results demonstrate that transition-metal doping and edge engineering provide effective routes to tailor the electronic properties of SnO monolayer, making it a promising candidate for future spintronic and nanoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Shaping Maximally Localized Wannier Functions via Discrete Adiabatic Transport

Maximally localized Wannier functions (MLWFs) are conventionally constructed by iteratively minimizing a spread functional over a high-dimensional gauge landscape. In this work, we present a non-variational constructive algorithm that unifies gauge smoothing and the eigenvalue problem of the projected position operator into a single deterministic framework. We demonstrate that discrete adiabatic transport across band degeneracies emerges naturally as an integral part of the solution procedure for the position eigenvectors. In this transport-aligned gauge, the Bloch overlaps exhibit an approximately linear phase dependence, allowing the Wannier centers to be extracted via deterministic fixed-point iterations and self-consistent updates rather than spread-functional minimization. Benchmark calculations for one- and two-dimensional systems yield spreads and orbital shapes in good agreement with standard minimization schemes. Furthermore, this analytical approach transparently isolates the physical origin of the $\mathcal{O}(L)$ mesh-dependent spread scaling ($L$ being the boundary seam resolution) observed in graphene, demonstrating that it is an intrinsic geometric manifestation of non-commuting projected position operators forcing finite gauge defects to accumulate along a one-dimensional boundary seam.

cond-mat.mtrl-sci↗

arXiv2026

Vacancy-driven inverse Lieb geometry: A general route to $d$-wave altermagnetism in two dimensions

Vacancy-induced structural reconstruction provides a general microscopic route to $d$-wave altermagnetism in two-dimensional systems. As a concrete realization, reconstructed $\mathrm{V_2X_2}$ ($\mathrm{X}=\mathrm{S}, \mathrm{Se}$) monolayers form an inverse Lieb magnetic network in which two inequivalent edge vanadium sites, related by $C_4$ lattice rotational symmetry and carrying opposite exchange fields, yield zero net magnetization despite broken time-reversal ($\mathcal{T}$) and combined inversion--time-reversal ($\mathcal{PT}$) symmetries. Structural stability is confirmed by formation energies, phonon spectra, and $ab$ $ initio$ molecular dynamics simulations at room temperature. A minimal tight-binding model, incorporating anisotropic second-order hopping between the inequivalent magnetic sites mediated by a nonmagnetic corner site, produces spin splitting with a $(\cos k_x - \cos k_y)$ form factor in quantitative agreement with first-principles calculations. The resulting spin splitting is strongly anisotropic, maximized near the $X$ and $Y$ high-symmetry points and exhibiting a symmetry-enforced nodal degeneracy at $M$, consistent with a $d_{x^2-y^2}$ altermagnetic form factor confirmed by the fourfold Fermi surface pattern. These findings establish vacancy-driven reconstruction of an inverse Lieb magnetic network as a general design principle for two-dimensional $d$-wave altermagnets.

cond-mat.str-el↗

arXiv2026

Non-Hermitian corner skin effect in a two-dimensional photonic crystal

We numerically study topological effects of electromagnetic (EM) waves in a two-dimensional (2D) non-Hermitian photonic crystal (PhC) composed of lossy magneto-optical materials. In this system, not only the EM wavefunctions but also the complex eigenfrequencies exhibit nontrivial topological properties. We demonstrate that the non-Hermitian skin effect, protected by point gaps in the complex eigenfrequency spectrum, emerges at both the edges and corners of truncated structures. This phenomenon has no counterpart in Hermitian systems. In addition, we identify non-Hermitian topological edge states originating from the nontrivial topology of the bulk bands. While most previous studies of non-Hermitian topology have focused on tight-binding models, our work addresses a continuous photonic system, providing a more realistic platform and offering a concrete route toward experimental realization of non-Hermitian effects.

physics.optics↗

arXiv2026

Topological Charges, Fermi Arcs, and Surface States of $K_4$ Crystal

We investigate the topological electronic properties of the $K_4$ crystal by constructing a tight-binding model. The bulk band structure hosts Weyl nodes with higher and conventional chiralities ($χ= \pm 2$ and $χ= \pm 1$) located at high-symmetry points in the Brillouin zone. Through analytical evaluation of the Berry curvature, we identify the positions and chiralities of these Weyl nodes. Furthermore, slab calculations for the (001) surface reveal Fermi arcs that connect Weyl nodes of opposite chirality, including those linking $χ= \pm 2$ nodes with pairs of $χ= \mp 1$ nodes. These results demonstrate that the $K_4$ crystal is a spinless Weyl semimetal featuring topologically protected surface states originating from multiple types of Weyl nodes.

cond-mat.mes-hall↗

arXiv2025

Strain Effects on Electronic Properties of Cobalt-Based Coordination Nanosheets

We theoretically study the strain effects on the electronic properties of cobalt-based benzenehexathiol (CoBHT) coordination nanosheets using first-principles calculations. Two distinct crystal structures, high-density structure (HDS) and low-density structure (LDS), are explored. Our results reveal that HDS behaves as a metal, while LDS exhibits semiconducting. Spin-polarized electronic band structures highlight the presence of energy band structures of Kagome lattice, and the inclusion of spin-orbit coupling (SOC) results in band gap openings at high-symmetric K points. Furthermore, we construct the tight-binding model to investigate the topological properties of CoBHT, demonstrating anomalous Hall conductivity driven by the intrinsic Berry curvature. The impact of uniaxial strain on the electronic and magnetic properties of CoBHT is also studied. Strain induces significant modifications in magnetic moments and density of states, particularly in the HDS. Anomalous Hall conductivity is enhanced under hole-doping conditions, suggesting that strain can be used to tailor the electronic properties of CoBHT for specific applications. Our findings underscore the potential of CoBHT nanosheets for use in next-generation electronic, optoelectronic, and catalytic devices with tunable properties through strain engineering.

cond-mat.mtrl-sci↗

arXiv2025

Optically induced spin Hall current in monolayer Janus NbSSe: A first-principles study

Monolayer Janus transition-metal dichalcogenides possess Ising- and Rashba-type spin-orbit-couplings (SOC), leading to intriguing spin splitting effects at K and K$'$, and around $Γ$ points across the wide energy range. Using first-principles calculations, we unveil these SOC characteristics in metallic Janus NbSSe and demonstrate its potential for optically controlled spin current eneration. On the basis of the symmetry of the system, we show that different linear polarized light can selectively drive spin currents of distinct spin components. Our findings establish NbSSe as a promising candidate for next-generation optospintronic technologies, which is offering a pathway toward the development of polarization-tunable spin-current sources.

cond-mat.mtrl-sci↗

arXiv2025

Optically induced spin Hall current in Janus transition-metal dichalcogenides

Monolayer Janus transition-metal dichalcogenides (TMDCs), such as WSeTe, exhibit Rashba-type spin-orbit coupling (SOC) due to broken out-of-plane mirror symmetry. Here, we theoretically demonstrate that pure spin Hall currents can be generated under light irradiation based on a tight-binding model. Rashba-type SOC plays a crucial role in determining the spin polarization direction and enhancing the generation efficiency of pure spin Hall currents. Our findings establish Janus TMDCs as promising materials for next-generation optospintronic devices.

cond-mat.mes-hall↗