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 YBa₂Cu₃O_y 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.