Harmonic Generation in InAs Nanowire Double Quantum Dots

InAs nanowires provide a useful platform for investigating the physics of confined electrons subjected to strong spin-orbit coupling. Using tunable, bottom-gated double quantum dots, we demonstrate electrical driving of single spin resonance.\footnote{S. Nadj-Perge {\it et al.}, Nature {\bf468}, 1084 (2010)}$^,$\footnote{M.D. Schroer {\it et al.}, Phys. Rev. Lett. {\bf107}, 176811 (2011)} We observe a standard spin response when the applied microwave frequency equals the Larmour frequency $f_{0}$. However, we also observe an anomalous signal at frequencies $f_n = f_0 / n$ for integer n up to n $\sim$5. This is equivalent to generation of harmonics of the spin resonance field. While a $f_0/2$ signal has observed,\footnote{E.A. Laird {\it et al.}, Phys. Rev. Lett. {\bf99}, 246601 (2007)} we believe this is the first observation of higher harmonics in spin resonance. Possible mechanisms will be discussed.\footnote{E.I. Rashba, arXiv:1110.6569 (2011)} Acknowledgements: Research supported by the Sloan and Packard Foundations, the NSF, and Army Research Office.