Limits on Spin-Charge Separation from $\mathit{h}/2\mathit{e}$ Fluxoids in Very Underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{6+\mathit{x}}$

Magnetic flux in superconductors is usually quantized in units of $h/2e$. Here we report scanning SQUID and scanning Hall probe studies of single fluxoids in high purity ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.35}$ crystals ( $T_c\lesssim 13\mathrm{K}$), extending flux quantization studies to a region of the cuprate phase diagram where the superfluid density is sufficiently low that novel behavior has been predicted. Some scenarios in which superconductivity results from spin-charge separation predict $h/e$ fluxoids in materials with low superfluid density. Our observations of only $h/2e$ fluxoids set limits on these theories.