Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety

In the search for
long-lived quantum coherence in spin systems,
vanadium­(IV) complexes have shown record phase memory times among
molecular systems. When nuclear spin-free ligands are employed, vanadium­(IV)
complexes can show at low temperature sufficiently long quantum coherence
times, Tm, to perform quantum operations,
but their use in real devices operating at room temperature is still
hampered by the rapid decrease of T1 caused
by the efficient spin–phonon coupling. In this work we have
investigated the effect of different coordination environments on
the magnetization dynamics and the quantum coherence of two vanadium­(IV)-based
potential molecular spin qubits in the solid state by introducing
a unique structural difference, i.e., an oxovanadium­(IV) in a square
pyramidal versus a vanadium­(IV) in an octahedral environment featuring
the same coordinating ligand, namely, the 1,3-dithiole-2-thione-4,5-dithiolate.
This investigation, performed by a combined approach of alternate
current (ac) susceptibility measurements and continuous wave (CW)
and pulsed electron paramagnetic resonance (EPR) spectroscopies revealed
that the effectiveness of the vanadyl moiety in enhancing quantum
coherence up to room temperature is related to a less effective mechanism
of spin–lattice relaxation that can be quantitatively evaluated
by the exponent n (ca. 3) of the temperature dependence
of the relaxation rate. A more rapid collapse is observed for the
non-oxo counterpart (n = 4) hampering the observation
of quantum coherence at room temperature. Record coherence time at
room temperature (1.04 μs) and Rabi oscillations are also observed
for the vanadyl derivative in a very high concentrated material (5
± 1%) as a result of the additional benefit provided by the use
of a nuclear spin-free ligand.