# Presentation: 632407

## Using Transport and Scanning Laser Microscopy  

- Study of Grain Boundaries
- Using Transport and Scanning Laser Microscopy

- California State University Long Beach

- Megumi Yamamoto

- Samuel Pottish
- Ryan G. James
- Jeremy L. Young
- Chuhee Kwon

- October 26th, 2007
- Lawrence Berkeley National Laboratory
- American Physical Society, California Section

## Background

- Coated Conductors
- YBa2Cu3O7- (YBCO) based multilayer materials developed for superconducting wires.
- The current transports through the network of grain boundaries.
- Grain Boundaries
- The interface between two crystals in a polycrystalline material and it acts as defects.

## Low Temperature Scanning Laser Microscopy (LTSLM)

-  A laser creates local heating in a sample
-  Measure the AC voltage induced by the local heating

-  The low temperature region images: (T < Tc)
-  With the DC bias current: IB ≈ Ic

- A laser creates local heating in superconducting state

-  Thermoelectric voltage δV :

- Map the superconducting dissipation

## Experiments: Setup

- Scanning Laser Microscopy (SLM)

## Results I:

- Ref) Hilgenkemp, *Rev. Mod. Phys.* (2002)

- Sample Geometry

- 24°Ca-doped YBCO
- on SrTiO3 bicrystal substrate

- Room temp.~

## Results II: Low Temperature SLM (LTSLM)

- Ca-doped YBCO at 84 K

-  I-V characteristics show step-like features in a wide range of temperatures.
-  I-V characteristics is related to the local dissipation pattern measured by LTSLM.
-  The local dissipation pattern stays the same within a step, and it changes with the step.
-  The same area appears more than once in SLM images.

## Results III: I-V Characteristics

-  For each temperature, take a derivative dV/dI
-  Find peak minima of the dV/dI
-  Find V, I of those minima

## Result IV: I-V Analysis

-   Fiske-steps are the likely cause for the step-like features in I-V.

- 77 K

## Summary 

- Measure a Ca-doped YBCO grain boundary junction on a 24° [100]-tilted SrTiO3 bicrystal substrate.
- Use transport and scanning laser microscopy techniques to investigate the characteristics of the grain boundary junction.
- We observed the relation between I-V and local dissipation pattern.
- Fiske-steps are a likely origin for the step-like behavior in I-V.

## Acknowledgments

- Chuhee Kwon
- Samuel Pottish
- Ryan G. James
- Jeremy L. Young  (CSULB)

-  Paul Barnes
-  Tim Haugen
-    (The Air Force Research Laboratory
-    AFRL/ PRPG)

- Funding
-  The Air Force Office of Scientific Research
-  Undergraduate Research and Creative Activity Scholarship
-    of Women and Philanthropy

## Scanning Laser Microscopy (SLM)

- Thermoelectric Scanning Laser Microscopy (TE-SLM)
- The normal state images with the DC bias current IB = 0
- Locate the grain boundary
- Variable Temperature Scanning Laser Microscopy (VTSLM)
- The superconducting transition images (T ~ Tc) with IB
- Map the Tc variation and current distribution in a grain boundary
- Low Temperature Scanning Laser Microscopy (LTSLM)
- The low temperature region images (T < Tc) with IB ≈ Ic
- Map the superconducting dissipation

- Three techniques:

- - A laser creates local heating in a sample
- - Measure the AC voltage induced by the local
-   heating

- Laser

## Scanning Laser Microscopy (SLM): I Thermoelectric Scanning Laser Microscopy (TE-SLM)

-  The normal state images
-     at room temperature: (T > Tc)
-  With no bias current: IB = 0

-  Thermoelectric voltage ΔV :

- For Seebeck constants of YBa2Cu3O7-δ at room temperature: Sa= - 2 mV/K                 		           Sb= 1 mV/K
-                                    Sc= 7-20 mV/K

- Locate the grain boundary

## Scanning Laser Microscopy (SLM) II:Variable Temperature Scanning Laser Microscopy (VTSLM)

-  The superconducting transition images:
-     (T ~ Tc)
-  With the DC bias current IB

- Map the local Tc* and dVmax
- in a grain boundary

-  Thermoelectric voltage δV :

- Derivative

## Scanning Laser Microscopy (SLM) III: Low Temperature Scanning Laser Microscopy (LTSLM)

-  The low temperature region images:
-    (T < Tc)
-  With the DC bias current: IB ≈ Ic

- A laser creates local heating
- in superconducting state

-  Thermoelectric voltage δV :

- Map the superconducting dissipation