BEC-BCS Crossover
BCS superfluidity
Superfluidity of boson was first discovered in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle ^4 He} system at a critical temperature of . This was connected to the formation of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle ^4 He} condensates. Superfluidity of fermions, the electrons, was first discovered in Mecury at a transition temperature Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_C \sim 4.2K} , which is known as the `superconductivity' of metals. \\ In the early age, there are two major confusions about the fermionic superfluidity
- what is the mechanism for superfluidity of fermions (electrons)?
It is intuitive to suggest that two electrons could form tightly bounded pairs (Schafroth pairs) and then form condensates. However, there was no known interaction which is strong enough to overcome the Coulomb repulsion.\\
- why does it happen at such low temperature compared with Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_F } (typically in metal)?
For bosonic case in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle ^4 He} , we can estimate the transition temperature Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_{BEC} } (assuming phase space density 1 and typical Helium density) to be Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_{BEC} \sim 3K} which is consistent with the experimental findings. However, the fermi temperature in a fermionic system in Mercury is much higher (10^4) than the observed superfluidity transition temperature.
The two puzzles remain unresolved until 1956 when Bardeen, Cooper and Schrieffer proposed the BCS theory.
Characteristic Temperature Scale
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