Background: Electron Spin Resonance


Magnetic resonance imaging is a very familiar technique for all of us who have torn a ligament since it is routinely used to image the inside of our bodies without harmful radiation. At its core lies nuclear magnetic resonance spectroscopy, which measures the magnetic properties of the tiny magnet that is inherent in every proton and hence every water molecule. This tiny magnet is also called a nuclear spin. The imaging resolution of MRI is good enough to see your ligament but requires billion and billions of hydrogen nuclear spins for each pixel.
Figure: ESR spectrum obtained on a low density of nitrogen (N) defects in diamond. The main ESR line at 0.374T corresponds to an electron spin at the resonance frequency of 10GHz. Two additional lines stem from the nuclear spin of N. Spectra obtained with our home-built ESR spectrometer that is optimized for surface science studies. Temperature T=5K.
Electron spin resonance (ESR) is a cousin to nuclear magnetic resonance and relies on the fact that certain atoms or small molecules can contain an electron spin, which is about 1000 times stronger than the nuclear spin. ESR is also used for imaging, albeit not in a hospital but rather in basic science research where it is a crucial tool to learn about the structure of molecules.

Nuclear and electron spin resonance techniques are also crucial for the coherent control of many quantum systems that are used for quantum sensing and quantum computation, including spins in quantum dots or defects in semiconductors and insulators.

The figure shows an ESR spectrum obtained on a small piece of diamond which contains some nitrogen (N) defects. Here nitrogen atoms replace some of the carbon atoms of the diamond and contain an electron spin. Three ESR lines are seen at different magnetic fields due to the additional nuclear spin of the nitrogen atom.
QNS widely employs ESR spectroscopy to control quantum spins:

1. ESR is combined with STM to perform ESR spectroscopy on individual spins on surfaces. QNS was first to measure ESR STM of a molecular spin as well as of multiple spins in double resonance spectroscopy.

2. Our surface science ESR spectrometer combines ensemble ESR with surface science and has a sensitivity to measure less than one layer of spins on samples that can be prepared in ultra-high vacuum.

3. We control the electron spin of nitrogen-vacancy (NV) centers and attach them to the tip of an AFM. This allows high-resolution quantum sensing at variable temperature including room temperature.
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