3He-4He Mixture Films

Helium Mixture Films

Most recent update: fall 2010


We are engaged in a long-term systematic study of the NMR and third sound properties of 3He-4He mixture films. Initial studies confirmed the Fermi gas character of such films at low temperatues for low 3He coverages [1], and observed the step-like structure of the magnetization as a function of 3He coverage [2]. More detailed measurements of the relaxation times and magnetization have allowed a determination of the energetics of the 3He in the 4He film environment [3,4], and the energies of the ground state and first excited state for the 3He are in good agreement with current theoretical calculations. Recent work has focused on the diffusion of the 3He in the 4He film, and has resulted in the observation of a dramatic change in the difusion coefficient as a function of 4He coverage over a narrow range of 4He coverage for 0.1 layers of 3He on the 4He film [5]. This mobility edge is apparently analogous to a metal-insulator transition in the case of a conducting system. Recent work documented this mobility edge [7] and compared the observations to theoretical work on localized Fermi particles. This recent work also uncovered an unexpected maximum in the diffusion coefficient as a function of 3He coverage[8].

Figure 1: The energy of the ground state and the first excited state for 3He in the environment of a 4He film as a function of bulk-density equivalent 4He layers. The smooth curves are theoretical predictions from density functional calculations (dashed) due to Treiner and his colleagues, and from variational calculations due to Krotscheck and his colleagues.

In parallel experiments we are investigating the velocity and damping of third sound in the mixture film system with a third sound resonator which is located in the same experimental cell as is the NMR resonator. Early results [6] from this work show interesting structure in the third sound velocity as a function of temperature which is not explained by current theory. An interesting transition of some sort is present in the film system near a temperature of 200 mK, with changes apparent in the velocity, the damping and the phase of the resonance. Presumably these changes are related to the configuration of the 3He in the film, although no theory which adequately explains the observations has been advanced. There is a gentle peak in the resonance frequency near 150 mK at the higher converages (which is not shown well in this figure) which has not been explained.
Figure 2: The third sound frequency, (a), and phase between the third sound driver and capacitive detector, (b), as a function of temperature. The frequency is shown for various thickness of 3He (0.099 [top] to 0.721 [bottom] monolayers) on a 5.0 (bulk equivalent) layer thickness 4He film. The phase is shown for several drive powers for the case of a 3He film of coverage 0.68 monolayers. From [6].

More recently we have initiated a study of the specific heat of the 3He on thin 4He films. This work is valuable in its own right, but especially so when viewed in the context of our previous NMR work on these films. Early results from the specific heat experiment[9] confirm the presence of a step in the specific heat in analogy with that seen in the magnetization. When this work is completed, we will be in a good position to determine the two most important Fermi liquid parameters for the two-dimensional 3He system [10].
Our most recent work involves a study of very thin helium 3He-4He mixture films on a hydrogen substrate. There we have observed a very unusual second decoupling of mass[11] from the substrate, but only in the presence of 3He. This requires further study.


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[9] P-C. Ho and R.B. Hallock, J. Low Temp. Phys. (QFS 2000, in press).
[10] R.B. Hallock, Physics Today 51, 30 (1998).
[11] P.T. Finley, P.S. Ebey and R.B. Hallock, "Unexpected Mass Decoupling for 3He - 4He Films on a Solid Hydrogen Substrate", Phys. Rev. Lett. 98, 265301 (2007).