Showing papers by "Shunsuke A. Sato published in 2021"
TL;DR: Li et al. as discussed by the authors demonstrate that light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the S r T i O 3 ground state, which has thus far only been achieved in out-of-equilibrium strongly excited conditions.
Abstract: Optical cavities confine light on a small region in space, which can result in a strong coupling of light with materials inside the cavity. This gives rise to new states where quantum fluctuations of light and matter can alter the properties of the material altogether. Here we demonstrate, based on first-principles calculations, that such light–matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the S r T i O 3 ground state, which has thus far only been achieved in out-of-equilibrium strongly excited conditions [X. Li et al., Science 364, 1079–1082 (2019) and T. F. Nova, A. S. Disa, M. Fechner, A. Cavalleri, Science 364, 1075–1079 (2019)]. This is a light–matter hybrid ground state which can only exist because of the coupling to the vacuum fluctuations of light, a photo ground state. The phase transition is accompanied by changes in the crystal structure, showing that fundamental ground state properties of materials can be controlled via strong light–matter coupling. Such a control of quantum states enables the tailoring of materials properties or even the design of novel materials purely by exposing them to confined light.
34 citations
TL;DR: Yoshikawa et al. as mentioned in this paper investigated high-order harmonic generation (HHG) in graphene with a quantum master equation approach and found that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light.
Abstract: We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al., Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation.
29 citations
TL;DR: In this paper, the authors combine time and angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet-Bloch states in the presence of scattering.
Abstract: Floquet theory has spawned many exciting possibilities for electronic structure control with light, with enormous potential for future applications. The experimental demonstration in solids, however, remains largely unrealized. In particular, the influence of scattering on the formation of Floquet-Bloch states remains poorly understood. Here we combine time- and angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet-Bloch states in the presence of scattering. We find that Floquet-Bloch states will be destroyed if scattering-activated by electronic excitations-prevents the Bloch electrons from following the driving field coherently. The two-level model also shows that Floquet-Bloch states reappear at high field intensities where energy exchange with the driving field dominates over energy dissipation to the bath. Our results clearly indicate the importance of long scattering times combined with strong driving fields for the successful realization of various Floquet phenomena.
28 citations
TL;DR: In this article, the authors apply attosecond transient reflection spectroscopy in a sequential two-foci geometry and observe sub-femtosecond dynamics of a core-level exciton in bulk MgF2 single crystals.
Abstract: The electro-optical properties of most semiconductors and insulators of technological interest are dominated by the presence of electron-hole quasi-particles, called excitons. The manipulation of excitons in dielectrics has recently received great attention, with possible applications in different fields including optoelectronics and photonics. Here, we apply attosecond transient reflection spectroscopy in a sequential two-foci geometry and observe sub-femtosecond dynamics of a core-level exciton in bulk MgF2 single crystals. Furthermore, we access absolute phase delays, which allow for an unambiguous comparison with theoretical calculations. Our results show that excitons surprisingly exhibit a dual atomic- and solid-like character, which manifests itself on different time scales. While the former is responsible for a femtosecond optical Stark effect, the latter dominates the attosecond excitonic response. Further theoretical investigation reveals a link with the exciton sub-femtosecond nanometric motion and allows us to envision a new route to control exciton dynamics in the close-to-petahertz regime. The capability to follow electron motion in solids is necessary to explore the ultimate speed limits of optical charge manipulation and signal processing in optoelectronic devices. Here, the authors reveal the sub-femtosecond dynamics of core excitons in MgF2 and find the dual atomic-solid nature of the exciton quasi-particle to deeply affect its ultrafast dynamics.
23 citations
TL;DR: In this article, the quantum paraelectric ground state of the lattice was accessed via a microscopic ab initio approach based on density functional theory, and it was shown that at low temperature the quantum fluctuations are strong enough to stabilize the ground state even though a classical description would predict a ferroelectric phase.
Abstract: We demonstrate how the quantum paraelectric ground state of ${\mathrm{SrTiO}}_{3}$ can be accessed via a microscopic ab initio approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in ${\mathrm{SrTiO}}_{3}$ are also well captured by the present microscopic framework.
16 citations
TL;DR: In this article, the authors review the recent development of the first-principles calculation for light-induced electron dynamics in solids by revising its application to recent attosecond experiments.
11 citations
TL;DR: In this paper, the quantum paraelectric ground state of SrTiO$_3$ can be accessed via a microscopic $ab~initio$ approach based on density functional theory.
Abstract: We demonstrate how the quantum paraelectric ground state of SrTiO$_3$ can be accessed via a microscopic $ab~initio$ approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in SrTiO$_3$ are also well captured by the present microscopic framework.
10 citations
TL;DR: In this paper, linear vibronic spectra in molecular systems can be simulated efficiently using first-principles approaches without relying on the explicit use of multiple Born-Oppenheimer potential ensembles.
Abstract: We show how linear vibronic spectra in molecular systems can be simulated efficiently using first-principles approaches without relying on the explicit use of multiple Born–Oppenheimer potential en...
9 citations
TL;DR: In this paper, the authors calculate the charge-transfer cross sections for the collision in an inverse collision framework, where the active electrons of the projectile are handled by applying an initial velocity to the Kohn-Sham orbitals via a Galilean boost, which ensures numerically converged final-time scattering states.
Abstract: We calculate the charge-transfer cross sections for the ${\mathrm{Ne}}^{2+}+\text{He}$ collision. To this end, we employ Ehrenfest molecular dynamics with time-dependent density-functional theory. The active electrons of the projectile are handled by applying an initial velocity to the Kohn-Sham orbitals via a Galilean boost. The dynamical calculations are performed in an inverse collision framework---the reference frame considers ${\mathrm{Ne}}^{2+}$ to be initially at rest, which ensures numerically converged final-time scattering states. The charge-transfer probabilities are extracted by extending the particle number projection technique to be able to handle the degenerate ${\mathrm{Ne}}^{2+}$ ion. Compared with experimental data available at 10--3000 keV, a fairly good agreement is found for the calculated single- and double-charge transfer cross sections, superior to other theoretical calculations for this ${\mathrm{Ne}}^{2+}+\text{He}$ collision. A time-resolved analysis of the charge-transfer probabilities finds that ionization to the continuum also takes place after the charge transfer has occurred. To account for it, the final scattering states should be followed for a long time, approximately 350 fs, until they stabilize.
5 citations
4 citations
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TL;DR: In this article, it was shown that light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the SrTiO$_3$ groundstate, which has thus far only been achieved in out-of-equilibrium strongly excited conditions.
Abstract: Optical cavities confine light on a small region in space which can result in a strong coupling of light with materials inside the cavity. This gives rise to new states where quantum fluctuations of light and matter can alter the properties of the material altogether. Here we demonstrate, based on first principles calculations, that such light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the SrTiO$_3$ groundstate, which has thus far only been achieved in out-of-equilibrium strongly excited conditions[1, 2]. This is a light-matter-hybrid groundstate which can only exist because of the coupling to the vacuum fluctuations of light, a "photo-groundstate". The phase transition is accompanied by changes in the crystal structure, showing that fundamental groundstate properties of materials can be controlled via strong light-matter coupling. Such a control of quantum states enables the tailoring of materials properties or even the design of novel materials purely by exposing them to confined light.
TL;DR: In this paper, a variational wave function ansatz based on a set of conditional wave function slices is proposed to determine the structural and time-dependent response properties of the hydrogen molecule.
Abstract: We demonstrate that a conditional wave function theory enables a unified and efficient treatment of the equilibrium structure and nonadiabatic dynamics of correlated electron-ion systems. The conditional decomposition of the many-body wave function formally recasts the full interacting wave function of a closed system as a set of lower-dimensional (conditional) coupled "slices". We formulate a variational wave function ansatz based on a set of conditional wave function slices and demonstrate its accuracy by determining the structural and time-dependent response properties of the hydrogen molecule. We then extend this approach to include time-dependent conditional wave functions and address paradigmatic nonequilibrium processes including strong-field molecular ionization, laser-driven proton transfer, and nuclear quantum effects induced by a conical intersection. This work paves the road for the application of conditional wave function theory in equilibrium and out-of-equilibrium ab initio molecular simulations of finite and extended systems.
TL;DR: In this article, the nonlinear electric conductivity of graphene was investigated under static electric fields for various chemical potential shifts, and the simulation results showed that, as the field strength increases, the effective conductivity is firstly suppressed, reflecting the depletion of effective carriers due to the large displacement in the Brillouin zone caused by the strong field.
Abstract: Based on the quantum master equation approach, the nonlinear electric conductivity of graphene is investigated under static electric fields for various chemical potential shifts. The simulation results show that, as the field strength increases, the effective conductivity is firstly suppressed, reflecting the depletion of effective carriers due to the large displacement in the Brillouin zone caused by the strong field. Then, as the field strength exceeds $1$~MV/m, the effective conductivity increases, overcoming the carrier depletion via the Landau--Zener tunneling process. Based on the nonlinear behavior of the conductivity, the charge transport induced by few-cycle THz pulses is further studied to elucidate the ultrafast optical control of electric current in matter.
21 Jun 2021
TL;DR: In this paper, the authors applied attosecond transient reflectivity spectroscopy to study the ultrafast response of a MgF2 single crystal and reported the first observation of sub-fs core-exciton dynamics.
Abstract: Excitons are electron-hole quasi-particles, which strongly influence the electro-optical properties of solids and hold the potential for a variety of applications [1] . Acquiring a detailed understanding of their dynamic nature is thus essential to promote their exploitation in advanced technological areas. In particular, the ultrafast processes unfolding at the femto- and attosecond domain are of primary relevance in view of the desired extension towards the petahertz regime. Here we applied attosecond transient reflectivity spectroscopy to study the ultrafast response of a MgF2 single crystal and report the first observation of sub-fs core-exciton dynamics [2] . Combined with simulations, our results allowed us to disentangle the dual atomic-solid nature of this fundamental quasi-particle.
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TL;DR: In this article, an extended Ptychographic Iterative Engine for eXcitons (ePIX) is proposed to reconstruct the main physical properties which determine the evolution of the quasi-particle with no prior knowledge of the exact relaxation dynamics or the pump temporal characteristics.
Abstract: The first step to gain optical control over the ultrafast processes initiated by light in solids is a correct identification of the physical mechanisms at play. Among them, exciton formation has been identified as a crucial phenomenon which deeply affects the electro-optical properties of most semiconductors and insulators of technological interest. While recent experiments based on attosecond spectroscopy techniques have demonstrated the possibility to observe the early-stage exciton dynamics, the description of the underlying exciton properties remains non-trivial. In this work we propose a new method called extended Ptychographic Iterative engine for eXcitons (ePIX), capable of reconstructing the main physical properties which determine the evolution of the quasi-particle with no prior knowledge of the exact relaxation dynamics or the pump temporal characteristics. By demonstrating its accuracy even when the exciton dynamics is comparable to the pump pulse duration, ePIX is established as a powerful approach to widen our knowledge of solid-state physics.
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TL;DR: In this paper, a numerical Brillouin-zone integration scheme for real-time propagation of electronic systems with time-dependent density functional theory is presented. But the authors focus on the performance of the decomposition scheme in both linear and nonlinear regimes by computing the linear optical properties of bulk silicon and high-order harmonic generation.
Abstract: We develop a numerical Brillouin-zone integration scheme for real-time propagation of electronic systems with time-dependent density functional theory. This scheme is based on the decomposition of a large simulation into a set of small independent simulations. The performance of the decomposition scheme is examined in both linear and nonlinear regimes by computing the linear optical properties of bulk silicon and high-order harmonic generation. The decomposition of a large simulation into a set of independent simulations can improve the efficiency of parallel computation by reducing communication and synchronization overhead and enhancing the portability of simulations across a relatively small cluster machine.
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TL;DR: In this article, the absorption spectrum of benzene was simulated in full dimensionality using time-dependent density functional theory at the multi-trajectory mean-field level, finding good qualitative agreement with experiment.
Abstract: We show how vibronic spectra in molecular systems can be simulated in an efficient and accurate way using first principles approaches without relying on the explicit use of multiple Born-Oppenheimer potential energy surfaces. We demonstrate and analyse the performance of mean field and beyond mean field dynamics techniques for the \ch{H_2} molecule in one-dimension, in the later case capturing the vibronic structure quite accurately, including quantum Franck-Condon effects. In a practical application of this methodology we simulate the absorption spectrum of benzene in full dimensionality using time-dependent density functional theory at the multi-trajectory mean-field level, finding good qualitative agreement with experiment. These results show promise for future applications of this methodology in capturing phenomena associated with vibronic coupling in more complex molecular, and potentially condensed phase systems.
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TL;DR: In this article, a variational wave function ansatz based on a set of conditional wave function slices is proposed to determine the structural and time-dependent response properties of the hydrogen molecule.
Abstract: We demonstrate that a conditional wavefunction theory enables a unified and efficient treatment of the equilibrium structure and nonadiabatic dynamics of correlated electron-ion systems. The conditional decomposition of the many-body wavefunction formally recasts the full interacting wavefunction of a closed system as a set of lower dimensional (conditional) coupled `slices'. We formulate a variational wavefunction ansatz based on a set of conditional wavefunction slices, and demonstrate its accuracy by determining the structural and time-dependent response properties of the hydrogen molecule. We then extend this approach to include time-dependent conditional wavefunctions, and address paradigmatic nonequilibrium processes including strong-field molecular ionization, laser driven proton transfer, and Berry phase effects induced by a conical intersection. This work paves the road for the application of conditional wavefunction theory in equilibrium and out of equilibrium ab-initio molecular simulations of finite and extended systems.