Theta Functions and Adiabatic Curvature on a Torus.

Let $M$ be a complex torus, $L_{\hat\mu}\to M$ be positive line bundles parametrized by $\hat \mu\in {\rm Pic}^0(M)$, and $E\to {\rm Pic}^0(M)$ be a vector bundle with $E|_{\hat\mu}\cong H^0(M, L_{\hat \mu})$. We endow the total family $\{L_{\hat\mu}\}_{\hat\mu}$ with a Hermitian metric that induces the $L^2$-metric on $H^0(M, L_{\hat \mu})$ hence on $E$. By using theta functions $\{\theta_m\}_{m}$ on $M\times M$ as a family of functions on the first factor $M$ with parameters in the second factor $M$, our computation of the full curvature tensor $\Theta_E$ of $E$ with respect to this $L^2$-metric shows that $\Theta_E$ is essentially an identity matrix multiplied by a constant $2$-form, which yields in particular the adiabatic curvature $c_1(E)$. After a natural base change $M\to \hat M$ so that $E\times_{\hat M} M:=E'$, we also obtain that $E'$ splits holomorphically into a direct sum of line bundles each of which is isomorphic to $L_{\hat\mu=0}^*$. Physically, the spaces $H^0(M, L_{\hat \mu})$ correspond to the lowest eigenvalue with respect to certain family of Hamiltonian operators on $M$ parametrized by $\hat\mu$ or in physical notation, by wave vectors $\bf k$.