Roton (physics)

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The roton is an excitation of superfluid fluids that was introduced by the Russian physicist Lev Landau . This quasiparticle describes the macroscopic rotational states of the superfluid liquid.

historical overview

The dispersion relation of helium II excitations.
For one has the linear increase of the phonons, after a maximum one finds the rotons as metastable excitation.

Landau's approach in 1941 to the consideration of helium II consisted in the quantization of the two macroscopic quantities, density and velocity of the liquid. In addition to the dispersion relation of the collective excitation, he also considered the heat capacity , derived the hydrodynamic equations of the quantum fluid and predicted the second sound . For low temperatures, he described the system as a two-fluid model, consisting of a superfluid and a normal-fluid component. The proportion of the superfluid component is temperature-dependent; this component disappears with the phase transition from helium II to helium I at the lambda point . As a result of this macroscopic theory, he distinguished phonons and rotons as two possible collective excitations of the entire quantum fluid. He ascribed the name Roton to Igor Evgenyevich Tamm .

Andronikashvili tried experimentally to test Landau's postulates in 1946. He measured the viscosity of helium II by lowering 100 wafer-thin, round aluminum foils approx. 13 μm thick at a distance of 0.2 mm into the helium, which were suspended in the center on a wire around which they could rotate. With this arrangement he had the greatest possible momentum transfer from the suspended foils to the liquid helium, which he wanted to set in rotation. He found that the superfluid component only moves at a certain minimum speed. His experiments aimed only at macroscopic sizes.

Richard Feynman proposed in the late 1950s to investigate the dispersion relation of the excitations by means of scattering experiments with neutrons . The results agreed with Landau's postulates.

The experiments of William Frank Vinen showed that the macroscopic rotational movement is quantized and that the rotons carry quanta of the angular momentum .

Individual evidence

  1. ^ Lew Landau : The theory of superfluidity of helium II . In: D. Ter Haar (Ed.): Collected Papers of LD Landau . S.  301 ( chapter 46 in Google Book Search - reprint of JPhys USSR 5, 71, 1941; JETP 11, 592, 1941).
  2. ↑ a b J. Wilks: The Theory of Liquid 4 He . In: Rep. Prog. Phys. tape 20 , 1957, pp. 38-85 , doi : 10.1088 / 0034-4885 / 20/1/302 ( University of Michigan [PDF; accessed December 14, 2014]). University of Michigan ( Memento of the original from September 24, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.pa.msu.edu
  3. ↑ E. Andronikashvili. J. Phys. USSR 10, 201 (1946) J. Exp. Theor. Phys. USSR 18, 429 (1946)
  4. ↑ KR Atkins: Liquid Helium . Cambridge University Press, 2014, ISBN 978-1-107-63890-7 ( limited preview in Google Book Search).
  5. ^ Richard P. Feynman : Superfluidity and Superconductivity . In: Rev. Mod. Phys. tape  29 , 1957, pp. 205 , doi : 10.1103 / RevModPhys.29.205 .
  6. ^ John Bardeen: Solid-State Physics: Accomplishments and Future Prospects . In: Sanborn Conner Brown (Ed.): Physics 50 Years Later . National Academies, 1873 ( limited preview in Google Book Search).
  7. ↑ Ludwig Bergmann, Clemens Schaefer: Textbook of Experimental Physics . Volume 5. Gases, Nanosystems, Liquids. Walter de Gruyter, 2005, ISBN 978-3-11-017484-7 ( limited preview in Google book search).
  8. ↑ WF Vinen: Detection of Single Quanta of Circulation in Rotating Helium II . In: Nature . tape 181 , 1958, pp. 1524–1525 , doi : 10.1038 / 1811524a0 ( abstract [accessed December 14, 2014]).
  9. ↑ WF Vinen: The Detection of Single Quanta of Circulation in Liquid Helium II . In: Proceedings A of the Royal Society . tape 260 , no. 1301 , 1961, pp. 218 , doi : 10.1098 / rspa.1961.0029 ( abstract [accessed December 14, 2014]).