Mechanism of superconductivity and electron-hole doping asymmetry in κ-type molecular conductors

Abstract

Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system κ -(BEDT-TTF) 2 X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with d xy -wave symmetry, which has similarities with high- T c cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with extended- s + wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high- T c cuprates.

Simultaneous Control of Bandfilling and Bandwidth in Electric Double-Layer Transistor Based on Organic Mott Insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl

“…The doping dependence of the chemical potential was nonmonotonic only on the electron-doped side [Figure 8b], implying the possibility of a phase separation between the Mott-insulating and filled band scenario. Calculations on a more detailed quarter-filled band model for the κ-BEDT-TTF salts also predict the doping-induced superconductivity, where the doping polarity alters the pairing symmetry (electron doping: extended 𝑠 + 𝑑 , hole doping: 𝑑 ) [20]. In addition, many quantum Monte Carlo calculations on the Hubbard model indicate the absence of superconductivity at near half filling [21,22], while superconductivity is predicted near quarter filling [23].…”

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

“…The unconventional nature of the superconductivity was characterized by the four-fold oscillation of C p T −1 in the angle-resolved heat capacity measurements, which demonstrated that the antiferromagnetic spin fluctuations play an important role for relatively high transition temperatures of the dimer-Mott superconductors. The symmetry change of Cooper pairs from d xy for κ-(BEDT-TTF) 2 Ag(CN) 2 H 2 O to d x2-y2 + s ± for the d[0,0] compound occurs inside the superconductive phase [28,[48][49][50][51][52][53]. The relation with the symmetry change and the crossover inside the superconducting phase are interesting subjects to be solved by heat capacity measurements, although it is still a speculative discussion at present.…”

Symmetry change of $d$-wave superconductivity in $κ$-type organic superconductors

“…Theoretical studies have also proposed d x 2 −y 2 +s ± - [12] and d xywave [13][14][15] symmetry depending on their models. Some recent theories based on a more realistic model [16][17][18][19] suggest that the two symmetries compete and the emergent symmetry is determined according to the ratio of transfer integrals. Nevertheless, the experimental evidence of the competition of the emergent symmetry is still absent.…”

“…To answer the question, we here compare the present results with the theoretical studies for the pairing mechanisms based on the antiferromagnetic spin fluctuations. In fact, recent theoretical calculations without the dimer approximation [16][17][18][19] pointed out that the d xyand d x 2 −y 2 +s ± -wave symmetries compete according to the transfer integrals in the dimer units and the geometric frustration of the dimer triangular lattice. Even though Guterding, etal.…”