Abstract
A change in ‘symmetry’ is often observed when matter undergoes a phase transition—the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a ‘pseudogap’ appears in the spectrum of electronic excitations1,2. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken5,6,7,8,9,10. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken11 below T*, which therefore corresponds to a phase transition.
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Acknowledgements
We thank M. Randeria for discussions, and T. Kubala, G. Rogers and M. Bissen for their help with the polarizer. This work was supported by the US NSF and the US Department of Energy—Basic Energy Sciences. The Synchrotron Radiation Center is supported by the NSF. S.R. is supported by the Swiss NSF.
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Kaminski, A., Rosenkranz, S., Fretwell, H. et al. Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor. Nature 416, 610–613 (2002). https://doi.org/10.1038/416610a
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DOI: https://doi.org/10.1038/416610a
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