Nuclear magnetic resonance on a single atom with a local probe

The nuclear spin is a prime candidate for quantum information applications due to its weak coupling to the environment and inherently long coherence times. However, this weak coupling also challenges the addressability of the nuclear spin. Here we demonstrate nuclear magnetic resonance (NMR) on a single on-surface atom using a local scanning probe. We employ an electron-nuclear double resonance measurement scheme and resolve nuclear spin transitions of a single 47Ti isotope with a nuclear spin of I = 5/2. The quadrupole interaction enables to resolve multiple NMR transitions, which are consistent with our eigenenergy calculations. Our experimental results indicate that the nuclear spin can be driven efficiently irrespective of its hybridization with the electron spin, which is required for direct control of the nuclear spin in the long-lifetime regime. This investigation of NMR on a single atom in a platform with atomic-scale control is a valuable development for other platforms deploying nuclear spins for characterization techniques or quantum information technology.

💡 Research Summary

In this work the authors demonstrate nuclear magnetic resonance (NMR) on a single on‑surface atom by combining scanning tunneling microscopy (STM) with electron‑nuclear double resonance (ENDOR). The system under study is a 47Ti atom (nuclear spin I = 5/2) adsorbed on oxygen sites of a bilayer MgO film grown on Ag(100). An external magnetic field perpendicular to the surface lifts the electron‑spin degeneracy (S = 1/2) and allows electron‑spin resonance (ESR) to be driven by a GHz‑frequency voltage applied to the tunnel junction. Spin‑dependent tunnelling current readout reveals six hyperfine‑split ESR peaks, each corresponding to a different nuclear spin projection mI. The relative heights of these peaks directly encode the time‑averaged nuclear‑spin populations, providing a built‑in readout channel for nuclear dynamics.

To address the limitation that conventional ESR‑STM readout is sensitive only to electron‑spin populations, the authors implement an ENDOR scheme: a second RF source in the MHz range is applied simultaneously with the ESR drive. When the MHz frequency matches a nuclear transition, the populations of the two involved nuclear states become equal, which manifests as a change in the heights of the corresponding ESR peaks. By fixing the ESR frequency on a chosen hyperfine line and sweeping the NMR frequency, the authors resolve multiple nuclear transitions. The quadrupole interaction (Q ≈ –2.8 MHz) splits each hyperfine line into two distinct NMR resonances, allowing the observation of at least four separate transitions for each electron‑spin orientation (mS = ↑, ↓). Fitting the measured transition energies with an effective spin Hamiltonian yields a hyperfine constant A = 132.1 ± 0.4 MHz, consistent with earlier reports, and a nuclear g‑factor of 0.37 ± 0.04, slightly larger than the bulk value (0.315) for 47Ti.

A systematic magnetic‑field study (0.2–1.4 T) shows that the NMR resonances persist even when the electron‑nuclear hybridization is strongly suppressed. The transition frequencies increase linearly with field, confirming their Zeeman origin, while the signal amplitudes depend on the ESR drive amplitude (V_ESR) because the electron‑spin state populations affect the efficiency of nuclear pumping. The authors also discuss possible driving mechanisms. Modulation of the hyperfine term would generate GHz‑scale transitions, inconsistent with the observed MHz frequencies. Quadrupole modulation, induced by the RF electric field, can in principle drive ΔmI = ±1 transitions but cannot account for all observed lines and is forbidden for certain transitions. The most plausible mechanism is an effective oscillating magnetic field generated by the RF electric field, analogous to the driving of electron spins in ESR‑STM via piezoelectric displacement, tunnelling‑barrier modulation, or spin‑orbit effects. This mechanism would be largely independent of the specific hyperfine or quadrupole constants, suggesting that STM‑based nuclear driving could be extended to other nuclear‑spin systems.

In summary, the paper provides the first clear demonstration of NMR on a single atom without relying on electron‑nuclear hybridization, using a local STM probe for both excitation and readout. The ability to address and manipulate individual nuclear spins with atomic‑scale spatial control opens new avenues for quantum information processing, nanoscale sensing, and quantum simulation platforms that exploit the long coherence times of nuclear spins while retaining the flexibility of STM‑based atomic engineering.