Enhancing the performance of potential molecular Qubits: insight into the phonons involved in the spin-lattice relaxation

The development of quantum mechanics led to the birth of the quantum computing, a new frontier of the computer science which exploits quantum phenomena such as the quantum states superposition and the entanglement to perform computation. The basic element of a quantum computer is the Qubit, a two-level quantum system that can be addressed whether in 0 or 1 , but also in a coherent superposition of these two states. Among the great variety of potential Qubit systems, molecular electron spins show interesting features as the great tunability of the electronic and magnetic properties as well as the possibility to interact with other Qubits by adopting an ad-hoc synthetic strategy. Despite these positive features, they suffer from relatively short coherence time, Tm, namely the time during which the quantum information stored inside the Qubit is maintained. The aim of this PhD work is the optimization of the molecular Qubits performances through the research of the key “elements” to enhance the spin-lattice relaxation time, T1, that sets the upper limit of Tm. The study has been focused on Vanadium(IV) coordination compounds whose magnetic properties are due to the single unpaired electron, resulting in a S= 1/2 spin value. This element shows a weak spin-orbit coupling and, consequently, a small spin-phonon coupling, that makes it particularly suitable for the purpose. Two classes of complexes have been taken into account: vanadyl-based molecules, which show the VO2+ metal centre in a square pyramidal coordination geometry, and V(IV)-based ones in which the metal centre is octahedrally coordinated. Several ligands have been used in order to test different structural effects on the spin relaxation. In this study we utilized different experimental techniques to investigate different aspects of the issue. We jointed standard magnetic techniques, as alternate-current (AC) susceptometry, continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) spectroscopy, and TeraHertz time-domain spectroscopy (THz-TDS) for the characterization of the phonons of the complexes in the crystal phase. Moreover, a deeper investigation of the spin-lattice relaxation mechanisms required a more exotic technique, that is time-resolved THz-pump EPR-probe (TR-THz-EPR) measurements, which exploits the free electron laser radiation as THz source at the Novosibirsk Free Electron Laser (NovoFEL) facility. The whole work has been supported by theoretical calculations that has been fundamental in the rationalization of several experimental results. The manuscript is organized in several parts and each of them includes different chapters. Part I provides a general introduction about Qubits and, in particular, those based on molecular electronic spins. In part II, many theoretical concepts that I consider to be essential in the comprehension of this work are discussed, such as the theory of the spin relaxation and the spin-phonon interaction, with a short mention to the spin-orbit coupling. In part III, the techniques based on custom setups, as THz-TDS setups and the NovoFEL equipment, as well as the samples preparation, are examined. Part IV, which represents the main one, is devoted to the results obtained during the PhD. It is divided in two chapters, according to the two main topics: the former contains the outcomes obtained by the combination of magnetic techniques and THz-TD spectroscopy, arranged according to the publications; while the latter includes the investigation of THz-induced effects on the spin dynamics, performed at NovoFEL. In part V, that is the last one, a summary of this work helps in getting an overall view of it.