Persistent current
In
In magnetized objects
In electromagnetism, all magnetizations can be seen as microscopic persistent currents. By definition a magnetization can be replaced by its corresponding microscopic form, which is an electric current density:
- .
This current is a bound current, not having any charge accumulation associated with it since it is
The converse is also true: any persistent electric current is divergence-free, and can therefore be represented instead by a magnetization. Therefore, in the macroscopic Maxwell's equations, it is purely a choice of mathematical convenience, whether to represent persistent currents as magnetization or vice versa. In the microscopic formulation of Maxwell's equations, however, does not appear and so any magnetizations must be instead represented by bound currents.
In superconductors
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In
In resistive conductors
Surprisingly, it is also possible to have tiny persistent currents inside resistive metals that are placed in a magnetic field, even in metals that are nominally "non-magnetic".[4] The current is the result of a
This type of persistent current is a mesoscopic low temperature effect: the magnitude of the current becomes appreciable when the size of the metallic system is reduced to the scale of the electron quantum phase coherence length and the thermal length. Persistent currents decrease with increasing temperature and will vanish exponentially above a temperature known as the Thouless temperature. This temperature scales as the inverse of the circuit diameter squared.[3] Consequently, it has been suggested that persistent currents could flow up to room temperature and above in nanometric metal structures such as metal (Au, Ag,...) nanoparticles. This hypothesis has been offered for explaining the singular magnetic properties of nanoparticles made of gold and other metals.[5] Unlike with superconductors, these persistent currents do not appear at zero magnetic field, as the current fluctuates symmetrically between positive and negative values; the magnetic field breaks that symmetry and allows a nonzero average current. Although the persistent current in an individual ring is largely unpredictable due to uncontrolled factors like the disorder configuration, it has a slight bias so that an average persistent current appears even for an
This kind of persistent current was first predicted to be experimentally observable in micrometer-scale rings in 1983 by Markus Büttiker, Yoseph Imry, and Rolf Landauer.[7] Because the effect requires the phase coherence of electrons around the entire ring, the current can not be observed when the ring is interrupted by an ammeter and thus the current must by measured indirectly through its magnetization. In fact, all metals exhibit some magnetization in magnetic fields due a combination of
Experimental evidence of the observation of persistent currents were first reported in 1990 by a research group at
"These are ordinary, non-superconducting metal rings, which we typically think of as resistors, yet these currents will flow forever, even in the absence of an applied voltage."
— Jack Harris, Associate Professor of Physics and Applied Physics at Yale.
The 2009 measurements both reported greater sensitivity to persistent currents than previous measurements and made several other improvements to persistent current detection. The scanning SQUID's ability to change the position of the SQUID detector relative to the ring sample allowed for a number of rings to be measured on one sample chip and better extraction of the current signal from background noise. The cantilever detector's mechanical detection technique made it possible to measure the rings in a clean electromagnetic environment over a large range of magnetic field and also to measure a number of rings on one sample chip.[11]
See also
- Aharonov–Bohm effect – Electromagnetic quantum-mechanical effect in regions of zero magnetic and electric field
- Byers-Yang theorem– Theorem in quantum mechanics
- Mesoscopic physics – Subdiscipline of condensed matter physics that deals with materials of an intermediate size
- Perpetual motion – Work being continuously done without an external input of energy
References
- S2CID 34374691.
- ^ File J, Mills, R Physical Review Letters 1963
- ^ S2CID 37548342.
- ^ "Measuring elusive "persistent current" that flows forever". R&D Daily. October 12, 2009.
- S2CID 28725119.
- ^ ISBN 978-0-521-85512-9.
- .
- PMID 10041570.
- ^ "Physicists Measure Elusive 'Persistent Current' That Flows Forever". ScienceDaily. October 12, 2009.
- S2CID 16760003.
- S2CID 9674177.