Interplay of order and disorder in two-dimensional critical systems with mixed boundary conditions

Abstract

In spin systems such as the Ising model, the local order and disorder can be characterized by the order-parameter and energy density profiles $\langle σ({\bf r}_1) \rangle$ and $\langle ε({\bf r}_2) \rangle$, respectively. Does increasing the order at ${\bf r}_1$ always decrease the disorder at ${\bf r}_2$? Does increasing the disorder at ${\bf r}_2$ always decrease the order at ${\bf r}_1$? The answer to these questions is contained in the cumulant response function $\langleσ({\bf r}_1) \, ε({\bf r}_2) \rangle^{(\rm cum)}$. This correlation function vanishes in the unbounded bulk but not in systems with fixed-spin boundary conditions. Using the universal operator-product expansion of $σ({\bf r}_1) \, ε({\bf r}_2)$ and exact results for the Ising model, we analyze $\langleσ({\bf r}_1) \, ε({\bf r}_2) \rangle^{(\rm cum)}$ in two-dimensional critical systems defined on the $x-y$ plane with mixed $+$ and $-$ boundary conditions. Particularly interesting behavior is found when either of the operators $σ$ or $ε$ is located on a ``zero line" in the $x-y$ plane, along which $\langleσ({\bf r})\rangle$ vanishes. Results for half-plane, triangular, and rectangular geometries are presented.