arXiv subjects

Takashi Oka

Publications and source records attributed to Takashi Oka.

At least 37 records · Page 2Linked to original sources

arXiv2022

Ellipticity control of terahertz high-harmonic generation in a Dirac semimetal

We report on terahertz high-harmonic generation in a Dirac semimetal as a function of the driving-pulse ellipticity and on a theoretical study of the field-driven intraband kinetics of massless Dirac fermions.Very efficient control of third-harmonic yield and polarization state is achieved in electron-doped Cd$_3$As$_2$ thin films at room temperature. The observed tunability is understood as resulting from terahertz-field driven intraband kinetics of the Dirac fermions. Our study paves the way for exploiting nonlinear optical properties of Dirac matter for applications in signal processing and optical communications.

cond-mat.mtrl-sci↗

arXiv2021

Nonlinear transverse current on $\mathcal{C}_{3v}$-symmetric honeycomb lattice

At nonlinear orders in the electric field, the vanishing of the Hall conductivity does not prevent the nonlinear component of the current from being transverse for selected field directions. We study electrons on $\mathcal{C}_{3v}$-symmetric honeycomb lattice for which the Hall conductivity vanishes at first and second order. Nevertheless, the second-order current component is transverse for fields perpendicular to the three mirror lines. The $\mathcal{C}_{3v}$ symmetry constrains the first-order and second-order conductivity tensors to have only one independent component each, which we calculate using the quantum kinetic equation. In linearly\hyp{}polarized oscillating field, the current has a zero\hyp{}frequency component switching sign upon $π/2$-rotation of the polarization angle.

cond-mat.mes-hall↗

arXiv2021

Nonadiabatic Nonlinear Optics and Quantum Geometry -- Application to the Twisted Schwinger Effect

We study the tunneling mechanism of nonlinear optical processes in solids induced by strong coherent laser fields. The theory is based on an extension of the Landau-Zener model with nonadiabatic geometric effects. In addition to the rectification effect known previously, we find two effects, namely perfect tunneling and counterdiabaticity at fast sweep speed. We apply this theory to the twisted Schwinger effect, i.e., nonadiabatic pair production of particles by rotating electric fields, and find a nonperturbative generation mechanism of the opto-valley polarization and photo-current in Dirac and Weyl fermions.

cond-mat.mes-hall↗

arXiv2021

Mott Memristors based on Field-Induced Carrier Avalanche Multiplication

We present a theory of Mott memristors whose working principle is the non-linear carrier avalanche multiplication in Mott insulators subject to strong electric fields. The internal state of the memristor, which determines its resistance, is encoded in the density of doublon and hole excitations in the Mott insulator. In the current-voltage characteristic, insulating and conducting states are separated by a negative-differential-resistance region, leading to hysteretic behavior. Under oscillating voltage, the response of a voltage-controlled, non-polar memristive system is obtained, with retarded current and pinched hysteresis loop. As a first step towards neuromorphic applications, we demonstrate self-sustained spiking oscillations in a circuit with a parallel capacitor. Being based on electronic excitations only, this memristor is up to several orders of magnitude faster than previous proposals relying on Joule heating or ionic drift.

cond-mat.str-el↗

arXiv2021

Photoinduced Nonperturbative Valley Polarization in Graphene

We investigate a valleytronic device based on graphene with charge separation at different sublattices and correspondingly at nonequivalent valleys. We characterize the maximality condition of valley polarization and investigate the parameters and conditions upon which we can coherently control the carriers and store data via valley degree of freedom. The valley polarization is controlled by the amplitude as well as the carrier envelope phase of the pulse and the curvature of the electron trajectory in the reciprocal space. When strong field excitation is taken into account, the optical selection rule in perturbative optics is replaced by the nonadiabatic geometric effects. As a result, a nonperturbative valley polarization in two dimensional Dirac materials is induced regardless of having an intrinsic bandgap. Microscopically, such a nonreciprocal response of graphene in the chiral electric field is encoded by the quantum Berry phase, as a (pseudo) magnetoelectric monopole.

cond-mat.mes-hall↗

arXiv2020

STM as a single Majorana detector of Kitaev's chiral spin liquid

In this letter, we propose a local detection scheme for the Majorana zero mode (MZM) carried by a vison in Kitaev's chiral spin liquid (CSL) using scanning tunneling microscopy (STM). The STM introduces a single Majorana into the system through hole/charge injection and the Majorana interacts with the MZM to form a stable composite object. We derive the exact analytical expression of single-hole Green's function in the Mott insulating limit of Kitaev's model and show that the differential conductance has split peaks, as a consequence of resonant tunneling through the vison-hole composite. The peak splitting scales with the binding energy of vison-hole composite, which is comparable to the Majorana gap in CSL, well within the reach of experimental observation.

cond-mat.str-el↗

arXiv2020

Nonperturbative Topological Current in Weyl and Dirac Semimetals in Laser Fields

We study non-perturbatively the anomalous Hall current and its high harmonics generated in Weyl and Dirac semimetals by strong elliptically polarized laser fields, in the context of kinetic theory. We find a novel crossover between perturbative and non-perturbative regimes characterized by the electric field strength $\mathcal{E}^{*}= \frac{μω}{ 2 e v_F}$ ($ω$: laser frequency, $μ$: Fermi energy, $v_F$: Fermi velocity). In the perturbative regime, the anomalous Hall current quadratically depends on the field strength ($\mathcal{E}$), whereas the higher order corrections, as well as high harmonics, vanish at zero temperature. In the non-perturbative regime, the anomalous Hall current saturates and decays as $(\log{\mathcal{E}})/\mathcal{E}$, while even-order high harmonics are generated when inplane rotational symmetry is broken. Based on the analytical solution of the Boltzmann equation, we reveal the topological origin of the sharp crossover: the Weyl monopole stays inside or moves outside of the Fermi sphere, respectively, during its fictitious motion in the pertubative or non-pertubative regimes. Our findings establish a new non-linear response intrinsically connected to topology, characteristic to Weyl and Dirac semimetals.

cond-mat.mes-hall↗

arXiv2020

$h/e$ Oscillations in Interlayer Transport of Delafossites

Transport of electrons in a bulk metal is usually well captured by their particle-like aspects, while their wave-like nature is commonly harder to observe. Microstructures can be are fully designed to reveal the quantum phase, for example mesoscopic metal rings resembling interferometers. Here we report a new type of phase coherent oscillation of the out-of-plane magnetoresistance in the layered delafossites PdCoO$_2$ and PtCoO$_2$. The oscillation period is equivalent to that determined by the magnetic flux quantum, $h/e$, threading an area defined by the atomic interlayer separation and the sample width. The phase of the electron wave function in these crystals appears remarkably robust over macroscopic length scales exceeding 10$μ$m and persisting up to elevated temperatures of $T$>50K. We show that, while the experimental signal cannot be explained in a standard Aharonov-Bohm analysis, it arises due to periodic field-modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in the delafossites, and identify a new form of quantum interference in solids.

cond-mat.mtrl-sci↗

arXiv2019

Wannier representation of Floquet topological states

A universal feature of topological insulators is that they cannot be adiabatically connected to an atomic limit, where individual lattice sites are completely decoupled. This property is intimately related to a topological obstruction to constructing a localized Wannier function from Bloch states of an insulator. Here we generalize this characterization of topological phases toward periodically driven systems. We show that nontrivial connectivity of hybrid Wannier centers in momentum space and time can characterize various types of topology in periodically driven systems, which include Floquet topological insulators, anomalous Floquet topological insulators with micromotion-induced boundary states, and gapless Floquet states realized with topological Floquet operators. In particular, nontrivial time dependence of hybrid Wannier centers indicates impossibility of continuous deformation of a driven system into an undriven insulator, and a topological Floquet operator implies an obstruction to constructing a generalized Wannier function which is localized in real and frequency spaces. Our results pave a way to a unified understanding of topological states in periodically driven systems as a topological obstruction in Floquet states.

cond-mat.mes-hall↗

arXiv2019

Non-perturbative high-harmonic generation in the three-dimensional Dirac semimetal Cd$_3$As$_2$

Harmonic generation is a general characteristic of driven nonlinear systems, and serves as an efficient tool for investigating the fundamental principles that govern the ultrafast nonlinear dynamics. In atomic gases, high-harmonic radiation is produced via a three-step process of ionization, acceleration, and recollision by strong-field infrared laser. This mechanism has been intensively investigated in the extreme ultraviolet and soft X-ray regions, forming the basis of attosecond research. In solid-state materials, which are characterized by crystalline symmetry and strong interactions, yielding of harmonics has just recently been reported. The observed high-harmonic generation was interpreted with fundamentally different mechanisms, such as interband tunneling combined with dynamical Bloch oscillations, intraband thermodynamics and nonlinear dynamics, and many-body electronic interactions. Here, in a distinctly different context of three-dimensional Dirac semimetal, we report on experimental observation of high-harmonic generation up to the seventh order driven by strong-field terahertz pulses. The observed non-perturbative high-harmonic generation is interpreted as a generic feature of terahertz-field driven nonlinear intraband kinetics of Dirac fermions. We anticipate that our results will trigger great interest in detection, manipulation, and coherent control of the nonlinear response in the vast family of three-dimensional Dirac and Weyl materials.

cond-mat.mtrl-sci↗

arXiv2019

Extraction of higher-order nonlinear electronic response to strong field excitation in solids using high harmonic generation

State-of-the-art experiments employ strong ultrafast optical fields to study the nonlinear response of electrons in solids on an attosecond time-scale. Notably, a recent experiment retrieved a 3rd order nonlinear susceptibility by comparing the nonlinear response induced by a strong laser field to a linear response induced by the otherwise identical weak field. In parallel, experiments have demonstrated high harmonic generation (HHG) in solids, a highly nonlinear process that until recently had only been observed in gases. The highly nonlinear nature of HHG has the potential to extract even higher order nonlinear susceptibility terms, and thereby characterize the entire response of the electronic system to strong field excitation. However, up till now, such characterization has been elusive due to a lack of direct correspondence between high harmonics and nonlinear susceptibilities. Here, we demonstrate a regime where such correspondence can be clearly made, extracting nonlinear susceptibilities (7th, 9th, and 11th) from sapphire of the same order as the measured high harmonics. The extracted high order susceptibilities show angular-resolved periodicities arising from variation in the band structure with crystal orientation. Nonlinear susceptibilities are key to ultrafast lightwave driven optoelectronics, allowing petahertz scaling manipulation of the signal. Our results open a door to multi-channel signal processing, controlled by laser polarization.

physics.optics↗

arXiv2019

Hidden kagome-lattice picture and origin of high conductivity in delafossite PtCoO$_2$

We study the electronic structure of delafossite PtCoO$_2$ to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that it is formed mainly by Pt $d$ orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structure: one originating from Pt $s+p_x/p_y$ orbitals, and the other from Pt $d_{3z^2-r^2}+d_{xy}/d_{x^2-y^2}$ orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt $s+p_x/p_y$ bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt $s+p_x/p_y$ band. More importantly, the kagome-like electronic structure gives rise to "orbital-momentum locking" on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in PtCoO$_2$.

cond-mat.mtrl-sci↗

arXiv2018

Dynamical synchronization transition in interacting electron systems

Synchronization is a ubiquitous phenomenon in nature and we propose its new perspective in ultrafast dynamics in interacting electron systems. In particular, using graphene irradiated by an intense bi-circular pulse laser as a prototypical and experimental viable example, we theoretically investigate how to selectively generate a coherent oscillation of electronic order such as charge density waves (CDW). The key is to use tailored fields that match the crystalline symmetry broken by the target order. After the pump, a macroscopic number of electrons start oscillating and coherence is built up through a transition. The resulting physics is detectable as a coherent light emission at the synchronization frequency and may be used as a purely electronic way of realizing Floquet states respecting exotic space time crystalline symmetries. In the process, we also explore possible flipping of existing static CDW orders and generation of higher harmonics. The general framework for the coherent electronic order is found to be analogous with the celebrated Kuramoto model, describing the classical synchronization of coupled pendulums.

cond-mat.str-el↗

arXiv2018

Spin Hall effect in 2D metallic delafossite PtCoO$_2$ and vicinity topology

The two-dimensional (2D) metal PtCoO$_2$ is renowned for the lowest room temperature resistivity among all oxides, close to that of the top two materials Ag and Cu. In addition, we theoretically predict a strong intrinsic spin Hall effect. This originates from six strongly-tilted Dirac cones that we find in the electronic structure near the Fermi surface, where a gap is opened by large spin-orbit coupling (SOC). This is underpinned by rich topological properties; in particular, the phenomenology of a mirror Chern metal is realized not exactly, but very accurately, on account of an approximate crystalline symmetry. We expect that such 'vicinity topology' to be a feature of relevance well beyond this material. Our Wilson loop analysis indicates further elaborate features such as fragile topology. These findings highlight PtCoO$_2$ as a promising material for spintronic applications as well as a platform to study the interplay of symmetry and topology.

cond-mat.mes-hall↗

arXiv2018

Floquet Engineering of Quantum Materials

Floquet engineering, the control of quantum systems using periodic driving, is an old concept in condensed matter physics, dating back to ideas such as the inverse Faraday effect. There is a renewed interest in this concept owing to the rapid developments in laser and ultrafast spectroscopy techniques and discovery and understanding of various "quantum materials" hosting interesting exotic quantum properties. Here, starting from a nontechnical introduction with emphasis on the Floquet picture and effective Hamiltonians, we review the recent applications of Floquet engineering in ultrafast, nonlinear phenomena in the solid state. In particular, Floquet topological states, application to ultrafast spintronics, and to strongly correlated electron systems are overviewed.

cond-mat.str-el↗

arXiv2017

Holographic Floquet states II: Floquet condensation of vector mesons in nonequilibrium phase diagram

With the aim to reveal universal features of hadronic matter and correlated Dirac insulators in strong AC-electric fields, we study the $\mathcal{N}=2$ supersymmetric QCD with a finite quark mass driven by a rotating electric field $\mathcal{E}_x+i\mathcal{E}_y= E e^{iΩt}$. The analysis is done in the holographically dual D3/D7 system in the co-rotating frame, effectively. The nonequilibrium phase diagram is determined from the threshold electric field at which the insulator phase breaks down to a conductive phase due to the AC version of the Schwinger mechanism. The external field induces a rotating current $\mathcal{J}_x + i \mathcal{J}_y = J e^{iΩt}$ originating from vacuum polarization and dissipative current in the insulating and conductive phases respectively. Intriguing features are observed as the frequency $Ω$ approaches resonance with the meson excitation energy $Ω_{\rm meson}$. There, the threshold minimizes and a condensate of vector mesons with oscillating current exists even in the zero driving field limit. This state, which we call Floquet condensate of vector mesons, is expected to be dynamically stable realizing a non-thermal fixed point that breaks time translational and reversal symmetries. Our finding has many similarities with exciton BEC discussed in solid state systems, where the semiconductor is to be replaced by materials hosting gapped Dirac electrons, e.g. 3D topological insulators or bismuth. Vector meson Floquet condensate may also have implications in the pre-thermalized dynamics in heavy ion collision experiments.

hep-th↗

arXiv2017

Probing and controlling spin chirality in Mott insulators by circularly polarized laser

Scalar spin chirality, a three-body spin correlation that breaks time-reversal symmetry, is revealed to couple directly to circularly polarized laser. This is shown by the Floquet formalism for the periodically driven repulsive Hubbard model with a strong-coupling expansion. A systematic derivation of the effective low-energy Hamiltonian for a spin degree of freedom reveals that the coupling constant for scalar spin chirality can become significant for a situation in which the driving frequency and the on-site interaction are comparable. This implies that the scalar chirality can be induced by circularly polarized lights, or that it can be used conversely for probing the chirality in Mott insulators as a circular dichroism.

cond-mat.str-el↗

arXiv2016

Emergent Weyl nodes and Fermi arcs in a Floquet Weyl semimetal

When a Dirac semimetal is subject to a circularly polarized laser, it is predicted that the Dirac cone splits into two Weyl nodes and a nonequilibrium transient state called the Floquet Weyl semimetal is realized. We focus on the previously unexplored low-frequency regime, where the upper and lower Dirac bands resonantly couple with each other through multi-photon processes, which is a realistic situation in solid state ultrafast pump-probe experiments. We find a series of new Weyl nodes emerging in pairs when the Floquet replica bands hybridize with each other. The nature of the Floquet Weyl semimetal with regard to the number, locations, and monopole charges of these Weyl nodes is highly tunable with the amplitude and frequency of the light. We derive an effective low energy theory using Brillouin-Wigner expansion and further regularize the theory on a cubic lattice. The monopole charges obtained from the low-energy Hamiltonian can be reconciled with the number of Fermi arcs on the lattice which we find numerically.

cond-mat.mes-hall↗