Phase Diagram of Phase Diagram of ’-(BEDT-TTF)’-(BEDT-TTF)22IClICl22 under High Pressure under High Pressure

based on the based on the First-Princples Electronic Structur Structure e

Hiori Kino, Hiroshi Kontani and Tsuyoshi MiyazakiJ. Phys. Soc. Jpn., 73, 25 (2004).

Experimental phase diagram

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

’-(BEDT-TTF)2ICl2

Taniguchi et al.

Electronic structure: First-Principles result

0GPa

4GPa

8GPa

12GPa

HOMO, HOMO-1: the HOMO of BEDT-TTF molecule

Pressure → increase dimensionality of the Fermi surface

van-Hove singularity at point: shift downward under pressures→ large DOS at EF

0.5eV

(Miyazaki)

Phys. Rev. B 68, 220511 (2003)

Purpose?

Understanding of

the phase diagram

origin of the superconductivity

origin of the high transition temperature

A Model

A tight binding Hamiltonian (Hubbard 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

iiieff

ksksksks nnUccH Only the HOMO band

and effective on-site Coulomb interaction(a dimer model)

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

(Original crystal structure: not square)

0.2 -0.20

Method

Approximation to include effects of Coulomb interaction: fluctuation exchange (FLEX)

Self-energy=↑

↓↓

↑

↑

Antiferromagnetism: Stoner criterionSuperconductivity: (linearized) Eliashberg equation

iiieff

ksksksks nnUccH

+

↑

↑

↑

↓

↓

↓

↑↓

↑

↑

↑

↓

↓

↓

+

Resultsc.f. Exp.

AF

SC

rapid increase of TN (P<4GPa) --- 1D suppress the AF orderbroad peak of TN (P=6GPa) --- nesting vector =(,0)decrease of TN (P>8GPa) --- 1D→2D, dimensional crossover, worse nestingshoulder of TN (P~10GPa) --- nesting vector (commensurate→incommensurate)emergence of SC (P>14GPa) --- origin AF fluctuation

AF: antiferromgetismSC: superconductivity

SC order parameter

SC orderFermi surfaces

SC order: singlet dxy,, no triplet

effects of U: Fermi surface nests better

+

-+

-0

0

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

Comparison of FS

12GPa

DFT (Miyazaki) Tight binding model

12GPa

Possible origin of high Tc

In increasing pressure,

Band width: largerDOS: stays large due to the tail of van Hove singularity

Calculated Tc: larger than that in the modeled simple-quasi-1D TMTSF salts.

0.2 -0.20

Fin.