Phase Diagram of bBEDTTTF2 ICl 2 under High

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

Experimental phase diagram b’-(BEDT-TTF)2 ICl 2 Onset superconducting transition temperature=14. 2 K (the highest among organic superconductors), SC nodes: unknown AFI at ambient pressure (commensurate vector: unknown) I phase under pressures: magnetic structures: unknown Taniguchi et al.

Electronic structure: First-Principles result (Miyazaki) 0. 5 e. V Phys. Rev. B 68, 220511 (2003) 0 GPa HOMO, HOMO-1: the HOMO of BEDT-TTF molecule 4 GPa Pressure → increase dimensionality of the Fermi surface 8 GPa 12 GPa van-Hove singularity at G point: shift downward under pressures → large DOS at EF

Purpose? Understanding of the phase diagram origin of the superconductivity origin of the high transition temperature

A Model Electronic structure near the EF: the HOMO of BEDT-TTF molecule, tight binding fit of the first-principles result superconductivity: (probably) next to the antiferromagnetic phase →on-site Coulomb interaction A tight binding Hamiltonian (Hubbard model) Only the HOMO band effective on-site Coulomb interaction (a dimer model)

Electronic structure Tigiht binding parameters: 0 -12 GPa: interpolation >12 GPa: linear extrapolation |t(p 1)| much larger than others band width: linear increase P>4 GPa 0. 2 0 -0. 2 DOS at EF: van-Hove singularity near EF Fermi surface: 1 D→ 2 D (Original crystal structure: not square)

Method Approximation to include effects of Coulomb interaction: fluctuation exchange (FLEX) ↑ ↓ ↓ Self-energy= + ↑ ↑ Antiferromagnetism: Stoner criterion Superconductivity: (linearized) Eliashberg equation ↑ ↓ ↑ ↓ ↑ ↓ + ↑ ↓

Results c. f. Exp. AF SC AF: antiferromgetism SC: superconductivity rapid increase of TN (P<4 GPa) --- 1 D suppress the AF order broad peak of TN (P=6 GPa) --- nesting vector =(p, 0) decrease of TN (P>8 GPa) --- 1 D→ 2 D, dimensional crossover, worse nesting shoulder of TN (P~10 GPa) --- nesting vector (commensurate→incommensurate) emergence of SC (P>14 GPa) --- origin AF fluctuation

SC order parameter Fermi surfaces SC order: singlet dxy, , no triplet effects of U: Fermi surface nests better - + 0 G + 0 (p, 0) -

Problems Theory: SC at ~14 GPa. Exp: SC at 8 GPa 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(p 1) v. s. other t

Comparison of FS DFT (Miyazaki) Tight binding model 12 GPa

Possible origin of high Tc Calculated Tc: larger than that in the modeled simple-quasi-1 D TMTSF salts. In increasing pressure, 0. 2 0 -0. 2 Band width: larger DOS: stays large due to the tail of van Hove singularity

Fin.