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The Weak Link Effect

in HTS

by Kevashen Govender &

Thashlan Pather

Definition of a Weak Link ?

A weak link is the conducting junction between bulk superconducting specimens, the critical current through which is much less than that in the electrodes themselves. It is specifically used to distinguish weakly linked superconducting structures.

Weak Link in HTS

  • Observed in Type 2 HTS
  • Grains not very good superconducting material
  • Impurity phases result with different Tc’s
  • Grains disconnect resulting in individual screening currents
  • Resistance now encountered with an input applied current
  • Affects production and fabrication of types of wires

Josephson Junction

  • Waveform of cooper pairs changes phase
  • Max current(Ic) that can flow through the weak links depends on:
  • Thickness and area
  • Type of superconducting material and temperature

Thanks Guys !!!

Applications of Weak Link Structures

Grain Boundaries

  • To exploit the beneficial effects of grain alignment,the coated conductor technologies have been developed
  • IBAD – Ion Beam Assisted Deposition
  • RABiTS - Rolling Assisted Biaxially Textured Substrate process
  • ISD -Inclined Substrate Deposition
  • The Josephson effect, which is due to weak links, has found wide usage. For example in the following areas:
  • SQUIDS
  • Precision metrology
  • RSFQ digital electronics
  • Superconducting tunnel junction detectors
  • Single-electron transistors
  • To increase the critical currents,
  • Two strategies can be implemented:
  • Align the grains along all major axes to within a few degrees
  • Jc can be increased by appropriate current doping

DC Josephson Current

  • The basic equation that govern the Josephson effect is:

DC Josephson Current

Grain Boundaries

Various Josephson Effects

  • DC Effect
  • The super current can flow through the barrier from one superconductor to another even without any applied voltage as long as it does not exceed a critical current.
  • AC Effect
  • When a voltage is applied across a junction the tunneling current can flow.
  • Superimposed on the dc current flows an addition ac current.

What is the Josephson Junction ?

Grain Boundaries

  • Junction between two superconductors separated by a thin insulator
  • Josephson predicted that tunneling may occur
  • Tunneling phenomena
  • SIS
  • NIS
  • The flowing current through this junction depends on a number of factors

Methods to Observe the "Effect"

  • FUNDAMENTAL METHOD:

Resistance/Temperature behaviour with a minute applied magnetic field

Josephson Effect

  • Meet the guy :

  • Born: 4-Jan-1940
  • Birthplace: Cardiff, Glamorgan, Wales
  • Occupation: Physicist

What does this all mean ?

  • Exhibition of The Josephson Effect
  • THz frequency sources
  • Deeper insight into macroscopic quantum effects
  • Detectors
  • Sensitive Magnetometers
  • Potentially useful in high speed digital logic

Resistance vs. Temperature

In General

Resistance vs. Temperature

Applied Magnetic Field

Resistance vs. Temperature

Ideal Case

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