Abstract: A highly promising area of research is harnessing electronic spins as qubits. As inherently quantum systems, electronic spins are ideal qubit candidates both for their modularity and their ease of manipulation with microwave radiation. Electronic defect sites, in particular nitrogen-vacancy sites in diamond, phosphorus defects in silicon, and double-vacancy sites in silicon carbide, are prominent examples. However, to create systems with the longest possible coherence times, we must continue to glean upon new insights into what drives decoherence and develop new materials design principles for qubit hosts.
The longitudinal electronic spin relaxation time constant, T1, relates to the spin-lattice relaxation time of the electronic spin. This parameter represents the maximum data storage time of an electronic spin. T2 is the spin echo dephasing time constant in the x-y plane and relates to the spin-spin relaxation time. While fundamentally, T2 represents the functional operating time of a qubit, we often find that T1 is the most restrictive parameter in practice, as T1 represents the theoretical upper limit to T2. The chemical properties leading to maximization of T1 remain an open question and indeed many recent studies have focused heavily on questions related to T1.
Materials design strategy has been used to introduce W5+ (d1), S = ½, I = 0, defect centers in single crystals of Ba2CaWO6-δ. The defects were characterized by measuring the spin-lattice (T1) and spin-spin relaxation (T2) times from T = 5 to 150 K. At T = 5 K, T1 = 310 ms and T2 = 4 μs, establishing the viability of these qubit candidates. Temperature-dependent octahedral tilt in the systems and the correlation of the T1 lifetimes obtained from pulse electron paramagnetic resonance with phonon modes obtained from the heat capacity data allow us to quantify the contribution of respective phonon modes to the spin-phonon coupling in the system.
Together, these results demonstrate that systematic defect generation in double perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for quantum. We extend this materials design strategy further to understand the tetragonal bonding in 2-D layered materials through hidden Jahn-Teller effects in Kagomé intermetallic MgCo6Ge6 and tuning the physical properties in single crystals of superconducting layered misfit compounds of NbSe2.