Hyperfine Interactions Curso 2008 Clase 5- Página-1 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Hyperfine Interactions The electric.

Apresentação em tema: "Hyperfine Interactions Curso 2008 Clase 5- Página-1 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Hyperfine Interactions The electric."— Transcrição da apresentação:

1 Hyperfine Interactions Curso 2008 Clase 5- Página-1 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Hyperfine Interactions The electric charges present in the nucleus interact with the electrons that surround it. The electric currents ( or the magnetic moments) associated with the electrons and the nuclei also interact. The main contributions to the interaction between the nucleus and its surrounding, involve the following nuclear moments: Electric part nuclear electric monopole moment, Ze. nuclear electric quadrupole moment, Q. Magnetic part nuclear magnetic dipole moment,  The interaction of the nuclear electric monopole moment with the electric field of the electrons is de Coulomb interaction, and does not concern us here.

2 Hyperfine Interactions Curso 2008 Clase 5- Página-2 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Hyperfine Interactions The other interactions between nuclei and electrons are called hyperfine interactions. Experimentally, it is observed that the hyperfine interactions are much weaker that other electronic, atomic or ionic interactions. For example, for rare earths: E LS ~ 10 4 k K E exch ~ 10 3 k K E CF ~ 10 2 k K E hf ~ 10 -4 k K The nuclei are characterized by the atomic number Z and by the mass number A. The angular momenta of the nucleons coupled in such way as to produce: I = 0 when ( and only when) both Z and A are even. I, the total nuclear angular momentum, measured in units of, can be integer or half- integer.

3 Hyperfine Interactions Curso 2008 Clase 5- Página-3 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Hyperfine Interactions The nuclei having nonzero have an associated magnetic dipole moment given by: where g I is the nuclear g-factor and  N is the nuclear magneton.  B is the Bohr magneton. The nuclear magnetic moment is also written: Here,  is the giromagnetic ratio. Since  N <<  B and the g-factors of nuclei and electrons are of the order of 1, it follows that the nuclear magnetic moments are much smaller than the ionic moments. For this reason, the nuclear magnetism of matter produces more subtle effects than the electronic magnetism. Every nucleus with I  0 has a magnetic dipolar moment. The nuclei that have I > 1/2 also possess a quadrupole moment Q, since their charge distributions lacks spherical symmetry.

4 Hyperfine Interactions Curso 2008 Clase 5- Página-4 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions The nuclei located in a solid interact with the electric charges in their neighborhood. These charges can be the electrons bound to the same atom or to neighbor atoms, conduction electrons and other nuclei. The interaction energy of a localized charge distribution with an electric potential produced by other charges is: The integration is made over the volume occupied by the nucleus. The potencial V(r) can be expanded in a Taylor series around the origin: Summing and substrating the term: where  ij is the Kronecker delta, we obtain:

5 Hyperfine Interactions Curso 2008 Clase 5- Página-5 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions Substituting in the integral:

6 Hyperfine Interactions Curso 2008 Clase 5- Página-6 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions Electrostatic energy of the nucleus taken as a point charge. The electric dipole moment of the nucleus is cero, because the center of mass and the center of charge coincides. This term only gives a displacement in the total energy. It does not depend on the nuclear orientation. Interaction between the nuclear quadrupole moment and the Electric Field Gradient

7 Hyperfine Interactions Curso 2008 Clase 5- Página-7 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions We will obtain the expression of the electric quadrupole interaction in quantum mechanics. We susbtitute the charge density , by the operator: The sum extends over the Z protons, at positions r k, with coordinates x ik. The quadrupole moment tensor becomes the tensorial operator:

8 Hyperfine Interactions Curso 2008 Clase 5- Página-8 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions The hamiltonian of the quadrupole interactions results: The matrix elements of this hamiltonian can be written in simple form using the Wigner -Eckart theorem. We apply this theorem to the matrix elements of the operators Q ij : C is a constant and  represents other quantum numbers besides I and m. The hamiltonian remains: where Q is a number, called electric quadrupole moment, defined as:

9 Hyperfine Interactions Curso 2008 Clase 5- Página-9 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions The Electric Field Gradient tensor is a symmetrical tensor Therefore, can be put in diagonal form. Using the Laplace equation ( ∇ 2 V = 0), it is posible to define the EFG tensor with only two values: The axes are chosen in such way that the EFG components satisfy: Taking the axes coincident with the principal axes of the EFG, the quadrupole hamiltonian becomes:

10 Hyperfine Interactions Curso 2008 Clase 5- Página-10 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions Examples I = 3/2 I = 5/2

11 Hyperfine Interactions Curso 2008 Clase 5- Página-11 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions Now, we will examinate the last term in W: This term involves the laplacian of V. Since some electrons have nonzero density at the nucleus, the potential satisfy the Poisson´s equation: Then,

12 Hyperfine Interactions Curso 2008 Clase 5- Página-12 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Electrostatic Interactions For a free ion of total angular momentum J, it can be shown that the interaction between the nuclear quadrupole moment and the EFG produced by the electrons is (Bleaney 1967): where is a number tabulated for each ion (for rare earths see Elliot and Stevens (1953))

13 Hyperfine Interactions Curso 2008 Clase 5- Página-13 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Magnetic Dipolar Interactions The magnetic dipolar hyperfine interaction may be written as the interaction of the nuclear magnetic moment with the magnetic field produced by extranuclear sources.

14 Hyperfine Interactions Curso 2008 Clase 5- Página-14 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Magnetic Dipolar Interactions

15 Hyperfine Interactions Curso 2008 Clase 5- Página-15 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Contribution of the Electronic Spin to the Magnetic Hyperfine Field

16 Hyperfine Interactions Curso 2008 Clase 5- Página-16 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Fermi contact term

17 Hyperfine Interactions Curso 2008 Clase 5- Página-17 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Core polarization field The incomplete shells (and the conduction electrons) may also give to another contribution to the hyperfine field, through the modification to the hyperfine field, through the modification of the radial distribution of the closed shells, thus producing a noncompensated spin density at the origin. This leads to an s magnetization equal to M' s (0) at the nucleus, and this term of the hyperfine field, called the core polatization field, acts through the contact term and is written This term is dominant in the hyperfine field of the S -state of rare earth ions, such as Gd +3 (-21T), and in the ions of d transition metals, such as Fe ( -27.5 T in metallic Fe). In the series of tripositive rare-earths ions, B cp es proporcional to the spin component of the of the total angular momentun J, given in Tesla, approximately by

18 Hyperfine Interactions Curso 2008 Clase 5- Página-18 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Orbital contribution to the magnetic hyperfine Field

19 Hyperfine Interactions Curso 2008 Clase 5- Página-19 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Orbital contribution to the magnetic hyperfine Field

20 Hyperfine Interactions Curso 2008 Clase 5- Página-20 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Free atom In the more general case A is the hyperfine tensor. The description of the inetraction in terms of the hyperfine field B hf in fact applies when A has uniaxial symmetry (A z = A >>A y, A y ) F is the hyperfine quantum number

21 Hyperfine Interactions Curso 2008 Clase 5- Página-21 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil. Contributions to B hf in the free ion

22 Hyperfine Interactions Curso 2008 Clase 5- Página-22 Centro Brasileiro de Pesquisas Física - Rio de Janeiro -Brasil.