under High Pressure

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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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

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

b

-
(BEDT
-
TTF)
2
ICl
2

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
G

point:

shift downward under pressures

→ large DOS at E
F

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
E
F
:

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








i
i
i
eff
ks
ks
ks
ks
n
n
U
c
c
H

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 E
F
: van
-
Hove singularity

near E
F


Fermi surface: 1D→2D

(Original crystal structure: not square)

0.2

-
0.2

0

Method

Approximation to include effects of Coulomb interaction:

fluctuation exchange (FLEX)

Self
-
energy=











Antiferromagnetism: Stoner criterion

Superconductivity: (linearized) Eliashberg equation








i
i
i
eff
ks
ks
ks
ks
n
n
U
c
c
H



+





























+

Results

c.f. Exp.

AF

SC

rapid increase of
T
N

(P<4GPa)
---

1D suppress the AF order

broad peak of
T
N

(P=6GPa)
---

nesting vector =(
p
,0)

decrease of
T
N

(P>8GPa)
---

1D→2D, dimensional crossover, worse nesting

shoulder of
T
N

(P~10GPa)
---

nesting vector (commensurate→incommensurate)

emergence of SC (P>14GPa)
---

origin AF fluctuation

AF: antiferromgetism

SC: superconductivity

SC order parameter

SC order

Fermi surfaces

SC order: singlet
d
xy,
, no triplet


effects of
U
: Fermi surface
nests better

(p,0)

G

+

-

+

-

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
T
c

In increasing pressure,


Band width: larger

DOS: stays large due to the tail of
van Hove singularity

Calculated

T
c
: larger than that in
the modeled simple
-
quasi
-
1D
TMTSF salts.

0.2

-
0.2

0

Fin.