Enter the Majorana Fermion

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Science  25 May 2012:
Vol. 336, Issue 6084, pp. 989-990
DOI: 10.1126/science.1223302

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All known fundamental particles are either bosons or fermions. Fermions are subject to the Pauli principle, which forbids two particles being in the same quantum state; bosons, by contrast, tend to bunch together in the same state. The same rule applies to the excitations of most solid-state systems, such as metals and semiconductors, which can be classified as fermionic or bosonic. However, sometimes excitations—quasiparticles—of a fundamentally different type emerge that resemble particles that hitherto have been considered only as a mathematical possibility. On page 1003 of this issue, Mourik et al. (1) report on a superconducting nanostructure that harbors such an exotic quasiparticle, a “Majorana bound state,” an excitation that can best be described as half a fermion. The Majorana bound state is named after the Italian physicist Ettore Majorana, who proposed an equation describing a fermionic particle with a real-valued wave function (2). In contrast to the standard (Dirac) fermion, which has a complex wave function, a particle with a real-valued wave function is equal to its own antiparticle. No fundamental particles are known to be Majorana fermions, although there are speculations that the neutrino is one (3). The reported bound state is a localized version of Majorana's fermion.