06-04-2011, 09:45 AM
Presented by:
SINUSHA.C
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Josephson Junction
► A Josephson junction is a type of electronic circuit capable of switching at very high speeds when operated at temperatures approaching absolute zero.
► The ability of certain materials to conduct electric current with practically zero resistance.
Operation of junction
► Assume Hamiltonian for the system can be written as a sum of two Hamiltonians
H = H0+ HT
where:
H0 = normal Hamiltonian for 2 isolated superconductors
HT = tunneling Hamiltonian
► So, this is "right" for tunneling links only.
Josephson Equations
► Consider the very simple example of two, identical superconductors separated by a thin insulator.
► (Typically about 1nm is sufficiently thin).
► Assume junction is sufficiently large in the x and y direction to ignore edge/boundary effects, and thick enough in z.
► The governing equations are:
DC Josephson Effect
► No Magnetic field:
A current flows, nut no voltage drop, up until the critical current.
Past the critical current, normal single electron tunneling is dominant.
With magnetic field:
Current is:
AC Josephson Effect
► With no magnetic field, static potential:
Integrating equation for Ф :
We can substitute into our other equation and get :
From this, we get a time varying current with frequency
AC Josephson Effect
► With varying potential:
Do similar analysis as static potential case.
It turns out that this has dc component
but when qVo/wħ = n (where n= integer)
Dc current has spikes at
regularly values of Vo
Total current has steps at these
points.
► Inverse AC Josephson Effect
► If the phase takes the form
φ(t) = φ0 + nωt + asin(ωt),
the voltage and current will be
The DC components will then be
Cooper pair
► Electron pairs coupling over range of hundreds of nanometers are called cooper pair.
► These coupled
electron can take character of boson and condense into ground state.
Applications
► Applications of Josephson Devices
Magnetic Sensors
Gradiometers
Oscilloscopes
Decoders
Analogue to Digital converters
Samplers
Oscillators
Microwave amplifiers
Sensors for biomedical, scientific and defense purposes
Digital circuit development for Integrated circuits
Microprocessors
Random Access Memories (RAM’s)