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# Orbital Angular Momentum - PowerPoint PPT Presentation

Orbital Angular Momentum . In classical mechanics, conservation of angular momentum L is sometimes treated by an effective (repulsive) potential Soon we will solve the 3D Schr. Eqn. The R equation will have an angular momentum term which arises from the Theta equation’s separation constant

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## Orbital Angular Momentum

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Orbital Angular Momentum
• In classical mechanics, conservation of angular momentum L is sometimes treated by an effective (repulsive) potential
• Soon we will solve the 3D Schr. Eqn. The R equation will have an angular momentum term which arises from the Theta equation’s separation constant
• eigenvalues and eigenfunctions for this can be found by solving the differential equation using series solutions
• but also can be solved algebraically. This starts by assuming L is conserved (true if V(r))

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Orbital Angular Momentum

z

f

• Look at the quantum mechanical angular momentum operator (classically this “causes” a rotation about a given axis)
• look at 3 components
• operators do not necessarily commute

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Side note Polar Coordinates
• Write down angular momentum components in polar coordinates (Supp 7-B on web,E&R App M)
• and with some trig manipulations
• but same equations will be seen when solving angular part of S.E. and so
• and know eigenvalues for L2 and Lz with spherical harmonics being eigenfunctions

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Commutation Relationships
• Look at all commutation relationships
• since they do not commute only one component of L can be an eigenfunction (be diagonalized) at any given time

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Commutation Relationships
• but there is another operator that can be simultaneously diagonalized (Casimir operator)

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Group Algebra
• The commutation relations, and the recognition that there are two operators that can both be diagonalized, allows the eigenvalues of angular momentum to be determined algebraically
• similar to what was done for harmonic oscillator
• an example of a group theory application. Also shows how angular momentum terms are combined
• the group theory results have applications beyond orbital angular momentum. Also apply to particle spin (which can have 1/2 integer values)
• Concepts later applied to particle theory: SU(2), SU(3), U(1), SO(10), susy, strings…..(usually continuous)…..and to solid state physics (often discrete)
• Sometimes group properties point to new physics (SU(2)-spin, SU(3)-gluons). But sometimes not (nature doesn’t have any particles with that group’s properties)

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Sidenote:Group Theory
• A very simplified introduction
• A set of objects form a group if a “combining” process can be defined such that
• 1. If A,B are group members so is AB
• 2. The group contains the identity AI=IA=A
• 3. There is an inverse in the group A-1A=I
• 4. Group is associative (AB)C=A(BC)
• group not necessarily commutative
• Abelian
• non-Abelian
• Can often represent a group in many ways. A table, a matrix, a definition of multiplication. They are then “isomorphic” or “homomorphic”

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Simple example
• Discrete group. Properties of group (its “arithmetic”) contained in Table
• Can represent each term by a number, and group combination is normal multiplication
• or can represent by matrices and use normal matrix multiplication

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Continuous (Lie) Group:Rotations
• Consider the rotation of a vector
• R is an orthogonal matrix (length of vector doesn’t change). All 3x3 real orthogonal matrices form a group O(3). Has 3 parameters (i.e. Euler angles)
• O(3) is non-Abelian
• assume angle change is small

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Rotations
• Also need a Unitary Transformation (doesn’t change “length”) for how a function is changed to a new function by the rotation
• U is the unitary operator. Do a Taylor expansion
• the angular momentum operator is the generator of the infinitesimal rotation

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For the Rotation group O(3) by inspection as:
• one gets a representation for angular momentum (notice none is diagonal; will diagonalize later)
• satisfies Group Algebra

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Group Algebra
• Another group SU(2) also satisfies same Algebra. 2x2 Unitary transformations (matrices) with det=1 (gives S=special). SU(n) has n2-1 parameters and so 3 parameters
• Usually use Pauli spin matrices to represent. Note O(3) gives integer solutions, SU(2) half-integer (and integer)

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Eigenvalues “Group Theory”
• Use the group algebra to determine the eigenvalues for the two diagonalized operators Lz and L2 (Already know the answer)
• Have constraints from “geometry”. eigenvalues of L2 are positive-definite. the “length” of the z-component can’t be greater than the total (and since z is arbitrary, reverse also true)
• The X and Y components aren’t 0 (except if L=0) but can’t be diagonalized and so ~indeterminate with a range of possible values

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Eigenvalues “Group Theory”
• Define raising and lowering operators (ignore Plank’s constant for now). “Raise” m-eigenvalue (Lz eigenvalue) while keeping l-eiganvalue fixed

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Eigenvalues “Group Theory”
• operates on a 1x2 “vector” (varying m) raising or lowering it

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Can also look at matrix representation for 3x3 orthogonal (real) matrices
• Choose Z component to be diagonal gives choice of matrices
• can write down (need sqrt(2) to normalize)
• and then work out X and Y components

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Eigenvalues
• Done in different ways (Gasior,Griffiths,Schiff)
• Start with two diagonalized operators Lz and L2.
• where m and l are not yet known
• Define raising and lowering operators (in m) and easy to work out some relations

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Eigenvalues
• Assume if g is eigenfunction of Lz and L2. ,L+g is also an eigenfunction
• new eigenvalues (and see raises and lowers value)

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Eigenvalues
• There must be a highest and lowest value as can’t have the z-component be greater than the total
• For highest state, let l be the maximum eigenvalue
• can easily show
• repeat for the lowest state
• eigenvalues of Lz go from -l to l in integer steps (N steps)

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Raising and Lowering Operators
• can also (see Gasior,Schiff) determine eigenvalues by looking at
• and show
• note values when l=m and l=-m
• very useful when adding together angular momentums and building up eigenfunctions. Gives Clebsch-Gordon coefficients

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Eigenfunctions in spherical coordinates
• if l=integer can determine eigenfunctions
• knowing the forms of the operators in spherical coordinates
• solve first
• and insert this into the second for the highest m state (m=l)

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Eigenfunctions in spherical coordinates
• solving
• gives
• then get other values of m (members of the multiplet) by using the lowering operator
• will obtain Y eigenfunctions (spherical harmonics) also by solving the associated Legendre equation
• note power of l: l=2 will have

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