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Hiori Kino, Hiroshi Kontani and Tsuyoshi Miyazaki

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.

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Hiori Kino, Hiroshi Kontani and Tsuyoshi Miyazaki

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  1. Phase Diagram of b’-(BEDT-TTF)2ICl2under High Pressure based on the First-Princples Electronic Structure Hiori Kino, Hiroshi Kontani and Tsuyoshi Miyazaki J. Phys. Soc. Jpn., 73, 25 (2004).

  2. Experimental phase diagram b’-(BEDT-TTF)2ICl2 Onset 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: unknown Taniguchi et al.

  3. (Miyazaki) Electronic structure: First-Principles result 0.5eV Phys. Rev. B 68, 220511 (2003) 0GPa HOMO, 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 EF 4GPa 8GPa 12GPa

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

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

  6. Electronic structure Tigiht 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: 1D→2D 0 -0.2 0.2 (Original crystal structure: not square)

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

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

  9. 0 + - (p,0) - 0 + SC order parameter Fermi surfaces SC order SC order: singlet dxy,, no triplet effects of U: Fermi surface nests better G

  10. Problems • Theory: 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

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

  12. Possible origin of high Tc Calculated Tc: larger than that in the modeled simple-quasi-1D TMTSF salts. In increasing pressure, Band width: larger DOS: stays large due to the tail of van Hove singularity 0 -0.2 0.2

  13. Fin.

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