Spin density wave

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The term spin density wave  (SDW) describes the state of the conduction electrons of some metals or superconductors in which the density of the electron spin is modulated in a wave-like manner.

Unlike spin waves (magnons) is in spin density waves not to suggestions , but a property of the ground state of the system.

Comparison with antiferromagnets

Spin density waves are similar to the ground state of antiferromagnets , in which the spin also differs from atom to atom.

In antiferromagnets, the spin of the atoms is mostly directed alternately up and down; this could be seen as a special case of a spin density wave whose wavelength is twice as large as the distance between the atomic layers ; the atoms of one layer are at the position of the "wave crest" (maximum spin in one direction), the atoms of the next layer are at the wave trough (maximum spin in the opposite direction).

In the case of spin density waves, on the other hand, the wavelength is generally incommensurable to the atomic lattice , i.e. not an integral or rational multiple of the atomic or layer spacing. As in simple antiferromagnets, the spins are only aligned along one axis.

Comparison with charge density waves

Spin density waves are similar to charge density waves : both result from an instability of the electron gas, in both the wavelength is determined by the Fermi surface of the conduction electrons, and in both the wavelength is generally incommensurable to the lattice. At values ​​of the electron momentum that correspond to the wavelength, there is a small band gap .

In contrast to charge density waves, however, with spin density waves the entire charge density is constant, the change in the density of the electrons with the different spin directions (“spin up” and “spin down”) therefore takes place in opposite directions.

Occur

Spin density waves occur u. a. on:

With high temperature superconductors

Spin density waves have also been observed in high-temperature superconductors , and it has been speculated whether they can be responsible for the occurrence of superconductivity in these materials . The idea behind this is that a moving charge causes the spin orientation of the atoms in the superconducting state to overturn. On its way through the solid, the charge creates a short-term spin flip in the neighboring atoms, which attracts another charge carrier . The coupling of the two charge carriers would be mediated by the spin flip instead of the phonons .

literature

  • George Grüner: The dynamics of spin-density waves . In: Reviews of Modern Physics . Vol. 66, 1994, pp. 1-24. Overview article with a focus on spin density waves in (TMTSF) 2 PF 6 . doi: 10.1103 / RevModPhys.66.1
  • Eric Fawcett: Spin-density-wave antiferromagnetism in chromium . In: Reviews of Modern Physics . Vol. 60, 1988, pp. 209-283. doi: 10.1103 / RevModPhys.60.209