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 Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. Measured values of {Delta}{sub CDW} at 4.2 K for 2{ital H}-TaSe{sub 2}, 2{ital H}-TaS{sub 2}, and 2{ital H}-NbSe{sub 2} are 80, 50, and 34 meV giving values of 2{Delta}{sub CDW}/{ital k}{sub {ital B}T{ital c}} equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of {Delta}{sub CDW} at 4.2 K in 1{ital T}-TaSe{sub 2} and 1{ital T}-TaS{sub 2} are {approximately}150 meV for both materials giving 2{Delta}{sub CDW}/{ital k}{sub {ital B}T{ital c}}{approx}5.8. STM scans of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} at 4.2 K resolve atoms on the BiO{sub {ital x}} layer and show possible variations in electronic structure. The energy gap determined from {ital I} versus {ital V} and {ital dI}/{ital dV} versus {ital V} curves is in the range 30--35 meV giving values of 2{Delta}/{ital k}{sub {ital B}T{ital c}}{approx}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.