Radio Frequency Scanning Tunneling Spectroscopy for Single-Molecule Spin Resonance

We probe nuclear and electron spins in a single molecule even beyond the electromagnetic dipole selection rules, at readily accessible magnetic fields (few mT) and temperatures (5 K) by resonant radio-frequency current from a scanning tunneling microscope. We achieve subnanometer spatial resolution combined with single-spin sensitivity, representing a 10 orders of magnitude improvement compared to existing magnetic resonance techniques. We demonstrate the successful resonant spectroscopy of the complete manifold of nuclear and electronic magnetic transitions of up to $\mathrm{\Delta }{I}_z=\pm 3$ and $\mathrm{\Delta }{J}_z=\pm 12$ of single quantum spins in a single molecule. Our method of resonant radio-frequency scanning tunneling spectroscopy offers, atom-by-atom, unprecedented analytical power and spin control with an impact on diverse fields of nanoscience and nanotechnology.

Figure 1

(a) Schematic experimental setup of radio frequency scanning tunneling spectroscopy (rf-STS) on a single molecule; a terbium-double decker molecule $[{\mathrm{TbPc}}_2{]}^0$ resides inside the STM tunnel junction formed by the tip and Au(111) substrate; the ${\mathrm{Tb}}^{3+}$ ion (red) is sandwiched between two phthalocyanine ligands (green); a static magnetic field is applied perpendicular to the substrate surface; dc and rf components of the tunneling current are simultaneously applied by separate sources. (b) STM topographic image of a ${\mathrm{TbPc}}_2$ molecule adsorbed on Au(111) at 5 K ($+1\mathrm{V}$, 100 pA); circles mark different tip positions over the ligand (1) and center (3) of the molecule as well as over the bare substrate surface (2). (c) $dI/dV$ spectrum of ${\mathrm{TbPc}}_2/\mathrm{Au}(111)$ at dc tunnel conditions measured at position 1; inset: characteristic Kondo signature of the neutral $[{\mathrm{TbPc}}_2{]}^0$ molecule on Au(111) observed by dc tunneling spectroscopy.

Figure 2

Nuclear and electron spin excitations in a single “terbium double decker” molecule by rf-STS. (a) Zeeman diagram of the lowest $J_{\mathrm{z}}=\pm 6$ substates of the $J=6$ electronic ground state of the ${\mathrm{Tb}}^{3+}$ ion in $[{\mathrm{TbPc}}_2{]}^{-}$; the hyperfine levels are labeled as $|J_z,I_z⟩$; yellow, green, and blue arrows mark purely nuclear hyperfine transitions, labeled 1–3; grey arrows mark purely electronic transitions labeled 4 and 4’; red arrows mark mixed electronic nuclear transitions, labeled 5 and $5^{\prime }$. (b) Comparison of the calculated hyperfine transition frequencies of ${\mathrm{TbPc}}_2$ (abscissa) with the experimental frequency values (ordinate) obtained by rf-STS on single ${\mathrm{TbPc}}_2$ molecules on Au(111) at a static magnetic field of 2.5 mT (left) and 16 mT (right) perpendicular to the sample surface; boxes (open square) mark purely nuclear transitions; triangles (open triangle) mark transitions with electronic spin flips; the dashed line represents ideal agreement as guide to the eye; selected transitions marked by arrows in (a) are labeled 1–5, 4’, and 5’; error bars are $\pm 10\%$ (horizontal) due to uncertainty of numerical parameters [20, 22], $\pm 0.05\mathrm{GHz}$ (vertical, open square) due to variable adsorption sites of different ${\mathrm{TbPc}}_2$ molecules studied, and $\pm 0.125\mathrm{GHz}$ (vertical, open triangle) due to the experimental uncertainty of the magnetic field of $\pm 0.5\mathrm{mT}$.

Figure 3

Single-molecule spin resonance spectra by rf-STS. Conductance ($dI/dV$) spectra obtained by rf-STS over single ${\mathrm{TbPc}}_2$ molecules adsorbed on Au(111) at 5 K, constant dc current between 0.5 and 2 nA, and $10\mathrm{MHz}/\mathrm{s}$ rf sweep rate in different frequency ranges; the data represent single-sweep spectra acquired within 30–60 s; labels 1–3 and $5^{\prime }$ relate to selected spin transitions marked by arrows in Fig. 2; to compensate for the observed tip-dependence of the signal intensity, the spectra are normalized to a constant peak-to-peak fluctuation amplitude of the baseline noise. (a) STM tip placed over ligand [position 1 in Fig. 1], static magnetic field of $B=2.5\mathrm{mT}$ applied perpendicular to the sample surface, dc sample bias voltage of $V_{\mathrm{T}}=+0.9\mathrm{V}$ for tunneling into unoccupied electronic state of ${\mathrm{TbPc}}_2$ ligand. (b) Same as (a) with $B=16\mathrm{mT}$. (c) Same as (a) but with tip placed over bare gold substrate [position 2 in Fig. 1]. (d) Same as (a) but with $V_{\mathrm{T}}=+0.7\mathrm{V}$ for suppressing tunneling into ${\mathrm{TbPc}}_2$ molecular orbitals. (e) Same as (a) with tip placed over the center of the adsorbed ${\mathrm{TbPc}}_2$ molecule [position 3 in Fig. 1].