Rare Earth Atoms on Thin MgO Films: From Single Atom Magnets to Quantum Bits

Abstract: Rare earth atoms are usually investigated in bulk studies through electronic transport or optical and microwave spectroscopic techniques. In this talk, we will focus on experiments in Scanning Tunneling Microscopy (STM), which offers the ability to study individual atoms and molecules on surfaces. We will start by looking at dysprosium and holmium on the oxygen binding site of MgO. Both elements have the unique property that they have bi-stable spin states: the magnetization can be up or down along the surface normal and does not relax (at temperatures below ~10K) [1]. Ho is a non-Kramers ion, which enables hybridization of the two low-lying electronic states. The O binding site has C4v symmetry and therefore enables quantum tunneling in Ho with an exceedingly small tunnel gap of only 10peV, which corresponds to about 1 kHz. Such fine measurements can be performed with STM by carefully ramping an external magnetic field through avoided level crossings [manuscript in preparation].

There is a strong international research effort in the area of quantum nanoscience where the concepts of quantum coherence, superposition, and entanglement of quantum states are exploited in solid-state and molecular systems [3]. In the second part of the talk, we will focus on such properties of rare earth atoms on MgO by looking at the example of Er. Er has one more f-electron than Ho but behaves completely differently. Like Dy, Er is a Kramers ion, which guarantees a two-fold degeneracy of the ground state in the absence of magnetic fields. Small fields lead to an energy splitting, which can be probed through electron spin resonance in STM [4] and turns Er on MgO into an interesting quantum bit [5].