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In Physics , proximity effects are group of effects where substances behave differently when near, or proximate, to one another. 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 . PROXIMITY EFFECT IN ELECTROMAGNETICS In electrical conductors, the Magnetic Field setup by a changing current affects charge currents in nearby conductors. This interaction attracts or repels the charges and causes the charge currents to concentrate in the cross-sections of the conductors. Known as proximity effect, this concentration reduces the effective Conductance of a conductor. If the conductors are a set of Coil windings, their currents have the same direction so the charges repel each other when the current changes. This causes them to concentrate in the centers of the wires, reducing the conductance and increasing loss of electrical energy to heat. This loss is of growing concern in power applications. If the conductors are a pair of audio Speaker Wire s, their currents have opposite direction so the charges attract each other. This causes them to migrate to the sides of the wires closest to the other wire. The wires' conductance dynamically changes with the audio signal, producing Distortion and degrading stereo imaging. Skin Effect describes a similar interaction but between charges in the same conductor. These charges always flow in the same direction so they repel each other when the current changes, distributing themselves evenly near and around the conductor's outer surface. 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. PROXIMITY EFFECT IN MUSIC 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. The same occurs with headphones, speakers, etc. 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 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 is largest when the normal material is a metal with a large diffusivity rather than an insulator (I). Proximity-effect suppression of 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 and S/I 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 and S/I 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. EXTERNAL LINKS
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