Limits on spin-charge separation from h/2e fluxoids in very underdoped YBa₂Cu₃O_6+x

Magnetic imaging experiments were performed on four single crystals of YBa2Cu3O6.35 with critical temperature (Tc) of 11-13 K. Over 170 fluxoids in the crystal samples were imaged with scanning superconducting quantum interference device (SQUID) microscopy and scanning Hall probe microscopy. All the observed fluxoids carried a single conventional flux quantum, h/2e, within experimental error. The results set a semiquantitative upper limit on the energy scale for spin-charge separation.