  We investigate whether the spin or charge degrees of freedom are responsible
for the nodal gap in underdoped cuprates by performing inelastic neutron
scattering and x-ray diffraction measurements on La$_{2-x}$Sr$_x$CuO$_4$, which
is on the edge of the antiferromagnetic phase. We found that fluctuating
incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass
dispersion exists even in this magnetic sample. The strongest component of
these fluctuations diminishes at the same temperature where the nodal gap
opens. X-ray scattering measurements on the same crystal show no signature of
charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the
electronic band of this cuprate opens due to fluctuating SDW with no
contribution from CDW.
