Quantum phase transition to unconventional multi-orbital superfluidity in optical lattices

TL;DR

This paper reports the discovery of a novel multi-orbital superfluid phase with a complex order parameter in ultracold quantum gases, revealing a second-order quantum phase transition and new orbital physics phenomena.

Contribution

It introduces the observation of a twisted superfluid phase with a complex order parameter in optical lattices, highlighting a new form of orbital superfluidity induced by interactions.

Findings

Observation of a second-order quantum phase transition from normal to twisted superfluid.

Identification of a complex order parameter with a continuously twisted phase.

Experimental results align with theoretical phase diagrams.

Abstract

Orbital physics plays a significant role for a vast number of important phenomena in complex condensed matter systems such as high-T$_c$ superconductivity and unconventional magnetism. In contrast, phenomena in superfluids -- especially in ultracold quantum gases -- are commonly well described by the lowest orbital and a real order parameter. Here, we report on the observation of a novel multi-orbital superfluid phase with a {\it complex} order parameter in binary spin mixtures. In this unconventional superfluid, the local phase angle of the complex order parameter is continuously twisted between neighboring lattice sites. The nature of this twisted superfluid quantum phase is an interaction-induced admixture of the p-orbital favored by the graphene-like band structure of the hexagonal optical lattice used in the experiment. We observe a second-order quantum phase transition between the normal superfluid (NSF) and the twisted superfluid phase (TSF) which is accompanied by a symmetry breaking in momentum space. The experimental results are consistent with calculated phase diagrams and reveal fundamentally new aspects of orbital superfluidity in quantum gas mixtures. Our studies might bridge the gap between conventional superfluidity and complex phenomena of orbital physics.