Unconventional Spin Density Waves in Dipolar Fermi Gases

TL;DR

This paper predicts the emergence of unconventional spin density waves with non-zero angular momentum in dipolar Fermi gases, revealing a new class of quantum phases with complex order parameters on optical lattices.

Contribution

It demonstrates, via functional renormalization group analysis, that spin-triplet $ ext{l}=1$ SDWs are generically formed in dipolar Fermi gases, a novel finding in quantum many-body physics.

Findings

Unconventional SDWs with $ ext{l}=1$ emerge in dipolar gases.

The SDWs have vector order parameters on lattice bonds.

Phase diagram shows competition with superfluid and charge density waves.

Abstract

The conventional spin density wave (SDW) phase (Overhauser, 1962), as found in antiferromagnetic metal for example (Fawcett 1988), can be described as a condensate of particle-hole pairs with zero angular momentum, $\ell=0$, analogous to a condensate of particle-particle pairs in conventional superconductors. While many unconventional superconductors with Cooper pairs of finite $\ell$ have been discovered, their counterparts, density waves with non-zero angular momenta, have only been hypothesized in two-dimensional electron systems (Nayak, 2000). Using an unbiased functional renormalization group analysis, we here show that spin-triplet particle-hole condensates with $\ell=1$ emerge generically in dipolar Fermi gases of atoms (Lu, Burdick, and Lev, 2012) or molecules (Ospelkaus et al., 2008; Wu et al.) on optical lattice. The order parameter of these exotic SDWs is a vector quantity in spin space, and, moreover, is defined on lattice bonds rather than on lattice sites. We determine the rich quantum phase diagram of dipolar fermions at half-filling as a function of the dipolar orientation, and discuss how these SDWs arise amidst competition with superfluid and charge density wave phases.