Evidence from scanning tunneling spectroscopy for magnetic-field-enhanced collective modes in the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$

We present scanning tunneling spectroscopic evidence for field-enhanced collective modes in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$. The observed spectra inside vortices exhibit two characteristic features: a pseudogap (V$_{CO }$= 31.5\underline {+}2.0 meV) larger than the superconducting gap ($\Delta _{SC }$= 20.0\underline {+}1.0 meV) and a subgap ($\Delta $' $\approx $ 7-10meV) smaller than $\Delta _{SC}$. Outside vortices, the spectra display a gap of $\Delta _{SC}$. As magnetic field increases, spectral weight rapidly shifts from $\Delta _{SC }$to V$_{CO}$ and $\Delta $'. The vortex state also reveals energy-independent conductance modulations with peridocities of 3.6 and 7.1 lattice constants along the Cu-O bonding direction and 9.5 lattice constants along the nodal direction. The energy-independent modulations differ fundamentally from energy-dispersive modes due to Bogoliubov quasiparticle scattering interferences and originate from field-enhanced collective modes of pair-, charge- and spin-density waves.