Effects of electron correlations and chemical pressures on superconductivity of β′′ -type organic compounds

Abstract

We investigate low-temperature electronic states of the series of organic conductors β ′′ -(BEDT-TTF)4[(H3O)M(C2O4)3]G, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, and M and G represent trivalent metal ions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of M and G give rise to systematic change in the cell parameters, especially in the b-axis length, which has positive correlation with the superconducting transition temperature Tc. Analyses of temperature and magnetic field dependences of the electrical resistance including the Shubnikov-de Haas oscillations elucidates that the variation of charge disproportionation, effective mass and the number of itinerant carriers, can be systematically explained by the change of the b-axis length. The changes of the transfer integrals induced by stretching/compressing the b-axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.Superconductivity dominated by electron correlations often appears in nearly half-filled electronic bands, where an antiferromagnetic Mott insulating state is easily formed by on-site Coulomb repulsion U . In such cases, the superconductive regions are located in close proximity to the magnetic Mott phases in electronic phase diagrams [1][2][3]. Therefore, unconventional pairing related to magnetic spin fluctuations has been suggested to provide an understanding of the mechanisms of the superconductivity. Indeed, highest critical temperatures T c are normally observed on the verge of the magnetic phases because the quantum fluctuation coming from the spin degree of freedom is strongly enhanced. High-T c cuprates[1], heavy fermion superconductors[2] and dimer-Mott type organic superconductors [3] have been discussed as such candidates of spin-fluctuation-mediated superconductors. Other degrees of freedom, such as orbital (multipole) [4,5] and charge [6], have been also proposed as origins of the pairing in some superconductors, e.g. iron-based compounds, cage compound, etc. However, it is difficult to examine the relationship between the quantum degrees of freedom and superconductivity because the number of such exotic superconductors is rather small and the pairing states are often in complicated situations due to the coexistence/competition of some degrees of freedom.Recently, β ′′ -type organic charge-transfer salts consisting of BEDT-TTF molecules with counter anions and guest solvent molecules have drawn extensive attention because they are expected to have a novel Cooper pairing mechanism related to charge degrees of freedom [6][7][8][9][10][11][12][13][14][15]. The critical temperatures T c of some β ′′ -salts are relatively high around 7-8 K, which is almost com-parable to the T c of well-known higher-T c dimer-Mott organic superconductors, κ-(BEDT-TTF) 2 X. Also, the strong-coupling ...

First Molecular Superconductor with the Tris(Oxalato)Aluminate Anion, β″-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, and Isostructural Tris(Oxalato)Cobaltate and Tris(Oxalato)Ruthenate Radical Cation Salts

“…Figure 6 shows the resistivity of β″-(BEDT-TTF)4(H3O)Ru(C2O4)3•G, where G = PhBr, PhCl, or PhF. Ru-PhBr for A = Kx(H3O)1-x has previously been studied by Prokhorova et al [29] with a sample-dependent Tc in the range 2.8-6.3 K. Resistivity measurements on our crystals of β″-(BEDT-TTF)4(H3O)Ru(C2O4)3.PhBr gave a Tc of 2.8 K, which was as expected based on the b axis length [31]. Upon reducing the size of G from PhBr to PhCl or PhF, no superconductivity was observed.…”

“…This salt (M = Al) is isostructural with bromobenzene salts where M = Fe, Ga, Rh, Ru, Mn, Cr. A relationship between the b axis length and superconducting T c has previously been observed in the Day series [31]. The b axis length of these bromobenzene salts at room temperature showed that the M = Fe salt had the longest b axis and also the highest T c of~3.8 K, while M = Al had the shortest b axis and the lowest T c of~2.5 K. We also reported the isostructural M = Co salt (Co-PhBr), which did not show superconductivity.…”

“…Figure 6 [31]. Upon reducing the size of G from PhBr to PhCl or PhF, no superconductivity was observed.…”

“…However, the b axis lengths in semiconducting Ru-PhCl and Ru-PhF were longer than that in superconducting Al-PhBr (Table 3). This indicates that other factors, such as the shape and the electric dipole of the guest molecule, may have minor influences even though the b axis length predominantly affects the electronic state, including the Tc [31]. .…”

“…A much more marked effect on the value of Tc is observed when changing the guest molecule, G. Changing M and G leads to a change in the length of the unit cell dimensions. A correlation between the b axis length and superconducting Tc has been observed through structural analysis[31]. The effect of chemical pressure through changing G and M is mainly attributed to the guest molecule, G, which is oriented with the R-group oriented in the b direction (Figure3).…”