Metallic Conduction and Carrier Localization in Two-Dimensional BEDO-TTF Charge-Transfer Solid Crystals

Manabu Ishikawa,Taishi Takenobu,Yongbing Shen,Hideki Yamochi,Shinji K Yoshina,Masahiro Yamashita,Akihiro Otsuka,Gunzi Saito,Motoki Matsuno,Yukihiro Yoshida,Seiu Katagiri,Hiroshi Ito

doi:10.3390/cryst12010023

Abstract

Charge-transfer salts based on bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF or BO for short) provide a stable two-dimensional (2D) metallic state, while the electrical resistance often shows an upturn at low temperatures below ~10 K. Such 2D weak carrier localization was first recognized for BO salts in the Langmuir–Blodgett films fabricated with fatty acids; however, it has not been characterized in charge-transfer solid crystals. In this paper, we discuss the carrier localization of two crystalline BO charge-transfer salts with or without magnetic ions at low temperatures through the analysis of the weak negative magnetoresistance. The phase coherence lengths deduced with temperature dependence are largely dominated by the electron–electron scattering mechanism. These results indicate that the resistivity upturn at low temperatures is caused by the 2D weak localization. Disorders causing elastic scattering within the metallic domains, such as those of terminal ethylene groups, should be suppressed to prevent the localization.

Highlights

Since the 1970s, molecular conductors with tetrathiafulvalene (TTF) derivatives have attracted attention for their rich variety of electronic phenomena such as metal–insulator transitions and superconductivity [1,2]
The phase coherence lengths are deduced with temperature dependence largely dominated by the electron–electron scattering mechanism. These values are comparable to those of other type of materials, such as LB films and polymer poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), implying a common origin of the inelastic scattering. These results indicate that the resistivity upturn at low temperatures is caused by the 2D weak localization, which confirms the 2D nature of the conduction layer made of BO molecules
Positive MR of metals was observed for the high field region above 4 T for both magnetic field directions; clear negative MR was observed up to 4 T only when the magnetic field was applied perpendicular to the 2D layer (θ = 90◦)

Summary

Introduction

Since the 1970s, molecular conductors with tetrathiafulvalene (TTF) derivatives have attracted attention for their rich variety of electronic phenomena such as metal–insulator transitions and superconductivity [1,2]. The bis(ethylenedioxy)-substituted derivative, BEDO-TTF (or BO for short, Figure 1a), in which the sulfur atoms in the outer six-membered ring of ET are replaced by oxygen atoms, has attracted attention as a building molecule. This donor molecule shows the strong tendency to create complexes of two-dimensional (2D) layers due to the self-aggregation ability caused by the intermolecular π–π interactions, along with the C–H···O hydrogen bonding [2,3].

Methods

Results

Conclusion