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}$${\mathrm{O}}_{8}$. Measured values of ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$ at 4.2 K for 2H-${\mathrm{TaSe}}_{2}$, 2H-${\mathrm{TaS}}_{2}$, and 2H-${\mathrm{NbSe}}_{2}$ are 80, 50, and 34 meV giving values of 2${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$ equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$ at 4.2 K in 1T-${\mathrm{TaSe}}_{2}$ and 1T-${\mathrm{TaS}}_{2}$ are \ensuremath{\sim}150 meV for both materials giving 2${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}5.8. STM scans of ${\mathrm{Bi}}_{2}$${\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 2\ensuremath{\Delta}/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}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.