Bootstrapping periodic quantum systems

Abstract

Periodic structures are ubiquitous in quantum many-body systems and quantum field theories, ranging from lattice models, compact spaces, to topological phenomena. However, previous bootstrap studies encountered technical challenges even for one-body periodic problems, such as a failure in determining the accurate dispersion relations for Bloch bands. In this work, we develop a new bootstrap procedure to resolve these issues, which does not make use of positivity constraints. We mainly consider a quantum particle in a periodic cosine potential. The same procedure also applies to a particle on a circle, where the role of the Bloch momentum $k$ is played by the boundary condition or the $θ$ angle. We unify the natural set of operators and the translation operator by a new set of operators $\{e^{inx} e^{iap} p^s\}$. To extract the Bloch momentum $k$, we further introduce a set of differential equations for $\langle{e^{inx} e^{iap} p^s}\rangle$ in the translation parameter $a$. At some fixed $a$, the boundary conditions can be determined accurately by analytic bootstrap techniques and matching conditions. After solving the differential equations, we impose certain reality conditions to determine the accurate dispersion relations, as well as the $k$ dependence of other physical quantities. We also investigate the case of noninteger $s$ using the Weyl integral in fractional calculus.