Quantum effect |
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| A thin layer of insulating material sandwiched between 2 superconducting layers. Electrons "tunnel" through this non-superconducting region in what is known as the "Josephson effect". Here a superconducting current flows even in the absence of an external voltage. (Named for Brian Josephson.) | |
| Although many properties of
superconductors can be described in macroscopic terms
such as resistivity, heat capacity, critical temperature,
etc., superconductivity is at base a quantum phenomenon
and several interesting quantum effects arise. In 1961, two groups working independently discovered flux quantization - the fact that the magnetic flux through a superconducting ring is an integer multiple of a flux quantum. The Cooper pairs of a superconductor can tunnel through a thin insulating layer between two superconductors. This is the basis for the Josephson junction which is used in high-speed switching devices. |
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http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scqua.html |
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| Two superconductors separated by a thin insulating layer can experience tunneling of Cooper pairs of electrons through the junction. The Cooper pairs on each side of the junction can be represented by a wavefunction similar to a free particle wavefunction. In the DC Josephson effect, a current proportional to the phase difference of the wavefunctions can flow in the junction in the absence of a voltage. In the AC Josephson effect, a Josephson junction will oscillate with a characteristic frequency which is proportional to the voltage across the junction. Since frequencies can be measured with great accuracy, a Josephson junction device has become the standard measure of voltage. | |
| The wavefunction which describes a Cooper pair of electrons in a superconductor is an exponential like the free particle wavefunction. In fact, all the Cooper pairs in a superconductor can be described by a single wavefunction in the absence of a current because all the pairs have the same phase - they are said to be "phase coherent" (Clarke). If two superconductors are separated by a thin insulating layer, then quantum mechanical tunneling can occur for the Cooper pairs without breaking up the pairs. Clarke envisions this condition as the wavefuntions for Cooper pairs on each side of the junction penetrating into the insulating region and "locking together" in phase. Under these conditions, a current will flow through the junction in the absence of an applied voltage (the DC Josephson effect). | |
| Josephson Voltage Standard : | |
| When a DC voltage is applied to a Josephson junction, an oscillation of frequency | |
f = 2 * e * Delta(V) / h |
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occurs at the
junction. Since this relationship of voltage to frequency
involves only fundamental constants and since frequency
can be measured with extreme accuracy, the Josephson
junction has become the standard voltage measurement. |
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| The Standard Volt : | |
The standard volt is now defined in terms of a Josephson junction oscillator. The oscillation frequency of a Josephson junction is given by |
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f = 2 * e * Delta(V) / h |
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| so the relationship between frequency and voltage across the junction depends only upon the fundamental constants e and h. For one microvolt applied to the junction the frequency is | |
f = 483.6 MHz |
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| The standard volt is now defined as the voltage required to produce a frequency of 483,597.9 GHz. | |
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html |
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