Anomalous Quasiparticle Reflection from the Surface of a 3He-4He Dilute Solution
Dimensionality is an important parameter in physics as one can see from the example of magnetism. Over the years, a large amount of research has been carried out on what happens to physical properties of a system when its dimensionality changes. Dimensionality could change from 3D to 2D to 1D and even down to 0D. However, interactions within a mixed-dimensional system, that is a system composed of sub-systems with different dimensions, has not been studied extensively. A mixture of 3He and 4He liquid offers a fairly unique opportunity to study such mixed-dimensional interactions. Researchers from the Department of Physics at KAIST, in collaboration with Japanese and Ukrainian low temperature physicists, examined such interactions and found a new type of surface scattering.
A liquid surface is free from defects or impurities and typically forms a perfectly specular surface, that is, a particle will bounce off from the surface with an identical incident angle and reflection angle. This is true for most mixtures of liquids also. Every rule is bound to have an exception, however. A research team led by Prof. Hyoungsoon Choi of the Department of Physics at KAIST, in collaboration with RIKEN in Japan and the Institute for Low Temperature Physics and Engineering in the Ukraine, recently found an exception to this phenomenon.
As is often the case in physics, quantum mechanics dictates this strange behavior.
When a small amount of liquid 3He is mixed with liquid 4He, i.e. in a 1:14 ratio or less, at temperatures well below 2 K, liquid 4He turns into a superfluid whereas liquid 3He remains a non-superfluid. Superfluid 4He has no viscosity and very little energy or entropy; it simply behaves like fluidic vacuum for 3He particles to swim inside freely.
3He particles in liquid 4He like to float to the free surface of the liquid. This is because the 3He atom is less massive than 4He and the quantum mechanical zero-point motion is larger for the lighter isotope. This zero-point motion forces the 3He atom to find room to jiggle around and that is the free surface of the liquid where it makes a contact with an empty space. When the free surface is saturated with roughly one monolayer of 3He, additional 3He particles find themselves inside the bulk of the liquid. This process results in two different types of liquid 3He in the 3He-4He mixture. One type is (i) a very dense two dimensional (2D) liquid of 3He situated on the free surface, and the other is (ii) a relatively dilute three dimensional (3D) bulk liquid.
Chemical potential of 2D dense liquid 3He and 3D dilute liquid 3He must be identical at equilibrium, but their kinetic energy components, usually called the degenerate Fermi energy, are vastly different due to the big negative binding energy on the surface.
In fact, 3He quasiparticles inside the 2D liquid has about a ten times greater kinetic energy than those inside the 3D liquid. Due to this stark contrast, when the 3He quasiparticles in the bulk reflect off of the free surface, they do not reflect specularly anymore. Instead they reflect diffusively, i.e. reflection angle is distributed over a wide range, irrespective of the incident angle. However, these are not typical diffusive reflections that happen on solid surfaces. These are a new and unique type of diffusive reflection, dubbed as “accommodated diffusive reflection” by the researchers.
This new process becomes possible because a small amount of in-plane momentum change of the 2D 3He quasiparticles have a much more dramatic effect in the relative momentum change of the 3D quasiparticles. When a 3He quasiparticle from the bulk approaches the surface, it can be accommodated by the surface and reaches an equilibrium with the 2D 3He. When it is ejected from the surface, it can have any amount of in-plane momentum available to the 2D 3He, hence, the reflection is diffusive. This is a unique and interesting example of an interaction within a composite system with mixed-dimensionality.
The nature of reflection, whether it is specular or diffusive, is probed with electrons floating on top of the 3He-4He liquid surface. Mobility of electrons on liquid helium can be accurately measured; impulses from the 3He quasiparticles bouncing off of the surface has a direct consequence on the electron mobility. The accommodated diffusive reflection imparts a considerably small amount of impulse to the electron, the electron mobility is enhanced. This interaction among 2D & 3D 3He quasiparticles may play an important role in the formation of 2D superfluid 3He which is yet to be found!
* lab webpage : http://absoluteze.ro