  The local application of mechanical stress in piezoelectric materials gives
rise to boundaries across which the electric polarization changes. Polarization
charges appear along such polar discontinuities and the ensuing electric fields
drive a charge reconstruction with the accumulation of free carriers at the
boundaries. This is particularly relevant for two-dimensional materials that
can sustain very large strains and display record piezoelectric responses. Here
we show by first-principles simulations the emergence of one-dimensional wires
of free electrons and holes along strain interfaces, taking SnSe as a
paradigmatic material. We complement this by developing a Schr\"odinger-Poisson
approach specifically designed for two-dimensional materials that it is able to
reproduce the ab-initio results and also to extend them to regimes of
parameters and system sizes that would be unaffordable in first principles
calculations. This model allows us to assess the degree of tunability for the
free charge in the wires coming from strain values and profiles, and to obtain
the critical size at which the interfaces start to be metallic.
