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Superconductivity: The Meissner Effect and Measurements of T c - - PowerPoint PPT Presentation
Superconductivity: The Meissner Effect and Measurements of T c - - PowerPoint PPT Presentation
Superconductivity: The Meissner Effect and Measurements of T c Edwin Ng | 6 April 2012 Properties of Superconductors A material with zero resistance Exhibits the Meissner Effect: B sc = 0 B ~ 0 past London penetration depth L
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Properties of Superconductors (cont.)
When B = 0, SC phase transition at Tc When B ≠ 0, transition occurs when (T, B)
satisfies
B0 is the limiting critical field at T = 0
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BCS Theory of Superconductivity
Bardeen, Cooper, and Schrieffer (1957) At low temperature, electrons couple into
Cooper pairs through vibrations (phonons) in the material lattice
The Cooper pairs condense into a mutual
ground state where currents can flow without resistance
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Meissner Effect: Tc Measurements
Put an SC into an AC driven solenoid and
measure the induced EMF at test coil
When T > Tc, get full flux of test coil When T < Tc, get no flux in SC region Can measure Tc by monitoring test coil flux
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Meissner Effect: Persistent Currents
Set up B ≠ 0 at T > Tc for a hollow cylinder of
SC material
After T < Tc, surface currents set up to cancel
field in material and B in cylinder
Field inside persists even after B is turned off Field inside quenches once T > Tc
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Overview of Probe I
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Tc Transition Curves: Vanadium
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Tc Transition Curves: Lead
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Tc Transition Curves: Lead
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Tc Transition Curves: Niobium
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Tc Fit Procedure
Find mean of constant regions Define midpoint as V0 with uncertainty δV0 Fit y = ax + b to transition region by least-
squares regression without uncertainties
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Tc Fit Procedure (cont.)
Define the deviation Uncertainty of Tc due to line width is E/a
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Tc Fit Procedure: Example
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Summary of Tc Transitions
Summary of Tc fitting procedure There is some hysteresis, esp. for Type II Error due to pumping and calibration curve
No error at 77 K ~0.5 K error at 4.2 K
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Tc Dependence on B for Vanadium
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Field Exclusion in Lead
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Overview of Probe 2
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Probe 2: Videos
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Conclusions
Observed Meissner effect in superconductors
Magnetic flux exclusion (Probe 1) Persistent current (Probe 2)
Used Meissner effect to measure Tc of various
superconducting samples
Observed B-dependent phase transition curve
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