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Magnetic catalysis
Magnetic catalysis is a physics phenomenon, which is defined as an enhancement of dynamical symmetry breaking by an external magnetic field in quantum field theory, used for the description of quantum particles/quasiparticles in subatomic physics as well as in condensed matter physics. The underlying phenomenon is a consequence of the strong tendency of a magnetic field to enhance binding of oppositely charged particles into bound states. The “catalyzing” effect comes from a partial restriction (dimensional reduction) of the motion of charged particles in the directions perpendicular to the direction of the magnetic field.
Commonly, the magnetic catalysis is specifically associated with spontaneous breaking of flavor or chiral symmetry in quantum field theory, which is enhanced or triggered by the presence of an external magnetic field.
==General description==
The underlying mechanism behind magnetic catalysis〔 (【引用サイトリンク】 title=Catalysis of Dynamical Flavor Symmetry Breaking by a Magnetic Field in 2 + 1 Dimensions )〕 is the dimensional reduction of low-energy charged spin-1/2 particles. (For a review, see 〔(【引用サイトリンク】 title=Magnetic Catalysis: A Review )〕) As a result of such a reduction, there exists a strong enhancement of the particle-antiparticle pairing responsible for symmetry breaking. For gauge theories in 3+1 dimensions, such as quantum electrodynamics and quantum chromodynamics, the dimensional reduction leads to an effective (1+1)-dimensional low-energy dynamics. (Here the dimensionality of space-time is written as D+1, where D stands for the number of space-like directions and 1 stands for a single time-like direction.) In simple terms, the dimensional reduction reflects the fact that the motion of charged particles is (partially) restricted in the two space-like directions perpendicular to the magnetic field. However, this orbital motion constraint alone is not sufficient (for example, there is no dimensional reduction for charged scalar particles, carrying spin 0, although their orbital motion is constrained in the same way.) It is also important that the fermions have spin 1/2 and, as follows from the Atiyah–Singer index theorem, their lowest Landau level states have an energy independent of the magnetic field. (The corresponding energy vanishes in the case of massless particles.) This is in contrast to the energies in the higher Landau levels, which are proportional to the square root of the magnetic field. Therefore, if the field is sufficiently strong, only the lowest Landau level states are dynamically accessible at low energies. The states in the higher Landau levels decouple and become almost irrelevant. The phenomenon of magnetic catalysis has applications in particle physics, nuclear physics and condensed matter physics.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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