Transcript Magneto-optical imaging of Superconductors Satyajit S .Banerjee Dept of Physics,

Magneto-optical imaging of
Superconductors
Satyajit S .Banerjee
Dept of Physics,
Indian Institute of Technology,
Kanpur, India
Principle of operation of MO
imaging
• Faraday Effect:
M
Light source
F = V Bz d
M
P A
P
A
M
Polariser
d
Z
Analyser
X
Y
Z
Transmission Mode
Reflection Mode MO
Polarized light
F = V Bz 2d
MO active layer
GGG
d
M
Sample
Protective layer
Z
Y
X
Reflecting layer
Types of MO active layers
• Type of MO active layer depends on the type
of experiments.
3
10
2
10
d
YIG
T(K)
EuTe
1
10
EuSe
0
10
10
-3
-2
10
-1
10
B(T)
0
10
1
10
MO imaging setup
Choice YIG : For high magnetic field resolution
and
Wide T range of application
Typical Faraday rotation: 0.06 deg/mT for
2-5 m thick indicators
I=IoSin2(2VdBz) or
I  Bz2
Sensitivity of the MO technique
• Field sensitivity is determined by the
Faraday rotation 2Vd & noise
For EuTe~20mT for Bi doped YIG ~ 0.15 mT
• Spatial resolution
Governed by thickness (d) + distance
between sample and MO active layer (z)
d
z
Sample
Sensitivity of the MO technique
• Temporal resolution
Governed by the Quantum efficiency and
the minimum exposure time permissible by
the imaging device like a video camera.
Temporal resolution ~ at best a few mSecs
In recent times there have been nearly two
to three order of magnitude improvement
in field, spatial and temporal resolution
Some basic ideas about vortices
a0~(0/B)1/2
At B = 1 T, a0~500 A0
 ~ 5 x 1010 vortices/cm2
2~5-10 nm
 r
ln   for r  
  
F 
exp   r  for r  
   
Loss of sensitivity in resolving
vortices with increasing dist.
With increasing distance of the
MO active layer from the
surface
of the superconductor causes
loss of the resolving power
for resolving vortices.
Applications of MO at Mesoscopic
length scales
• Observing the Meissner effect in superconductors
Strong meissner screening currents on surface
YBCO, 10 K, field of 10 G
•Observing the Critical state
YBCO, 70 K, field of 100mT
B
0
x
B(r )  J c (r )
Phase transitions in the vortex
state
Similarities between ice to water transition & Vortex
solid to liquid transition
213.4
liquid
213.3
213.2
solid
213.1
B~0.2G
B~0.1%B
213.0
vor  B
H a = 240 Oe
58.35
58.40
58.45
58.50
T [K]
ordered
58.55
disordered
kB T
solid
liquid
Source of noise in MOI
B(x)
»1 G
Static:
• Indicator inhomogeneities and defects
• CCD pixel variations
• Light inhomogeneities
Dynamic:
• CCD noise
• Light fluctuations
• Vibrations
Fundamental noise:
• Photon shot noise
Differential MOI imaging
dc field B = 100 G
• Equilibrium magnetization step B  0.1 G
• Desired resolution ~0.01 G
• Required signal/noise 100/0.01=104
• Photon shot noise N/N = (N)1/2  N=108
photons/pixel
• CCD full well capacity ~105 electrons  ~103 frames
• Reduce static noise by differential process:
500
n up
Ha+Ha
Ha
600
700
n up
static noise  Ha
n~10
500
n down
…~100 times
600
differential
static noise  Ha
700
Observation of melting in MOI
Dept. of Condensed
Matter Physics Weizmann Institute
Of Science
image scan
temperature
213.4
liquid
213.3
213.2
A
solid
213.1
B~0.2G
B~0.1%B
small large small
213.0
58.35
58.40
P
58.45
F
F=
B
H a = 240 Oe
58.50
58.55
T [K]
Difference image:
light source
Solid
(no change in B)
MO indicator
mirror
Liquid
change in B already occurred
N
S
Movie of melting in a HTSC
superconductor
Phase diagram of melting
10 5
Hliquid
c2
10 4
solid
B [G]
disordered
10 3
second
magnetization
peak
10 2
quasi-ordered-lattice
(Bragg glass)
10 1
0
20
40
T [K]
60
80
100
Effect of disorder on melting
Sample Bi2Sr2CaCu2O8 (BSCCO), Tc ~ 89-90 K
SST mask
Columnar defects
90 m
Melting phase diagram in presence
of disorder
200
S. S. Banerjee et al, Phys. Rev. Lett. 90, 87004 (2003)
Melting with
disorder
B(G)
150
Vortex
Liquid ?
100
50
0
50
Porous
vortex solid
Melting without
disorder
60
70
T(K)
80
90
Imaging transport current
distribution using MOI
Inversion
scheme
Wijngaarden
et al
PRB54,
6742 (96)
Schematic of self field
image one should see
Self field generated by I
(Biot-Savarts law)
Sample with uniform I distribution
Fixed (MO Image with I+) - (MO Image with I-) = Difference Image
H,T
Can detect self field down to 0.1 mA
Two to three orders of magnitude improvement in sensitivity
S. S. Banerjee et al, Phys. Rev. Lett. 93, 97002 (2004)
Some examples :Surface barrier
-I
-V
+V
0.5 mm
+I
BSCCO crystal
30mA, 75K, 25G
Self-induced field
Current distribution
BG)
(
Imaging current distribution in the
vortex liquid phase
200
CD
Bm
150
nanoliquid
0
homogeneous
liquid
Bdl
Bm
100
50
0
50
B = 60 G
60
70
80
90
TK)
(
Unirradiated
NL
Irradiated
S. S. Banerjee et al, Phys. Rev. Lett. 93, 97002 (2004)
Micron-submicron resolution
• Single vortex imaging with MO
Conventional MO indicator:
Prof. Tom Johansens Group,
Oslo, Norway
MO layer
GGG
d
M
Protective layer
Sample
Reflecting layer
Latest MO indicator:
GGG
M
Sample
Dynamics of single vortices
Interaction of magnetic
Domain walls with
vortices
Nanosecond temporal resolution
Paul Leidere’s group, University of Konstadz, Germany
Application of MO in different areas
of condensed matter physics
L.E.Helseth et al,
PRL 91, 208302 (2003)
Manipulating magnetic beads
Dilute magnetic semiconductors (Mn doped GaAs)
U. Welp et al., PRL 90, 167206 (2003)
Summary
• Two orders of magnitude improvements in
spatial, temporal and magnetic field
sensitivity.
• Improvement in transport current detection
capability
• Enormous potential for investing the
physics of magnetic response in a diverse
class of materials.
Acknowledgements
Prof Eli Zeldov, Israel
Prof Yossi Yeshurun, Israel.
Prof. Marcin Konczykowski,France
Prof. Kees van der Beek, France
Prof. Tsuyoshi Tamegai, Japan
Prof. M. Indenbom, Russia
Prof Tom Johansen, Oslo
Prof. Paul Leiderer, Germany
Prof. A. A. Polyanski, USA
Prof. Vlasko Vlasov, USA
Prof. U. Welp, USA
Prof. Larbalestier, USA
Prof. H. Brandt