  Quasi-particle picture in a magnetic field is pursued for dynamical spin and
charge correlation functions of the one-dimensional supersymmetric t-J model
with inverse-square interaction. With use of exact diagonalization and the
asymptotic Bethe-ansatz equations for finite systems, excitation contents of
relevant excited states are identified which are valid in the thermodynamic
limit. The excitation contents are composed of spinons, antispinons, holons and
antiholons obeying fractional statistics. Both longitudinal and transverse
components of the dynamical spin structure factor are independent of the
electron density in the region where only quasi-particles with spin degrees of
freedom (spinons and antispinons) contribute. The dynamical charge structure
factor does not depend on the spin-polarization density in the region where
only quasi-particles with charge (holons and antiholons) are excited. These
features indicate the strong spin-charge separation in dynamics, reflecting the
high symmetry of the model.
