Helium Droplets: A Nanoscale Cryostat for High Resolution Spectroscopy and Studies of Quantized Vorticity

In this paper we discuss helium droplets as a nanoscale cryostat for a variety of fundamental experiments in condensed matter physics. Specifically, we describe our recent work on the spectroscopy of silver atoms, europium atoms and C60in helium droplets and compare the helium droplet, as a matrix for low temperature studies of complex systems, with traditional matrix and gas phase techniques. Further, we discuss our recent work on the production of ultra-cold metal clusters of silver, indium and europium embedded in helium droplets at a temperature (T=0.37K) two orders of magnitude lower than previously achieved in beams of free metal clusters. This work opens the door to high resolution spectroscopic studies of metal clusters and, possibly, high resolution studies of the size dependence of their superconducting properties. We further speculate on a series of experiments where we plan to use standard spectroscopic methods, developed in recent years, to exploit the helium droplet for studies of the existence, stability and dynamics of quantized vortices. Helium droplets may be the ideal system for such studies due to the complete absence of pinning sites that plague many similar experiments performed in bulk helium.