Quantum Fluids Research Group

A Quantum Fluid can show macroscopic quantum behaviour. For example, if a rotational state is set up in such a fluid then it can persist indefinitely - it appears as though the liquid has zero viscosity, i.e. it is superfluid. This orbital motion is similar to the familiar electron orbitals in an atom except that the radius is macroscopic in a superfluid. This quantum behaviour is a consequence of Bose-Einstein condensation. Very few systems show this, liquid ⁴He is the most accessible but it has recently been reported that it occurs in small numbers of rubidium atoms and it is considered to exist in neutron stars. However, only liquid helium is superfluid on Earth.

At Exeter we are studying quantum evaporation, the non-wetting of alkali metals and film flow in a quantum solid. Liquid ⁴He is the only liquid in which the evaporation process can be studied at the atomic level. This is because the evaporation is caused by phonons and rotons which can be created in the liquid as a ballistic beam. When a sufficiently energetic phonon or roton reaches the free surface of the liquid ⁴He, it is destroyed and all the energy is given to the atom which escapes the liquid. This quantum evaporation process is analogous to the photo-electric effect and it is a direct proof that phonons are quantised. The properties of quantum evaporation are being studied, including the difficult problem of measuring the quantum efficiency of the evaporation. However the process of quantum evaporation is well enough understood for us to use it to study, in a depth that was impossible before, the properties of phonons and rotons. Their behaviour shows many surprises, for example a cloud of low energy phonons can convert a major part of its energy into high energy phonons (this does not violate thermodynamics!).

Liquid ⁴He wets most materials but it does not wet cesium or rubidium and so these systems are excellent for studying the phenomena of non-wetting and pre-wetting which are difficult to do with classical fluids. We have done an experiment that shows cesium is particularly free of ⁴He at temperatures well below the wetting temperature. We have measured the contact angle between liquid helium and cesium and have shown that it goes to zero in a first order phase transition at the wetting temperature.The magnitude of the contact angle at low temperatures indicates ripplons (quantised capillary waves) at the cesium-helium interface; this work is continuing and we are planning to measure the contact angle by optical imaging. On rubidium we have measured the wetting temperature and showed that the two ⁴He films of different thicknesses that coexist are probably both superfluid. Solid hydrogen is a quantum solid with a large zero-point energy. This means that the molecules on the surface solid hydrogen do not stay at fixed lattice sites as with normal solids do but move around to go to the lowest free energy configuration. We have developed a model that shows that the process is activated diffusion.

Quantum fluids research gives the possibility of discovering new phenomena and tackling original and fundamental physics. There are a number of groups in England studying Quantum Fluids and we enjoy a high international reputation. Our PhD students have little problem in getting good jobs.

Our current projects are:

1. We have recently predicted a method to form a free standing slab of liquid helium so both sides are accessible. The aim is to fire helium atoms at the top side and measure the atom flux coming from the other side. When an atom condenses on the top side there is probability that it will produce a roton. This roton will then travel through the liquid helium to the other side where there is a probability that it will evaporate an atom. From measurements of the incident and emitted fluxes we will obtain the product of these probabilities. These can then be compared with the results of theory.

2. ³He forms a two-dimensional layer on the surface of liquid ⁴He. This is a two-dimensional Fermi system. The ³He atoms that are quantum evaporated from liquid ⁴He probably come from the outer regions of the surface density profile as the atoms are not scattered as they evaporate. However the excitations which evaporate them are modified by this change in density. By seeing where the phonon-atom interaction takes place we hope to learn about this modification. We use ³He atoms which occupy surface states and we measure the relative probabilities of ³He and ⁴He evaporation. Furthermore, we can investigate the two-dimensional Fermi system of the ³He.

3. The interface between liquid ⁴He and cesium is very interesting. A pure cesium surface presents the weakest binding substrate for liquid ⁴He and we have recently shown that the liquid ⁴He at this interface behaves very similarly to the free liquid surface. This is a very surprising result and is obtained from measuring the contact angle of ⁴He on cesium below the wetting temperature. We are currently studying ³He bound states at this interface and have designed a new experiment to measure the contact angle at the lower wetting temperature.

4. The surface of solid H₂ is much more mobile than other solids.We have studied films of hydrogen which are four layers thick, and by creating holes in these films we can watch their return to equilibrium.This is done by evaporating molecules and measuring the flux with a bolometer.We have developed a model that accounts for these measurements which shows that the hydrogen molecules move by thermally activated diffusion.

5. We have a theoretical collaboration with the University of Kharkov, Ukraine. We are currently developing a model for the creation of high energy phonons in liquid helium. A propagating cloud of low energy phonons creates high energy phonons by phonon-phonon up-scattering and once they reach a certain critical energy they are relatively stable.The high energy phonons are lost out of the back of the cloud as their group velocity is less than that of the low energy phonons. This process is very efficient in transforming energy from low to high energy phonons which agrees with experiments.

6. The quantum efficiency of evaporation has been predicted but it is difficult to measure. If a phonon is incident on the free surface of ⁴He it can reflect, evaporate an atom or perhaps create a riplon and another phonon. As we cannot measure the flux of phonons directly we have to detect reflected phonons and show that the remaining phonons evaporate the atoms that we detect. In this way we can put a lower limit on the quantum efficiency of the process. This is a crucial number to compare with theoretical calculations.

Our philosophy is to answer current fundamental questions about quantum fluids. To do this we design unique experiments and construct theoretical models. This is an area in which an individual can be creative. We have many contacts world-wide, from Russia to Australia including France, Germany, Finland, Italy, Ukraine and U.S.A. We keep in touch by email, visits to each others' laboratories, international workshops and conferences. We are very much part of a world-wide exploration of quantum fluids.

We are mainly funded by EPSRC and currently have two substantial grants from them. Other grants are from the EU.

We have a well-equipped laboratories including five dilution refrigerators of various sizes so that each research project has ready access to millikelvin temperatures. The detection and data-capture electronics is state of the art and enables us to detect subtle effects. We have a range of vacuum evaporators for producing thin-film devices used in the experiments and an ultra high vacuum system for preparing atomically clean alkali metal surfaces. The School has recently installed a new liquefier so there is a ready supply of liquid helium for experiments. The group enjoys the support of a technician who helps build new experiments.

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