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: Light source M M P A M Polariser d X Y F = V B z d Z Z Analyser Transmission Mode
Reflection Mode MO Polarized light F = V Bz 2 d MO active layer GGG M Sample Protective layer Z Y X d Reflecting layer
Types of MO active layers • Type of MO active layer depends on the type of experiments. d YIG Eu. Te Eu. Se
MO imaging setup Choice YIG : For high magnetic field resolution and Wide T range of application Typical Faraday rotation: 0. 06 deg/m. T for 2 -5 m thick indicators I=Io. Sin 2(2 Vd. Bz) or I Bz 2
Sensitivity of the MO technique • Field sensitivity is determined by the Faraday rotation 2 Vd & noise For Eu. Te~20 m. T for Bi doped YIG ~ 0. 15 m. T • 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 m. Secs 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 a 0~( 0/B)1/2 At B = 1 T, a 0~500 A 0 ~ 5 x 1010 vortices/cm 2 2 ~5 -10 nm
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 100 m. T B 0 x
Phase transitions in the vortex state Similarities between ice to water transition & Vortex solid to liquid transition B(G) 213. 4 liquid 213. 3 213. 2 solid 213. 1 B~0. 2 G 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 k. BT 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: Ha+ Ha Ha n~10 n up static noise Ha n down …~100 times differential static noise Ha
Observation of melting in MOI Dept. of Condensed Matter Physics Weizmann Institute Of Science image temperature scan B(G) 213. 4 liquid 213. 3 213. 2 A solid 213. 1 B~0. 2 G B~0. 1%B small large small 213. 0 58. 35 58. 40 P light source 58. 45 F F= B H a = 240 Oe 58. 50 T [K] 58. 55 Difference image: 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 dep solid second magnetization peak 10 2 first -ord er tran sitio n disordered ng i inn 10 4 B [G] liquid Hc 2 10 3 quasi-ordered-lattice (Bragg glass) 10 1 0 20 40 T [K] 60 80 100
Effect of disorder on melting Sample Bi 2 Sr 2 Ca. Cu 2 O 8 (BSCCO), Tc ~ 89 -90 K SST mask Columnar defects 90 mm
Melting phase diagram in presence of disorder S. S. Banerjee et al, Phys. Rev. Lett. 90, 87004 (2003) Vortex Liquid ? Porous vortex solid
Imaging transport current distribution using MOI Inversion scheme Wijngaarden et al PRB 54, 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 I H, T Can detect self field down to 0. 1 m. A 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 30 m. A, 75 K, 25 G Self-induced field Current distribution
Imaging current distribution in the vortex liquid phase Unirradiated NL Irradiated S. S. Banerjee et al, Phys. Rev. Lett. 93, 97002 (2004)
Micron-submicron resolution Prof. Tom Johansens Group, Oslo, Norway • Single vortex imaging with MO MO layer Conventional MO indicator: 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, Germa
Application of MO in different areas of condensed matter physics ute magnetic semiconductors (Mn doped Ga. As) 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, 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
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