Superfluidity within a small helium-4 cluster: the microscopic andronikashvili experiment

Area of Science:

• Quantum Fluids
• Molecular Spectroscopy
• Low-Temperature Physics

Background:

• Superfluidity is a quantum mechanical phenomenon observed in certain fluids at very low temperatures.
• Helium-4 and helium-3 are isotopes of helium known to exhibit superfluidity under specific conditions.
• Molecular spectroscopy provides insights into the behavior and interactions of molecules within different environments.

Purpose of the Study:

• To investigate the rotational behavior of oxygen carbon sulfide (OCS) molecules within superfluid helium-4 and nonsuperfluid helium-3 droplets.
• To determine if the rotational spectrum of OCS can serve as a microscopic probe of superfluidity.
• To identify the minimum number of dopant atoms required to induce superfluidity in helium-3 droplets.

Main Methods:

• Infrared spectroscopy was employed to measure the spectrum of single OCS molecules.
• Experiments were conducted using large, pure helium-4 and helium-3 droplets (approximately 10^4 atoms each).
• Helium-4 atoms were incrementally added to helium-3 droplets to observe changes in the OCS spectrum.

Main Results:

• Sharp, distinct rotational lines of OCS were observed in helium-4 droplets, indicating free molecular rotation.
• A broad, featureless peak was observed for OCS in helium-3 droplets, suggesting hindered rotation.
• The addition of approximately 60 helium-4 atoms to helium-3 droplets resulted in the reappearance of sharp rotational lines.

Conclusions:

• The observed differences in OCS rotational spectra provide microscopic evidence for superfluidity in helium droplets.
• Free rotation of OCS molecules is a direct indicator of the superfluid properties of the surrounding helium.
• A minimum of 60 helium-4 atoms appears necessary to induce superfluidity in helium-3 droplets.