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07-04-2009, 11:37 PM
The 20th century saw many developments in the field of electronics because of basically two reasons
1. The development of transistors, which forms the basics of everything that is electronics.
2. The development of IC, which helped in the fabrication of fast, compact & sophisticated electronic circuits.
In the 21st century we are going to see some radical changes in the approach towards electronics. These are :
1. The replacement of semiconducting devices with superconducting devices.
2. The use of new classical theories in physics like the relative physics & quantum mechanics to explain various phenomenon, application & working of electronic devices.
The first step to integrate the previously separate branches, electronics &super conductivity was done by the scientist called Brian Josephson by the invention of the JJ in the year 1962 for which he received the Nobel prize in the year 1973.The analysis of the device is impossible using classical theories of physics. The device has immense potential & numerous applications in almost all fields of applied electronics.
The Josephson junction (JJ) is basically an insulator sandwitched between the two semiconductor layers. Hence the device is also called as a SIS (Superconductor-Insulator-superconductor). A tunneling phenomenon called Josephson tunneling takes place through the insulator when the thickness of the insulator is very thin (less than 1.5 nm) and the insulator turns into a superconductor due to the tunneling of charge carriers from the 1st to the 2nd super conductor; through the insulator.
To explain the working of the device we need to analyze the principles of superconductivity & the principles of tunneling. The superconductivity is explained in terms of BCS theory & tunneling in terms of the uncertainity principle.
It is a remarkable property in which there is a complete loss of resistivity in a metal or alloy, usually at temperature close to the absolute zero & this property was discovered by Kammerlingh Onnes. As perfect conductors, superconductors will carry current without resistance loss, i.e, the current applied will persist forever without any loss of power. These materials are also perfect diamagnetic & magnet placed above the super conductor will levitate under its own magnetic field.
Low temperature superconductors exhibit property at temperature near-250?C. LBCO &certain alloys of La & Ba shows this property near 35k, RBa2Cu3O7, Bi2Sr2ca2Cu3O10 can show the property near 90k. Thallium based & mercury based cuprates can show superconductivity at 134k. Progress in the development of high temperature superconductivity & particular cuprate based superconductors has made significant advances. Some organic compounds have lately been developed as Superconductors.
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06-04-2011, 09:45 AM
Qpres_SQUID.ppt (Size: 1.27 MB / Downloads: 50)
► 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
H0 = normal Hamiltonian for 2 isolated superconductors
HT = tunneling Hamiltonian
► So, this is "right" for tunneling links only.
► 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:
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
► 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
► 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 of Josephson Devices
Analogue to Digital converters
Sensors for biomedical, scientific and defense purposes
Digital circuit development for Integrated circuits
Random Access Memories (RAM’s)
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24-02-2012, 12:11 PM
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