An international team of experimental and theoretical physicists from Austria and Switzerland has investigated what happens when a dipolar supersolid is brought out of balance.
Ilzhöfer et al. discovered a soft form of a supersolid. Image credit: Harald Ritsch / IQOQI Innsbruck.
Predicted five decades ago, supersolidity is an exotic phase of matter where superfluidity and crystalline order coexist.
In this state, atoms are arranged in a crystalline pattern while at the same time behaving like a superfluid, in which particles move without friction.
In 2017, two teams of physicists from MIT and ETH Zurich independently turned a Bose-Einstein condensate into a supersolid.
Two years later, University of Innsbruck physicists experimentally realized such a state using ultracold quantum gases of highly magnetic lanthanide atoms.
“Due to quantum effects, a very cold gas of atoms can spontaneously develop both a crystalline order of a solid crystal and particle flow like a superfluid quantum liquid, i.e. a fluid able to flow without any friction,” said senior author Dr. Francesca Ferlaino, a researcher in the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences and the Department of Experimental Physics at the University of Innsbruck.
“Much simplified, a dipolar supersolid can be imagined as a chain of quantum droplets which communicate with each other via a superfluid background bath,” added co-author Dr. Thierry Giamarchi, a theoretical physicist at the University of Geneva.
In the new research, Dr. Ferlaino, Dr. Giamarchi and their colleagues studied how a supersolid state reacts if the superfluid bath between the droplets is drained by control of the external magnetic field.
“We were able to show that without the bath the droplets quickly lose knowledge about each other and start to behave like small independent quantum systems — they diphase,” said co-author Dr. Maximilian Sohmen, also from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences and the Department of Experimental Physics at the University of Innsbruck.
“The supersolid turns into a normal solid.”
Surprisingly, the scientists were also able to reverse this dephasing process.
When they replenished the background bath, the droplets renewed their communication by particle tunneling and re-established supersolidity.
“This solid, however, is still ‘soft,’ it can wobble and support many collective excitations, called phonons,” said first author Dr. Philipp Ilzhöfer, from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences.
“This makes this state a very interesting but complex subject of study with strong connections to solid-state physics and other fields.”
The results were published in the journal Nature Physics.
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P. Ilzhöfer et al. Phase coherence in out-of-equilibrium supersolid states of ultracold dipolar atoms. Nat. Phys, published online January 4, 2021; doi: 10.1038/s41567-020-01100-3
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