Atomically Layered Helium Films at Ultralow Temperatures: Model Systems for Realizing Quantum Materials

This year is also the 50th anniversary of the discovery of exfoliated graphite as a particularly uniform substrate (Thomy and Duval in J Chim Phys 66:1966, 1969. 10.1051/jcp/196966s21966, J Chim Phys 67:286, 1970. 10.1051/jcp/1970670286, J Chim Phys 67:1101, 1970. 10.1051/jcp/1970671101). In this article, we focus on the study of helium films on graphite-based substrates at ultralow temperatures. We provide a flavour of the historical development of this subject and a perspective on the current status. We discuss how atomically layered helium films provide model systems for the realization of a broad range of quantum materials of generic significance. Future prospects arising from new techniques and new substrates will also be discussed.

Defects on cylinders: superfluid helium films and bacterial cell walls

There is a deep analogy between the physics of crystalline solids and the behaviour of superfluids, dating back to the pioneering work of Phillip Anderson, Paul Martin, and others. The stiffness to shear deformations in a periodic crystal resembles the super-fluid density that controls the behaviour of supercurrents in neutral superfluids such as He4. Dislocations in solids have a close analogy with quantized vortices in superfluids. Remarkable recent experiments on the way rod-shaped bacteria elongate their cell walls have focused attention on the dynamics and interactions of point-like dislocation defects in partially-ordered cylindrical crystalline monolayers. In these lectures, we review the physics of superfluid helium films on cylinders and discuss how confinement in one direction affects vortex interactions with supercurrents. Although there are similarities with the way dislocations respond to strains on cylinders, important differences emerge due to the vector nature of the topological charges characterizing the dislocations.

Early Work on Defect Driven Phase Transitions

This article summarizes the early history of the theory of phase transitions driven by topological defects, such as vortices in superfluid helium films or dislocations and disclinations in two-dimensional solids. We start with a review of our two earliest papers, pointing out their errors and omissions as well as their insights. We then describe the work, partly done by Kosterlitz but mostly done by other people, which corrected these oversights, and applied these ideas to experimental systems, and to numerical and experimental simulations.