Editing BCS theory (section)

===Underlying evidence===
The hyperphysics website pages at [[Georgia State University]] summarize some key background to BCS theory as follows:<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/solids/bcs.html|title=BCS Theory of Superconductivity|website=hyperphysics.phy-astr.gsu.edu|access-date=16 April 2018}}</ref>

* '''Evidence of a [[band gap]] at the Fermi level''' (described as "a key piece in the puzzle")
: the existence of a critical temperature and critical magnetic field implied a band gap, and suggested a [[phase transition]], but single [[electron]]s are forbidden from condensing to the same energy level by the [[Pauli exclusion principle]]. The site comments that "a drastic change in conductivity demanded a drastic change in electron behavior". Conceivably, pairs of electrons might perhaps act like [[boson]]s instead, which are bound by [[Bose–Einstein statistics|different condensate rules]] and do not have the same limitation. 
*'''Isotope effect on the critical temperature, suggesting lattice interactions'''
: The [[Debye frequency]] of phonons in a lattice is proportional to the inverse of the square root of the mass of lattice ions. It was shown that the superconducting transition temperature of mercury indeed showed the same dependence, by substituting the most abundant natural [[isotopes of mercury|mercury isotope]], <sup>202</sup>Hg, with a different isotope, <sup>198</sup>Hg.<ref name=maxwell1950>{{cite journal|last1=Maxwell|first1=Emanuel|title=Isotope Effect in the Superconductivity of Mercury|journal=Physical Review|volume=78|issue=4|pages=477|doi=10.1103/PhysRev.78.477|bibcode = 1950PhRv...78..477M |year=1950}}</ref>
* '''An [[Exponential growth|exponential rise]] in [[heat capacity]] near the critical temperature for some superconductors'''
: An exponential increase in heat capacity near the critical temperature also suggests an energy bandgap for the superconducting material. As superconducting [[vanadium]] is warmed toward its critical temperature, its heat capacity increases greatly in a very few degrees; this suggests an energy gap being bridged by thermal energy.
* '''The lessening of the measured energy gap towards the critical temperature'''
: This suggests a type of situation where some kind of [[binding energy]] exists but it is gradually weakened as the temperature increases toward the critical temperature. A binding energy suggests two or more particles or other entities that are bound together in the superconducting state. This helped to support the idea of bound particles – specifically electron pairs – and together with the above helped to paint a general picture of paired electrons and their lattice interactions.