Physics news on the Internet: August 2024

States combining superfluidity and crystalline periodicity are called superfluid solid states (SSSs). The possibility of their existence was predicted theoretically by A F Andreev and I M Lifshitz (Zh. Eksp. Teor. Fiz. 56 2057 (1969) [Sov. Phys. JETP 29 1107 (1969)]) and G V Chester (Phys. Rev. A 2 256 (1970)) and A J Leggett (Phys. Rev. Lett. 25 1543 (1970)). The conditions for an SSS to appear are the presence of a roton minimum on the dispersion curve and quantum stabilization. An observation of SSSs in solid helium-4 was reported in 2004, but the result has not been confirmed. The SSS was reliably discovered in 2009 in rubidium atom gas in a periodic potential generated by the radiation field. The possibility of the roton minimum and SSS occurring was also predicted without an external periodic field. Three independent teams of researchers confirmed this prediction when they discovered the SSS in Bose±Einstein condensates of gases whose atoms have large magnetic dipole moments. An absorption image of atomic wave interference was obtained in a condensate cloud upon its free expansion. This was how periodically located phase-coherent drops of Bose±Einstein condensate were observed. Two groups guided by G Modugno (University of Florence, Italy) and T Pfau (University of Stuttgart, Germany) examined the Dy isotope. The SSS lifetime measured by them was equal to 30 ms. This time was limited by three-body losses. 3D-modeling based on the generalized Gross±Pitaevsky equation showed good agreement between theory and experiment. F Ferlaino (University of Innsbruck, Austria) and her colleagues revealed SSSs in the gas of Er and Dy isotopes. An SSS lasts 30 ms in Er and 150 ms in Dy. Sources: Phys. Rev. Lett. 122 153601 (2019) Phys. Rev. X 9 011051, 021012 (2019) https://doi.org/10.1103/PhysRevLett.122.130405 https://doi.org/10.1103/PhysRevX.9.011051 https://doi.org/10.1103/PhysRevX.9.021012