λ点

λ点是氦从一般流体氦(I)相变到超流体氦(II)的温度，在1标准大气压下约为2.17 K。氦（I）和氦（II）可以共存的最低压力是在He气体−He(I)−He(II)的三相点，是在2.1768 K（−270.9732 °C）及5.048 kPa（0.04982 atm），是该温度下的饱和蒸气压（若在气封（英语：Hermetic seal）的容器内，纯氦气会在液体表面形成热平衡）^[1]。氦(I)和氦(II)可以共存的最高压力是立方晶系氦固体−He(I)−He(II)的三相点，位在1.762 K（−271.388 °C）, 29.725 atm（3,011.9 kPa）^[2]。

λ点的名称是因为在上述温度范围内描绘比热容和温度的图时（在上述的压力下，例如一大气压力），会出现希腊文的字母λ。当温度接近λ点时，其比热容会到达其峰值，只有在零重力时才能准确量测到可以说明比热容发散的临界指数（为了要让流体在一体积内的密度是均匀的）。曾在1992年太空船的酬载中量过比λ点低2 nK时的热容^[3]。

未解决的物理问题：说明He-4在超流体形变时热容临界指数

α

理论值和实际值之间的差异原因^[4]

热容的图上有出现峰值，而附近的斜率很大，但在该点的值不会趋近无限大，在形变点前后的值都是有限值^[3]。热容在峰值附近的行为可以用 $C\approx A_{\pm }t^{-\alpha }+B_{\pm }$ 公式表示，其中 $t=|1-T/T_{c}|$ 是对比温度（reduced temperature）， $T_{c}$ 是Λ点温度， $A_{\pm },B_{\pm }$ 是常数（在形变点前和形变点后各有一组值）， $α$ 为临界指数： $\alpha =-0.0127(3)$ ^[3]^[5]。因为在超流体相变时，该指数为负，因此比热仍为有限值

临界指数 $α$ 的实际值和最精准的理论判定技术所得值之间，仍有很大的差异^[6]^[4]，这些技术^[7]^[8]^[9]包括高温膨胀技术、蒙地卡罗方法以及Conformal bootstrapping（英语：Conformal bootstrapping）。

参考资料[编辑]

^ Donnelly, Russell J.; Barenghi, Carlo F. The Observed Properties of Liquid Helium at the Saturated Vapor Pressure. Journal of Physical and Chemical Reference Data. 1998, 27 (6): 1217–1274. Bibcode:1998JPCRD..27.1217D. doi:10.1063/1.556028.
^ Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. Thermodynamic properties of ⁴He. II. The bcc phase and the P-T and VT phase diagrams below 2 K. Journal of Low Temperature Physics. April 1976, 23 (1): 63–102. Bibcode:1976JLTP...23...63H. doi:10.1007/BF00117245.
^ ^3.0 ^3.1 ^3.2 Lipa, J.A.; Swanson, D. R.; Nissen, J. A.; Chui, T. C. P.; Israelsson, U. E. Heat Capacity and Thermal Relaxation of Bulk Helium very near the Lambda Point. Physical Review Letters. 1996, 76 (6): 944–7. Bibcode:1996PhRvL..76..944L. doi:10.1103/PhysRevLett.76.944.
^ ^4.0 ^4.1 Rychkov, Slava. Conformal bootstrap and the λ-point specific heat experimental anomaly. Journal Club for Condensed Matter Physics. 2020-01-31. doi:10.36471/JCCM_January_2020_02  （英语）.
^ Lipa, J. A.; Nissen, J. A.; Stricker, D. A.; Swanson, D. R.; Chui, T. C. P. Specific heat of liquid helium in zero gravity very near the lambda point. Physical Review B. 2003-11-14, 68 (17): 174518. Bibcode:2003PhRvB..68q4518L. S2CID 55646571. arXiv:cond-mat/0310163 . doi:10.1103/PhysRevB.68.174518.
^ Vicari, Ettore. Critical phenomena and renormalization-group flow of multi-parameter Phi4 theories. Proceedings of the XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007) (Regensburg, Germany: Sissa Medialab). 2008-03-21, 42: 023. doi:10.22323/1.042.0023  （英语）.
^ Campostrini, Massimo; Hasenbusch, Martin; Pelissetto, Andrea; Vicari, Ettore. Theoretical estimates of the critical exponents of the superfluid transition in $^{4}\mathrm{He}$ by lattice methods. Physical Review B. 2006-10-06, 74 (14): 144506. S2CID 118924734. arXiv:cond-mat/0605083 . doi:10.1103/PhysRevB.74.144506.
^ Hasenbusch, Martin. Monte Carlo study of an improved clock model in three dimensions. Physical Review B. 2019-12-26, 100 (22): 224517. Bibcode:2019PhRvB.100v4517H. ISSN 2469-9950. S2CID 204509042. arXiv:1910.05916 . doi:10.1103/PhysRevB.100.224517.
^ 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. Bibcode:2020JHEP...06..142C. S2CID 208910721. arXiv:1912.03324 . doi:10.1007/JHEP06(2020)142.

外部链接[编辑]

What is superfluidity?

[Donnelly-1] Donnelly, Russell J.; Barenghi, Carlo F. The Observed Properties of Liquid Helium at the Saturated Vapor Pressure. Journal of Physical and Chemical Reference Data. 1998, 27 (6): 1217–1274. Bibcode:1998JPCRD..27.1217D. doi:10.1063/1.556028.

[Hoffer-2] Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. Thermodynamic properties of ⁴He. II. The bcc phase and the P-T and VT phase diagrams below 2 K. Journal of Low Temperature Physics. April 1976, 23 (1): 63–102. Bibcode:1976JLTP...23...63H. doi:10.1007/BF00117245.

[JPL-3] 3.0 ^3.1 ^3.2 Lipa, J.A.; Swanson, D. R.; Nissen, J. A.; Chui, T. C. P.; Israelsson, U. E. Heat Capacity and Thermal Relaxation of Bulk Helium very near the Lambda Point. Physical Review Letters. 1996, 76 (6): 944–7. Bibcode:1996PhRvL..76..944L. doi:10.1103/PhysRevLett.76.944.

[Rychkov-4] 4.0 ^4.1 Rychkov, Slava. Conformal bootstrap and the λ-point specific heat experimental anomaly. Journal Club for Condensed Matter Physics. 2020-01-31. doi:10.36471/JCCM_January_2020_02  （英语）.

[5] Lipa, J. A.; Nissen, J. A.; Stricker, D. A.; Swanson, D. R.; Chui, T. C. P. Specific heat of liquid helium in zero gravity very near the lambda point. Physical Review B. 2003-11-14, 68 (17): 174518. Bibcode:2003PhRvB..68q4518L. S2CID 55646571. arXiv:cond-mat/0310163 . doi:10.1103/PhysRevB.68.174518.

[6] Vicari, Ettore. Critical phenomena and renormalization-group flow of multi-parameter Phi4 theories. Proceedings of the XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007) (Regensburg, Germany: Sissa Medialab). 2008-03-21, 42: 023. doi:10.22323/1.042.0023  （英语）.

[7] Campostrini, Massimo; Hasenbusch, Martin; Pelissetto, Andrea; Vicari, Ettore. Theoretical estimates of the critical exponents of the superfluid transition in $^{4}\mathrm{He}$ by lattice methods. Physical Review B. 2006-10-06, 74 (14): 144506. S2CID 118924734. arXiv:cond-mat/0605083 . doi:10.1103/PhysRevB.74.144506.

[8] Hasenbusch, Martin. Monte Carlo study of an improved clock model in three dimensions. Physical Review B. 2019-12-26, 100 (22): 224517. Bibcode:2019PhRvB.100v4517H. ISSN 2469-9950. S2CID 204509042. arXiv:1910.05916 . doi:10.1103/PhysRevB.100.224517.

[9] 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. Bibcode:2020JHEP...06..142C. S2CID 208910721. arXiv:1912.03324 . doi:10.1007/JHEP06(2020)142.

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