Helium nanodroplets (HeNDs) are ideal model systems for exploring quantum hydrodynamics in self-contained, isolated superfluids at the nanoscale. However, the dynamical properties of individual droplets are experimentally challenging to study due to the difficulties in probing and manipulating the superfluid. In this project we will develop a new method that allows control over the rotational states of HeNDs for the first time. This is to be achieved by collision of a helium droplet beam with low-energy ions, which generates angular momentum and creates quantized vortices in the superfluid. A particle deposition method will then be applied to probe the vortices, which utilizes nanoparticles pinned to the vortex lines to image vortices. By this research we will unveil new physics of quantum fluids and will foster possibilities for the applications of HeNDs in nanoscience and nanotechnology. Consequently, this project will have significant scientific and technological impact.
Quantized circulation of superfluids, manifesting as topological defects, is one of the most remarkable macroscopic showcases of quantum mechanics. Although quantized vortices have been a hot research topic since the 1930s, only until recently have quantized vortices been successfully imaged by introducing diluted H2 gas  and metal atoms  into in bulk superfluid helium. For its nanoscale counterparts, HeNDs, direct imaging of quantized vortices has only been recently achieved by Gomez et al . The vortex density (Nv) was found to be 105 times greater than in bulk superfluid helium.
In a recent experiment we used a Cs+ ion source to impact on HeNDs. An off-centre collision event between a Cs+ ion and a droplet can result in an angular momentum, creating a quantized vortex array in the droplet. After doping with Ag atoms, we observed regular patterns in TEM images that are consistent with the presence of quantized vortex arrays in HeNDs (see Fig. 1, with one Ag particle in each vortex). The vortex density is ~ 6 1013/m2, which is very similar to that observed by Gomez et al .
The angular momentum of a HeND generated by the incident Cs+ ion clearly depends on the momentum of the ion; hence, this technique offers a unique opportunity to explore the control over the rotational states of superfluid helium. The key scientific questions that can be addressed therefore include 1) how to control the vortex density in quantum fluid? 2) how the vortex density varies within the HeNDs; 3) how the vortex density depends on HeND sizes and 4) what is the lower limit for a HeND to contain a vortex and what is the upper limit of vortex density? Regarding applications in nanoscience, the control over vortex arrays can potentially become a highly versatile technique to form patterned nanoparticle arrays on a surface with no need of highly sophisticated and expensive lithography techniques, which will also be explored in the project.
A purpose-built HeND apparatus for the fabrication of nanomaterials is readily available for this research, which is incorporated with a high-flux low-energy Cs+ ion source. The HeNDs, after collision with the Cs+ ions, will be doped with metals and then deposited on TEM grids for ex situ investigations.
Academic entry requirements
UK Bachelor Degree with at least 2:1 in a relevant subject or overseas equivalent.
University of Leicester English language requirements apply (where applicable).
UK/EU applicants only.
When applying, please ensure we have received all of the following required documents by Tuesday 4th February 2020:
- Application Form
- 2 academic references
- Undergraduate transcripts
If you have completed your undergraduate degree, we will also require your undergraduate degree certificate.
If you have completed a postgraduate degree, we will also require your transcripts and degree certificate.
If we do not have the required documents by the closing date, your application may not be considered for the studentship.
Please refer to guidance at - https://le.ac.uk/study/research-degrees/funded-opportunities/chem-gta-2020
1. G. P. Bewley, D. P. Lathrop, K. R. Sreenivasan, Nature 441, 588 (2006).
2. V. Lebedev, P. Moroshkin, B. Grobety, E. Gordon, A. Weis, J. Low Temp. Phys. 165, 166 (2011).
3. L. F. Gomez, K. R. Ferguson, J. P. Cryan, et al., Science 345, 6199 (2014).