New Collective Modes in the Superconducting Ground States in the Gauge   Theory Description of the Cuprates
  In the slave boson mean field treatment of the t-J model, the ground state for small doping is a d-wave superconductor. A conventional superconductor has collective modes associated with the amplitude and phase of the pairing order parameter. Here, the hopping matrix element $\chi_{ij}$ is also a mean field order parameter. We therefore expect that new collective modes will be introduced with these additional complex degrees of freedom. We compute the new collective modes and their spectral functions by numerically diagonalizing the matrix which describes the fluctuations about the mean field solution. We also show that the SU(2) gauge theory formulation allows us to predict and classify these collective modes. Indeed, the SU(2) formulation is essential in order to avoid spurious collective modes as the doping $x$ goes to zero. The most important new collective modes are the $\theta$ mode and the longitudinal and transverse $\phi$ gauge modes. The $\theta$ mode corresponds to fluctuations of the staggered flux phase and creates orbital current fluctuations. The $\phi$ gauge modes correspond to out-of-phase fluctuations of the {\em amplitudes} of the pairing and the hopping matrix elements. We compute the neutron scattering cross-section which couples to the $\theta$ mode and inelastic X-ray scattering cross-section which couples to the fluctuation in the real part of $\chi_{ij}$. In addition, we show that the latter fluctuation at $(\pi,\pi)$ may be coupled to the buckling phonon mode in the LTO phase of LSCO and may be detected optically. Experimental searches of these collective modes will serve as important tests of this line of attack on the high T$_c$ problem. 
Patrick A. Lee and Naoto Nagaosa