Josephson Junction

[seminar class]

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)