ESR-STM: Mechanism and magnetic resonance of nuclei.

by Mr. Michael Averbukh

BGU

at Special seminar

Thu, 15 Sep 2022, 11:00
Sacta-Rashi Building for Physics (54), room 207

Abstract

ESR -STM noise spectroscopy is a single spin ESR technique. This technique was shown to be effective on paramagnetic defects, atoms and molecules. Also non - magnetic molecules can be ionized and becoming paramagnetic and detected with ESR-STM. The physical mechanism for this technique was not understood for a very long time. Recently it was shown that when there are 2 neighboring spins at the STM tunneling junction, the interference between the tunneling through these two spins creates an ESR-STM signal (at room temperature and without rf excitation). The main prediction of this model was that in addition to the Larmor frequency signal of both spins - we get another signal which is the difference between these 2 spectra. We show experimentally that such a difference is observed at 4 different experiments - in different STMs, different tips and different environments - that confirm the mechanism.
One of the most important ESR-STM observations was the hyperfine peaks signals. These enabled the access of the ESR-STM technique into single nuclear magnetic resonance. The first experiment was a single spin ENDOR experiment where an rf irradiation at the frequency of the nuclear transitions (half of the hyperfine frequency) was shown to change the amplitude of the ESR hyperfine peaks. This was done on several different ESR - STM spectra on Si, Cu and 14N atoms. However, in this case, the magnetic field was internal (applied by the coupled electron) and normally much larger than the external field. In order to make a real NMR measurement it was necessary to change the experimental procedures. In this experiment sequential Ionization by Scanning Tunnelling Microscope (STM) bias voltage pulses of a molecule with anisotropic hyperfine interaction, results in a temporal transformation from diamagnetic to paramagnetic. The effective magnetic field on the nucleus (in the diamagnetic state) and on the electron (in the paramagnetic state) are not parallel. The precession of the nucleus in the diamagnetic state is modulating the polarization of the hyperfine states and the matrix elements for the hyperfine transitions. An NMR spectrum was observed in 14N nucleus in a tempo molecule on gold covered with graphene oxide, and of 1H nuclei in toluene deposited on a clean gold substrate. A model that explains the observations quantitatively is presented. The technique can work both on magnetic and non-magnetic molecules.

Created on 24-08-2022 by Meidan, Dganit (dganit)
Updaded on 24-08-2022 by Meidan, Dganit (dganit)