Electric‐Field‐Driven Spin Resonance by On‐Surface Exchange Coupling to a Single‐Atom Magnet

Abstract

Coherent control of individual atomic and molecular spins on surfaces has recently been demonstrated by using electron spin resonance (ESR) in a scanning tunneling microscope (STM). Here, a combined experimental and modeling study of the ESR of a single hydrogenated Ti atom that is exchange‐coupled to a Fe adatom positioned 0.6–0.8 nm away by means of atom manipulation is presented. Continuous wave and pulsed ESR of the Ti spin show a Rabi rate with two contributions, one from the tip and the other from the Fe, whose spin interactions with Ti are modulated by the radio‐frequency electric field. The Fe contribution is comparable to the tip, as revealed by its dominance when the tip is retracted, and tunable using a vector magnetic field. The new ESR scheme allows on‐surface individual spins to be addressed and coherently controlled without the need for magnetic interaction with a tip. This study establishes a feasible implementation of spin‐based multi‐qubit systems on surfaces.

Double-Resonance Spectroscopy of Coupled Electron Spins on a Surface

“…Third, we positioned the tip on Ti-2 and attempted to remotely drive Ti-1, which showed no ESR signal, most likely due to the weakness of the interaction with Fe. These control experiments confirm that the Ti-1 atom in the tunnel junction can be used to read the population of the quantum states of the remote spin and that the Fe atom is indeed needed to enable coherent driving of the remote spin …”

“…Considering the interactions J 1,tip between Ti-1 and the tip and J 2,Fe between Ti-2 and Fe as additional sources of magnetic fields, B tip and B Fe , a time-independent Hamiltonian can be written as …”

“…The best fits yield a combination of Rabi rates and relaxation times of the two spins, given reasonable bounds and starting points as detailed below. For the Rabi rates, from recent pulsed measurements of similar structures, we set the fitting start points to be around 20 MHz for tip-induced driving of the sensor spin and around 25 MHz for Fe-induced driving of the remote spin . This leaves us only with the determination of the lifetimes T 1 (1) and T 1 (2) , which appear in the collapse operators.…”

“…The ESR of the sensor spin was driven by the inhomogeneous magnetic field generated from the coupling with the tip. , In contrast, the remote spin was driven by its interaction with the nearby Fe. It was recently found that the coupling J 2,Fe provides a magnetic field gradient at the position of Ti-2, generating an effective time-varying field when coupled with the RF bias voltage, which enabled coherent driving of the Ti-2 spin . In the driving and measurement of the Ti ESR, the spin of the Fe atom ( S = 2) remained aligned perpendicular to the MgO surface due to its large uniaxial magnetic anisotropy and only flipped up and down infrequently − compared to the Rabi rates of the Ti spins.…”

“…We simultaneously and individually drive the ESR of two coupled, hydrogenated Ti ( S = 1/2) atoms on a MgO surface . To remotely drive the spin outside the tunnel junction, we placed a single-atom magnet in close proximity, which provides a spatially inhomogeneous magnetic field to generate a transverse driving field when coupled with a radio frequency (RF) electric field . By controlling the frequency of the applied RF field, we can selectively address all transitions, which enables us to remotely drive the ESR of an atomic spin outside the tunnel junction.…”