spinexchange

Spinexchange refers to processes in which angular momentum associated with spin is transferred between particles or subsystems without a net change in total spin angular momentum. In physics, spin exchange is a consequence of the quantum mechanical exchange interaction, which arises from the indistinguishability of identical fermions and the Coulomb repulsion between electrons. This interaction couples spins on nearby particles, yielding an energy difference between spin configurations.

In many-body systems this is captured by the Heisenberg exchange model, with a Hamiltonian H = -∑ Jij Si·Sj, where Jij are exchange integrals determined by the electronic structure. The sign of J determines whether parallel (ferromagnetic) or antiparallel (antiferromagnetic) alignment is energetically favored.

Spin-exchange optical pumping (SEOP) uses resonant light to polarize alkali-metal electrons, which then transfer angular momentum to noble-gas nuclei via spin-exchange collisions. The technique is used to produce hyperpolarized 3He and 129Xe for magnetic resonance imaging and neutron scattering experiments.

In ultracold atomic gases, spin-exchange collisions between atoms can redistribute spin populations and are central to studies of quantum magnetism, spinor condensates, and quantum simulations. At high density and low magnetic field, spin-exchange relaxation-free (SERF) regimes enable highly sensitive magnetometers.

In chemistry and molecular physics, spin exchange also describes exchange coupling between unpaired electrons in radical pairs and diradicals, influencing reaction yields and magnetic field effects on chemical dynamics.

Spin exchange is a broad and cross-disciplinary concept with both fundamental and applied relevance.