Proximity effect
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- This article is about the physics phenomenon of Proximity Effect. For other uses, see: Proximity Effect (disambiguation)
In physics, proximity effects are group of effects where substances behave differently when near, or proximate, to one another.
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[edit] Proximity effect in atomic physics
At the atomic level, when two atoms come into proximity, the highest energy, or valence, orbitals of the atoms change substantially and the electrons on the two atoms reorganize. One way to probe a correlated state is through the proximity effect. This phenomenon occurs when the correlations present in one degenerate system "leak" into another one with which it is in chemical equilibrium. See also quantum tunneling, Casimir effect, van der Waals force.
[edit] Proximity effect in electromagnetics
A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. When an AC current flows through an isolated conductor, it creates an associated alternating magnetic field. The alternating magnetic field induces eddy currents within the conductor, altering the overall distribution of current flowing through the conductor. As a result, an AC current preferentially flows through the outer portion (or skin) of the conductor, a phenomenon called skin effect. If similar currents are also flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the conductor will be constrained to smaller regions. The resulting current crowding is termed proximity effect. The combination of skin and proximity effects significantly increases the AC resistance of the conductor when compared to its resistance to a DC current. At higher frequencies, the AC resistance of a conductor can easily exceed ten times its DC resistance. The additional resistance increases electrical losses which, in turn, generate undesireable heating. Proximity and skin effects significantly complicate the design of efficient transformers operating at high frequencies within switching power supplies.
Proximity effect can also occur within electrical cables. For example, if the conductors are a pair of audio speaker wires, their currents have opposite direction, and currents will preferentially flow along the sides of the wires that are facing each other. The AC resistance of the wires will dynamically change (slightly) along with the audio signal. Some believe that this will potentially introduce distortion and degrade stereo imaging. However, it can be shown that, for reasonable conductor sizes, spacing, and length, this effect is so small as to have an immeasurable practical impact on audio quality.
[edit] Proximity effect in electron beam lithography
When an electron beam is incident on a material, the electrons are not destroyed but are scattered both elastically (with angle changes but without energy loss) and inelastically (with energy loss). The elastically scattered electrons generally have sufficient energy to travel a large distance. Those which head back toward the source are called the back-scattered electrons. The inelastically scattered electrons generate additional radiation quanta through their energy loss, including X-rays, Auger electrons, and low-energy ejected electrons (also called secondary electrons). The range of the back-scattered electrons is much larger than the range of the secondary or Auger electrons due to their higher energy.
Back-scattered electrons often cause features written by electron beam lithography to be wider in densely patterned areas. Most electron-beam lithography systems compensate for this pattern dependence by reducing the dose in densely patterned regions compared to isolated features. The compensation cannot completely remove the fundamentally large difference in dose sensitivity between isolated and nested features.
[edit] Proximity effect in audio
The exaggeration of low-frequency sounds in a directional microphone when it is very near the sound source. An increase in bass response occurs when the sound source is near the microphone, a loss in bass response is experienced as the microphone is moved away.
[edit] Superconducting proximity effect
The term "proximity effect" is used in the field of superconductivity to describe phenomena that occur when a superconductor (S) is placed in contact with a "normal" (N) non-superconductor. Typically the critical temperature <math>T_{c}</math> of the superconductor is suppressed and signs of weak superconductivity are observed in the normal material. The superconducting proximity effect (SPE) is caused by diffusion of Cooper pairs into the normal material, and by the diffusion of electronic excitations in the superconductor. As a contact effect, the SPE is closely related to thermoelectric phenomena like the Peltier effect or the formation of pn junctions in semiconductors. The proximity effect enhancement of <math>T_c</math> is largest when the normal material is a metal with a large diffusivity rather than an insulator (I). Proximity-effect suppression of <math>T_c</math> in a superconductor is largest when the normal material is ferromagnetic, as the presence of the internal magnetic field weakens superconductivity (Cooper pairs breaking).
The study of S/N, S/I and S/S' (S' is lower superconductor) bilayers and multilayers has been a particularly active area of SPE research. The behavior of the compound structure in the direction parallel to the interface differs from that perpendicular to the interface. In type II superconductors exposed to a magnetic field parallel to the interface, vortex defects will preferentially nucleate in the N or I layers and a discontinuity in behavior is observed when an increasing field forces them into the S layers. In type I superconductors, flux will similarly first penetrate N layers. Similar qualitative changes in behavior do not occur when a magnetic field is applied perpendicular to the S/I or S/N interface. In S/N and S/I multilayers at low temperatures, the long penetration depths and coherence lengths of the Cooper pairs will allow the S layers to maintain a mutual, three-dimensional quantum state. As temperature is increased, communication between the S layers is destroyed resulting in a crossover to two-dimensional behavior. The anisotropic behavior of S/N, S/I and S/S' bilayers and multilayers has served as a basis for understanding the far more complex critical field phenomena observed in the highly anisotropic cuprate high-temperature superconductors.
[edit] See also
[edit] External links
- Proximity Effect music technology glossary
- Skin Effect, Proximity Effect, and Litz Wire Electromagnetic effects
- Skin and Proximity Effects and HiFi Cables
[edit] References
- Terman, F.E. Radio Engineers' Handbook, McGraw-Hill 1943 -- details electromagnetic proximity and skin effects
- Superconductivity of Metals and Alloys by P.G. de Gennes, ISBN 0-201-40842-2, a textbook which devotes significant space to the superconducting proximity effect.de:Nahbesprechungseffekt
