Electron Localization near the Mott Transition in the Organic Superconductorκ−(BEDT−TTF)2Cu[N

Abstract

The effect of disorder on the electronic properties near the Mott transition is studied in an organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br, which is systematically irradiated by X-ray. We observe that X-ray irradiation causes Anderson-type electron localization due to molecular disorder. The resistivity at low temperatures demonstrates variable range hopping conduction with Coulomb interaction. The experimental results show clearly that the electron localization by disorder is enhanced by the Coulomb interaction near the Mott transition.PACS numbers: 74.70. Kn, 71.30.+h, 72.15.Rn Metal-insulator (MI) transitions are of considerable importance for strongly correlated electron systems. Among the various types of the MI transitions, the Mott transition due to electron-electron (e-e) interactions is one of the most attractive phenomena [1]. A Mott insulator derives from the large on-site Coulomb energy with respect to the bandwidth. The electrons in the Mott insulator are localized on the individual sites to minimize the mutual Coulomb repulsion, which results in the opening of a charge gap at the Fermi level. Another way of the electron localization originates from the interference of the electron wave functions due to randomness. This is the Anderson insulator derived by introducing disorder into the material [2]. In contrast to Mott insulators, in Anderson insulators, there is no opening of a gap in the density of states in principle. Since the randomness in the correlated electron system is essentially important in real materials, systematic studies of disorder effects are desired in systems nearby a Mott transition for understanding their physical properties. The e-e interaction effects in disordered systems have been considered in the weak localization (WL) effect of electrons [3,4]. Recently, several theoretical studies have been performed in consideration of the Mott transition [5][6][7], but there has been few experimental approaches so far [8] because of limited suitable materials and ways of introducing disorder.Organic charge-transfer salts based on a donor molecule bis(ethylenedithio)-tetrathiafulvalene (abbreviated as BEDT-TTF) have been recognized as highly correlated electron systems [9]. Among them, κ-(BEDT-TTF) 2 X, where X is anion molecule, has attracted considerable attention as a bandwidth-controlled Mott transition system [10]. One can control the strength of electron correlation relative to the bandwidth by small pressure [11] or partial molecule substitution [12], which leads to a Mott insulator -metal transition. Recently, X-ray irradiation effects in κ-(BEDT-TTF) 2 X have been examined [13][14][15]. It has been known that X-ray irradiation causes molecular disorder and this remains permanently in organic materials [16]. In case of the superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 (hereafter κ-NCS), the molecular disorder causes an increase of the residual resistivity and then suppresses the superconductivity [13]. On the other hand in case of the Mott insulator κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]...

Mott-Anderson Transition in Molecular Conductors: Influence of Randomness on Strongly Correlated Electrons in the κ-(BEDT-TTF)2X System

“…In the case of the X = Cu(NCS) 2 sample, the characteristic hump structure around 100 K is suppressed [18] and the ρ(T) curves intersect a single point at approximately T = 50 K and ρ ~ These changes are reproduced well in line with the previous report on the inter-plane resistivity measurements [18]. In contrast to the X = Cu(NCS) 2 sample, a rather low irradiation dose for the X = Cu[N(CN) 2 ]Br sample induces a drastic change of the temperature dependence of the resistivity [31]. The resistivity hump at 100 K is completely suppressed and the residual resistivity ρ 0 increases rapidly with the irradiation time.…”

“…However, further irradiation leads to increase in the resistivity above approximately 200 h irradiation. This anomalous resistivity behavior is closely related to the critical randomness of localizing insulating states discussed in the latter section [31]. 2 X at 300 K [20].…”

“…In this section, we focus on the randomness effect in the insulating state of the X-ray irradiated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, which shows a localization MI transition induced by disorder [31,33]. In the insulating temperature region (10 K < T < 40 K) with t irr > 200 h (Figure 4b), the resistivity follows accurately the Arrhenius law described as ρ(T) = ρ a exp(E a /k B T), where ρ a is the resistivity extrapolated to the high-temperature limit and E a is the characteristic energy.…”

“…We investigated the X-ray irradiation effect widely in κ-(BEDT-TTF) 2 X showing superconductivity or Mott insulating state from the viewpoint of the relation between the correlated electronic states and randomness [20,[27][28][29][30][31][32][33][34]. Recently, we found that the weak molecular disorder introduced by X-ray irradiation of the organic superconductor κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br induced the Anderson-type localization insulating state from the strongly correlated metallic/superconducting state [31].…”

Mott metal-insulator transition induced by utilizing a glasslike structural ordering in low-dimensional molecular conductors

“…Besides the possibility to tune the interaction strength for studying fundamental aspects of the Mott transition, an aspect of considerable recent theoretical and experimental interest in this field of research relates to the interplay of correlation effects and disorder (Mott-Anderson scenario), see e.g. [8][9][10][11][12] .…”