Formation of Fermi arcs at $T\ll T_c$ in the vicinity of $d$-wave nodes of structurally inhomogeneous YBa$_{2}$Cu$_{3}$O$_{6.92}$ HTSCs

The influence of structural inhomogeneity on a superconducting gap near $d$-wave nodes is co-nsidered for optimally doped YBa$_{2}$Cu$_{3}$O$_{6.92}$ HTSCs. To this aim, specific heat in the temperature range of $T$ = 2 – 10 K at magnetic fields $H$ = 0 – 9T is analyzed for a series of fine-crystalline samples with different degrees of controlled structural disorder. Information about specific features of the superconducting gap near $d$-wave nodes in structurally disordered samples is obtained. It is shown that $d$-wave nodes (typical of an ideal crystal structure) change under the influence of structural disorder with the formation of small Fermi arcs, which are not covered by the superconducting gap, in the vicinity of the nodal point at $T\ll T_c$. Note that the gap is retained at $T\le T_c$ in other directions of the Brillouin zone and coexists with Fermi arcs. This transformation is accompanied by an increase in the nodal slope, a decrease in the Volovik effect, and creation of the metal-type linear term $\gamma(0)T$ in the temperature dependence of the specific heat, which should not exist for superconductors with an ideal crystal structure; its nature has not yet been established.