Energy gaps measured by scanning tunneling microscopy

A scanning tunneling microscope (STM) has been used to measure energy gaps in the charge-density-wave (CDW) phases of the layer-structure dichalcogenides and in the high-temperature superconductor Bi₂{Sr}₂CaCu₂{O}₈$. Measured values of ${{{Δ}}}CDW$ at 4.2 K for 2H-${TaSe}₂$, 2H-${TaS}₂$, and 2H-${NbSe}₂$ are 80, 50, and 34 meV giving values of 2${{{Δ}}}CDW$/${k}BTc equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of ΔCDW at 4.2 K in 1T-TaSe₂ and 1T-TaS₂ are {~}150 meV for both materials giving 2ΔCDW/kB{T}c5.8. STM scans of{Bi}₂Sr₂{CaCu}₂O₈ at 4.2 K resolve atoms on the BiOₓ layer and show possible variations in electronic structure. The energy gap determined from I versus V and dI/dV versus V curves is in the range 30--35 meV giving values of 2{Δ}/kB{T}c${}8. Spectroscopy measurements with the STM can exhibit large zero-bias anomalies which complicate the analysis of the energy-gap structure, but adequate separation has been accomplished.