  By exploiting density-matrix renormalization group techniques, we investigate
the dynamical spin structure factor of a spin-1/2 Heisenberg chain with
ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor
exchange interactions in an applied magnetic field. In a field-induced spin
nematic regime, we find gapless longitudinal and gapped transverse spin
excitation spectra, in accordance with quasi-long-ranged longitudinal and
short-ranged transverse spin correlations, respectively. The gapless point
coincides with the dominant longitudinal spin correlation, whereas the gap
position exhibits a characteristic field dependence contradicting the dominant
transverse spin correlation.
