会社名など
E. Bauer et al, Phys. Rev. Lett. 92. 027003 (2004)
M. Yogi et al. Phys. Rev. Lett. 93, 027003 (2004)
Kitaoka Laboratory
Takuya Fujii
Unconventional superconductivity in Noncentrosymmetric Heavy-
Fermion superconductors
2
Content
Introduction
Heavy Fermion & Fermi Liquid
Symmetry of SC state
Inversion symmetry
Experimental Data in CePt3Si
Summary
3
Heavy Fermion & Fermi Liquid
conduction electron
4f-electrons localize at atom.
Magnetic order Fermi liquid state by heavy electrons
Kondo effect
conduction electron
rare earth ion
RKKY interaction
4f-electrons are itinerant.
Introduction
4
Phase diagram of heavy fermion systems
Unconventional SC around QCP mediated by magnetic fluctuations.
Tem
per
atu
re (
K)
0
Pressure
QCPMagnetic order
Non Fermi liquid
Fermi Liquid
Introduction
5
Pair function
)',( )()( ' ' ssχkΦcckF skskSS
orbital spin
Spin part Orbital part
Singlet
(S=0)
antisymmetric symmetric (s,d..)
Triplet
(S=1)
symmetric antisymmetric (p,f..)
)( )( kΦkΦ
)( )( kΦkΦ
Introduction
BCS-SC High-TC cuprateCe based HF-SC
UPt3,Sr2RuO4
6
Centrosymmetric vs Noncentrosymmetric H-F SC
Pt(1)
Pt(2)
CePt3SiCeRhSi3
Centrosymmetric Noncentrosymmetric
Cooper pair
materials (ex)CeCu2Si2 CePt3Si, UIr, CeRhSi3
CeCu2Si2
Si
Cu
Ce
Introduction
±
7
Noncentrosymmetric CePt3Si
the first heavy fermion SC without a center of symmetry
1) SC ; cooper pairs formed by heavy quasiparticles (TC=0.75K)
2) AFM ; coexistence with SC on a microscopic scale (TN=2.2K)
3) High Hc2(0)4) Anomalous behavior in 1/T1 of NMR
Pt(1)
Pt(2)
Novel SC state caused by lack of inversion symmetry ?
key experimental features
8
Resistivity & susceptibility
TN=2.2K
TC=0.75K
T.Yasuda et al., J.Phys Soc.Jpn.73, 1657 (2004)
Coexistence of SC & AFM
9
Specific heat Cp
TC=0.75K
TN=2.2K
Cp/T=γ=390 [mJ/molK2] m∝ *
λ- like anomaly at 2.2 K :
onset of long range magnetic order
jump around 0.75K:
the transition into a SC phase
C/T =γ+AT2
Coexistence of SC & AFM
10
Microscopic evidence of AFM & SC
N.Metoki ,J.Phys.: Cond. Mat 16 L207 (2004)
Ce
Coexistence of SC & AFM on a microscopic scale
Pt(2)
11
H(T)-T(K) phase diagram
Hc2(0) 5T > > Hp 1THp =1.84* TC
HP: Pauli-Clogston limiting field
H=0 H≠0
Spin-Singlet H=HP
High Hc2(0)
SC
P-C Limit
12
0.1
1
(1/T
1T )
/ (1/
T 1T)
TC
0.2 1T / TC
LaPt3Si (La-NQR)
BCS fitting2/kBTC = 3.6
CePt3Si ( ~ 1T)BW
2/kBTC = 4
line-node2/kB
195Pt nuclear spin-relaxation rate 1/T1
LaPt3Si (La-NQR)s-wave
CePt3Si (~1T)BW (full gap)
p-wave
LaPt3Si ;BCS s-wave model is applicable.
CePt3Si ;Small jump at Tc.However, overall T dependence is different from p-wave & BCS s-wave.
Unusual T dependence of 1/T1T ⇒ s+p-wave pairing SC ?
13
Noncentrosymmetric heavy fermion Ce-compounds
T (
K)
0
Pressure
AFM
SC
CePt3SiCeRhSi3
Pt(1)
Pt(2)
CePt3Si
CeRhSi3: Pressured-induced SC
14
Noncentrosymmetric heavy fermion Ce-compounds
CePt3Si CeIrSi3 CeRhSi3
TN(K)
(P=0)
2.2 5.0 1.6
TC(K) 0.75 1.6
(2.5GPa)
0.85
(1.62GPa)
HC2(T) 5 11.1
(2.5GPa)
7
(1.62GPa)
γ(mJ/molK2)
(P=0)
390 120 120
Novel SC states caused
by lack of inversion symmetry & high HC2(0) !?
High Hc2(0)
Fe26 Co27 Ni28
Ru44 Rh45 Pd46
Os76 Ir77 Pt78
15
Summary
The SC exists even in the AFM state in CePt3Si.
Novel cooper pairing state with the two-component order
parameter composed of spin-singlet and spin-triplet pairing
components may be realized in CePt3Si.
Noncentrosymmetric heavy fermion Ce-compounds may be
thought of as unconventional SC from the aspect of high
HC2(0).