Fermionic spin liquid analysis of the paramagnetic state in volborthite

Recently, thermal Hall effect has been observed in the paramagnetic state of volborthite, which consists of distorted kagome layers with $S=1/2$ local moments. Despite the appearance of magnetic order below $1\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the response to external magnetic field and unusual properties of the paramagnetic state above $1\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ suggest possible realization of exotic quantum phases. Motivated by these discoveries, we investigate possible spin liquid phases with fermionic spinon excitations in a nonsymmorphic version of the kagome lattice, which belongs to the two-dimensional crystallographic group $p2gg$. This nonsymmorphic structure is consistent with the spin model obtained in the density functional theory calculation. Using projective symmetry group analysis and fermionic parton mean field theory, we identify twelve distinct ${\mathbb{Z}}_{2}$ spin liquid states, four of which are found to have correspondence in the eight Schwinger boson spin liquid states we classified earlier. We focus on the four fermionic states with bosonic counterpart and find that the spectrum of their corresponding root $U(1)$ states features spinon Fermi surface. The existence of spinon Fermi surface in candidate spin liquid states may offer a possible explanation of the finite thermal Hall conductivity observed in volborthite.