On homology concordance in contractible manifolds and two bridge links

Let ${\widehat{\mathcal{C}}}_{\mathbb{Z}}$ be the group consists of manifold-knot pairs $(Y,K)$ modulo homology concordance, where $Y$ is an integer homology sphere bounding an integer homology ball, and let ${\mathcal{C}}_{\mathbb{Z}}$ be the subgroup consisting of pairs $(S^3,K)$. Dai-Hom-Stoffregen-Truong show that the quotient group ${\widehat{\mathcal{C}}}_{\mathbb{Z}}\mathrm{/}{\mathcal{C}}_{\mathbb{Z}}$ admits a ${\mathbb{Z}}^{\mathrm{\infty }}$-summand. In this paper, we improve the result by showing that there exists a family $\{(Y,K_m){\}}_{m>1}$ generating the ${\mathbb{Z}}^{\mathrm{\infty }}$-summand where $Y$ is the boundary of a smooth contractible $4$-manifold. In fact, we give a $\mathbb{Z}$-count of such families. The examples are constructed using a family of knots obtained by blowing down a component of a two-bridge link. They are studied in Jonathan Hales’s thesis. Using the algorithm due to Ozsváth, Szabó and Hales we give a classification of the knot Floer homology of a larger family of such knots, that might be of independent interest.