Enhancing Coherence with a Clock Transition and Dynamical Decoupling in the Cr$_7$Mn Molecular Nanomagnet

Abstract

Molecular magnets are attractive as spin qubits due to their chemical tunability, addressability through electron-spin resonance techniques, and long coherence times. Clock transitions (CTs), for which the system is immune to the effect of magnetic-field fluctuations to first order, provide a method to enhance the coherence time $T_2$, and to reveal mechanisms of decoherence that are not due to such fluctuations. Here we investigate two variants of Cr$_7$Mn, a spin-1 molecular nanomagnet, at fields near a zero-field CT. We find that at temperatures $\le$2 K, $T_2\sim1$ $\mu$s at the CT using a Hahn-echo pulse sequence. Away from the CT, electron-spin-echo envelope modulation (ESEEM) oscillations due to coupling to nuclear spins are observed and have a $T_2$ as high as $1.35$ $\mu$s, indicating a distinct mechanism of coherence preservation. Dynamical decoupling with the CPMG pulse sequence yields $T_2\sim\!2.8$ $\mu$s at the CT and up to $\sim\!3.6$ $\mu$s in the ESEEM regime along with a demodulation of the oscillatory behavior. The experimental values of $T_2$ are largely independent of the degree of dilution of the molecules in solvent or whether the solvent is deuterated, indicating that much of the decoherence and ESEEM arises from sources within the molecules themselves. To account for decoherence, we develop a model that includes not only field fluctuations but also fluctuations in the CT transition frequency itself. Our results can be well explained by treating the environment as a combination of noise at the nuclear Larmor precession frequency and $1/f$ noise in the transverse anisotropy parameter $E$. Such information about the microscopic origins of decoherence can aid the rational design of molecular-based spin qubits.