---
title: "Landau’s helium: describe the liquid’s excitations"
description: "How did collective excitations connect heat, sound and superfluid motion?"
canonical_url: "https://sandboxphysics.com/stories/condensed-matter/landau-superfluid-helium/"
language: "en"
page_type: "story"
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  zh-CN: "https://sandboxphysics.com/zh/stories/condensed-matter/landau-superfluid-helium/"
last_modified: "2026-10-23"
---

[Cold liquids and superconducting order](https://sandboxphysics.com/stories/condensed-matter/#collective)

Materials · Measurements · Explanation

# Landau’s helium: describe the liquid’s excitations

How did collective excitations connect heat, sound and superfluid motion?

Sandbox Physics · 2026-10-23

Nobel contribution: 1962-1

1. Excite the liquid
2. Assign energy to collective motion
3. Compare heat and sound
4. Test the excitation spectrum

The excitation spectrum constrains ideal dissipationless flow. Vortex nucleation and boundaries can cause flow to fail earlier.

## The relevant particles were collective excitations

Landau’s 1941 theory organized low-energy motion in liquid helium through quasiparticles. Phonon-like excitations and a roton branch represented motions of the strongly interacting liquid, rather than independent helium atoms.

The 1962 award recognized a much broader body of condensed-matter theory, especially liquid helium. The helium account illustrates how an effective description can make quantitative thermodynamic predictions.

Sources: [2]

## Energy and momentum constrain possible dissipation

A moving fluid can lose energy by creating an excitation only when both energy and momentum allow it. Minimizing excitation energy divided by momentum gives the Landau threshold for that mechanism. It is a criterion for excitation creation, not a guaranteed engineering flow speed.

$$
v_{\mathrm L}=\min_{p>0}\frac{\epsilon(p)}{p}
$$

Sources: [2]

## Heat and neutron scattering tested different predictions

The two-fluid description connects an entropy-carrying normal component to a superfluid component. Second sound probes coupled thermal motion. Neutron scattering later tested excitation energies more directly, providing information different from a narrow-channel flow measurement.

Sources: [2]

## Dilute-gas occupation is useful but incomplete context

The linked occupation model explains quantum statistics, and the crossover model discusses pairing. Dense helium needs interactions and collective excitations beyond either simple picture. Condensate fraction and superfluid density answer different questions and should not be identified.

Sources: [2] [1] [3]

Continue with the physics

## Try the related models

### [Pairing across the crossover](https://sandboxphysics.com/labs/bec-bcs-crossover/)

Compare weak pairs with tightly bound pairs.

The teaching crossover does not predict a cuprate mechanism or helium-three order parameter.

### [Quantum gas occupation](https://sandboxphysics.com/labs/quantum-gas-occupation/)

Lower temperature and compare Bose and Fermi occupations.

Equilibrium occupations alone do not reconstruct a time-of-flight absorption image or establish superfluidity.

## Sources and reading notes

1. [Leggett: Superfluid helium-3, the early days as seen by a theorist (2003)](https://www.nobelprize.org/uploads/2018/06/leggett-lecture.pdf)

   Spin-triplet order and nuclear resonance connect the new liquid phases to microscopic pairing.

2. [1962 award presentation: Landau’s liquid-helium theories](https://www.nobelprize.org/prizes/physics/1962/ceremony-speech/)

   Official contemporary account of collective excitations, neutron tests and second sound.

3. [Nobel Prize in Physics 1962: official award](https://www.nobelprize.org/prizes/physics/1962/summary/)

   Official attribution and award year. The observation or theoretical paper may be much earlier.

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