We present measurements demonstrating that copper-oxide high-temperature superconductors can exhibit broken time-reversal symmetry in the absence of external magnetic fields. Using ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ microbridges, we observe a pronounced superconducting diode effect at 100 K under strictly zero-field conditions. This nonreciprocal response remains unchanged in magnetic fields up to $\pm 100$ Oe. Our results are consistent with recent reports of zero-field diode behavior in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ and together they indicate that time-reversal symmetry breaking may be an intrinsic property of the cuprate superconducting state. These findings provide an additional experimental constraint on theoretical descriptions of high-temperature superconductivity and motivate further investigations of possible time-reversal-symmetry-breaking phenomena in cuprates.

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• Superconductivity
• Cuprates
• Diodes
• Resistivity measurements