Electric control of spins is a significant goal in solid-state physics, promising increased efficiency in information processing. Optimizing this efficiency requires scaling spintronics to the atomic level. Scanning tunneling microscopy (STM), particularly when combined with electron spin resonance spectroscopy (ESR), provides an ideal platform for achieving atomic-scale spin control. ESR-STM enhances ESR sensitivity to atomic-scale spin systems and improves STM energy resolution. The applied bias voltage in ESR-STM creates a strong electric field between the tip and sample, offering a potential for direct spin manipulation. While bias voltage is a readily tunable parameter in ESR experiments, its use for direct spin manipulation has been unexplored until now. This study utilizes the bias voltage to directly manipulate spin transitions, aiming to achieve sizeable atomic-scale electrical spin control.

Several decades of research have focused on controlling spin and magnetic properties using electric fields, with various approaches explored. These include spin transistors, the spin Hall effect, dopants in silicon, and magnetic molecules. Electric field control offers advantages in scalability and switching speed compared to magnetic field control. Previous work has investigated spin-electric coupling in various systems, including molecular magnets and bulk materials, but achieving this control at the atomic scale has been a major challenge. ESR-STM has emerged as a powerful technique for probing and manipulating individual spins, but its potential for direct electric field control of spin transitions has been largely untapped.

The experiments were performed using a commercial Unisoku USM-1300 STM modified for high-frequency operation. TiH molecules adsorbed on a MgO layer on a Ag(100) substrate were studied. ESR signals were acquired by sweeping the external magnetic field at various bias voltages, while maintaining a constant microwave frequency (61.545 GHz) and amplitude (20 mV). Measurements were conducted at 310 mK with the magnetic field perpendicular to the sample surface. Different setpoint currents were used to investigate the effect of tip-sample distance on the spin-electric coupling. Data analysis involved fitting the ESR peaks to extract the g-factor and tip magnetic field as functions of bias voltage. Similar experiments were conducted on TiH molecules adsorbed on different sites and dimers to examine interactions. The setup involved cleaning the Ag(100) substrate, growing MgO, and depositing Fe and Ti atoms, with Ti naturally hydrating to form TiH molecules. ESR-sensitive tips were created by picking up Fe atoms. Dimers were either naturally occurring or created by atom manipulation.

The study observed significant bias voltage-dependent shifts in the ESR signal of TiH molecules, far exceeding previously reported values in bulk systems. The shift is attributed to the electric field in the tunnel junction, which causes a displacement of the TiH molecule, altering both the g-factor and the effective magnetic field from the STM tip. Quantitative analysis revealed that both the g-factor and the tip field increase monotonically with increasing bias voltage. The magnitude of the effect was stronger at higher setpoint currents, consistent with a stronger electric field at smaller tip-sample distances. Analysis of the data at different frequencies allowed for the extraction of g-factor and tip field dependencies on bias voltage. Effective frequency shifts of 0.83 GHz/V and 4.3 GHz/V were calculated for the g-factor and tip field, respectively, at a setpoint current of 250 pA—orders of magnitude larger than previously reported in bulk systems. This is attributed to the extremely strong electric field in the tunnel junction. Direct manipulation via spin-electric coupling (SEC) was demonstrated in two types of TiH dimers with varying intermolecular distances. In strongly coupled dimers, three transitions were identified, and their positions were tuned using bias voltage. A clock transition, only visible when g-factors differ, was identified even at zero bias voltage. In weakly coupled dimers, an avoided crossing between singlet and triplet states was observed and tuned. By adjusting both bias voltage and tip-sample distance, the avoided crossing was successfully moved near zero bias voltage, minimizing tunneling current and maximizing coherence time. This demonstration highlights the potential to use bias voltage for manipulating the coherent evolution of entangled states.

The findings demonstrate the effectiveness of electric field control over spin transitions at the atomic scale, significantly exceeding previous observations in bulk materials. The strong electric field in the STM junction is key to this enhanced control. The ability to manipulate both the g-factor and the tip field provides a powerful tool for tuning spin properties. The observation of controllable transitions in coupled TiH dimers showcases the potential for using this technique to engineer complex spin interactions. The ability to tune the avoided crossing to zero bias voltage, where coherence is maximized, points to significant advantages for quantum information processing applications. The observed SEC is a general phenomenon, likely applicable to other atoms and molecules, suggesting wide applicability. This research offers insights for understanding and optimizing SEC in both atomic-scale and bulk systems.

This study successfully demonstrated electric control of spin transitions in single TiH molecules and coupled dimers using ESR-STM. The results highlight the potential of using bias voltage as a powerful tool for manipulating spin systems at the atomic scale. This approach significantly enhances control over spin transitions, opening up new possibilities for quantum information processing and the investigation of spin-electric coupling in various systems. Future research could focus on exploring different atomic and molecular systems to optimize SEC, investigating higher-order spins, and applying this technique to time-resolved experiments for studying the dynamics of spin systems.

The study focuses on specific TiH molecules and dimers on MgO substrates. The observed spin-electric coupling might vary with different molecules, substrates, or adsorption sites. The strong electric field in the STM junction could introduce limitations or artifacts in the measurements. Further experiments are needed to validate the generalizability of these findings to other systems.