Fractional vortices as evidence of time-reversal symmetry breaking in high-temperature superconductors.
We argue that recent experiments by Kirtley et al. may show evidence of time-reversal symmetry breaking in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{O}_{7}$ at crystal grain boundaries. We illustrate this through a Ginzburg-Landau model calculation. Further experimental tests are proposed.