Aim
To demonstrate the MRI of individual atoms on a surface to subangstrom spatial resolution.
Apparatus
Cryogenic scanning tunneling microscope, Electron spin resonance, Magnetic tip, RF oscillator, Ti$_\text{O}$ atom samples, MnO$_2$ surface
Introduction
A clear image of Strontium was captured in 2018 by a group of students from Oxford [2]. The primary idea is to design the magnetic field gradient to locally tune the resonant frequency of spin ensembles for capturing using MRI. Though low temperature STM has subnanometre spatial resolution, their energy resolution is limited in tunneling spectroscopy experiments due to thermal broadening. In this paper, a new method was proposed to image the magnetic field to subangstrom resolution.
Setup
Single adatoms are adsorbed on $MnO_2$ surface. Fe atoms form the spin cluster on tip apex of STM. A magnetic field gradient of $\SI{0.3}{\tesla}{\angstrom}^{-1}$ is formed over the sample which is four to five orders of magnitude larger than in other scanning field gradient experiments. This is a consequence of the close proximity between the magnetic tip and the surface atom.
Principle
Single adatoms adsorbed on a surface are electrically probed using an STM tip above the atom. The Tunnel-current-assisted ESR is performed by applying an RF frequency which induces spin transitions between the ground and excited states due to Zeeman effect. At the resonant frequency, when $f$ of RF Voltage $V_{RF} \approx f_0$, the frequency of the atom, a peak in tunnel current is observed. The proximity of magnetic STM tip also exerts a force due to the field $B_{tip}$, to the adatoms, which further shifts the Zeeman splitting levels. This is a local phenomena and such spatial variations in resonant frequency can be utilized to acquire the information of both magnetic field lines and topographic image simultaneously. Lateral scanning along $x$ and $y$ directions, while keeping a fixed vertical tip-atom distance $z$, the MRI scan shows a crescent pattern at different $V_{RF}$. The slice patterns are a function of $z$, vertical tip-sample distance, and $f$ of $V_{RF}$ which basically control the tunnel current $I$.
Also, it was found that the resonant slices change drastically for different magnetic tips. Importantly, for any given STM tip, the resonant slice pattern also changes for different types of atoms on the surface. The magnetic interactions between the tip and the atom, in particular magnetic dipolar and exchange interactions help in acheiving the additional resolution needed.
Sources of Noise
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Thermal noise: Though the sample is cooled to ultra-low tempratures, the noise would be present nevertheless.
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The tunnel current induces a shot noise.
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Pink noise: Since the resonant conditions are typically acheived at low frequencies at ultra-cold temperatures, $1/f$ dependence of pink noise plays a major role in determining the resolutions of image obtained.
Applications
The potential applications of atomic-scale MRI include imaging biomolecules with unprecedented resolution, revealing the spin structure of atoms, molecules and solids, and giving site-dependent control in quantum simulators and spin networks.