SpinSpin

Spinspin, sometimes written spin-spin, refers to the interaction between the intrinsic angular momenta (spins) of particles in a system. It includes both exchange interactions and magnetic dipole-dipole couplings, and it influences magnetic properties, energy level structure, and spectroscopic signals.

Mechanisms of spin-spin interactions arise from different physical origins. Exchange coupling is a short-range effect caused by quantum mechanical overlap of particle wavefunctions and tends to align or anti-align spins depending on the system. Magnetic dipole-dipole coupling originates from the magnetic fields produced by spinning particles and falls off with distance as 1/r^3, with a strength that depends on the relative orientation of the spins and their connecting vector. In a simplified two-spin model, the effective Hamiltonian includes an exchange term proportional to J S1·S2 and a dipolar term that reflects the dipole-dipole interaction.

In practice, spin-spin interactions are central to many spectroscopic techniques and magnetic phenomena. In NMR and ESR spectroscopy, they cause multiplet structures via J-coupling and contribute to line broadening or anisotropic line shapes through dipolar couplings, especially in solids or restricted molecular motion. Spin-spin interactions also influence spin relaxation and coherence times, which are important for understanding dynamic processes in materials and biological systems.

Applications of spin-spin interactions span molecular structure determination, materials science, and quantum information processing. By controlling or exploiting spin-spin couplings, researchers implement two-qubit gates and study spin dynamics in condensed matter, polymers, and biomolecules. The term spin-spin is standard across physics and chemistry, though spinspin may appear in certain databases or as shorthand in literature.