Voltage Dependent Symmetry Breaking Effects in the Spin Excitation Spectrum of Spin-$\frac{1}{2}$ Paramagnetic Molecular Junctions Driven Out of Equilibrium
In the present work, we consider a spin 1/2 paramagnetic dimer embedded in a magnetic tunnel junction driven out of equilibrium by means of an applied voltage and an applied temperature gradient, in the presence of an external magnetic bias, which can be turn on/off. Here, we derive the spin excitation spectrum for a two spin 1/2 system analytically and show that an external magnetic field is required to lift the degeneracy in the triplet state, whether both spin units experience the magnetic field in the same direction or in opposite staggered directions. We show that an applied bias voltage in the absence of a magnetic field, transfers the magnetization from the magnetic leads into the spin units embedded in the molecule, hence breaking a symmetry and lifting the triplet degeneracy in the absence of external magnetization. From this theoretical demonstration, we then propose two schemes of magnetic driving to lift spin degeneracy in the spin excitation spectrum, which we named PMD (Parallel Magnetic Driving) and AMD (Anti-Parallel Magnetic Driving). We argue that a symmetry breaking is then required to project the spin configuration of a dimer into quantum transport measurements, and hence propose different schemes to detect the spin triplet state and the spin singlet state using differential conductivity measurements in the non-degenerate non-symmetric configuration of the spin dimer, and compare our results with the ones reported in the literature.