Measuring the quantum geometry of Bloch bands with current noise

Single-particle states in electronic Bloch bands form a Riemannian manifold whose geometric properties are described by two gauge invariant tensors, one being symmetric and the other being antisymmetric, that can be combined into the so-called Fubini-Study metric tensor of the projective Hilbert space. The latter directly controls the Hall conductivity. Here we show that the symmetric part of the Fubini-Study metric tensor also has measurable consequences by demonstrating that it enters the current noise spectrum. In particular, we show that a nonvanishing equilibrium current noise spectrum at zero temperature is unavoidable whenever Wannier states have nonzero minimum spread, the latter being quantifiable by the symmetric part of the Fubini-Study metric tensor. We illustrate our results by three examples: (1) atomic layers of hexagonal boron nitride, (2) graphene, and (3) the surface states of three-dimensional topological insulators when gapped by magnetic dopants.

Figure 1

Current noise spectrum computed using Eq. (13) and Hamiltonian (15) for longitudinal currents in boron nitride. It is a direct measure for the Fubini-Study metric tensor times the density of states averaged over equal-energy contours in the BZ. The largest contributions stem from the massive Dirac cones directly above the band gap of $\omega =2{\mu }_{\mathrm{s}}=5.8\mathrm{eV}$ and from the van Hove singularities at $\omega =2\sqrt{t^2+{\mu }_{\mathrm{s}}^2}=8.23\mathrm{eV}$. The anisotropy is attributed to the fact that the honeycomb lattice lacks a fourfold rotational symmetry. Insets: Distributions of the components of the Fubini-Study metric tensor over the BZ.