Energy gaps measured by scanning tunneling microscopy

Abstract

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 ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. Measured values of ${\mathrm{\Delta }}_{\mathrm{CDW}}$ at 4.2 K for 2H-${\mathrm{TaSe}}_2$, 2H-${\mathrm{TaS}}_2$, and 2H-${\mathrm{NbSe}}_2$ are 80, 50, and 34 meV giving values of 2${\mathrm{\Delta }}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$ equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of ${\mathrm{\Delta }}_{\mathrm{CDW}}$ at 4.2 K in 1T-${\mathrm{TaSe}}_2$ and 1T-${\mathrm{TaS}}_2$ are ∼150 meV for both materials giving 2${\mathrm{\Delta }}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$≊5.8. STM scans of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ at 4.2 K resolve atoms on the ${\mathrm{BiO}}_{\mathit{x}}$ 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 2Δ/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$≊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.