Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{I}}_{3}$ irradiated by linearly polarized light

Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt $α − {(BEDT − TTF)}_{2} I_{3}$ irradiated by linearly polarized light

Keisuke Kitayama, Masahito Mochizuki, Yasuhiro Tanaka, and Masao Ogata

Phys. Rev. B 104, 075127 – Published 16 August 2021

Abstract

Prolonged experimental attempts to find magnetic monopoles (i.e., elementary particles with an isolated magnetic charge in three dimensions) have not yet been successful despite intensive efforts made since Dirac's proposal in 1931. Particle physicists have predicted the possible collision and pair annihilation of two magnetic charges with opposite signs. However, if such annihilation exists, its experimental observation would be difficult because its energy scale is predicted to be tremendously high ( $\sim 10^{16}$ GeV). In the present work, we theoretically predict using the Floquet theory that a pair of slightly gapped Dirac-cone bands in a weakly charge-ordered organic conductor $α - {(BEDT - TTF)}_{2} I_{3}$ , which behave as magnetic charges with opposite signs in the momentum space, exhibit pair annihilation under irradiation with linearly polarized light. This photoinduced pair annihilation is accompanied by a nontopological phase transition to the Floquet normal insulator phase in contrast to the well-known circularly polarized-light-induced topological phase transition to the Floquet Chern insulator phase. We discuss that $α - {(BEDT - TTF)}_{2} I_{3}$ has a peculiar band structure capable of realizing a suitable experimental condition (i.e., off-resonant condition) and a charge-ordered state providing a required staggered site potential and thereby provides a rare example of materials that can be used to observe the predicted pair annihilation phenomenon. The feasibility of experimental observation is also discussed.

Received 23 April 2021
Accepted 4 August 2021

DOI:https://doi.org/10.1103/PhysRevB.104.075127

Physics Subject Headings (PhySH)

Photoinduced effect

Dirac semimetal Floquet systems Organic compounds

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Keisuke Kitayama ^1,*, Masahito Mochizuki ², Yasuhiro Tanaka ², and Masao Ogata ^1,3

¹Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
²Department of Applied Physics, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
³Trans-scale Quantum Science Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

^*kitayama@hosi.phys.s.u-tokyo.ac.jp

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Vol. 104, Iss. 7 — 15 August 2021

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Figure 1
(a) Crystal structure of the BEDT-TTF layer in $α$ -(BEDT- ${TTF)}_{2} I_{3}$ whose unit cell (dashed rectangle) contains four molecules (A, $A^{'}$ , B, and C). Transfer integrals for a tight-binding model of this compound are also shown. (b) Schematic illustration of $α$ -(BEDT- ${TTF)}_{2} I_{3}$ irradiated with linearly polarized light. The ac electric field of light is given in the form $- E^{ω} cos (ω τ) (cos θ, sin θ)$ , where $E^{ω}$ and $θ$ are the amplitude and polarization angle.
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Figure 2
[(a)–(d)] Quasienergy band structures of the photodriven $α$ -(BEDT- ${TTF)}_{2} I_{3}$ under irradiation with linearly polarized light for various light amplitudes $E^{ω}$ : (a) $E^{ω}$ =0 (unirradiated case), (b) $E^{ω}$ =12 MV/cm, (c) $E^{ω}$ =14 MV/cm, and (d) $E^{ω}$ =15 MV/cm. [(e)-(h)] Berry curvature of the fourth band in the photodriven $α$ -(BEDT- ${TTF)}_{2} I_{3}$ . The sharp peaks with opposite signs indicate the existence of positive and negative magnetic charges at the momentum points corresponding to the gapped Dirac points. The frequency and the polarization angle of light are fixed at $ℏ ω = 0.6$ eV and $θ = 45^{\circ}$ , respectively. The pair annihilation of magnetic charges is observed with increasing $E^{ω}$ .
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Figure 3
(a) Phase diagram for nonequilibrium steady states in the photodriven $α$ -(BEDT- ${TTF)}_{2} I_{3}$ under irradiation with linearly polarized light, in the plane of the amplitude $E^{ω}$ and the polarization angle $θ$ of the light. (b), (c) Color maps of the calculated two types of energy gap, (b) $E_{gap}$ and (c) ${\tilde{E}}_{gap}$ , defined in Eqs. (18) and (19), respectively. The light frequency $ω$ is fixed at $ℏ ω$ =0.6 eV.
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Figure 4
Schematic illustrations of two possible types of band structure for $E_{gap} < 0$ and ${\tilde{E}}_{gap} = 0$ , both of which are expected to behave as a semimetal. (a) Band structure in which the upper band crosses the Fermi level at momenta far from the Dirac cones. (b) Band structure with overtilted Dirac cones where the upper (lower) cone band is located below (above) the Fermi level. Both cases are assigned to the type-II Dirac semimetal phase in the phase diagram in Fig. 3.
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Figure 5
(a) Photoinduced renormalization factors $J_{0} (A_{a} / 2), J_{0} (A_{+})$ , and $J_{0} (A_{-})$ for the transfer integrals in $α$ -(BEDT- ${TTF)}_{2} I_{3}$ irradiated with linearly polarized light as functions of the light polarization angle $θ$ . The light amplitude and frequency are fixed at $E^{ω}$ =15 MV/cm and $ℏ ω$ =0.6 eV, respectively. (b) Trajectories of the positive and negative emergent magnetic charges at the Dirac points with increasing light amplitude $E^{ω}$ in the momentum space.
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Figure 6
(a) Phase diagram for nonequilibrium steady states in the photodriven $α$ -(BEDT- ${TTF)}_{2} I_{3}$ under irradiation with linearly polarized light in the plane of the amplitude $E^{ω}$ and the frequency $ω$ of light. (b), (c) Color maps of the calculated two types of energy gap, (b) $E_{gap}$ and (c) ${\tilde{E}}_{gap}$ defined in Eqs. (18) and (19), respectively. The inset of (c) shows the $E^{ω}$ dependence of ${\tilde{E}}_{gap}$ for selected light frequencies. The polarization angle $θ$ of light is fixed at $θ$ = $45^{\circ}$ .
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Physical Review B

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Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt $α − {(BEDT − TTF)}_{2} I_{3}$ irradiated by linearly polarized light

Keisuke Kitayama, Masahito Mochizuki, Yasuhiro Tanaka, and Masao Ogata

Phys. Rev. B 104, 075127 – Published 16 August 2021

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Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt α−(BEDT−TTF)2I3 irradiated by linearly polarized light

Keisuke Kitayama, Masahito Mochizuki, Yasuhiro Tanaka, and Masao Ogata

Phys. Rev. B 104, 075127 – Published 16 August 2021

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Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt $α − {(BEDT − TTF)}_{2} I_{3}$ irradiated by linearly polarized light