Quantum transport of topological spin solitons in a one-dimensional organic ferroelectric

Letter

Quantum transport of topological spin solitons in a one-dimensional organic ferroelectric

S. Imajo, A. Miyake, R. Kurihara, M. Tokunaga, K. Kindo, S. Horiuchi, and F. Kagawa

Phys. Rev. B 103, L201117 – Published 21 May 2021

Abstract

We report the dielectric, magnetic, and ultrasonic properties of a one-dimensional organic salt $TTF − {QBr}_{3} I$ . These indicate that $TTF − {QBr}_{3} I$ shows a ferroelectric spin-Peierls (FSP) state in a quantum critical regime. In the FSP state, coupling of charge, spin, and lattice leads to emergent excitation of spin solitons as topological defects. Amazingly, the solitons are highly mobile even at low temperatures, although they are normally stationary because of pinning. Our results suggest that strong quantum fluctuations enhanced near a quantum critical point enable soliton motion governed by athermal relaxation. This indicates the realization of quantum topological transport at ambient pressure.

Received 25 November 2020
Revised 5 March 2021
Accepted 12 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.L201117

Physics Subject Headings (PhySH)

Ferroelectricity Peierls transition Quantum criticality Topological defects

1-dimensional systems

DC susceptibility measurements Dielectric spectroscopy Magnetization measurements Ultrasound techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Imajo ^1,*, A. Miyake¹, R. Kurihara¹, M. Tokunaga¹, K. Kindo¹, S. Horiuchi ², and F. Kagawa^3,4

¹Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
²Research Institute of Advanced Electronics and Photonics (RIAEP), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
³RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
⁴Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

^*imajo@issp.u-tokyo.ac.jp

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Issue

Vol. 103, Iss. 20 — 15 May 2021

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Images

Figure 1
(a) Crystal structure viewed along the $a$ axis, and chemical forms of TTF and ${QBr}_{3} I$ molecules. The dashed boxes signify the one-dimensional chains along the $a$ axis (red) and $b$ axis (blue). [(b) and (c)] Schematic illustrations of the arrangement of the $D^{+}$ and $A^{-}$ molecules in (b) the paraelectric paramagnetic state and (c) the FSP state. The green arrows represent the magnetic spin. The red and blue arrows signify the directions of the electric dipoles in the $D^{+} A^{-}$ dimers. In the FSP state shown in (c), two degenerate patterns occur, patterns 1 and 2, according to the direction of the dipole moments. (d) Creation of spin solitons at the ferroelectric DWs in the FSP state.
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Figure 2
(a) Temperature dependence of the dielectric permittivity in 1 kHz (red) and 100 kHz (light blue) ac electric fields. The reported data of TTF- ${QBr}_{4}$ (light green) are also shown on the right axis. The dotted curves are the fits to the Barrett formula with the parameters mentioned in the text. [(b) and (c)] 1/ $ε_{r}$ vs $T$ plot of $TTF − {QBr}_{3} I$ data (b) and TTF- ${QBr}_{4}$ data (c) shown in (a). (d) Magnetization curves up to 30 T at 4.2 K. The blue curve represents the total magnetization of $TTF − {QBr}_{3} I$ , while the red curve denotes the magnetization obtained by subtracting the paramagnetic component displayed by the dotted curve. (e) Total and subtracted magnetic susceptibility as a function of temperature. The susceptibility is obtained by subtracting the Curie-type paramagnetic component as mentioned in the text and Ref. [24]. Since the subtraction is valid only around 4.2 K, the higher-temperature data are shown as the dashed curve. The purple curve presents the data for TTF- ${QBr}_{4}$ . The arrows signify the FSP transition temperatures. (f) Temperature dependence of the elastic constant for longitudinal ultrasonic waves along the $b$ axis $C_{L}$ . The dashed curve is a background curve estimated based on the normal elastic stiffening [35]. The inset displays the additional component related to the FSP transition derived by subtracting the background. (g) Relative change in temperature-dependent ultrasonic attenuation $Δ α$ plotted in a semilogarithmic plot. The data of imaginary part of the dielectric permittivity $ε_{i}$ at 1 kHz are also shown on the right axis.
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Figure 3
Schematic illustration of the temperature-pressure phase diagram for the FSP system. $TTF − {QBr}_{3} I$ is positioned in the quantum critical region located around the QCP.
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Figure 4
(a) Permittivity-frequency profiles at 1.4 and 4.2 K. The dotted curves denote fits to the Cole-Cole type relaxation described in the figure. (b) Relaxation time as a function of inverse temperature. The blue line shows the Arrhenius-type linear dependence of the classical relaxation, whereas the orange line is the constant relaxation of the quantum tunneling. The dotted curve indicates a simple approximation of the crossover between the classical and quantum regime obtained by the Wentzel-Kramers-Brillouin model. (c) Schematic energy landscape describing the disassociation and recombination of the dimer accompanied by the annihilation and creation of spin solitons. The parameters $d$ , $Δ$ , and $m_{eff}$ denote the unit cell distance, activation energy and effective mass of the spin solitons, respectively. $τ_{athermal}$ and $τ_{thermal}$ are the relaxation times of the quantum tunneling process and thermal activation process crossing the potential.
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