Scanning tunneling spectroscopic evidence for a magnetic field-revealed microscopic order in the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$

We present spatially resolved scanning tunneling spectroscopic measurements of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ as a function of magnetic field and at T$<<$T$_{C}$. The observed \textit{intra}-vortex quasiparticle (QP) spectra appear pseudogap (PG)-like, with an energy gap of V$_{PG}\approx $32meV. The value of V$_{PG}$ is significantly larger than the observed \textit{inter}-vortex superconducting (SC) gap, $\Delta _{SC}$=20meV, and equal to the incommensurate spin fluctuation gap observed by neutron scattering. We also observe a secondary and less pronounced intra-vortex gap at $\Delta $'$\sim $7-10meV. Fourier transformation of QP spectra reveals two sets of non-dispersive, field-enhanced conductance modulations with periods of 3.4\underline {+}0.5 and 7.3\underline {+}0.5 lattice constants. Energy histograms of QP spectra show a significant shift from SC to primarily PG-like spectra and a growing enhancement of spectral weight at $\Delta $' as magnetic field increases, implying a significant interplay between SC and a field-enhanced microscopic order. Ref.: Beyer, \textit{et.al. }[arxiv:0808.3016].