Anderson impurity model

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The Anderson impurity model, named after Philip Warren Anderson, is a Hamiltonian that is used to describe magnetic impurities embedded in metals.[1] It is often applied to the description of Kondo effect-type problems,[2] such as heavy fermion systems[3] and Kondo insulators[citation needed]. In its simplest form, the model contains a term describing the kinetic energy of the conduction electrons, a two-level term with an on-site Coulomb repulsion that models the impurity energy levels, and a hybridization term that couples conduction and impurity orbitals. For a single impurity, the Hamiltonian takes the form[1]

,

where the operator is the annihilation operator of a conduction electron, and is the annihilation operator for the impurity, is the conduction electron wavevector, and labels the spin. The on–site Coulomb repulsion is , and gives the hybridization.

Regimes[edit]

The model yields several regimes that depend on the relationship of the impurity energy levels to the Fermi level :

  • The empty orbital regime for or , which has no local moment.
  • The intermediate regime for or .
  • The local moment regime for , which yields a magnetic moment at the impurity.

In the local moment regime, the magnetic moment is present at the impurity site. However, for low enough temperature, the moment is Kondo screened to give non-magnetic many-body singlet state.[2][3]

Heavy-fermion systems[edit]

For heavy-fermion systems, a lattice of impurities is described by the periodic Anderson model.[3] The one-dimensional model is

,

where is the position of impurity site , and is the impurity creation operator (used instead of by convention for heavy-fermion systems). The hybridization term allows f-orbital electrons in heavy fermion systems to interact, although they are separated by a distance greater than the Hill limit.

Other variants[edit]

There are other variants of the Anderson model, such as the SU(4) Anderson model[citation needed], which is used to describe impurities which have an orbital, as well as a spin, degree of freedom. This is relevant in carbon nanotube quantum dot systems. The SU(4) Anderson model Hamiltonian is

,

where and label the orbital degree of freedom (which can take one of two values), and represents the number operator for the impurity.

See also[edit]

References[edit]

  1. ^ a b Anderson, P. W. (1961). "Localized Magnetic States in Metals". Phys. Rev. 124 (1): 41–53. Bibcode:1961PhRv..124...41A. doi:10.1103/PhysRev.124.41.
  2. ^ a b Schrieffer, J.R.; Wolff, P.A. (September 1966). "Relation between the Anderson and Kondo Hamiltonians". Physical Review. 149 (2): 491–492. Bibcode:1966PhRv..149..491S. doi:10.1103/PhysRev.149.491. S2CID 55838235.
  3. ^ a b c Hewson, A. C. (1993). The Kondo Problem to Heavy Fermions. New York: Cambridge University Press.