Signatures of anomalous symmetry breaking in the cuprates

The temperature versus doping phase diagram of the cupratehigh-Tc superconductorsfeatures an enigmatic pseudogap region whose microscopic origin remains a subjectof intensive study. Experimentally resolving its symmetry properties isimperative for narrowing down the list of possible explanations. In this talk I will give an overview of how optical second harmonic generation (SHG) can be used as a sensitive probe of symmetrybreaking, and recap the ways it has been used to solve outstanding problems in condensed matter physics. I will then describe how we have been applying SHG polarimetry and spectroscopy to interrogate the cuprate pseudogap. In particular, I will discuss our data on YBa2Cu3Oy [1],which show an order parameter-like increase in SHG intensity below the pseudogap temperature T* across abroad range of doping levels. I will then focus on our more recent results on a model parent cuprate Sr2CuO2Cl2 [2],where evidence of anomalous broken symmetries surprisingly also exists. Possible connections between these observations will be speculated upon. [1] L.Zhao, C. A. Belvin, R. Liang, D. A. Bonn, W. N. Hardy, N. P. Armitage and D.Hsieh, “A global inversion-symmetry-broken phase inside the pseudogap region ofYBa2Cu3Oy,” Nature Phys. 13, 250 (2017).