1 A. Buzdin University of Bordeaux I and Institut Universitaire de France Non-uniform (FFLO) states...
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A. BuzdinUniversity of Bordeaux I and Institut Universitaire de France
Non-uniform (FFLO) states and quantum oscillations in superconductors and
superfluid ultracold Fermi gases
ECRYS-2011, August 15-27, 2011 Cargèse , France
in collaboration with L. Bulaevskii, J. P. Brison, M. Houzet, Y. Matsuda, T. Shibaushi, H. Shimahara, D.
Denisov, A. Melnikov, A. Samokhvalov
1. Singlet superconductivity destruction by the magnetic field:
- The main mechanisms - Origin of FFLO state.
2. Experimental evidences of FFLO state.
3. Exactly solvable models of FFLO state.
4. Vortices in FFLO state. Role of the crystal structure.
5. Supefluid ultracold Fermi gases with imbalanced state populations: one more candidate for FFLO state?
Outline
1. Singlet superconductivity destruction by the magnetic field.
• Orbital effect (Lorentz force)
B
-pFL
p
FL
• Paramagnetic effect (singlet pair)
Sz=+1/2 Sz=-1/2
μBH~Δ~Tc cTsSI
Electromagneticmechanism
(breakdown of Cooper pairsby magnetic field
induced by magnetic moment)
Exchange interaction
Normal
MeissnerT
B
Hc2
Tc
Abrikosov Lattice
Hc2 0
2 2
Vortex
Orbital effect
(x coherence length)
Flux quantum
(l penetration length )
F0=hc/2e=2.07x10-7Oe ・ cm2z
B
D
Vortex lattice in NbSe2
(STM)
B
-pFL
p
FL
Pc
orbc
H
H
2
22 1~
F
Superconductivity is destroyed by magnetic field
Orbital effect (Vortices)
Zeeman effect of spin (Pauli paramagnetism)
1
2N H 2
1
2N(0)2
N 1
2(g B )2 N (0) B
Pc g
H
2
2
20
2 2
orbcH
Maki parameter
Usually the influence of Pauli paramagnetic effect is negligibly small
B
-pFL
p
FL
Superconducting order parameter behavior under paramagnetic effect
Standard Ginzburg-Landau functional:
...2222 aF
The minimum energy corresponds to Ψ=const
The coefficients of GL functional are functions of the Zeeman field h= μBH !
Modified Ginzburg-Landau functional ! :
422
24
1
b
maF
The non-uniform state Ψ~exp(iqr) will correspond to minimum energy and higher transition temperature
q
F
q0
242 )( qqqaF
Ψ~exp(iqr) - Fulde-Ferrell-Larkin-Ovchinnikov state (1964).Only in pure superconductors and in the rather narrow region.
FFLO inventors
Fulde and Ferrell
Larkin and Ovchinnikov
E
kkF +dkF
kF -dkF
The total momentum of the Cooper pair is -(kF -dkF)+ (kF -dkF)=2 dkF
ky
kx
E
Conventional pairing
FFLO pairing
( k ,-k )
( k ,-k+q )
pairing between Zeeman split parts of the Fermi surface
Cooper pairs have a single non-vanishing center of mass momentum
k
-k
-k k
ky
kx
E
q
q~gmBH/vF
-k+q
q
-k+q
k
k
Pairing of electrons with opposite spins and momenta unfavourable :
But :
• the upper critical field is increased
• Sensivity to the disorder and to the orbital effect:
(clean limit)
At T = 0, Zeeman energy compensation is exact in 1d, partial in 2d and 3d.
if
0.2 0.4 0.6 0.8 1cTT /0.56
0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
cTT /
/HB1d SC2d SC3d SC
0.56
FF state uniform
(r) (q)exp(iqr)
LO state spatially nonuniform
(r) 0 cos(qr)
~1
q
+
+
+
The SC order parameter performs one-dimensional spatial modulations along H, forming planar nodes
available for pairing
depaired
( k ,-k )
( k ,-k+q )
May be 1st order transition at
0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
cTT /
/BB
0.56
...62*222*
2242222
aF
Modified Ginzburg-Landau functional :
2. Experimental evidences of FFLO state.
• Unusual form of Hc2(T) dependence• Change of the form of the NMR
spectrum• Anomalies in altrasound absorbtion• Unusual behaviour of magnetization• Change of anisotropy ….
CSH or D
CSH or D
BEDT-TTF (donor molecule)
BEDT-TTF layer
Cu[N(CN)2]Br layer
15 Å
k-(BEDT-TTF)2Cu(NCS)2 (Tc=10.4K)Organic superconductor
Layered structure Suppression of orbital effects in H parallel to the planes
Talk of Stuart Brownabout FFLO in this
compound!
18
Anomalous in-plane anisotropy of the onset of SC in
(TMTSF)2ClO4
S.Yonezawa, S.Kusaba, Y.Maeno, P.Auban-Senzier, C.Pasquier, K.Bechgaard, and D. Jerome, Phys. Rev. Lett. 100, 117002 (2008)
Field induced superconductivity (FISC) in an organic compound
S. Uji et al., Nature 410 908 (2001)
L. Balicas et al., PRL 87 067002 (2001)
FISC
Metal
Insulator AF
-(BETS)2FeCl4
c-axis (in-plane) resistivity
Other example:
Temperature [K]
Cri
tica
l fie
ld H
c2 [
Tesl
a]
Jaccarino-Peter effect
Eu-Sn Molybdenum chalcogenide
H. Meul et al, 1984
Zeeman energy Exchange energy between conduction electrons in the BETS layers and magnetic ions Fe3+ (S=5/2)
For some reason J > 0 : the paramagnetic effect is suppressed at
S
S(Eu0.75Sn0.25Mo6S7.2Se0.8)
Strong evidence of inhomogeneous FFLO phasein CeCoIn5
2nd order
2nd
1st
1st
H// ab
H// c
H-T phase diagram of CeCoIn5
Pauli paramagnetically limited superconducting state
New high field phase of the flux line lattice in CeCoIn5
This 2nd order phase transition is characterized by a structural transition of the flux line lattice
Ultrasound and NMR results are consistent with the FFLO state which predicts a segmentation of the flux line lattice
24
In ferromagnet ( in presence of exchange field) the equation for superconducting order parameter is different
042 aIts solution corresponds to the order parameter which decays with oscillations! Ψ~exp[-(q1 ± iq2 )x]
x
Ψ
Order parameter changes its sign!
Wave-vectors are complex!They are complex conjugate and
we can have a real Ψ.
Proximity effect in a ferromagnet ?
In the usual case (normal metal):
4maq where, is TTfor solution and ,04
1c
2 qxem
a
Many new effects in
S/F heterostructures!
25
Remarkable effects come from the possible shift of sign of the wave function in the ferromagnet, allowing the possibility of a « π-coupling » between the two superconductors (π-phase difference instead of the usual zero-phase difference)
SF
S
S F
« phase » « 0 phase »
S F S
S/F bilayer nmhD ff )101(/
h-exchange field,Df-diffusion constant
26
S-F-S Josephson junction in the clean/dirty limit
S SF
Damping oscillating dependence of the critical current Ic as the function of the parameter =hdF /vF has been predicted.
(Buzdin, Bulaevskii and Panjukov, JETP Lett. 81)
h- exchange field in the ferromagnet,dF - its thicknessIc
E(φ)=- Ic (Φ0/2πc) cosφ
J(φ)=Icsinφ
27
Critical current density vs. F-layer thickness (V.A.Oboznov et al., PRL, 2006)Collaboration with V. Ryazanov group from ISSP, Chernogolovka
-stateI=Icsin
I=Icsin(+ )= - Icsin()
Nb-Cu0.47Ni0.53-Nb junctions
Ic=Ic0exp(-dF/F1) |cos (dF /F2) + sin (dF /F2)|
dF>> F1
“0”-state
F2 >F1
“0”-state-state
0
Spin-flip scattering decreases thedecaying length and increases the oscillation period.
28
Cluster Designs (Ryazanov et al.)
2 x 2
6 x 6
fully-frustrated checkerboard-frustrated
fully-frustrated unfrustrated checkerboard-frustrated
30m
29
T
T = 1.7K T = 4.2KT = 2.75K
Scanning SQUID Microscope images(Ryazanov et al., Nature Physics, 2008))
T
Ic
FFLO State in Neutron Star
R.Casalbuoni and G.Nardulli Rev. Mod. Phys. (2004)
Glitches
Color superconductivity Bose-Einstein-Condensate
Vortices
Supefluid ultracold Fermi gases with imbalanced state populations: one more candidate for FFLO state?
Massachusetts Institute of Technology: M.W. Zwierlein, A. Schirotzek, C. H. Schunck, W.Ketterle (2006)
Rice University, Houston: Guthrie B. Partridge, Wenhui Li, Ramsey I. Kamar, Yean-an Liao, Randall G. Hulet (2006)
3. Exactly solvable models of FFLO state.
FFLO phase in the case of pure paramagnetic interaction and BCS limit
Exact solution for the 1D and quasi-1D superconductors ! (Buzdin , Tugushev 1983)
• The FFLO phase is the soliton lattice,
first proposed by Brazovskii, Gordyunin and Kirova in 1980 for polyacetylene.
0.2
0.4
0.6
0.8
1
cTT /
/HB1d SC
0.56
),/()( 0 kxsnx
20 HTat B
x)(x
Magnetic moment
Spin-Peierls transitions - e.g. CuGeO3
TSP=14.2 K
Cu
O Ge
Chains direction
)()1( naxu nn
In 2D superconductors
( k ,-k )
Y.Matsuda and H.Shimahara J.Phys. Soc. Jpn (2007)
In 3D superconductors
The transition to the FFLO state is 1st order. The sequence of phases is similar to 2D case. Houzet et al. 1999; Mora et al. 2002
4. Vortices in FFLO state. Role of the crystal structure.
FFLO phase in the case of paramagnetic and orbital effect (3D BCS limit) – upper critical field
Lowest m=0 Landau level solution, Gruenberg and Gunter, 1966
Pc
orbc
H
H
2
22 FFLO exists for Maki parameter α>1.8.
For Maki parameter α>9 the highest Landau level solutions are realized – Buzdin and Brison, 1996.
Note : The system with elliptic Fermi surface can be tranformed by scaling transformation to ihe isotropic one.Sure the direction of the magnetic field will be changed.
FFLO phase in 2D superconductors in the tilted magnetic field - upper critical field
Highest Landau level solutions are realized – Bulaevskii, 1974; Buzdin and Brison, 1996; Houzet and Buzdin, 2000.
q
B
Exotic vortex lattice structures in tilted magnetic field
Near the tricritical point, the characteristic length is
Microscopic derivation of the Ginzburg-Landau functional :
Instability toward FFLO state
Instability toward 1st order transition
Validity:
• large scale for spatial variation of : vicinity of T *
small orbital effect, introduced with
• we neglect diamagnetic screening currents (high- limit)
Generalized Ginzburg-Landau functional
Next orders are important :
• 2nd order phase transition at
higher Landau levels
• Near the transition: minimization of the free energy with solutions in the form
gauge
Parametrizes all vortex lattice structures at a given Landau level N
is the unit cell
All of them are decribed in the subset :
__ 1st order transition__
2nd order transition
__ 2nd order transition in the paramagnetic limit
Temperature
-2 -1 0
Mag
netic
fie
ld
0
1
2
3
Tricritical point
n=1
n=0
n=2Landau levels
Triangular lattice
squareTriangular
Rectang.
Rectangular
Analysis of phase diagram :
• cascade of 2nd and 1st order transitions between S and N phases
• 1st order transitions within the S phase
• exotic vortex lattice structures
At Landau levels n > 0, we find structures with several points of vanishment of the order parameter in the unit cell and with different winding numbers w = 1, 2 …
43
Order parameter distribution for the asymmetric and square lattices with Landau level n=1.
The dark zones correspond to the maximum of the order parameter and the white zones to its minimum.
44
Intrinsic vortex pinning in LOFF phase for parallel field orientation
Δn= Δ0cos(qr+αn)exp(iφn(r))
t – transfer integral
Josephson coupling between layers is modulated
Fn,n+1=[-I0cos(αn_- αn+1)+I2cos(qr) cos(αn_+ αn+1)] cos(φn- φn+1)
φn- φn+1=2πxHs/Φ0 + πn s-interlayer distance, x-coordinate along q
Quasi-2D heavy fermion
(Tc=2.3K)
Strong antiferromagnetic
fluctuation
d-wave symmetry
Tetragonal symmetry
CeIn3
CoIn2
CeIn3
CeCoIn5
z
x yisotropic part
Modified Ginzburg-Landau functional:
jjj iAc
e
2
z
=/2xy-plane modulation
=0.5 arccos(z/(-
z))
~
=0z-axis modulation
=2z
~
~
No orbital effect
Modulation diagram in the case of the absence of the orbital effect (pure paramagnetic limit).
Areas with different patterns correspond to different orientation of the wave-vector modulation. The phase diagram does not depend on the εx value.
),~( z
Magnetic field along z axis
Modulation diagram in the case when the magnetic field applied along z axis. There are 3 areas on the diagram corresponding to 3 types of the solution for modulation vector qz and
Landau level n. Modulation direction is always parallel to the applied field and εx here is treated as a constant.
z
=2z
qz>0, n>0
qz=max, n=0
qz=0, n=max
z=
x
=x
~
~
~
H||z
Magnetic field along z axis
z
=2z
qz>0, n>0
qz=max, n=0
qz=0, n=max
z=
x
=x
~
~
~
z
=/2xy-plane modulation
=0.5 arccos(z/(-
z))
~
=0z-axis modulation
=2z
~
~
z
z
intermediate
xy
xy
intermediate
Magnetic field along x axis
Modulation diagram (ἕ, εz) in the case when the magnetic field is applied along x axis. There are three areas on the diagram corresponding to different types of the solution for modulation vector qx and Landau level n. Modulation direction is always parallel to the applied field. The choice of the intersection point is
determined by the coefficient εx.
z
qx=max, n=0
qx>0, n>0
qx0, n=max
=-x
~
=3z-2
x
~
=-3z
~ ~
H||x
Magnetic field along x axis
z
qx=max, n=0
qx>0, n>0
qx0, n=max
=-x
~
=3z-2
x
~
=-3z
~ ~
z
=/2xy-plane modulation
=0.5 arccos(z/(-
z))
~
=0z-axis modulation
=2z
~
~
z
z
intermediate
xy
xy
intermediate
Small angle neutron scattering from the vortex lattice for H //c
A.D.Bianch et al.Science (2007)
FFLO? Neutron form factor
Neutron form factor seems to be consisitent with FFLO state
The crystal structure effects influence on the FFLO state is very important.
The FFLO states with higher Landau level solutions could naturally exist in real 3D compounds (without any restrictions to the value of Maki parameter).
Wave vector of FFLO modulation along the magnetic field could be zero.
In the presence of the orbital effect the system tries in some way to reproduce optimal directions of the FFLO modulation by varying the Landau level index n and wave-vector of the modulation along the field.
5. Supefluid ultracold Fermi gases with imbalanced state populations: one more candidate for FFLO state?
Massachusetts Institute of Technology: M.W. Zwierlein, A. Schirotzek, C. H. Schunck, W.Ketterle (2006)
Rice University, Houston: Guthrie B. Partridge, Wenhui Li, Ramsey I. Kamar, Yean-an Liao, Randall G. Hulet (2006)
Experimental system: Fermionic 6Li atoms cooled in magnetic and optical traps (mixture of the two lowest hyperfine states with different populations)
Experimental result: phase separation
Supefluid core
Normal gas
rf transitions76 MHz
hyperfine states
Rotating supefluid ultracold Fermi gases in a trap
MIT: Ketterle et al (2005)
Fermion condensate
Laser beams
Coils generating magnetic field
Images of vortex lattices
Vortex as a test for superfluidity
Questions:
1. What is the effect of confinement (finite system size) on FFLO states?
2. Effect of rotation on FFLO states in a trap (effect of magnetic field on FFLO state in a small superconducting sample).
3. Possible quantum oscillation effects.
Multiply-connected systemsSuperconducting thin-wall cylinder
Multiquantum vortices
Superconductor with a columnar defect or hole
A.Bezryadin, A.I. Buzdin, B. Pannetier (1994)
Little-Parks effect. Switching between the vortex states.
Ф/Фo
-1 1
0
L=-1 L=0 L=1
-ΔTc/Tc0
Ф/Фo
Tc (H) oscillations
sv
e
c 0
Examples of quantum oscillation effects.
Tc (H) oscillations
Mesoscopic samples dimensions ~ several coherence lengths
Simply-connected systemsO.Buisson et al (1990)R.Benoist, W.Zwerger (1997)V.A.Schweigert, F.M.Peeters (1998)H.T.Jadallah, J.Rubinstein, Sternberg (1999)
H
Origin of Tc oscillations: Transitions between the states
with different vorticity L
Examples of quantum oscillation effects.
Little-Parks effect.
iLer || L - Vorticity (orbital momentum)
R~ξ
Multiquantum vortices
FFLO states and Tc(H) oscillations in infinite 2D superconductors
A.I. Buzdin, M.L. Kulic (1984)
Examples of quantum oscillation effects.
Hz
H|| ~ Hp
200k
H za
Model: Modified Ginzburg-Landau functional (2D)
Trapping potential
dxdyDDrVaF )||||||))((( 2222
2)( 22rMrV
H
T
S
N
FFLO
Range of validity: vicinity of tricritical
point
FFLO instability
Confinement mechanisms:
1. Zero trapping potential. Boundary condition at the system edge
2. nonzero trapping potential
],[2 riMD
cAieD
2
0Dn
)( 0cTTa
FFLO states in a 2D mesoscopic superconducting disk
Hz
r
z H|| ~ Hp
R~ξ
Interplay between the system size, magnetic length, and FFLO length scale
Perpendicular magnetic field component Hz = 0
Eigenvalue problem:
Wave number of FFLO instability
The critical temperature:
H - T Phase diagram: Hz = 0
T
H
L=1
L=2L=0
FFLO state
H ↑ → L↓
),(2 0 THk
Tilted magnetic field: Hz ≠ 0
Eigenvalue problem:
0
2
z
a
HR
Field induced superconductivity
Tilted magnetic field: Hz ≠ 0
Transitions with large jumps in vorticity
Vortex solutions beyond the range of FFLO instability. Critical field of the vortex entry.
Hz
r
z H|| ~ Hp
dxdyDDF )||||( 2242
221
T
S
N
FFLO
Beyond the range of FFLO
instability
21 We focus on the limit
Condition of the first vortex entry:
0)0()1( LFLF
iLe(H*,T*)
||H
Condition of the first vortex entry:
0ln432
ln 322
2
m
m
m
R
R
R
),max( 21 m
0
2
RH z
21
2
42
m
Limiting cases:
12 1
lnR
1m
R
32
2
R
12
ln1
R
RH z
341
RH z
Change in the scaling law
12
||* HH
FFLO states in a trapping potential
Interplay between the rotation effect, confinement, and FFLO instability
FFLO states in a 2D system in a parabolic trapping potential (no rotation)
0)(2 20
24 v
20 k40ka
60
20 2/ kMv
FFLO length scale
Temperature shift
(Trapping frequency)2
Fourier transform:
qde qqi
2
rk
0 Dimensionless coordinate
242
2
0 2qqq
v
xq 1224 412)( xqqqU
2xq e
0
1
v
q
)(qU
0L
00 v
FFLO states in a 2D system in a parabolic trapping potential (no rotation)
Phase diagram
021 v
0v
1
Condensate wave function
40
0
022
0)4cos( vrkerkrk
10 rk
rk0
Suppression of wave function oscillations by the increase in the trapping frequency
12
4/10
v
=Number of observable oscillations
FFLO states in a rotating 2D gas in a parabolic trapping potential.
0)(2 20
24 LLL fvfDfD
2
2 1
a
LD
0
)()(n
nLnL ucf
nLanLnLnL uLLnuquD )1||2(222
Expansion in eigenfunctions of the problem without trap:
nm
mnmnnLnL ccvcqq )2( 42
0
30 duuvv nLmLnm
20 k40ka
60
20 2/ kMv
202 kMa
FFLO length scale
Temperature shift
(Trapping frequency)2
rotation frequency
iLL erfr )()(
rk
0 Dimensionless coordinate
2200
0)12(4)12)(4(max
LLvLv aaaa
L
FFLO states in a rotating 2D gas in a parabolic trapping potential.Suppression of quantum oscillations by the increase in the trapping frequency.
First-order perturbation theory:
rotation induced superfluid phase
73
Conclusions
• There are strong experimental evidences of the existence of the the FFLO state in organic layered superconductors and in heavy fermion superconductor CeCoIn5
• FFLO –type modulation of the superconducting order parameter plays an important role in uperconductor-ferromagnet heterostructures. The p-junction realization in S/F/S structures is quite a general phenomenon.
• The interplay between FFLO modulation and orbital effect results in new type of the vortex structures, non-monotonic critical field behavior in layered superconductor in tilted field.
• Special behavior of fluctuations near the FFLO transition – vanishing stiffness.
• FFLO phase in ultracold Fermi gases with imbalanced state populations?