Abstract
To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically designed europium(II)‐based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i. e. metal‐cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low‐energy metal‐displacing vibrations in mediating the spin‐lattice relaxation times (T 1). The temperature dependence of T 1 can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS2 solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin‐vibration coupling in these EMF‐based qubit candidates.
Highlights
Electronic spins promise a yet untapped potential as nanoscale memories, both as classical bits[1] and as quantum bits.[2]
We report four novel spin qubits based on divalent monoeuropium endohedral metallofullerenes (EMFs) using a combination of experimental and theoretical characterization techniques, including density functional theory (DFT) to model molecular vibrations, pulsed electron paramagnetic resonance (EPR) to study the spin dynamics, and complete active space self-consistent field theory (CASSCF) to model the evolution of the spin energy levels along with vibrational distortions
As is typical for EMFs, while the structures consist in pentagons and hexagons, the overall geometry presents in most cases a low symmetry, and the metal ion – having ample room inside the carbon cage – is attached to a wall; in particular, it is bonded to a specific site that varies from case to case and depends non-trivially on the structure of the carbon cage.[13,14]
Summary
Chemical strategies to extend T2 include the design of molecular architectures that are free from nuclear spins,[5] dilution within a diamagnetic matrix,[6] or choosing a crystal field Hamiltonian which allows for “atomic clock transitions”, that in turn protect the spin states from magnetic noise.[7] a necessary condition for a long T2 is a long T1; in simple terms, preserving quantum information is only possible if the classical memory is preserved. We report four novel spin qubits based on divalent monoeuropium EMFs using a combination of experimental and theoretical characterization techniques, including density functional theory (DFT) to model molecular vibrations, pulsed EPR to study the spin dynamics, and complete active space self-consistent field theory (CASSCF) to model the evolution of the spin energy levels along with vibrational distortions
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