Phase Diagram of b-(BEDT-TTF)2ICl2 under High Pressure based on the First-Princples Electronic Structure Hiori Kino, Hiroshi Kontani and Tsuyoshi MiyazakiJ. Phys. Soc. Jpn., 73, 25 (2004).

Experimental phase diagramOnset superconducting transition temperature=14.2K (the highest among organic superconductors),SC nodes: unknown

AFI at ambient pressure (commensurate vector: unknown)

I phase under pressures: magnetic structures: unknownb-(BEDT-TTF)2ICl2Taniguchi et al.

Electronic structure: First-Principles result0GPa4GPa8GPa12GPaHOMO, HOMO-1: the HOMO of BEDT-TTF molecule

Pressure increase dimensionality of the Fermi surface

van-Hove singularity at G point: shift downward under pressures large DOS at EF0.5eV(Miyazaki)Phys. Rev. B 68, 220511 (2003)

Purpose?Understanding of the phase diagramorigin of the superconductivityorigin of the high transition temperature

A ModelA tight binding Hamiltonian (Hubbard model)Electronic structure near the EF:the HOMO of BEDT-TTF molecule, tight binding fit of the first-principles resultsuperconductivity: (probably) next to the antiferromagnetic phaseon-site Coulomb interactionOnly the HOMO bandand effective on-site Coulomb interaction(a dimer model)

Electronic structureTigiht binding parameters:0-12GPa: interpolation>12GPa: linear extrapolation

|t(p1)| much larger than others

band width: linear increase P>4GPa

DOS at EF: van-Hove singularity near EF

Fermi surface: 1D2D(Original crystal structure: not square)0.2-0.20

MethodApproximation to include effects of Coulomb interaction: fluctuation exchange (FLEX)Self-energy=Antiferromagnetism: Stoner criterionSuperconductivity: (linearized) Eliashberg equation++

Resultsc.f. Exp. AFSCrapid increase of TN (P8GPa) --- 1D2D, dimensional crossover, worse nestingshoulder of TN (P~10GPa) --- nesting vector (commensurateincommensurate)emergence of SC (P>14GPa) --- origin AF fluctuationAF: antiferromgetismSC: superconductivity

SC order parameterSC orderFermi surfacesSC order: singlet dxy,, no triplet

effects of U: Fermi surface nests betterG

ProblemsTheory: SC at ~14GPa. Exp: SC at 8GPa

Origin of this discrepancy: worse tight binding fit under pressures --- position of van Hove Singularity. A Model Hamiltonian (A dimer model): worse for higher pressures. t(p1) v.s. other t

Comparison of FS12GPaDFT (Miyazaki)Tight binding model12GPa

Possible origin of high TcIn increasing pressure,

Band width: largerDOS: stays large due to the tail of van Hove singularityCalculated Tc: larger than that in the modeled simple-quasi-1D TMTSF salts.0.2-0.20

Fin.