Electron paramagnetic resonance of individual atoms on a surface

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Science  23 Oct 2015:
Vol. 350, Issue 6259, pp. 417-420
DOI: 10.1126/science.aac8703

EPR, one atom at a time

Electron paramagnetic resonance (EPR) usually detects atoms with unpaired electrons as ensemble averages. Baumann et al. used a spin-polarized scanning tunneling microscope tip to measure EPR spectra of single iron atoms adsorbed on a magnesium oxide surface at cryogenic temperatures. The measurement depends on the atomic orbital symmetry; no signal was observed for cobalt atoms under the same conditions

Science, this issue p. 417


We combined the high-energy resolution of conventional spin resonance (here ~10 nano–electron volts) with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. We drove the spin resonance with an oscillating electric field (20 to 30 gigahertz) between tip and sample. The readout of the Fe atom’s quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. We determine an energy relaxation time of T1 ≈ 100 microseconds and a phase-coherence time of T2 ≈ 210 nanoseconds. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening.

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