Scanning tunneling spectroscopic evidence for magnetic field-induced microscopic orders in the high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$

Abstract

We report spatially resolved tunneling spectroscopic evidence for field-induced microscopic orders in a high-$T_c$ superconductor $\rm YBa_2Cu_3O_{7-\delta}$. The spectral characteristics inside vortices reveal a pseudogap ($V_{\rm CO}$) larger than the superconducting gap ($\Delta\_{\rm SC}$) as well as a subgap ($\Delta\^{\prime}$) smaller than $\Delta\_{\rm SC}$, and the spectral weight shifts steadily from $\Delta_{\rm SC}$ to $V_{\rm CO}$ and $\Delta\^{\prime}$ upon increasing magnetic field. Additionally, energy-independent conductance modulations at 3.6 and 7.1 lattice constants along the Cu-O bonding directions and at 9.5 lattice constants along the nodal directions are manifested in the vortex state. These wave-vectors differ fundamentally from the strongly dispersive modes due to Bogoliubov quasiparticle scattering interferences and may be associated with field-induced microscopic orders of pair-, charge- and spin-density waves.