Lambda point explained

The lambda point is the temperature at which normal fluid helium (helium I) makes the transition to superfluid helium II (approximately 2.17 K at 1 atmosphere). The lowest pressure at which He-I and He-II can coexist is the vapor−He-I−He-II triple point at 2.1768K and 5.0418kPa, which is the "saturated vapor pressure" at that temperature (pure helium gas in thermal equilibrium over the liquid surface, in a hermetic container).^[1] The highest pressure at which He-I and He-II can coexist is the bcc−He-I−He-II triple point with a helium solid at 1.762K, 29.725atm.^[2]

λ . 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 a Space Shuttle payload in 1992.^[3]

Although the heat capacity has a peak, it does not tend towards infinity (contrary to what the graph may suggest), but has finite limiting values when approaching the transition from above and below. The behavior of the heat capacity near the peak is described by the formula

C ≈ A _{\pm} t ^{-\alpha} + B _{\pm} where t = | 1- T/T _{c|} is the reduced temperature, T _{c} is the Lambda point temperature, A _{\pm,B} _{\pm} are constants (different above and below the transition temperature), and is the critical exponent: \alpha =-0 . 0127 ( 3 ) .^[4] Since this exponent is negative for the superfluid transition, specific heat remains finite.^[5]

The quoted experimental value of is in a significant disagreement^{[6] [7]} with the most precise theoretical determinations^{[8] [9] [10]} coming from high temperature expansion techniques, Monte Carlo methods and the conformal bootstrap.

See also

• Lambda point refrigerator

External links

• What is superfluidity?

Notes and References

1. The Observed Properties of Liquid Helium at the Saturated Vapor Pressure . Russell J.. Donnelly. Carlo F.. Barenghi . Journal of Physical and Chemical Reference Data. 1998. 27. 6. 1217–1274. 10.1063/1.556028. 1998JPCRD..27.1217D .
2. Thermodynamic properties of ⁴He. II. The bcc phase and the P-T and VT phase diagrams below 2 K . J. K.. Hoffer. W. R.. Gardner. C. G.. Waterfield. N. E.. Phillips. Journal of Low Temperature Physics. April 1976. 23. 1. 63–102. 10.1007/BF00117245. 1976JLTP...23...63H . 120473493.
3. Heat Capacity and Thermal Relaxation of Bulk Helium very near the Lambda Point . J.A.. Lipa. D. R.. Swanson. J. A.. Nissen. T. C. P.. Chui. U. E.. Israelsson. Physical Review Letters. 1996. 76. 6. 944–7. 10.1103/PhysRevLett.76.944. 1996PhRvL..76..944L. 10061591. 2060/19950007794. 29876364. free.
4. Lipa. J. A.. Nissen. J. A.. Stricker. D. A.. Swanson. D. R.. Chui. T. C. P.. 2003-11-14. Specific heat of liquid helium in zero gravity very near the lambda point. Physical Review B. 68. 17. 174518. 10.1103/PhysRevB.68.174518. 2003PhRvB..68q4518L. cond-mat/0310163. 55646571.
5. For other phase transitions \alpha may be negative (e.g. \alpha ≈ +0 . 1 for the liquid-vapor critical point which has Ising critical exponents). For those phase transitions specific heat does tend to infinity.
6. Vicari. Ettore. 2008-03-21. Critical phenomena and renormalization-group flow of multi-parameter Phi4 theories. Proceedings of the XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007). 42. en. Regensburg, Germany. Sissa Medialab. 023. 10.22323/1.042.0023. free.
7. Rychkov. Slava. 2020-01-31. Conformal bootstrap and the λ-point specific heat experimental anomaly. Journal Club for Condensed Matter Physics. en. 10.36471/JCCM_January_2020_02. free.
8. Campostrini. Massimo. Hasenbusch. Martin. Pelissetto. Andrea. Vicari. Ettore. 2006-10-06. Theoretical estimates of the critical exponents of the superfluid transition in $^\mathrm$ by lattice methods. Physical Review B. 74. 14. 144506. 10.1103/PhysRevB.74.144506. cond-mat/0605083. 118924734.
9. Hasenbusch. Martin. 2019-12-26. Monte Carlo study of an improved clock model in three dimensions. 1910.05916. Physical Review B. 100. 22. 224517. 10.1103/PhysRevB.100.224517. 2469-9950. 2019PhRvB.100v4517H. 204509042.
10. Chester. Shai M.. Landry. Walter. Liu. Junyu. Poland. David. Simmons-Duffin. David. Su. Ning. Vichi. Alessandro. Carving out OPE space and precise O(2) model critical exponents. Journal of High Energy Physics. 2020. 2020. 6. 142. 10.1007/JHEP06(2020)142. 1912.03324. 2020JHEP...06..142C. 208910721.