Magneto-optical imaging of Superconductors

Satyajit S .BanerjeeDept of Physics,

Indian Institute of Technology,Kanpur, India

Principle of operation of MO imaging

• Faraday Effect:

P A

Light source

Polariser

d

MM

M

Analyser

ZY

X

Z

P A

F = V Bz d

Transmission Mode

Reflection Mode MO

Polarized light

GGGM

Sample

F = V Bz 2d

d

Protective layerReflecting layer

MO active layer

Z

Y X

Types of MO active layers

• Type of MO active layer depends on the type of experiments.

10-1-2-31010101010

3

2

1

10

10

10

010

T(K

)

B(T)

YIG

EuSe

EuTe

d

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

Samplez

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

rforr

rforr

F

exp

ln

Loss of sensitivity in resolving vortices with increasing dist.

With increasing distance of the MO active layer from the surfaceof the superconductor causesloss of the resolving powerfor resolving vortices.

Applications of MO at Mesoscopic length scales

• Observing the Meissner effect in superconductors

•Observing the Critical stateYBCO, 10 K, field of 10 G

YBCO, 70 K, field of 100mT

Strong meissner screening currents on surface

B

x0

)()( rJrB c

Phase transitions in the vortex state

Similarities between ice to water transition & Vortex solid to liquid transition

213.0

213.1

213.2

213.3

213.4

58.35 58.40 58.45 58.50 58.55

T [ K ]

H a = 240 Oe

liquid

solid

B~0.2GB~0.1%BB

(G) vor B

solid

kBT

ordered

liquid

disordered

Source of noise in MOI

Dynamic:

• CCD noise• Light fluctuations• Vibrations

Fundamental noise:

• Photon shot noise

Static:

• Indicator inhomogeneities and defects• CCD pixel variations • Light inhomogeneities

B(x) » 1 G

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:

…~100 timesn~10 n downHa

Ha+Ha

n upn up

0.0

2000.0

4000.0

6000.0

8000.0

10000.0

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 800

differential static noise Ha

static noise Ha

Observation of melting in MOI

P

A

image

light source

mirrorMO indicator

SN

largesmall small

FF=

B

temperature scan

213.0

213.1

213.2

213.3

213.4

58.35 58.40 58.45 58.50 58.55

T [ K ]

H a = 240 Oe

liquid

solidB~0.2G

B~0.1%BB(G

)

Difference image:

Solid(no change in B)

Liquidchange in B already occurred

Dept. of CondensedMatter Physics Weizmann Institute Of Science

Movie of melting in a HTSC superconductor

Phase diagram of melting

101

102

103

104

105

0 20 40 60 80 100

first-ordertransition

secondmagnetization

peak

Hc2

T [ K ]

B

[ G ]

depinningdisordered

quasi-ordered-lattice(Bragg glass)

liquid

solid

Effect of disorder on meltingSample Bi2Sr2CaCu2O8 (BSCCO), Tc ~ 89-90 K

SST maskSST mask 9090 mmColumnar defects

50 60 70 80 900

50

100

150

200 Melting withdisorder

Melting withoutdisorder

T(K)

B(G

)

Melting phase diagram in presence of disorder

Porous vortex solid

VortexLiquid ?

S. S. Banerjee et al, Phys. Rev. Lett. 90, 87004 (2003)

Imaging transport current distribution using MOI

(MO Image with I+) - (MO Image with I-) = Difference Image FixedH,T

Sample with uniform I distribution

Self field generated by I(Biot-Savarts law)

Schematic of self fieldimage one should see

InversionschemeWijngaardenet alPRB54,6742 (96)

Can detect self field down to 0.1 mATwo to three orders of magnitude improvement in sensitivity

S. S. Banerjee et al, Phys. Rev. Lett. 93, 97002 (2004)

Some examples :Surface barrier

BSCCO crystalBSCCO crystal

0.5 mm

Current distributionCurrent distributionSelf-induced fieldSelf-induced field

30mA, 75K, 25G 30mA, 75K, 25G

-- I I

+ I+ I

-- V V

+ V+ V

Imaging current distribution in the vortex liquid phase

Irradiated

Unirradiated

NL

50 60 70 80 900

50

100

150

200

0Bm

CDBm

homogeneous liquid

nanoliquid

B = 60 G

B (G

)

T (K)

Bdl

S. S. Banerjee et al, Phys. Rev. Lett. 93, 97002 (2004)

Micron-submicron resolution

• Single vortex imaging with MO

GGG M

Sample

d

Reflecting layerProtective layer

Conventional MO indicator:

Latest MO indicator:

GGG M

Sample

MO layer

Prof. Tom Johansens Group,Oslo, Norway

Dynamics of single vortices

Interaction of magneticDomain walls with

vortices

Nanosecond temporal resolution

Paul Leidere’s group, University of Konstadz, Germany

Application of MO in different areas of condensed matter physics

Dilute magnetic semiconductors (Mn doped GaAs)U. Welp et al., PRL 90, 167206 (2003)

L.E.Helseth et al,PRL 91, 208302 (2003)

Manipulating magnetic beads

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, IsraelProf Yossi Yeshurun, Israel.

Prof. Marcin Konczykowski,FranceProf. Kees van der Beek, FranceProf. Tsuyoshi Tamegai, Japan

Prof. M. Indenbom, RussiaProf Tom Johansen, Oslo

Prof. Paul Leiderer, GermanyProf. A. A. Polyanski, USAProf. Vlasko Vlasov, USA

Prof. U. Welp, USAProf. Larbalestier, USA

Prof. H. Brandt