Mentions: Figure 1A shows a constant current image of CoH complexes on h-BN/Rh(111). The lattice mismatch between the Rh(111) substrate and the single monolayer of h-BN results in a strongly corrugated surface with 3.2-nm periodicity, on which the CoH complexes appear as bright protrusions. A clear indication of hydrogen adsorption on the tip apex is the sharp change in tip height, reduced by 20 pm (Fig. 1A, red dashes), while imaging the h-BN/Rh(111) surface in constant current mode (21). Figure 1B shows constant current images of an individual CoH complex located near the rim-valley boundary of h-BN/Rh(111) that is imaged with a hydrogen-functionalized tip. At low junction conductance (G = IS/VS = 1.61 × 10−4G0; G0 = 77.48 μS; the quantum of conductance), corresponding to relatively large tip-sample separations z, the increased contrast due to the hydrogen in the junction partially overlaps a CoH complex (Fig. 1B, bottom panel). As G is increased, this boundary region transitions to a noise-speckled circle with a brighter appearance, that is, larger z height, to compensate for an overall increase in the conductance. Given the strong G dependence within this narrow range, these results hint that the observed contrast is not solely due to the local topography but is also due to mechanical and electronic changes in the junction. These images are qualitatively similar to measurements of undercoordinated metal adatoms in the presence of adsorbed hydrogen (22, 23). Because the hydrogen content of the CoHx complex governs the spin state (5), dI/dV spectroscopy was performed while varying the setpoint conductance G with the tip positioned over the central region. At the lowest conductance, G = 6.45 × 10−4G0 (Fig. 1C, bottom curve), the spectra show two symmetric steps around zero bias with increasing differential conductance. These steps originate from the inelastic spin excitations of a CoH complex with a total spin S = 1, where magnetocrystalline anisotropy has removed the 3d-level degeneracy. Increasing G by decreasing the tip-sample separation z results in progressively unstable spectra until the emergence of a stable zero bias peak at G = 12.9 × 10−4G0, identified as an S = 1/2 CoH2 Kondo resonance (5). This transition is fully reversible, and the initial S = 1 total spin state is restored when the junction conductance is reduced (see fig. S1). We observe a metastable state, when G is between 8 × 10−4G0 and 11 × 10−4G0, where the hydride complex randomly transitions between the S = 1 and S = 1/2 states on a time scale of 100 ms. The change in tip-sample separation for this conductance range corresponds to a Δz of less than 25 pm. Note that this metastable behavior does not depend on the bias voltage during the spectroscopic measurement (see also fig. S2). Differential conductance (dI/dV) spectroscopy not only identifies the spin state but also aids in the interpretation of the STM images in Fig. 1B. The constant current images in Fig. 1B were obtained over a bias range (0.3 to 1.6 mV) where the topographic appearance is closely linked to the features in the dI/dV measurements and, therefore, at small bias voltages, is dominated by the Kondo resonance.
A hydrogen-functionalized scanning probe tip is used to reversibly switch the total spin of a cobalt hydride complex.
No MeSH data available.