Interplay of charge-density waves and superconductivity in the organic conductorβ″−(BEDT-TTF)2AuBr2

Abstract

We studied the low-temperature electrical properties of the two-dimensional organic conductor ␤Љ-͑BEDT-TTF͒ 2 AuBr 2 , whose electronic structure is similar to that of the pressure-induced superconductor ␤Љ-͑DODHT͒ 2 PF 6 having a charge-ordered transition at 255 K at ambient pressure. We found that the ground state of ␤Љ-͑BEDT-TTF͒ 2 AuBr 2 at ambient pressure has a charge-density-wave phase below 6.5 K accompanied by x-ray superlattice spots. We also discovered superconductivity in the vicinity of the charge-densitywave phase by applying uniaxial strain. Based on the obtained results, we propose a unified electronic phase diagram that suggests the relationships among the charge-ordered insulating state, the superconducting phase, and the charge-density-wave phase in ␤Љ-type organic conductors. The phase diagram shows that the superconducting phase is located near the charge-density-wave phase, which suggests that spin fluctuations play a minor role in the mechanism of the superconductivity and sheds light on the possibility of charge-fluctuationmediated superconductivity in this type of organic conductor.

From charge- and spin-ordering to superconductivity in the organic charge-transfer solids

“…β -(ET) 2 AuBr 2 stays metallic until low temperatures. SC has not been found under isotropic pressure, but a T c of approximately 1 K was found under conditions of uniaxial strain [352,384]. AuBr 2 has some unusual properties at low temperatures that could indicate a weak CO state: At ambient pressure there is a kink in the resistivity at 110 K followed by a slight upturn at around 8 K that is suppressed by pressure [384].…”

“…AuBr 2 has some unusual properties at low temperatures that could indicate a weak CO state: At ambient pressure there is a kink in the resistivity at 110 K followed by a slight upturn at around 8 K that is suppressed by pressure [384]. X-ray superlattice reflections are observed at low temperatures, indicating bond ordering below 6.5 K [352]; optical studies are consistent with a gapped state coexisting with free carriers [385]. There is also a steep decrease in the susceptibility at low temperatures indicating a non-magnetic ground state [384].…”

Vibronic activation of molecular vibrational overtones in the infrared spectra of charge-ordered organic conductors

“…1,2 The Wigner-crystallization-like phase transition has been found to occur in a number of mixed-valency charge-transfer complexes with the 3/4-filled electron system, and has drawn the attention of researchers because of its intriguing properties, such as various kinds of nonlinear conductions, [3][4][5][6] light-induced phase transitions, 7-10 unconventional ferroelectricity, [11][12][13] and a possible relation with superconductivity. [14][15][16][17][18][19][20] As we will discuss in a following section, the anomaly activated along with the charge ordering is ascribed to an overtone of a molecular vibration, activated by a vibronic coupling effect. It has been widely known for organic conductors that a strong vibronic effect called electron-molecular vibration (e-mv) coupling significantly affects the spectroscopic properties.…”

A Database for Crystalline Organic Conductors and Superconductors

“…Partially driven by their low dimensionality, this class of materials exhibits a variety of competing electronic behaviors [1][2][3][4][5] including metallic conductivity, Mott insulators [6][7][8] , antiferromagnetic states [9][10][11], and superconductivity [12,13]. Other forms of long range charge order have also been observed, such as charge density waves (CDWs) [14][15][16] and spin density waves (SDWs) [17,18]. More exotic long range order, such as the quantum hall effect [18] and some of the first believable evidence for field induced inhomogeneous superconductivity (the FFLO state) [19][20][21][22][23] were also found in COM.…”