91̽»¨

Abstract:

Coherent control of individual molecular spins in nano-devices is a pivotal prerequisite for fulfilling the potential promised by molecular spintronics. By applying electric field pulses during time-resolved electron spin resonance measurements, we measure the sensitivity of the spin in several antiferromagnetic molecular nanomagnets to external electric fields. We find a linear electric field dependence of the spin states in Cr$_7$Mn, an antiferromagnetic ring with a ground-state spin of $S=1$, and in a frustrated Cu$_3$ triangle, both with coefficients of about $2~\mathrm{rad}\, \mathrm{s}^{-1} / \mathrm{V} \mathrm{m}^{-1}$. Conversely, the antiferromagnetic ring Cr$_7$Ni, isomorphic with Cr$_7$Mn but with $S=1/2$, does not exhibit a detectable effect. We propose that the spin-electric field coupling may be used for selectively controlling individual molecules embedded in nanodevices.

Abstract:

This thesis details three studies performed with the aim of deepening our understanding of how nuclear and electronic spins can be manipulated such that they might be used in quantum information processing.

I start by discussing the possibility of using pulses of static electric fields to coherently control qudits implemented on molecular magnets. The success of this control depends on the level of spin-electric coupling (SEC) which reflects how the respective Hamiltonian changes with the application of an electric field. I present our research on a family of Mn(II)-containing molecules in which the systematic control of SEC is realised by varying the coordination environment of their spin centre. Their trigonal bipyramidal molecular structure with C₃ symmetry leads to a significant molecular electric dipole moment. Due to this, as well as high polarisability of the ligands, an applied electric field induces enhanced structural distortions. This gives rise to significant experimentally observed SEC, which is further rationalised by wavefunction theoretical calculations.

I then discuss the SEC in a molecular magnet [Yb (trensal)], which similarly possesses C₃ symmetry, but instead of manganese, this molecule contains a rare-earth ion of ytterbium (III). At cryogenic temperatures, [Yb (trensal)] can be described by an effective spin-1/2 Hamiltonian. However, our study shows that the significant values of SEC exhibited by [Yb (trensal)] can be only explained if the Hamiltonian is additionally equipped with the extended Stevens operators. The unique property of [Yb (trensal)] is that it demonstrates linear SEC even when the E-field is oriented perpendicularly to the C₃-axis of the molecule, and that this perpendicular SEC is of the same order of magnitude as the parallel effect.

In the third study, I show how, by using electron-nuclear double resonance, we implement a logical qubit encoded on four states of an I = 3/2 nuclear spin hyperfine-coupled to an S = 1/2 electron spin qubit. The encoding protects against the dominant decoherence mechanism in such systems – fluctuations of the quantizing magnetic field. We explore the dynamics of the encoded state both under a controlled application of the fluctuation and under natural decoherence processes. Our results confirm the potential of these proposals for practical, implementable, fault-tolerant quantum memories.