スライド 1

(Review Article)
Nature Physics Volume: 6, Pages: 645–658 Year published: (2010)
10.1038/NPHYS1759
Kitaoka Lab.
Mariko Nitta
Contents
• Introduction
- Discovery of Fe-pnictide superconductivity
- Four structures of Fe-pnictide superconductivity
- Common and different features between Fe-pnictide and cuprate
- Phase diagram of Fe-pnictide superconductivity
- Superconducting energy gap and its symmetry
• Experiments & Results
- Jump in electronic specific heat at SC transition
- Thermal conductivity
- Relaxation rate 1/T1
• Summary
Introduction
Discovery of Fe-pnictide superconductivity in 2008
Fe:ferro magnetic element
Fe
As
O
La
1111 system
LaFeAsO1-y
Tcmax～28 K
Superconductivity
magnetism
Introduction
Four structures of Fe-pnictide superconductivity
all systems have
FeAs layer
distance
between
FeAs layers
Block layer
11 system
111 system
122 system
1111 system
FeSe
LiFeAs
BaFe2As2
LaFeAsO1-y
Introduction
Common and different features between Fe-pnictide and cuprate
FeAs layer
(1)2D layered structure
(2)electron/hole doping causes
superconductivity
(3)non-doped state shows magnetism
CuO2 layer
122 system
BaFe2As2
La2-xSrxCuO4
(LSCO)
E
E
electron doping
εF
k
multi band
parent compound
Fe-pnictide ; metal
Cuprate ; Mott insulator
εF
k
single band
Introduction
Fe-pnictide superconductivity
BaFe2As2
Tetragonal
Tc(K)
Tc(K)
Tetragonal
AFM
Orthor
AFM
Ba1-xKxFe2As2
BaFe2-xCoxAs2
BaFe2As2-xPx
T0
Orthor
SC
apply pressure
SC
doping level
Lattice shrinking &
electron/hole doping
Tetragonal
Superconductivity appears
Orthorhombic
Introduction
superconducting energy gap and its symmetry
  ei
E
εF
ele
hole
multi band
  ei
nodal s+-wave
k
s+-wave
  ei
d wave
Sign changing between hole band and electron band
Experiment (1)
Jump in electronic specific heat at SC transition
S
SN
SS
Tc
C
CS
CN
Tc
Energy gap : large
→Jump in specific heat at SC transition temperature Tc
C T
S
T
Results (1)
Jump in electronic specific heat at SC transition
K-doped Ba122
gap size : large
⊿C/Tc(mJ mol-1K-2)
2L0 / kBTc ~ 7.5
Tc increases with gap size
Co-doped Ba122
gap size : small
2L0 / kBTc ~ 5.0
Tc(K)
L
BCS type SC 20 / kBTc ~ 3.52
Experiment (2)
Thermal conductivity
F
HOT
HEAT
conduction electron
COOL
Carrier : electron and phonon
Conduction electron is responsible for the thermal conductivity
Thermal conductivity measurement at 0K
electron state at Fermi level
Experiment (2)
Thermal conductivity ～Doppler shift～
D E 
Full gap
NS(E) full gap
E  E  vs p
vs p
N0
EF EF +Δ0
NS(E) nodal gap
D E 
nodal gap
F
E
F
E
vs p
N0
EF EF +Δ0
Results (2)
Ni-doped
10%
Co-doped
K-doped
4.8%
Co-doped
Magnetic field (H/Hc2)
E  E  vs p
Thermal conductivity (normalized)
Thermal conductivity (normalized)
Thermal conductivity
KFe2As2
Co-doped
P-doped
residual DOS at εF
particular for nodal ga
Magnetic field (H/Hc2)
Full gap
vs p
nodal gap
Experiment (3)
Relaxation rate 1/T1 by NMR
H0
What isT1??
T1 ~ spin-lattice relaxation time
I
e
Release the energy
H0
spin-lattice interaction
nuclear spin
Energytransfer
electronic spin
Time constant T1
1/T1 measurement is a good probe for Fermi surface !
Results (3)
compare 1/T1 in1111,122 K-doped,122 P-doped
“Ba122 K-doped”
“La1111”
Fe
Ba
Fe
As
As
O
La
K
“Ba122 P-doped”
Ba
Fe
As
P
Results (3)
compare 1/T1 in1111,122 K-doped,122 P-doped
“Ba122 K-doped”
“La1111”
1000
100
10
LaFeAsO0.7 (Tc = 24 K)
57
Fe-NMR
H = 6.309 T
Tc
57
Ba0.6K0.4Fe2As2
75
-1
75As-
NQR
0.1
～ T33
~T
0.01
2-D line node
57
-1
1/T1 ( s )
～T
1
As-NMR
100
FeNMR
1/T1 [sec ]
“Ba122 P-doped”
Fe-NMR
10
1
~T
3
~T5
0.1
2D/kBTc = 5
1E-3
10
100
0.01
10
100
Temperature [K]
T(K)
two-gapped type s wave
(unconventional SC)
nodal two gap
Possible Scenario
Ba1-xKxFe2As2
LaFeAsO
BaFe2As2-xPx
(LaFePO)
nodal two gap
Full gap
As
Fe
P
Fe
As and P-height relate to gap symmetry???
Summary
mechanism of Fe-pnictide superconductors ？
gap symmetry … full gap s+- type
or
nodal gap s+-type
Full gap
La1111
Tcmax~28K
nodal two gap
E
electronic structure is described by εF
multi-band picture
Ba122 P-doped
Tcmax~30K
k

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