Advanced Solid State Physics

16.4 Microscopic Theories

16.4.1 BCS Theory of Superconductivity

We briefly discuss the fundamental concepts of the BCS theory of superconductivity as developed
byBardeen, CooperandSchriefferin 1957. The applications of BCS theory range from the
description of superfluidous He^3 to type I and type II metallic superconductors, and even some high-
temperature superconductors. Moreover, the BCS theory proved to be very important for our modern
understanding of theoretical solid state physics since many concepts appearing in the framework of this
theory reappeared in the context of numerous further effects and phenomena, such as the fractional
quantum Hall effect, topological insulators or even in nuclear physics.

The basic ingredient to a successful microscopic description is the realization that at sufficiently low
temperatures two electrons form an energetically favored state through electron - phonon - electron
interaction, called aCooperpair. This attractive interaction between the electrons yields a ground
state which is separated from the excited states by a band gap, as experimentally observed, see
Sec. 16.2.3. Also the fact that phonons play a crucial role in the formation of this ground state
was observed experimentally through the isotope effect, Sec. 16.2.4. Most of the thermodynamic
properties of superconductors are a direct consequence of the occurrence of a band gap, thus a profound
understanding ofCooperpairs is essential in a microscopic description of superconductivity.

16.4.2 Some BCS Results

We briefly summarize some of the most remarkable results from the BCS theory

(i) Attractive interaction between electrons leads to the formation ofCooperpairs. The ground-

state is separated from the excited state by an energy gap. With the help of this energy gap

one can explain phenomena such as the critical field, thermal properties and electromagnetic

properties.

(ii) The electron - lattice - electron interaction can be understood in an intuitive way with the help

of the following simplified picture: One electron interacts with the lattice and deforms it, the

second electron is affected by the lattice deformation and moves therefore in the field of the

first electron. In particular, the two electrons interact through the lattice deformation. In this

attractive process the total energy is lowered.

(iii) TheLondon equations, and therefore also the penetration depth as well as the coherence

length, are a direct consequence of the BCS theory. Since theLondonequations account for

theMeissner - Ochsenfeldeffect, also this experimental fact is implicitly accounted for within

the microscopic theory.

(iv) It is (correctly) predicted that the transition temperature is influenced by the electron density

of the orbitals at the Fermienergy of one spin, D(εF) and the electron lattice interaction

parameterV which is connected to the electrical resistance at room temperature. In particular

caseV D(εF) 1 (V attractive) the BCS theory predicts

Tc= 1.14Θ exp

(

−

1

V D(εF)

)

, (309)