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.
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