Electron Spin Resonance of Heisenberg Spin Chains of Four Ti Atoms

Heisenberg spin chain (HSC) is a one-dimensional circular array of spins, where the nearest neighbors are exchange-coupled. Quantum transitions between the energy eigenstates of such spin chains are well understood by the creation and annihilation of spin wave quanta. Using a low temperature (1.1K) scanning tunneling microscope (STM), we composed two circular spin chains of four hydrogenated Ti atoms, spin-1/2 atomic species, on MgO surface by atom manipulation techniques. The nearest neighbor couplings are antiferromagnetic and tuned to be 6 and 25 GHz by interatomic distance control. We performed electron spin resonance (ESR) combined with the STM on the spin chains. The measured ESR spectra show a remarkable consistency with the quantum transition probabilities calculated from the Heisenberg spin Hamiltonian, which identifies the energy eigenstates of the spin chains. We discuss the experimental data by employing the spin wave theories. This work suggests that our Ti spin chains deserve an ideal exchange-coupled quantum spin system. In addition, this work demonstrates an experimental approach plausible to creation and control of quantum phenomena in artificial spin structures.