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We present here an exact version of our response (dated April 27) to Wernsdorfers correspondence submitted to Nature Physics on March 31, 2008. After consultation with a referee, Nature Physics chose not publish any part of this exchange. We would therefore like to point out that our original study has now been considered favorably by four separate referees chosen by Nature Physics. Unfortunately, Wernsdorfer subsequently posted two further variations of his correspondence on this archive (arXiv:0804.1246v1 and arXiv:0804.1246v2). We note that aspects of the most recent posting (dated after submission of our response) contradict the version submitted to Nature Physics. However, none of the revisions add weight to Wernsdorfers original correspondence.
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Ramsey et al. [Nature Phys. 4, 277-281 (2008)] report the observation of quantum interference associated with tunnelling trajectories between states of different total spin length in a dimeric molecular nanomagnet. They argue that the interference is a consequence of the unique characteristics of a molecular Mn12 wheel, which behaves as a molecular dimer with weak ferromagnetic exchange coupling. We show here that the data published by Ramsey et al. are not consistent and unfortunately mostly wrong. We show further that the Landau-Zener (LZ) formula, which links the tunnel probability with the tunnel splitting, can only be applied in a well-defined experimental region, which lays outside the region accessed by Ramsey and colleagues. Only a lower-limit estimate of the tunnel splitting can be obtained, showing that the observed transition cannot be explained with the dimer model. We also present other shortcomings of the paper questioning the dimer model, and that the alignment of the magnetic field is crucial for observing quantum interference.
We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6%.
Qubits based on the singlet (S) and the triplet (T0, T+) states in double quantum dots have been demonstrated in separate experiments. It has been recently proposed theoretically that under certain conditions a quantum interference could occur from the interplay between these two qubit species. Here we report experiments and modeling which confirm these theoretical predictions and identify the conditions under which this interference occurs. Density matrix calculations show that the interference pattern manifests primarily via the occupation of the common singlet state. The S/T0 qubit is found to have a much longer coherence time as compared to the S/T+ qubit.
Here we respond briefly to a comment on our work in arXiv:cond-mat/0405501.
We study spin tunneling in a magnetic nanoparticle with biaxial anisotropy that is free to rotate about its anisotropy axis. Exact instanton of the coupled equations of motion is found that connects degenerate classical energy minima. We show that mechanical freedom of the particle renormalizes magnetic anisotropy and increases the tunnel splitting.
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