Signatures of two gaps in the spin susceptibility of a cuprate superconductor

A fundamental obstacle to understanding high-temperature superconducting cuprates is that the occurrence of superconductivity hinders the observation of the normal-state properties at low temperature. One important property illustrating this issue is the spin susceptibility: its decrease upon cooling in the normal state is considered as evidence of pseudogap behaviour. However, unambiguous interpretation of this decrease has been impossible, as the crucial low-temperature data inevitably reflect the superconducting pairing rather than the normal state. Here we measure the spin susceptibility of YBa2Cu3Oy at low temperature while suppressing superconductivity in high magnetic field. We found that there are two thermally activated contributions, each of which comes from a different gap, alongside a residual component due to gapless excitations. We relate these two distinct gaps to short-range charge density waves and to the formation of singlets, as occurs in certain quantum spin systems. Both phenomena contribute to the pseudogap at low temperature, supplementing the short-lived antiferromagnetism that initiates pseudogap behaviour at high temperatures. We, therefore, propose that the pseudogap should be regarded as a composite property and that, when not undergoing spin-stripe ordering, underdoped cuprates tend to form short-range spin singlets. Measurements of the spin susceptibility in a model cuprate reveal the presence of two distinct gaps underlying the pseudogap behaviour. One gap is attributed to charge density waves and the other to the predicted formation of spin singlets.