Gating Orbital Memory with an Atomic Donor

Orbital memory is defined by two stable valencies that can be electrically switched and read out. To explore the influence of an electric field on orbital memory, we studied the distance-dependent influence of an atomic Cu donor on the state favorability of an individual Co atom on black phosphorus. Using low temperature scanning tunneling microscopy and spectroscopy, we characterized the electronic properties of individual Cu donors, corroborating this behavior with ab initio calculations based on density functional theory. We studied the influence of an individual donor on the charging energy and stochastic behavior of an individual Co atom. We found a strong impact on the state favorability in the stochastic limit. These findings provide quantitative information about the influence of local electric fields on atomic orbital memory.

Figure 1

(a) STM image of individual Co and Cu atoms on the surface of BP. ($V_S=-400\mathrm{mV}$, $I_t=20\mathrm{pA}$, $\mathrm{scalebar}=5\mathrm{nm}$). (b)–(d) STM images of the charge density of three Cu species: (b) Cu residing in a hollow site (${\mathrm{Cu}}_{\mathrm{H}}$), (c) hydrogenated Cu residing in a hollow site (${\mathrm{CuH}}_{\mathrm{H}}$) and (d) Cu residing on a top site (${\mathrm{Cu}}_{\mathrm{T}}$). ($V_S=-400\mathrm{mV}$, $I_t=60\mathrm{pA}$, $\mathrm{scalebar}=1\mathrm{nm}$). (e)–(g) Ab initio calculations of the relaxed charge density of (e) a ${\mathrm{Cu}}_{\mathrm{H}}$ atom, (f) ${\mathrm{CuH}}_{\mathrm{H}}$, and (g) a ${\mathrm{Cu}}_{\mathrm{T}}$ atom. (h)–(j) Schematics of the relaxed atomic adsorption geometries of (h) ${\mathrm{Cu}}_{\mathrm{H}}$, (i) ${\mathrm{CuH}}_{\mathrm{H}}$, and (j) ${\mathrm{Cu}}_{\mathrm{T}}$.

Figure 2

(a) $dI/dV$ spectrum taken on an isolated ${\mathrm{Co}}_{\mathrm{low}}$ atom (distance to nearest Cu species $r>10\mathrm{nm}$). Inset: STM image of the isolated ${\mathrm{Co}}_{\mathrm{low}}$ atom ($V_S=-400\mathrm{mV}$, $I_t=60\mathrm{pA}$). (b) $dI/dV$ spectrum taken on a ${\mathrm{Co}}_{\mathrm{low}}$ atom in the vicinity of ${\mathrm{CuH}}_{\mathrm{H}}$ (distance $r=3.8\mathrm{nm}$). The shift of the ionization peak with respect to the isolated atom is indicated by $\mathrm{\Delta }{V}_r$ and equals 118 mV. Inset: STM image of the ${\mathrm{Co}}_{\mathrm{low}}\text{−}{\mathrm{CuH}}_{\mathrm{H}}$ pair. The red $X$ in both insets (a)–(b) mark the locations where the $dI/dV$ spectra were taken. ($V_S=-400\mathrm{mV}$, $I_t=60\mathrm{pA}$).

Figure 3

Shift of the ionization peak energy $\mathrm{\Delta }{V}_r$ of single ${\mathrm{Co}}_{\mathrm{low}}$ atoms as a function of distance to a Cu species $r$. The shift is defined with respect to the peak energy of an isolated atom: $\mathrm{\Delta }{V}_r=V(r)-V(r>10\mathrm{nm})$. Different colors represent different microtips. The solid line is a fit to the Yukawa potential, with parameters $g=4.88$; $r_c=1.93\mathrm{nm}$. In the inset, a schematic band diagram shows downward band bending of the Co energy level (which is pulled through $E_F$ by TIBB) in the presence of a Cu species.

Figure 4

(a) Two-state telegraph noise signal of an isolated Co atom (nearest Cu donor at $r=14.7\mathrm{nm}$), with ${\mathrm{Co}}_{\mathrm{low}}$ in purple and ${\mathrm{Co}}_{\mathrm{high}}$ in blue. (b)–(c) State lifetime $\tau$ of (b) ${\mathrm{Co}}_{\mathrm{high}}$ and (c) ${\mathrm{Co}}_{\mathrm{low}}$ as a function of $r$. Different symbols represent different microtips and different colors represent different biases (500 and 550 mV). In the insets, STM images of the corresponding states ${\mathrm{Co}}_{\mathrm{high}}$ and ${\mathrm{Co}}_{\mathrm{low}}$ are displayed ($V_S=-400\mathrm{mV}$, $I_t=60\mathrm{pA}$).