Metallization of the Organic Conductor (TTM-TTP)I₃with a Highly One-Dimensional Half-Filled Band under Pressure beyond 7 GPa

Abstract

It is theoretically predicted that the ground state of the one-dimensional (1D) half-filled system is the Mott insulator at any positive on-site Coulomb interaction U . In this paper, we demonstrate that (TTM-TTP)I3 with a highly 1D half-filled energy band is almost metallized by the application of pressure beyond 7 GPa. We find that the metal-insulator transition temperature decreases linearly with increasing pressure and is directing towards 0 K near 10 GPa. Above 5.7 GPa, the metallic behaviors are observed in the high-temperature region for 70 < T < 300 K, in which the temperature dependence of the resistivity is described by ρ(T ) ∝ T α with α ∼ 0.3. The nature of the metallic state is discussed in terms of the Tomonaga-Luttinger liquid described by the Hubbard model. KEYWORDS: (TTM-TTP)I3, organic conductor, one-dimensional system, half-filling, high-pressure, Tomonaga-Luttinger liquid, Hubbard model, Kρ IntroductionMetal-insulator (MI) transitions by Coulomb interaction are accompanied with huge resistance change and are widely observed in strongly correlated systems. Many theories of strongly correlated systems begin with a Hubbard model because of its simplicity.1) The Hubbard model is the prototypical model used for the description of correlated fermions in a variety of materials, ranging from high-T c superconductors to heavy fermion compounds and organic molecular crystals. In spite of its apparent simplicity, there is still no general solution, or even a consensus on its fundamental properties for the Hubbard model. Notable exceptions are the cases of oneand infinite-dimensions.2, 3) In one-dimension, an exact solution is available.4) Especially, the one-dimensional (1D) half-filled case is special but important because it corresponds to the strong coupling limit of the Hubbard model, i.e., U → ∞. 5) That is, no matter how small U is, the ground state is the Mott insulator. Thus metallization of the 1D half-filled system is one of the most challenging problems in the strongly correlated electronic systems.In this paper, we report the metallization of (TTM-TTP)I 3 with a highly 1D half-filled band, 6) where TTM-TTP stands for 2,5-bis[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene [see Fig. 1(a)]. 7,8) In the interchain directions, the TTM-TTP molecules cannot approach close to each other because of the steric hindrance of the terminal -SCH 3 groups.7) According to the band structure calculation, the interchain interactions, t a , t b (< 0.01 eV) are less than 1/20 of the intrachain interaction, t c = 0.26 eV, which is much more onedimensional than the typical quasi-1D Bechgaard salts.

Recent progress in high-pressure studies on organic conductors

“…Since the discovery of superconductivity in β -(BEDT-TTF) 2 ICl 2 [9], various insulators, stable to pressure due to the electron correlation, were metallized under pressure and a variety of fascinating physical phenomena were found. Examples include the anomalous metallic state for (TTM-TTP)I 3 with quasi-one-dimensional half-filled band [18][19][20], the possible quantum critical point in the reduced π-donor system (MeDH-TTP) 2 AsF 6 [21][22][23], metallization of the Peierls insulator TTT-TCNQ [24], new aspect in unified T -P phase diagram in (TMTTF) 2 X (X = AsF 6 or SbF 6 ) [25][26][27], and the robust superconducting state against pressure and non-Fermi liquid behavior in the doped Mott system κ-(BEDT-TTF) 4 Hg 2.89 Br 8 [28]. These observations provide us new problems, and the pressure will increasingly play a more important role the field of organic conductors in the future.…”

“…Taking advantage of the softness of organics, Yasuzuka et al [18][19][20] studied the temperature dependence of the resistivity in (TTM-TTP)I 3 under pressure, where TTM-TTP [43,44]. Figure 5(a) shows the structure of the TTM-TTP donor molecule [46,47].…”

“…Hence, pressures above 1 GPa are required for the metallization of (TTM-TTP)I 3 . Figure 6 shows the temperature dependence of resistivity under pressures up to 8.0 GPa [18][19][20]. At ambient pressure, the resistivity gradually increases with decreasing temperature above T MI (= 128 K), and then rises sharply below T MI .…”

“…At higher pressures, the so-called 'dimensional crossover', which is often discussed in Bechgaard salts, is the key physics for exotic metals such as the Tomonaga-Luttinger liquid state. To consider the dimensionality of (TTM-TTP)I 3 under pressures above 5.7 GPa, Yasuzuka et al [18][19][20] discussed the dimensional crossover temperature T x (= t ⊥ /π ) at 8 GPa. From the band structure calculation, T x was estimated as 37 K at ambient pressure.…”