查看“︁Λ点”︁的源代码

{{noteTA| T= zh-cn:λ点; zh-tw:λ點; zh-hk:λ點;}}
[[File:Lambda transition.svg|thumb|250px|比熱容和不同溫度的圖]]
'''λ點'''是[[氦]]從一般流體氦(I)相變到[[超流體]]氦(II)的溫度，在1[[标准大气压]]下約為2.17 [[开尔文|K]]。氦（I）和氦（II）可以共存的最低壓力是在He氣體−He(I)−He(II)的[[三相点]]，是在{{convert|2.1768|K|C}}及{{convert|5.048|kPa|atm|abbr=on}}，是該溫度下的飽和[[蒸氣壓]]（若在{{le|氣封|Hermetic seal}}的容器內，純氦氣會在液體表面形成熱平衡）<ref name=Donnelly>{{cite journal| title=The Observed Properties of Liquid Helium at the Saturated Vapor Pressure | first1=Russell J.| last1=Donnelly| first2=Carlo F.| last2=Barenghi| | journal=Journal of Physical and Chemical Reference Data| year=1998| volume=27| issue=6| pages=1217-1274| doi=10.1063/1.556028|bibcode = 1998JPCRD..27.1217D }}</ref>。氦(I)和氦(II)可以共存的最高壓力是[[立方晶系]]氦固體−He(I)−He(II)的三相点，位在{{convert|1.762|K|C}}, {{convert|29.725|atm|kPa|abbr=on}}<ref name=Hoffer>{{cite journal| title=Thermodynamic properties of <sup>4</sup>He. II. The bcc phase and the P-T and VT phase diagrams below 2 K | first1=J. K.| last1=Hoffer| first2=W. R.| last2=Gardner| first3=C. G.| last3=Waterfield| first4=N. E.| last4=Phillips| | journal=Journal of Low Temperature Physics| date=April 1976| volume=23| issue=1| pages=63-102| doi=10.1007/BF00117245|bibcode = 1976JLTP...23...63H }}</ref>。

λ點的名稱是因為在上述溫度範圍內描繪[[比熱容]]和[[溫度]]的圖時（在上述的壓力下，例如一大氣壓力），會出現[[希腊语|希腊文]]的字母[[λ]]。當溫度接近λ點時，其比熱容會到達其峰值，只有在零重力時才能準確量測到可以說明比熱容發散的臨界指數（為了要讓流體在一體積內的密度是均勻的）。曾在1992年太空船的酬載中量過比λ點低2&nbsp;nK時的熱容<ref name=JPL>{{cite journal| title=Heat Capacity and Thermal Relaxation of Bulk Helium very near the Lambda Point | first1=J.A.| last1=Lipa| first2=D. R.| last2=Swanson| first3=J. A.| last3=Nissen| first4=T. C. P.| last4=Chui| first5=U. E.| last5=Israelsson| journal=Physical Review Letters| year=1996| volume=76| issue=6| pages=944-7| doi=10.1103/PhysRevLett.76.944|bibcode = 1996PhRvL..76..944L }}</ref>。
{{unsolved|物理|說明He-4在超流體形變時熱容臨界指數{{math|''α''}}理論值和實際值之間的差異原因<ref name="Rychkov"/>
}}

熱容的圖上有出現峰值，而附近的斜率很大，但在該點的值不會趨近[[無限大]]，在形變點前後的值都是有限值<ref name=JPL />。熱容在峰值附近的行為可以用<math>C\approx A_\pm t^{-\alpha}+B_\pm</math>公式表示，其中<math>t=|1-T/T_c|</math>是對比溫度（reduced temperature），<math>T_c</math>是Λ点溫度，<math>A_\pm,B_\pm</math>是常數（在形變點前和形變點後各有一組值），{{math|''α''}}為[[临界指数]]：<math>\alpha=-0.0127(3)</math><ref name=JPL /><ref>{{Cite journal|last1=Lipa|first1=J. A.|last2=Nissen|first2=J. A.|last3=Stricker|first3=D. A.|last4=Swanson|first4=D. R.|last5=Chui|first5=T. C. P.|date=2003-11-14|title=Specific heat of liquid helium in zero gravity very near the lambda point|journal=Physical Review B|volume=68|issue=17|pages=174518|doi=10.1103/PhysRevB.68.174518|bibcode=2003PhRvB..68q4518L|arxiv=cond-mat/0310163|s2cid=55646571}}</ref>。因為在超流體相變時，該指數為負，因此比熱仍為有限值

临界指数{{math|''α''}}的實際值和最精準的理論判定技術所得值之間，仍有很大的差異<ref>{{Cite journal|last=Vicari|first=Ettore|date=2008-03-21|title=Critical phenomena and renormalization-group flow of multi-parameter Phi4 theories|journal=Proceedings of the XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007)|volume=42|language=en|location=Regensburg, Germany|publisher=Sissa Medialab|pages=023|doi=10.22323/1.042.0023|doi-access=free}}</ref><ref name="Rychkov">{{Cite journal|last=Rychkov|first=Slava|date=2020-01-31|title=Conformal bootstrap and the λ-point specific heat experimental anomaly|url=https://www.condmatjclub.org/?p=4037|journal=Journal Club for Condensed Matter Physics|language=en|doi=10.36471/JCCM_January_2020_02|doi-access=free|access-date=2024-01-22|archive-date=2020-06-09|archive-url=https://web.archive.org/web/20200609002316/https://www.condmatjclub.org/?p=4037|dead-url=no}}</ref>，這些技術<ref>{{Cite journal|last1=Campostrini|first1=Massimo|last2=Hasenbusch|first2=Martin|last3=Pelissetto|first3=Andrea|last4=Vicari|first4=Ettore|date=2006-10-06|title=Theoretical estimates of the critical exponents of the superfluid transition in $^{4}\mathrm{He}$ by lattice methods|journal=Physical Review B|volume=74|issue=14|pages=144506|doi=10.1103/PhysRevB.74.144506|arxiv=cond-mat/0605083|s2cid=118924734}}</ref><ref>{{Cite journal|last=Hasenbusch|first=Martin|date=2019-12-26|title=Monte Carlo study of an improved clock model in three dimensions|arxiv=1910.05916|journal=Physical Review B|volume=100|issue=22|pages=224517|doi=10.1103/PhysRevB.100.224517|issn=2469-9950|bibcode=2019PhRvB.100v4517H|s2cid=204509042}}</ref><ref>{{cite journal|last1=Chester|first1=Shai M.|last2=Landry|first2=Walter|last3=Liu|first3=Junyu|last4=Poland|first4=David|last5=Simmons-Duffin|first5=David|last6=Su|first6=Ning|last7=Vichi|first7=Alessandro|title=Carving out OPE space and precise O(2) model critical exponents|journal=Journal of High Energy Physics|year=2020|volume=2020|issue=6|page=142|doi=10.1007/JHEP06(2020)142|arxiv=1912.03324|bibcode=2020JHEP...06..142C|s2cid=208910721}}</ref>包括高溫膨脹技術、[[蒙地卡羅方法]]以及{{le|Conformal bootstrapping|Conformal bootstrapping}}。

== 相關條目 ==
* {{le|Λ点冷凍機|Lambda point refrigerator|λ點冷凍機}}

== 參考資料 ==
{{reflist}}

== 外部連結 ==
* [https://web.archive.org/web/20130927063921/http://www.egglescliffe.org.uk/physics/supercond/supfluid/superfluids.html What is superfluidity?]

{{物质状态}}
[[Category:阈温]]