Vestigial Order Melting of a Chiral Atomic Superfluid in a Double-Valley Optical Lattice

TL;DR

This paper experimentally investigates thermal phase transitions in a Floquet-engineered double-valley bandstructure with ultracold atoms, revealing a sequence of vestigial order melting in a chiral superfluid driven by temperature and frequency tuning.

Contribution

It demonstrates the first experimental observation of vestigial order melting in a Floquet-engineered chiral superfluid in a double-valley optical lattice.

Findings

Superfluid transition temperature is higher than Ising transition temperature.

Ising transition temperature decreases near resonance frequency.

Two phase transitions merge into one at far detuning.

Abstract

Quantum simulations of vestigial orders in multi-orbital superfluids have been attracting continuous research interests in both cold atoms and condensed matter systems, as it provides valuable insights into the high-temperature superconductivity. Here, we experimentally investigate thermal phase transitions in a Floquet-engineered double-valley bandstructure realized with ultracold bosonic atoms in a shaken optical lattice. The system exhibits both U(1) and time-reversal $\mathbb{Z}_2$ symmetries, and in the ground state, it forms a chiral superfluid with the Bose-Einstein condensation at a single valley.By tuning the temperature, we observe a vestigial order melting of the chiral superfluid: first into a paramagnetic superfluid, and then into a normal phase. We measure the superfluid and Ising transition temperatures across a range of driving frequencies, and find that the critical temperature of the superfluid transition is always higher than that of Ising transition within the studied frequency range. As the frequency approaches the resonance, the Ising transition temperature decreases, while the superfluid transition temperature remains almost unaffected. The two phase transitions merge into a single transition at far detuning. Our experimental results imply rich quantum many-body physics as an interplay of quantum and thermal fluctuations in periodically driven quantum systems.