Abstract
A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. Magnetic atoms on surfaces probed through scanning tunnelling microscopy provide an excellent platform to investigate coupled impurities, but typical high Kondo temperatures prevent field-dependent studies from being performed, rendering large parts of the phase space inaccessible. We present a study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo temperature of only 2.6 K. The pairs are designed to have interaction strengths similar to the Kondo temperature. By applying a sufficiently strong magnetic field, we are able to access a new phase in which the two coupled impurities are simultaneously screened. Comparison of differential conductance spectra taken on the atoms to simulated curves, calculated using a third-order transport model, allows us to independently determine the degree of Kondo screening in each phase.
Highlights
A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena
A single Co atom on Cu2N can be described by an effective spin with magnitude S 1⁄4 3/2, and the crystal field induced by the surface is such that the two lowest energy states have magnetization m 1⁄4 ±1/2, degenerate in absence of an external magnetic field[4]
Its differential conductance spectrum shows a sharp peak at zero excitation voltage, corresponding to a Kondo resonance with a Kondo temperature TK 1⁄4 2.6±0.2 K, equivalent to a characteristic Kondo energy eK 1⁄4 kBTK 1⁄4 0.22±0.02 meV, which is much lower than for single atoms directly on[2,3] or inside a metal[23]. This peak splits when a magnetic field is applied due to the Zeeman effect removing the degeneracy of the states with m 1⁄4 ±1/2
Summary
A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. The coupling of individual magnetic atoms to the itinerant host electrons of a metal substrate can lead to the formation of a correlated Kondo state in which the magnetic moment is effectively reduced[1]. This has been shown for 3d-atoms on bare metal surfaces[2,3] as well as on thin decoupling layers[4,5] and leads to a strong spectroscopic feature at the Fermi energy. Pairs of magnetic atoms are considerably more complex because in addition to the Kondo coupling they can couple to each other through exchange interactions mediated by the substrate electrons. We find that when in AFM coupled dimers the field exactly cancels the exchange interaction, Kondo resonances are re-established, signifying a phase transition through the two-impurity Kondo-screened phase
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