Heavy dibaryons $\Xi^{(*)}_{cc}\Xi^{(*)}_{cc}$ and $\Xi^{(*)}_{bb}\Xi^{(*)}_{bb}$

We systematically investigate the dibaryons $\Xi^{(*)}_{cc}\Xi^{(*)}_{cc}$ (di-$\Xi_{cc}$) and $\Xi^{(*)}_{bb}\Xi^{(*)}_{bb}$ (di-$\Xi_{bb}$), with various isospin-spin configurations $I(J^P)$ in a nonrelativistic quark model. For the di-$\Xi_{cc}$ system, only the single channels $\Xi_{cc}\Xi^*_{cc}$ and $\Xi^*_{cc}\Xi^*_{cc}$ with $0(1^+)$ are capable of forming deuteronlike bound states, with the $\sigma$ meson exchange playing a decisive role. Those states have binding energies of approximately $-1.5$ MeV and $-3.3$ MeV and sizes of 2.37 fm and 1.87 fm, respectively. The coupled channel effect in the di-$\Xi_{cc}$ system with $0(1^+)$ enhances the attraction. As a result, this di-$\Xi_{cc}$ system can establish a deuteronlike configuration, with the binding energy of $-7.5$ MeV relative to the threshold $\Xi_{cc}\Xi_{cc}$ and the size of approximately 1.40 fm. For the di-$\Xi_{bb}$ system, the single channels with $0(1^+)$, $0(2^+)$, and $0(3^+)$ can give rise to deuteronlike bound states with binding energies ranging from $-6.1$ MeV to $-14.3$ MeV. Additionally, the di-$\Xi_{bb}$ system with $1(0^+)$ and $1(2^+)$ can also establish deuteronlike bound states with binding energies of around $-0.5$ MeV. When considering the coupled channel effect in the di-$\Xi_{bb}$ system with $0(1^+)$, a compact hexaquark state is formed, exhibiting a binding energy of $-21.2$ MeV relative to the threshold $\Xi_{bb}\Xi_{bb}$ and a size of 0.53 fm. In this state, the $\pi$ meson exchange provides a very powerful attractive force. The meson exchange interactions in the quark model is dispensable in the di-$\Xi_{bb}$ bound states, except for $\Xi_{bb}^*\Xi_{bb}^*$ with $1(0^+)$.