Vibration-assisted tunneling through single Au adatoms on two-dimensional WSe2

Scanning tunneling microscopy (STM) at 5 K was used to study individual Au atoms adsorbed on the surface of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>WS</a:mi><a:msub><a:mi mathvariant="normal">e</a:mi><a:mn>2</a:mn></a:msub></a:mrow></a:math> layer grown on epitaxial graphene. In line with theoretical predictions, scanning tunneling spectroscopy measurements reveal that the weakly bound adatom gives rise to an electronic state within the energy band gap of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>WS</c:mi><c:msub><c:mi mathvariant="normal">e</c:mi><c:mn>2</c:mn></c:msub></c:mrow></c:math> layer. Adatoms in different surface locations show different gap-state energy values that follow a random distribution around the Fermi level of the sample with a standard deviation of ∼50 meV. The location-dependent shift is attributed to spatial variations in disorder potential. Tunneling via the gap state is accompanied by vibrational excitations as apparent from pronounced sideband peaks in the conductance spectra with Poisson-distributed intensities indicating significant electron-phonon coupling with a Huang-Rhys factor of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>S</e:mi></e:mrow></e:math> ≅ 2.8. STM tunneling through single Au adatoms on two-dimensional <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>WS</f:mi><f:msub><f:mi mathvariant="normal">e</f:mi><f:mn>2</f:mn></f:msub></f:mrow></f:math> constitutes a model case of resonant double-barrier tunneling accompanied by strong coupling to vibrational degrees of freedom.