Spin decoherence of magnetic atoms on surfaces

Abstract

We review the problem of spin decoherence of magnetic atoms deposited on a surface. Recent breakthroughs in scanning tunnelling microscopy (STM) make it possible to probe the spin dynamics of individual atoms, either isolated or integrated in nanoengineered spin structures. Transport pump and probe techniques with spin polarized tips permit measuring the spin relaxation time T 1 , while novel demonstration of electrically driven STM single spin resonance has provided a direct measurement of the spin coherence time T 2 of an individual magnetic adatom. Here we address the problem of spin decoherence from the theoretical point of view. First we provide a short general overview of decoherence in open quantum systems and we discuss with some detail ambiguities that arise in the case of degenerate spectra, relevant for magnetic atoms. Second, we address the physical mechanisms that allows probing the spin coherence of magnetic atoms on surfaces. Third, we discuss the main spin decoherence mechanisms at work on a surface, most notably, Kondo interaction, but also spin-phonon coupling and dephasing by Johnson noise. Finally, we briefly discuss the implications in the broader context of quantum technologies.

Electrical spin manipulation in graphene nanostructures

“…Both T 1 and T 2 depend a lot on whether the nanographenes are deposited on top of a conductor or an insulator. In the former case, exchange interaction with the electrons in the conductor will be the dominant spin relaxation and decoherence mechanism [57].…”

“…We assume that the nuclear spins undergoes a stochastic motion with a white-noise spectrum with correlation time τ . Under these assumptions, the T 2 dephasing time for the electronic transitions due to their hyperfine interaction with the edge hydrogen atoms is [57,58] T…”

“…This equation is valid as long as τ is the shortest time scale in the problem [57,58]. In particular, τ ω…”

General scheme for stable single and multiatom nanomagnets according to symmetry selection rules

“…However, there is an ambiguity in the definition of the pointer basis when the atom presents pairs of degenerate states (which is the case when the atom has no GSS and applied magnetic field). There are indications [43] that the states of the pointer basis are those with maximum magnitude of the average magnetization, as the dephasing due to the scattering is the largest for these states. Thus we are allowed to assume that the pointer basis coincides with the atomic eigenstates considered in the previous sections, with well defined quasispin.…”

Spin-1 two-impurity Kondo problem on a lattice

“…When non-zero, it splits the S z = ±1 pair. The transverse anisotropy is particularly important in the context of spin-state life-times and decoherence 71 . The magnetic anisotropy hence determines the effective impurity degrees of freedom and the possible physical mechanisms for their quenching at low temperatures.…”