arXiv subjects

Akihito Kato

Publications and source records attributed to Akihito Kato.

12 recordsLinked to original sources

arXiv2026

Effective electron coupling to phonon mechanical angular momentum in helical systems

In chiral crystals, two types of phonon angular momenta have been introduced. One is crystal angular momentum (CAM) arising from the rotational or screw-rotational symmetry and the other is mechanical angular momentum (MAM) associated with the circular motion of atomic displacements about equilibrium positions. Recently, the electron-phonon coupling that respects the screw-rotational symmetry is derived, whereby the CAM between electrons and phonons is interconverted. Here, we show that, in addition to CAM, MAM can also be converted to the electronic degrees of freedom by deriving a second-order perturbative Hamiltonian proportional to phonon MAM. This finding highlights that the electronic motion is directly affected by phonon MAM, and consequently, that phonon degrees of freedom can play a crucial role in phenomena related to electronic orbital polarization.

cond-mat.mtrl-sci↗

arXiv2026

Nonlocal current-response theory of structured-light dichroism

We develop a nonlocal minimal-coupling theory of structured-light dichroism. Optical absorption is written as a bilinear functional of the incident vector potential and the nonlocal current-response kernel, retaining the spatial structure of optical vortex beams and other inhomogeneous fields. Circular dichroism, helical dichroism, and helical circular dichroism are treated as distinct reversal channels that project different reversal-odd components of the same microscopic response. The resulting signal is resolved into symmetry, tensor, and angular-mode sectors. Single azimuthal-mode fields probe diagonal OAM-channel components, whereas mixed modes access off-diagonal mode-space coherence through interference between different OAM channels. The relative polarization of the mixed field further selects scalar, axial-vector, or rank-2 tensor sectors of the response. This decomposition gives selection rules for high-symmetry systems and a diagnostic scheme for low-symmetry nanophotonic structures, finite systems, and extended materials, where several angular-channel responses may coexist. It also connects the nonlocal current-response formulation to local-gradient descriptions based on spatial dispersion, optical chirality, and tensorial anisotropy.

physics.optics↗

arXiv2026

Optical vortex probe of loop-current chirality in moiré materials

We propose a symmetry-resolved optical probe of intrinsic loop-current chirality in moiré materials, with twisted bilayer graphene as a representative realization. Interlayer interference generates chiral electronic circulation on triangular plaquettes, giving rise to an intrinsic geometric chirality that enters the second-order response through a $C_3$-selected angular harmonic of the Berry curvature and can be isolated by the orbital-angular-momentum difference $Δ\ell$ of interfering optical vortex beams. When moiré $C_3$ symmetry is preserved, the intrinsic contribution appears in the $Δ\ell=3$ channel of the helicity-dependent dc photocurrent, whereas $C_3$-breaking perturbations activate additional channels. These results establish angular-momentum-resolved nonlinear optics as a route to probing geometric chirality in moiré and other symmetry-engineered quantum materials.

cond-mat.mes-hall↗

arXiv2026

Microscopic Theory of Chiral-Phonon-Induced Orbital Selectivity in Helical Crystals

We present a microscopic theory of chirality-induced orbital selectivity (CIOS) in helical crystals, in which truly chiral phonons selectively transfer angular momentum to electronic orbital degrees of freedom. For a threefold helical crystal with line-group symmetry $L3_1$, we show that phonon-induced local rotations generate a rotational electron-phonon interaction proportional to $\hat{L}^{\pm}$, which drives the orbital transfer $m_{\ell}\to m_{\ell}-m_{s}$ in accordance with crystal angular momentum (CAM) conservation, where $m_{s}=\pm 1$ denotes the eigenvalue of the phonon rotational mode. Evaluating $\langle\hat{L}^{z}\rangle$ to leading order in perturbation theory, we find that the orbital response is suppressed near the $Γ$ point and the BZ boundary, and enhanced at intermediate wave vectors -- a feature intimately tied to the degeneracy structure of the phonon bands.

cond-mat.other↗

arXiv2025

Electron-Chiral Phonon Coupling, Crystal Angular Momentum, and Phonon Chirality

We explicitly derive the wavefunctions of chiral phonons propagating along the helical axis in chiral crystals and clarify the characteristics of electron-phonon interactions in chiral helical crystals. In particular, we elucidate how the conservation of not only the crystal momentum (CM) but also the crystal angular momentum (CAM) manifests in the interaction vertex. This formulation provides a microscopic framework for describing physical processes involving chiral phonons. Furthermore, we construct a phononic analogue of Zilch, a known measure of chirality carried by light, and discuss its relationship with phonon angular momentum.

cond-mat.mtrl-sci↗

arXiv2024

Description of molecular chirality and its analysis with high harmonic generation

To clarify the microscopic origin of chirality-induced optical effect, we develop an analytical method that extracts the chiral part of the Hamiltonian of molecular electronic states. We demonstrate this method in a model chiral molecule consisting of two helically stacked $N$-sided regular polygons, and compare it with numerical calculation for chiral discrimination via high-harmonic generation (HHG) of the same molecule. The discrimination signal here is the Kuhn $g$ factor, the difference between the harmonic intensity from the bicircular laser field and that from its reflected laser field normalized by their average. The $g$ factor is a pseudoscalar quantity that reflects the chirality of the molecule. As a result, we find that the $g$ factor becomes large over a wide range of harmonic orders making HHG suitable for chiral discrimination. We further find that, to increase the difference of harmonic intensity from the above two fields, the unnormalized $g$ factor, the increase of capacity to generate the longitudinal dipole moment is more advantageous than maximizing the transverse-to-longitudinal conversion efficiency via the optimization of molecular chirality. We speculate this criteria may be extended to other optical and current-induced processes relevant to the chiral molecules and materials.

cond-mat.mtrl-sci↗

arXiv2023

Note on Angular Momentum of Phonons in Chiral Crystals

Phonon angular momentum in chiral materials has been widely studied in spintronics and condensed matter physics. In chiral crystals, this is not the conserved quantity in contrast to the pseudo-angular momentum. To highlight this point and to understand the behavior of the angular momentum, we reexamined phonon dispersion theory based on the irreducible representation of helix and found the distinction of these angular momentum is originated from chirality.

cond-mat.mtrl-sci↗

arXiv2022

Chirality-Induced Spin Filtering in Pseudo Jahn-Teller Molecules

Chirality-induced spin selectivity (CISS) refers to an ability to induce a spin polarization of an electron transmitted through chiral materials. An important experimental observation is that incredibly large spin polarization is realized at room temperature even for organic molecules that have weak spin-orbit coupling (SOC), although SOC is the only interaction that can manipulate the electrons' spins in the setups. Therefore, the mechanism of the CISS needs to be constructed in a way insensitive to or enhancing the magnitude of the SOC strength. In this paper, we describe a theoretical study of CISS with a model chiral molecule that belongs to the point group $\mathrm{C}_3$. In this molecule, electronic translational and rotational degrees of freedom for an injected electron are coupled to one another via the nuclear vibrational mode with a pseudo Jahn-Teller effect. By properly taking the molecular symmetry as well as the time-reversal symmetry into account and classifying the molecular ground states by their angular- and spin-momentum quantum numbers, we show that the chiral molecule can act as an efficient spin filter. The efficiency of this spin filtering can be nearly independent of the SOC strength in this model, while it well exceeds the spin polarization relying solely on the SOC. The nuclear vibrations turned out to have the role of not only mediating the translation-rotation coupling, but also enhancing the spin-filtering efficiency.

cond-mat.mtrl-sci↗

arXiv2018

Non-Markovian quantum-classical ratchet for ultrafast long-range electron-hole separation in condensed phases

In organic photovoltaic systems, a photogenerated molecular exciton in the donor domain dissociates into a hole and an electron at the donor/acceptor heterojunction, and subsequently separate into free charge carriers that can be extracted as photocurrents. The recombination of the once-separated electron and hole is a major loss mechanism in photovoltaic systems, which controls their performance. Hence, efficient photovoltaic systems need built-in ratchet mechanisms, namely, ultrafast charge separation and retarded charge recombination. In order to obtain insight into the internal working of the experimentally observed ultrafast long-range charge separation and protection against charge recombination, we theoretically investigate a potential ratchet mechanism arising from the combination of quantum delocalization and its destruction by performing numerically accurate quantum-dynamics calculations on a model system. It is demonstrated that the non-Markovian effect originating from the slow polaron formation strongly suppresses the electron transfer reaction back to the interfacial charge-transfer state stabilized at the donor/accepter interface and that it plays a critical role in maintaining the long-range electron--hole separation.

cond-mat.mes-hall↗

arXiv2018

Hierarchical Equations of Motion Approach to Quantum Thermodynamics

We present a theoretical framework to investigate quantum thermodynamic processes under non-Markovian system-bath interactions on the basis of the hierarchical equations of motion (HEOM) approach, which is convenient to carry out numerically "exact" calculations. This formalism is valuable because it can be used to treat not only strong system-bath coupling but also system-bath correlation or entanglement, which will be essential to characterize the heat transport between the system and quantum heat baths. Using this formalism, we demonstrated an importance of the thermodynamic effect from the tri-partite correlations (TPC) for a two-level heat transfer model and a three-level autonomous heat engine model under the conditions that the conventional quantum master equation approaches are failed. Our numerical calculations show that TPC contributions, which distinguish the heat current from the energy current, have to be take into account to satisfy the thermodynamic laws.

quant-ph↗

arXiv2017

Quantum Heat Current under Non-perturbative and Non-Markovian Conditions: Applications to Heat Machines

We consider a quantum system strongly coupled to multiple heat baths at different temperatures. Quantum heat transport phenomena in this system are investigated using two definitions of the heat current, one in terms of the system energy, and the other in terms of the bath energy. When we consider correlations among system-bath interactions (CASBI) -- which have a purely quantum mechanical origin -- the definition in terms of the bath energy becomes different. We found that CASBI are necessary to maintain the consistency of the heat current with thermodynamic laws in the case of strong system-bath coupling. However, within the context of the quantum master equation approach, both of these definitions are identical. Through a numerical investigation, we demonstrate this point for a non-equilibrium spin-boson model and a three-level heat engine model using the reduced hierarchal equations of motion approach under strongly coupled and non-Markovian conditions. We observe cyclic behavior of the heat currents and the work performed by the heat engine,and we find that their phases depend on the system-bath coupling strength. Through consideration of the bath heat current, we show that the efficiency of the heat engine decreases as the strength of the system-bath coupling increases, due to the CASBI contribution. In the case of a large system-bath coupling, the efficiency increases further if the bath temperature is decreased, with fixed the ratio of the bath temperatures, due to the discretized nature of energy eigenstates. This is also considered to be a unique feature of quantum heat engines.

quant-ph↗

arXiv2013

Quantum Suppression of Ratchet Rectification in a Brownian System Driven by a Biharmonic Force

We rigorously investigate the quantum dissipative dynamics of a ratchet system described by a periodic potential model based on the Caldeira-Leggett Hamiltonian with a biharmonic force. In this model, we use the reduced hierarchy equations of motion in the Wigner space representation. These equations represent a generalization of the Gaussian-Markovian quantum Fokker-Planck equation introduced by Tanimura and Wolynes (1991), which was formulated to study non-Markovian and non-perturbative thermal effects at finite temperature. This formalism allows us to treat both the classical limit and the tunneling regimes, and it is helpful for identifying purely quantum mechanical effects through the time evolution of the Wigner distribution. We carried out extensive calculations of the classical and quantum currents for various temperatures, coupling strengths, and barrier heights. Our results reveal that at low temperature, while the quantum current is larger than the classical current in the case of a high barrier, the opposite is true in the case of a low barrier. We find that this behavior results from the fact that the tunneling enhances the current in the case of a high barrier, while it suppresses the current in the case of a low barrier. This is because the effect of the ratchet potential is weak in the case of a low barrier, due to the large dispersion of the distribution introduced by tunneling. This causes the spatio-temporal asymmetry, which is necessary for ratchet current, to be weak, and as a result, the net current is suppressed.

quant-ph↗