Thelambda point is thetemperature at which normal fluidhelium (helium I) makes the transition tosuperfluid state (helium II). At pressure of 1atmosphere, the transition occurs at approximately 2.17K. The lowest pressure at which He-I and He-II can coexist is the vapor−He-I−He-IItriple point at 2.1768 K (−270.9732 °C) and 5.0418 kPa (0.049759 atm), which is the "saturatedvapor pressure" at that temperature (pure helium gas in thermal equilibrium over the liquid surface, in ahermetic container).^[1] The highest pressure at which He-I and He-II can coexist is thebcc−He-I−He-II triple point with a helium solid at 1.762 K (−271.388 °C), 29.725 atm (3,011.9 kPa).^[2]

The point's name derives from the graph (pictured) that results from plotting thespecific heat capacity as a function oftemperature (for a given pressure in the above range, in the example shown, at 1 atmosphere), which resembles theGreek letterlambda${\displaystyle \lambda }$. The specific heat capacity has a sharp peak as the temperature approaches the lambda point. The tip of the peak is so sharp that a critical exponent characterizing the divergence of the heat capacity can be measured precisely only in zero gravity, to provide a uniform density over a substantial volume of fluid. Hence, the heat capacity was measured within 2 nK below the transition in an experiment included in aSpace Shuttle payload in 1992.^[3]

Although the heat capacity has a peak, it does not tend towardsinfinity (contrary to what the graph may suggest), but has finite limiting values when approaching the transition from above and below.^[3] The behavior of the heat capacity near the peak is described by the formula${\displaystyle C\approx A_{\pm }{t}^{-\alpha }+B_{\pm }}$ where${\displaystyle t=|1-T/T_c|}$ is the reduced temperature,${\displaystyle T_c}$ is the Lambda point temperature,${\displaystyle A_{\pm },B_{\pm }}$ are constants (different above and below the transition temperature), andα is thecritical exponent:${\displaystyle \alpha =-0.0127(3)}$.^[3][5] Since this exponent is negative for the superfluid transition, specific heat remains finite.^[6]

The quoted experimental value ofα is in a significant disagreement^[7][4] with the most precise theoretical determinations^[8][9][10] coming from high temperature expansion techniques,Monte Carlo methods and theconformal bootstrap.

• Lambda point refrigerator

• What is superfluidity?

• Threshold temperatures
• Superfluidity
• Liquid helium

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