Ferroelectric Mott-Hubbard Phase of Organic(TMTTF)2XConductors

Abstract

We present experimental evidences for a ferro-electric transition in the family of quasi-one-dimensional conductors (T M T T F ) 2 X. We interpret this new transition in the frame of the combined Mott-Hubbard state taking into account the double action of the spontaneous charge disproportionation on the T M T T F molecular stacks and of the X anionic potentials.PACS numbers: 71.30 +h, 7127 +a, 77.80.Bh, 7145.Lr Low dimensional electronic systems serve as a workshop on problems of strong correlations. The richest opportunities have been opened by the family of charge transfer salts M 2 X formed of stacks of organic molecules tetramethyltetrathiofulvalene (M = T MT T F ) or tetramethyltetraselenafulvalene (M = T MT SF ) with anions X = P F 6 , AsF 6 , SbF 6 , SCN, etc as counterions. The ions sit in loose cavities delimited by the methyl groups of the organic molecules. These materials show almost all known electronic phases: a metal, a paramagnetic insulator (PI), spin/charge density waves (SDW/CDW), the spin-Peierls state and finally the superconducting state [1]. In parallel, there is a set of several different structural types due to anion ordering (AO) which are slight arrangements in X chains [2]. Their tran-1 sition temperatures T a ≈ 100 − 200K are much higher than that corresponding to electronic transitions occurring in the range of T c ≈ 1 − 10K.Very recently, in the already very rich phase diagram of these salts, a new phase transition has been discovered, being revealed by a huge anomaly in the low frequency dielectric constant ǫ ′ at T 0 = 70K in (T MT T F ) 2 P F 6 and at 100K in (T MT T F ) 2 AsF 6 [3]. Very soon NMR studies have proved [4] that the transitions at T 0 are related to charge disproportionation, also called charge ordering (CO), dividing molecules within in two non equivalent species. Previous reports on T MT T F salts with larger anions such as SbF 6 have suggested the occurrence of a phase transition, sometimes called " structureless" transition, . So, the identification of the nature of this " structureless" q = 0 transition was still unsolved. We show hereafter that ǫ ′ in (T MT T F ) 2 SbF 6 , as in the case of (T MT T F ) 2 P F 6 and (T MT T F ) 2 AsF 6 , exhibits a huge divergence at 154K. Dielectric experiments are providing thus an unique access to the anomalous collective properties of this mysterious transition identifying the resulting state as a never expected ferroelectric (FE) one.In the high temperature range T > T a , T 0 the counterions already have a profound effect on the electronic properties: the anions dimerize the intermolecular spacing of the TMTTF molecules along the stacks. As a result, the conduction band is split into a filled lower band separa ted from a half-filled upper band by a dimerization gap, leading to Mott-Hubbard charge localization. The effects brought by the AOs with q = 0 have been previously discussed ( [2,9] and refs. therein). The new effects of combined CO-AO with q = 0 are 2 the main topic of the present work. Empirically one ...

Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band

“…Although the pattern of the CO has not been determined directly, the observation of ferroelectric behavior below T CO is consistent with¯1010t ype CO in this region. 5,23 With further lowering of T, most ͑TMTTF͒ 2 X undergo transitions to the AFM1 or SP phase ͑with X = Br a possible exception, see below͒. X = SbF 6 at low T lies in the AFM1 region, 9 with a very high T CO and relatively low Néel temperature T N = 8 K. As the schematic phase diagram indicates, pressure suppresses both T CO and T N in this region.…”

“…24 from exact diagonalization calculations. The inclusion of interactions with counterions may enhance the Wigner crystal ordering 5,23 for some ͑TMTTF͒ 2 X. We will discuss this point further below, and argue that it is important in the AFM1 region of the phase diagram.…”

“…Spatial broken symmetries in the quasi-one-dimensional ͑quasi-1D͒ 1 4 -filled organic charge-transfer solids ͑CTSs͒ have been of strong experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21][22][23][24][25][26] interests. The broken symmetry states include charge order ͑hereafter CO, this is usually accompanied by intramolecular distortions͒, intermolecular lattice distortions ͑hereafter bond order wave or BOW͒, antiferromagnetism ͑AFM͒, and spinPeierls ͑SP͒ order.…”

Ordering phenomena in quasi-one-dimensional organic conductors

“…The first indications of CO came from dielectric measurements in the radiofrequency range (Nad et al 1999, Monceau et al 2001) where a divergency of the low-frequency dielectric constant was observed at a certain temperature T CO . This behavior is well known from ferroelectric transitions, where also a dielectric catastrophe is observed due to a softening of the lattice.…”

“…However, taking electron-lattice coupling into account other charge configurations become possible (Mazumdar et al 2000). In particular the ferroelectric response observed by measurements of dielectric permittivity requires a coupling term between the electrons in the stack and the charged counterions (Monceau et al 2001). The phase diagram (Fig.…”