  Recent measurements of quasiparticles in hole-doped cuprates reveal highly
unusual features: 1) the doping-independent Fermi velocity, 2) two energy
scales in the quasiparticle spectral function, and 3) a suppression of the low
energy spectral weight near the zone center. The underlying mechanism is under
hot debate. We addressed these important issues by a novel two-mode variational
Monte Carlo (VMC) study of the t-J model. We obtained results in agreement with
the experiments but without invoking extrinsic effects. Besides, we resolved a
long standing issue of the sum rule for quasiparticle spectral weights in VMC
studies. The electron doped case was also discussed.
