Magnetic field effects on the CDW and SC states in -(BEDT-TTF) 2 KHg(SCN) 4
description
Transcript of Magnetic field effects on the CDW and SC states in -(BEDT-TTF) 2 KHg(SCN) 4
Magnetic field effects on the CDW and SC states in -(BEDT-TTF)2KHg(SCN)4
Dieter Andres, Sebastian Jakob, Werner Biberacher, Karl Neumaier and Mark Kartsovnik
Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, Garching, Germany
Ilya SheikinLaboratoire National des Champs Magnétiques Intenses, Grenoble, France
Harald MüllerEuropean Synchrotron Radiation Facility, Grenoble, France
Natalia KushchInstitute of Problems of Chemical Physics, Chernogolovka, Russia
c
a
-(BEDT-TTF)2KHg(SCN)4: basic featuresS
CHBEDT-TTF molecule:bis(ethylenedithio)-tetrathiafulvalene
a
b
r||(300K) 10 - 20 mW•cm r/r|| ~ 104 - 105 ra/rc 2
r (300K) / r (1.4K) ~ 102 t ||/t 670 ,coh/|| 2.210-6
T. Mori et al., Bull. Chem. Soc. Jpn. 1990;R. Rousseau et al., J. Phys. I (France) 1996;P. Foury-Leylekian et al., PRB 2010
-(BEDT-TTF)2KHg(SCN)4: basic features
2D Fermi surface
CDW formation at 8 K
Nesting instability of the Fermi surface
Q
very low!!
small DCDW kBTCDW high sensitivity to external conditions: pressure, magnetic field
[P. Foury-Leylekian et al., PRB 2010]
Q+
Q-
CDW in a magnetic field
Pauli paramagnetic effect: suppresses CDW [W. Dieterich & P. Fulde, 1973]
2mBB/hvF
B
Q- < Q+
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.5
1.0
1.5
2.0
TCDW/TCDW(0), exp
Phase diagram of -(BEDT-TTF)2KHg(SCN)4
P. Christ, W. Biberacher, M.K., et al., JETP Lett. 2000
~ 23 T
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
TCDW/TCDW(0)
Theory: A. Buzdin & V. Tugushev, JETP 1983 D. Zanchi et al., PRB 1996; P. Grigoriev & D. Lyubshin , PRB 2005
CDWx
CDW0
NM
CDW in a magnetic field
Orbital effect (requires an imperfectly nested FS): stimulates CDW );2cos(2)cos(2)( yyyyFxF katkatkkv ---k Ftt
4( vt + t ’ ) / F
k y
c
k x
~ t’/ v F
-c
)0'(DWDW tTT
CDW in a magnetic field
Orbital effect (requires an imperfectly nested FS): stimulates CDW );2cos(2)cos(2)( yyyyFxF katkatkkv ---k Ftt
4( vt + t ’ ) / F
k y
c
k x
~ t’/ v F
-c
I n a magnetic field:
zFy Bve
dtdk
;
zFyc Bvae
Dy ~ 1/Bz
electrons become effectively more 1D
lB = 2vF/c
Dy =
ay(4
t /
c )
Real space orbit:
)0'(DWDW tTT
0 2 4 6 8 100
5
10
15
20
25
30
2.3 kbar
3.6 kbar
1.8 kbar
0 kbar
B [T
]T [K]
D. Andres, M.K., et al., PRB 2001
-(BEDT-TTF)2KHg(SCN)4
CDW in a magnetic field
Orbital effect (requires an imperfectly nested FS): stimulates CDW
Theory:
D. Zanchi et al., PRB 1996
0 2 4 6 8 100
5
10
15
20
25
30
2.3 kbar
3.6 kbar
1.8 kbar
0 kbar
B [T
]T [K]
D. Andres, M.K., et al., PRB 2001
-(BEDT-TTF)2KHg(SCN)4
CDW in a magnetic field
Orbital effect (requires an imperfectly nested FS): stimulates CDW
Theory:
D. Zanchi et al., PRB 1996
FICDW at t’ > t’ * ???L. Gor’kov & A. Lebed, J. Phys. Lett. (Paris) 1984
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
The “slow oscillations”
appear at P Pc 2.5 kbar
approximately periodic with 1/B
SdHo
display a weak hysteresis
P = 3 kbar
Positions of the FICDW transitions can be fitted with t 0.5 meV
[A. Lebed, PRL 2010]
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
A. Kornilov et al., PRB 2002
FICDW in -(BEDT-TTF)2KHg(SCN)4
FISDW in (TMTSF)2PF6
A. Lebed, JETP Lett. 2003
FICDW is weaker than FISDW due to the paramagnetic effect!
Superconductivity vs. CDW
0.05 0.10 0.150.0
0.5
1.0
3.03.5 4.0
R(
T )/R
(16
0mK
)
T (K)
P [kbar] =
0.05 0.10 0.150.0
0.5
1.0
2 kbar1.5 kbar
0 kbar
R(
T )/R
(30
0mK
)
T (K)
0.05 0.10 0.15
60.0
63.3
66.7
R (
Ohm
)
T (K)
P = 0 kbar; sample #1
Sample #2:zero resistancebut no Meissner!
0.05 0.10 0.150
1
2
3
R (
Ohm
)T (K)
P = 0 kbar sample #2R
(O
hm)
0 1 2 3 4
0.01
0.1
1
10
SC
Normal metalCDW
T ( K
)P ( kbar )
R 0 R = 0
Resistance at zero field
See also: H. Ito et al., SSC 85 1005 (1993) – inhomogeneous superconductivity at P = 0
Superconductivity vs. CDW
0.1 0.2 0.3 0.435
40
45
50
0.1 0.2 0.3 0.445
50
55
60
65
P = 2 kbar0.5 mA
2 mA
0.05 mA
3.0 kbar
R (W)
T (K)
3.5 kbar
R (W)
T (K)
P =
0 2 445
50
55
60
0 2 435
40
45
50
P = 2 kbar
R (W)
B (mT)
180 mK 160 mK 140 mK 120 mK 100 mK
110 mK 100 mK 95 mK
B (mT)
P = 3.5 kbar
R (W)
Onset of superconductivity
The SC onset temperature is 3 times higher in the SC/CDW coexistence region!
Superconductivity vs. CDW
Onset of superconductivity
0 2 40.0
0.2
0.4
T (K
)
P (kbar)
CDWNormal metal
SC
CDW+SC
R = 0R 0
Superconductivity in a magnetic field; P > Pc
Critical field layers
0 20 40 60 80 1000
1
2
3
4
5
6
B (m
T)
T (mK)
3.0 kbar 3.5 kbar 4.0 kbar
at P = 3 kbar: x||(0) 250 nm
cf. mean free path 1mm
Superconductivity in a magnetic field; P > Pc
Critical field // layers
0 20 40 60 80 100 1200
100
200
300
400
Mag
netic
fiel
d (m
T)
Temperature (mK)
R(T)R(H)
GL: Hc2 (Tc-T )
Hp0: Chandrasekhar-Clogston paramagnetic limit
dHc2/dT 12 T/K
x(0) = 1.0 nm d/2;
x||(0)/ x(0) 250!
1.6Hp0
Superconductivity in a magnetic field; P > Pc
-4 -2 0 2 4
10
100
102 mK 90 mK 40 mK
Hc2
(,T
)/H
c2(0
,T )
90o -
Direct manifestation of the paramagnetic pair-breaking!
Summary
0 2 40.0
0.2
0.4
T (K
)
P (kbar)
CDWNormal metal
SC
-4 -2 0 2 4
10
100
102 mK 90 mK 40 mK
Hc2
(,T
)/H
c2(0
,T )
90o -
CDW state:
• rich phase diagram due to the interplay of
competing Pauli paramagnetic and orbital
effects of magnetic field
SC state:
• at P < Pc: coexists with the CDW state; the
SC onset temperature is drastically increased
in the coexistence region;
• at P > Pc: bulk SC state with a highly
anisotropic Hc2 near Tc(0) and a clear
manifestation of paramagnetic pair-breaking
at H // layers.
CDW in a magnetic field
Field-induced density wave transitions, t’>t’*:
B
ky
a y
kx
Q
-a y
kF-kF
N=
0
1
234567
B
Q x
2k F
Q k NG
G ea B
x F = 2 + ,
= /y z
Qx = 2kF + NG,
G = eayBz/
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
2QP = MG
N = 3,4 2,3
1,2
0,1
0
Commensurate splitting (A. Bjelis et al., 1999; A. Lebed, 2003) “Spin-zero”
2QP = (M + 1/2)G
with M - integer
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
N = 5 4 3 2
1
0
4N = 332
1
2
1
0
0
no Pauli effect (FISDW) Pauli effect on (FICDW)
Qx = 2kF + NG Qx = 2kF QP + NG
G = 2eayBz/QP = 2mBB/vF
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
4N = 332
1
2
1
0
0
no Pauli effect (FISDW) Pauli effect on (FICDW)
Qx = 2kF + NG Qx = 2kF QP + NG
A. Lebed, JETP Lett. 78, 138 (2003)
G = 2eayBz/QP = 2mBB/vF
CDW in a magnetic field
Field-induced CDW (FICDW) transitions
m
cos1222/1
F
BP
veaG
QMy
Spin-zero condition:
vF 1.2105 m/s
The SC onset temperature is 3 times higher in the SC/CDW coexistence region!
Superconductivity vs. CDW
Onset of superconductivity
0 2 40.0
0.2
0.4
T (K
)
P (kbar)
CDWNormal metal
SC
CDW+SC
R = 0R 0 Ginzburg-Levanyuk
parameter:
Gi(2) ~ 10-5
Low Tc weak fluctuations!
Superconductivity in a magnetic field; P > Pc
0
20
40
60
0.05 0.10 0.15 0.20
P = 2 kbar
B:
2.9 mT
0 mT
1.5 mT
15.2 mT
4.4 mT
T (K)
R (W
)
0
20
40
60
0 5 10 15
112 mK80 mK60.8 mK44 mK27 mK23 mK
Bk
P = 2 kbar
B (T)
R
(W)
B (mT)
Critical field layers
Superconductivity in a magnetic field; P > Pc
Critical field // layers
0.04 0.06 0.08 0.10 0.120
5
10
15
20
R (
Ohm
)
T (K)
m0H (mT):
270150210
60
0
Tc