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The Two-Fluid Theory and Second Sound in Liquid Helium

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The two-fluid theory and second sound in feature liquid helium Russell J. Donnelly Laszlo Tisza’s contributions to our understanding of superfluid helium are often overshadowed by Lev Landau’s, but Tisza’s insights are still paying dividends—and not just for helium. Russell Donnelly is a professor of physics at the University of Oregon in Eugene. Early research with liquid helium showed that it had re- dissipation. The uncondensed atoms, in contrast, constituted markable new properties, especially below the so-called a normal fluid that was responsible for phenomena such as lambda transition near 2.2 K, where there is a sharp, narrow the damping of pendulums immersed in the fluid. That rev- peak in the specific heat (see figure 1a). The liquid below the olutionary idea demanded a “two-fluid” set of equations of transition was named helium II to distinguish it from the liq- motion and, among other things, predicted not only the exis- uid above the transition, called helium I. Three properties dis- tence of ordinary sound—that is, fluctuations in the density covered in the late 1930s were particularly puzzling. First, a of the fluid—but also fluctuations in entropy or temperature, test tube lowered partly into a bath of a b helium II will gradually fill by means of 120 a thin film of liquid helium that flows Roton branch without friction up the tube’s outer wall. Second, in the thermomechanical effect, if two containers are connected by a 80 Phonon branch very thin tube that can block any vis- Helium II Helium I cous fluid, an increase in temperature in Δ one container will be accompanied by a 40 ENERGY rise in pressure, as seen by a higher liq- Roton energy uid level in that container. Third is the gap viscosity paradox: The oscillations of a SPECIFIC HEAT (cal/gK) torsion pendulum in helium II will 0 012345 MOMENTUM gradually decay with an apparent vis- TEMPERATURE (K) cosity about one-tenth that of air, but if c d liquid helium is made to flow through a 20 very fine tube, it will do so with no ob- servable pressure drop—the apparent viscosity is not only small, it is zero! Maxons Laszlo Tisza was captivated by an Δ idea being pushed in those days by Fritz ENERGY Roton 10 R− R+ London that the transition to helium II Phonons energy gap Δ was a manifestation of Bose–Einstein ENERGY (K) Rotons condensation (BEC). In 1925 Albert Ein- p 0 stein had generalized a calculation for MOMENTUM photons by Satyendra Nath Bose to Phonons massive particles, such as atoms; he Figure 1. Properties of liquid helium. (a) The 0 p found that for an ideal gas of identical, specific heat of liquid helium has an anomalous 00 4.0 −1 integer-spin particles below a certain jump near 2.2 K. The “lambda transition” between MOMENTUM (Å ) temperature, a macroscopic fraction of the superfluid phase (helium II) at lower temperatures and the normal phase (helium I) the particles accumulates, or condenses, at higher temperatures is so named because the anomaly resembles the Greek letter λ. in the lowest- energy single- particle (b) Lev Landau’s 1941 dispersion curve for the excitations in superfluid helium II had two state with zero momentum. After one of branches: a linear curve for phonons, with a slope corresponding to the velocity of his discussions with London and in- sound, and a quadratic curve with an energy gap Δ for a new class of excitations he spired by the recently discovered ef- called rotons. (c) Landau modified the curve in 1947 so that phonons (low- momentum fects, Tisza had the idea that the Bose- excitations) and rotons (excitations near the minimum at momentum p ) were parts of a condensed fraction of helium II formed 0 single excitation branch. (d) An experimentally measured dispersion curve obtained by a superfluid that could pass through inelastic neutron scattering. The curve’s exact form varies with temperature and pressure. narrow tubes and thin films without 34 October 2009 Physics Today © 2009 American Institute of Physics, S-0031-9228-0910-020-0 Box 1. The velocities of first and second sound in helium II In Laszlo Tisza’s two-fluid model of helium II, the normal and superfluid components each have their own independent den- ρ ρ sity ( n and s) and velocity (vn and vs). The total density of the liquid is the sum of the densities of the two components: ΔP ρ ρ ρ = n + s. For the two-fluid system, the equations of motion can be linearized (as is appropriate for sound waves) to obtain n s n s n s n s s s n n s s s n v ρ s n n s n n s n n n s n n ∂ n s s s n s n s s s s n n 2 n n n n s s n n n n s s ρρη=−∇∇∇PST + + v s s n s s s n s nst ρ n s s s s s s s ∂ n n n n n n n n s s s n n n n s n s n s n n s s n s s n n s n s n s n s n s n n v ρ n n s s n s s n s s n s s ∂ s s n n n s n s ρρ=−∇∇PST. − s n s n s n s n ss∂t ρ Here, P is pressure, S entropy, T temperature, and η viscosity. If the viscous term is omitted, then after some manipulation two wave equations emerge: ΔT ∂2ρ =,∇2P ∂t 2 ∂2SρS2 n s s s s s n n n n n s s s s s n n n s 2 s n s s s n s n n n s n s s s n s n n = ∇ T. n s n s n s n s n s n s n s n s n s n ∂t 2 ρ s n s n s n s n s n s n s n s n s n s n n s n s n s n s n s n s n s n s n s n s n s s s n s n n n s n s s s n s n n n s s s s s n n n n n s s s s s n n n These equations produce two velocities of “sound”: 2 ∂PTSρ 2 2 s 2 and s on any vertical line represent schematically the relative u ≈ S ,.uT≈ ∇ 1 ( ∂ρCρ( 2 portions of the normal and superfluid components; the total n number of symbols on a vertical line represents the total density The derivation is valid only if the heat capacities at constant of the fluid. The waves are traveling horizontally. Here, first pressure and constant volume (denoted by C) are nearly equal, sound’s fluctuations in density are driven by changes in pres- u as they are in liquid helium. The velocity 1 is the usual expres- sure, and the two components move in phase with each other. sion for the speed of sound. The expression for the second- Second sound’s fluctuations in entropy (carried by the normal u 3 sound velocity 2 was first given by Lev Landau. fluid) are driven here by changes in temperature; the two com- The figure contrasts the waves constituting first sound (top) ponents are out of phase, and the density remains constant to and second sound (bottom). The proportions of the symbols n first order. (Figure adapted from ref. 10.) which were given the designation “second sound” by Russian put forward by Landau3 in 1941 and then modified in 1947 physicist Lev Landau (see box 1). By 1938 Tisza’s and Lon- (see figures 1b and 1c and box 2). Phonons are quantized, col- don’s papers had at least qualitatively explained all the exper- lisionless sound waves (not to be confused with ordinary, hy- imental observations available at the time: the viscosity para- drodynamic sound waves) and occur in crystals as well as su- dox, frictionless film flow, and the thermomechanical effect.1 perfluid helium. Rotons are higher- energy excitations, and their properties are still under study. An experimental quasi- A mixture of two fluids particle spectrum is shown in figure 1d. Tisza’s two-fluid hypothesis eventually became the basis for Today the most fundamental aspect of a superfluid is understanding the behavior of helium II. (For a recent history considered to be the Bose–Einstein condensate. The conden- of ideas on superfluidity in helium II, see reference 2.) The sate is described by a quantum mechanical wavefunction that flow of helium II acts as if it were a mixture of two fluids. characterizes a macroscopic number of atoms in a single One, called the superfluid, has no viscosity or entropy and quantum state—a coherent matter wave. The coherence of can flow without dissipation through extremely narrow the wavefunction leads to, among other things, the quantiza- channels. The other, called the normal fluid, does have a finite tion of circulation: In particular, if a container of superfluid viscosity η and carries all the entropy S. The liquid is pictured is rotated, quantized vortex lines will appear arrayed parallel to be all normal fluid at the lambda transition and all super- to the axis of rotation, an entire subject in itself.
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