| dc.description.abstract | Liquid helium surfaces display fascinating phenomena resulting from the interaction be-
tween helium atoms and excess electrons. These interactions are characterized by (i) a
long-range attraction due to the finite polarizability of helium atoms and (ii) a short-range
repulsion due to the Pauli exclusion principle. Due to the combined potential, the excess
free electrons with energy less than 1 eV form a two-dimensional electron layer on the
liquid helium surface.
Beyond a critical electron density, the liquid helium surface breaks to form Multi-
electron Bubbles (MEBs), essentially cavities filled with many electrons in liquid helium.
MEBs provide a versatile platform for exploring the properties of interacting electrons in
two dimensions and under curvature in a regime of densities that has not been studied be-
fore.
When the number of electrons is large (> 1000) in an MEB, the energetics of the bubble
are primarily governed by classical electrostatic forces and liquid surface tension, with bub-
ble sizes ranging between 1-100 microns. For fewer electrons, the quantum confinement
energy becomes appreciable, and therefore, the bubbles are classified as single electron
bubbles (SEBs) or few (<20) electron bubbles (FEBs). These objects are typically of sizes
of a few nm and provide a quantum mechanics textbook example of particle(s) in a flexi-
ble box. In this thesis, we have performed detailed studies of all these different species of
bubbles using various experimental techniques.
Characterising the different species of electron bubbles requires distinct measurement
techniques primarily due to their size variations. MEBs, with sizes of tens of micrometres,
can be effectively visualised using high-speed cameras, allowing researchers to analyse
their behaviour. However, SEBs and FEBs, with sizes of a few nanometers, cannot be directly imaged
using conventional light scattering method. We employ the cavitation technique to address
this, leveraging sound waves to induce cavitation in the bubbles. The cavitated bubbles
can then be imaged using conventional imaging setups, enabling researchers to study their
properties and behaviour.
After concisely introducing the various experimental techniques in this thesis, we will delve
into the main results obtained during my PhD. These results focus primarily on studying
the dynamics and properties of different species in liquid helium-4. The outline of the re-
sults presented in this thesis during the duration of my PhD is discussed in the following
sections.
The study of MEBs has traditionally been challenging due to their movement in the liquid
and fast collapse time. The collapse is essentially limited by the rate at which the vapour
inside the MEBs condenses. We engineered convective fluid flow inside the experimental
chamber to counter the condensation rate by placing heaters at different cell parts. We thus
were able to control their collapse rate to a reasonable degree. Subsequently, we used an
rf point Paul trap to localize the MEBs. Numerical simulations with adjustable parameters
like frequency, amplitudes of rf, and dc voltages were used to simulate the MEB trajecto-
ries, including the medium’s viscous drag. Specific combinations of these parameters were
found to trap MEBs of particular sizes, with experimental results showing good agreement
with simulations. The planar trapping geometry offers better optical access, scalability,
and ease of loading MEBs compared to the previously used 3D Paul traps. In the first
study, we used the trap to measure the rate of loss of electrons when MEBs are made to
impact against a solid substrate. Our results provide an understanding of the charge loss
mechanism previously observed but not understood in experiments with charged thin films
of helium. In a second study, we directly excite, observe, and measure the electron-ripplon
coupling in MEBs using externally applied electric fields.
Next, we will discuss an experimental technique to study SEBs and FEBs. We used
a cylindrical transducer to cavitate (and thus image) a single electron bubble located on
the focal line. We developed a new method to quantify the pressure required to cavitate
the bubbles. Subsequently, we used a Fresnel zone plate to amplify the sound signal at the
primary focal point and create multiple secondary maxima with a larger focal volume. This
technique enables us to discover a new species of FEB, possibly containing 12 electrons,
that has not been observed before. | en_US |
