Scanning-probe-microscopy studies of superlattice structures and density-wave structures in 2H-NbSe2, 2H-TaSe2, and 2H-TaS2 induced by Fe doping.

Abstract

The intercalation of Fe into the van der Waals gap in the 2H phase transition-metal dichalcogenides ${\mathrm{NbSe}}_{2}$, ${\mathrm{TaSe}}_{2}$, and ${\mathrm{TaS}}_{2}$ produces many interesting electronic, magnetic, and structural effects. The scanning tunneling microscope (STM) and atomic force microscope (AFM) prove to be very sensitive to these changes and we report a wide range of results as a function of Fe concentration. All three materials support similar 3${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}3${\mathbf{a}}_{0}$ charge-density-wave (CDW) structures in the pure state at low temperatures. At low concentrations of Fe the CDW superlattice is still strong at 4.2 K and persists to high concentrations of Fe. At high concentrations, the Fe becomes ordered in the octahedral holes in the van der Waals gaps, and superlattices of the form 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ and \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ are observed. These can be detected at both 300 and 4.2 K. STM spectroscopy at 4.2 K shows that in 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{NbSe}}_{2}$ and 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaSe}}_{2}$ the energy gap in the electronic spectrum is initially reduced, but stabilizes at higher Fe concentrations and remains well defined for the ordered 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ phase. A transition from a CDW to a mixed CDW and spin-density-wave state is indicated, since these high Fe concentration phases are antiferromagnetic. In 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaS}}_{2}$ both 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ and \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ superlattices are observed. The 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ regions show a large energy gap, while the \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ do not. The latter phase is ferromagnetic and would not be expected to exhibit a gap. The development of the electronic structures over the entire range of Fe concentrations has been followed by STM and AFM and can be tracked in detail.