Topological charge transport by mobile dielectric-ferroelectric domain walls

Abstract

The concept of topology has been widely applied in condensed matter physics, leading to the identification of peculiar electronic states on three-dimensional (3D) surfaces or 2D lines separating topologically distinctive regions. In the systems explored so far, the topological boundaries are built-in walls; thus, their motional degrees of freedom, which potentially bring about new paradigms, have been experimentally inaccessible. Here, working with a quasi-1D organic material with a charge-transfer instability, we show that mobile neutral-ionic (dielectric-ferroelectric) domain boundaries with topological charges carry strongly 1D-confined and anomalously large electrical conduction with an energy gap much smaller than the one-particle excitation gap. This consequence is further supported by nuclear magnetic resonance detection of spin solitons, which are required for steady current of topological charges. The present observation of topological charge transport may open a new channel for broad charge transport–related phenomena such as thermoelectric effects.

Experimental Verification of Charge Soliton Excitations in the Ionic Mott-Peierls Ferroelectric, TTF-CA

“…Our previous study demonstrated that the resistivity minimum results from the NI domain wall (NIDW) excitations arising around NI crossover pressure [Fig. 1] [12]. The present experiment with the piston-cylinder pressure cell confirmed the pressure-insensitive resistivity previously suggested by the experiment using the cubic anvil apparatus [33].…”

“…Topological excitations in one dimension (1D) are zerodimensional defects behaving like particles. They are known as solitons and domain walls, which occasionally cause unconventional electrical and magnetic properties [1][2][3][4][5][6][7][8][9][10][11][12]. Notably, the solitons expected to emerge in the neutral-ionic (NI) transition material, tetrathiafulvalenep-chloranil (TTF-CA), are of profound interest in that they can be elementary excitations responsible for electrical and magnetic properties instead of electrons in a Mott-Peierls system, in which charge, spin, and lattice are strongly entangled [13][14][15].…”

“…Significantly, the I para phase is suggested to host two types of solitons, charge and spin solitons, that emerge on the boundaries of the oppositely polarized domains [Fig. 1], both of which are predicted to contribute to electrical conduction [12][13][14][15]. The pressure-insensitive electrical conductivity possibly stems from the peculiar nature of the solitons; however, the experimental verification of the existence of the charge soliton has not yet been done.…”

Ionic Charge-Transfer Liquid Crystals Formed by Alternating Supramolecular Copolymerization of Liquid π-Donors and TCNQ

“…These findings have led to the development of related materials and applications (Frére and Skabara, 2005 ; Pauliukaite et al, 2007 ; Piek et al, 2018 ; Fujihara et al, 2020 ). On the other hand, another type of ICT complexes, i.e., those consist of alternately stacked arrays of the ionized D and A molecules, have recently been attracted attention because they are expected to serve as potential platforms for designing advanced materials such as organic ferroelectrics (Horiuchi and Tokura, 2008 ; Kobayashi et al, 2012 ; Takehara et al, 2019 ), multiferroics (Kagawa et al, 2010 ; Wang and Zhang, 2020 ), and photovoltaics (Nakamura et al, 2017 ). In contrast to the conventional neutral CT complexes, reports on ICT complexes have been largely limited to crystalline solids.…”

“…The vast majority of existing CT complexes reported to date fall into the class of neutral CT complexes with smaller ρ CT values (0 < ρ CT < 0.5), and only those having large ρ CT values (0.5 < ρ CT ≤ 1) are referred to as ICT complexes. The neutral to ionic transition of CT complexes has been induced by applying pressure (Torrance et al, 1981b ; Takehara et al, 2019 ) or at a considerably lower temperature of 81 K for a tetrathiafluvalne- p -chloranil complex (Torrance et al, 1981a ; Okamoto et al, 1991 ). These conditions severely limit the applicability of ICT complexes for device applications.…”