Spin density wave
The term density wave describes the state of the conduction electrons of some metals or superconductors in which the density of the electron spin is modulated wave-shaped. In contrast to spin waves ( magnons ) are in these waves to any suggestions, but a property of the ground state of the system.
Spin density waves are similar to the ground state of the antiferromagnet, where also the spin of atom to atom is different. In antiferromagnets, it is usually so that the spin of the atoms is directed alternately upward and downward; This could be viewed as a special case of a spin density wave whose wavelength is twice as large as the distance between the atom layers; the atoms of one layer are at the position of " wave crest " (maximum spin in one direction), the atoms of the next layer in the trough (maximum spin in the opposite direction ). In spin-density waves is the wavelength in general incommensurable to the atomic lattice, ie not an integer or a rational multiple of the atomic or location distances. The spins are oriented as in simple antiferromagnets, along one axis only.
Spin density waves are similar to the charge density waves, both resulting from an instability of the electron gas at the two wavelengths is determined by the Fermi surface of the conduction electrons, and at both the wavelength of the grating is in general incommensurate. At values of the electron pulse, corresponding to the wavelength, there is a small band gap. In contrast to the charge density waves on the overall charge density is constant spin density waves, the change in the density of electrons with different spin directions ( "spin up" and " down spin " ) thus takes place in opposite directions.
Spin density waves are among others the metal chromium below the Neel temperature of 311 Kelvin, with many chrome alloys and the Bechgaard salt ( TMTSF ) 2PF6, an organic conductor, under 12 K on ( TMTSF stands for Tetramethyltetraselenofulvalen ).
Even at high temperature superconductors spin density waves were observed and it was speculated that the spin density waves can also be responsible for the occurrence of superconductivity in these materials. The idea behind this is that a moving charge can tip over in the superconducting state, the spin orientation of the atoms. On its way through the solid, the charge thus produced a short spin flip at the neighboring atoms, which attracts an additional charge carriers. Thus, the coupling of the two charge carriers through the spin flip would give instead of phonons.