  A major obstacle to understanding high-Tc cuprates is that superconductivity
precludes observing normal-state properties at low temperatures. One prime
example is the normal-state spin susceptibility: although its decrease upon
cooling far above Tc typifies pseudogap behavior, its behavior at low
temperatures is generally unknown. Here, our measurements in high magnetic
fields expose the spin susceptibility of YBa2Cu3Oy down to low temperatures.
Even though superconductivity is suppressed by the field, we uncover two
thermally-activated contributions alongside a residual susceptibility at T=0
due to gapless excitations. We relate these two distinct gaps to short-range
charge-density waves and to the formation of spin singlets similar to those
found in certain quantum spin systems. These phenomena thus collectively
contribute to the pseudogap in the spin susceptibility at low temperature,
supplementing short-lived antiferromagnetism known to initiate pseudogap
behavior at high temperatures. We therefore propose that the pseudogap should
be regarded as a composite property.
