Double-Resonance Spectroscopy of Coupled Electron Spins on a Surface

Abstract

Scanning–tunneling microscopy (STM) combined with electron spin resonance (ESR) has enabled single-spin spectroscopy with nanoelectronvolt energy resolution and angstrom-scale spatial resolution, which allows quantum sensing and magnetic resonance imaging at the atomic scale. Extending this spectroscopic tool to a study of multiple spins, however, is nontrivial due to the extreme locality of the STM tunnel junction. Here we demonstrate double electron–electron spin resonance spectroscopy in an STM for two coupled atomic spins by simultaneously and independently driving them using two continuous-wave radio frequency voltages. We show the ability to drive and detect the resonance of a spin that is remote from the tunnel junction while read-out is achieved via the spin in the tunnel junction. Open quantum system simulations for two coupled spins reproduce all double-resonance spectra and further reveal a relaxation time of the remote spin that is longer by an order of magnitude than that of the local spin in the tunnel junction. Our technique can be applied to quantum-coherent multi-spin sensing, simulation, and manipulation in engineered spin structures on surfaces.

All-Electrical Driving and Probing of Dressed States in a Single Spin

“…We performed experiments on a designed nanostructure composed of two Ti atoms (each having an electron spin of S = 1/2), hereafter referred to as Ti-1 and Ti-2, forming a pair of coupled electron spins. These spins were placed on a bilayer of MgO grown on a single crystalline Ag substrate (Figure a). , The electron spin resonance (ESR) spectra of Ti-1 (Figure b) and Ti-2 (Figure c) were obtained by using continuous-wave single- and dual-frequency ESR, respectively . From the spectra, we obtained the respective resonance frequencies of the coupled spins as well as the strength of their exchange interaction J .…”

“…The peak splitting Δ f again showed a linear dependence on V RF2 , resulting in a Rabi rate of the Ti-2 spin, Ω (2) (Figure c). Here, the Autler–Townes doublets in the double resonance spectroscopy allowed a direct measure of the strength of the remote driving of a spin outside the tunnel junction, as reported in our previous work. , Note that the Rabi rate of the Ti-2 spin is comparable to that of the Ti-1 spin in the tunnel junction, demonstrating that our tailored quantum spin structure provides an effective way to remotely control spins outside the STM tunnel junction.…”

“…We performed experiments in an ultrahigh vacuum ( P base < 10 –10 mbar) low-temperature STM at a base temperature of 0.4 K (USM1300, Unisoku), equipped with 3 He cryogenics, two-axis superconducting magnets, and RF-compatible signal cables. The sample preparation procedures are described in detail elsewhere. , Fe and Ti atoms were deposited on an atomically flat surface of 2 ML thick MgO on Ag(100) substrate, which is precooled at the STM stage. All measurements were performed on Ti atoms at bridge sites of the MgO surface (that is, in the middle of two oxygen sites).…”

“…(a) STM topographic image of an engineered nanostructure composed of two coupled spins: Ti-1 (in the tunnel junction), Ti-2 (outside the tunnel junction), and one Fe atom . The Ti-1 to Ti-2 spacing is 1.22 nm, and Ti-2 to Fe is 0.59 nm ( I DC = 10 ⁡ pA , V DC = 100 ⁡ mV ) . (b, c) Single and double resonance ESR spectra obtained from the structure in (a).…”

“…In this work, we demonstrate the creation and read-out of dressed states of single spins in a scanning tunneling microscope. , We utilized a tailored atomic scale nanostructure of two weakly coupled electron spins (Figure a) in which multi-spin control was achieved through a spin in the STM tunnel junction using the double resonance technique. , Both spins were subjected to the applied radio frequency (RF) fields and selectively driven by tuning the RF frequency into resonance. − Resonant driving of one spin resulted in the dressing of its states. Instead of using an optical transition to a third level, resonant transitions of the other spin were then used to probe the dressed states of the first spin in our experimental scheme, enabling both creation and read-out of the dressed states to be performed all electrically with two RF voltages of moderate amplitude.…”