What is symmetry? Immunity (of aspects of a system) to a possible change - PowerPoint PPT Presentation

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What is symmetry? Immunity (of aspects of a system) to a possible change. The natural language of Symmetry - Group Theory.

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What is symmetry? Immunity (of aspects of a system) to a possible change

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What is symmetry?

Immunity (of aspects of a system) to a possible change

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The natural language of Symmetry - Group Theory

We need a super mathematics in which the operations are as unknown as the quantities they operate on, and a super-mathematician who does not know what he is doing when he performs these operations. Such a super-mathematics is the Theory of Groups.

- Sir Arthur Stanley Eddington

  • GROUP = set of objects (denoted ‘G’) that can be combined by a binary operation (called group multiplication - denoted by )

  • ELEMENTS = the objects that form the group (generally denoted by ‘g’)

  • GENERATORS = Minimal set of elements that can be used to obtain (via group multiplication) all elements of the group


  • Must be closed under multiplication () - if a,b are in G then ab is also in G

  • Must contain identity (the ‘do nothing’ element) - call it ‘E’

  • Inverse of each element must also be part of group (gg -1 = E)

  • Multiplication must be associative - a  (b  c) = (a  b)  c [not necessarily commutative]

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Ex. Of continuous group (also Lie gp.)

Group of all Rotations in 2D space - SO(2) group

Det(U) = 1

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Lie Groups

  • Lie Group:A group whose elements can be parameterized by a finite number of parameters i.e. continuous group where:1.If g(ai)  g(bi) = g(ci) then - ci is an analytical fn. of ai and bi . 2.The group manifold is differentiable. ( 1 and 2 are actually equivalent)

  • Group Generators:Because of above conditions, any element can be generated by a Taylor expansion and expressed as :

  • (where we have generalized for N parameters).

  • Convention:Call A1, A2 ,etc. As the generators (local behavior determined by these).

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Lie Algebras

  • Commutation is def as : [A,B] = AB - BA

  • If generators (Ai) are closed under commutation, i.e. then they form a Lie Algebra.

Generators and physical reality

  • Hermitian conjugate:Atake transpose of matrix and complex conjugate of elements

  • U = eiA ------ if U is unitary , A must be hermitian

U U = 1

A = A

Hermitian operators ~ observables with real eigenvalues in QM

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Symmetry : restated in terms of Group Theory

State of a system:|[Dirac notation]

Transformation:U| = |[Action on state]

Linear Transformation: U ( | + | ) = U| + U| [distributive]

Composition:U1U2( | ) = U1(U2| ) = U1 |

Transformation group:If U1 , U2 , ... , Un obey the group rules, they form a group (under composition)

Action on operator:U U -1 (symmetry transformation)

Again, What is Symmetry?

Symmetry is the invariance of a system under the action of a group

U U -1 = 

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Why use Symmetry in physics?

  • 1.Conservation Laws (Noether’s Theorem):

  • 2.Dynamics of system:

    • Hamiltonian ~ total energy operator

    • Many-body problems: know Hamiltonian, but full system too complex to solve

    • Low energy modes:All microscopic interactions not significantCollective modes more important

    • Need effective Hamiltonian

For every continuous symmetry of the laws of physics, there must exist a conservation law.

Use symmetry principles to constrain general form of effective Hamiltonian + strength parameters ~ usually fitted from experiment

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High TC Superconductivity

  • The Cuprates (ex. Lanthanum + Strontium doping)

CuO4 lattice

  • BCS or New mechanism? - d-wave pairing with long-range order.

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The procedure - 1

1.Find relevant degrees of freedom for system

2.Associate second-quantized operators with them (i.e. Combinations of creation and annihilation operators)

3.If these are closed under commutation, they form a Lie Algebra which is associated with a group ~ symmetry group of system.

Subgroup:A subset of the group that satisfies the group requirements among themselves ~ G  A.

Direct product & subgroup chain:G = A1 A2  A3 ... if (1) elements of different subgroups commute and (2) g = a1 a2 a3 ... (uniquely )

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The Procedure - 2

4.Identify the subgroups and subgroup chains ~ these define the dynamical symmetries of the system. (next slide.)

5.Within each subgroup, find products of generators that commute with all generators ~ these are Casimir operators - Ci. [Ci ,A] = 0 CiA = ACi ACiA-1 = Ci

6.Since we know that effective Hamiltonian must (to some degree of approximation) also be invariant ~ use casimirs to construct Hamiltonian

7.The most general Hamiltonian is a linear combination of the Casimir invariants of the subgroup chains -

 =  aiCi

where the coefficients are strength parameters (experimental fit)

 Ci’s are invariant under the action of the group !!

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Dynamical symmetries and Subgroup Chains


Physical implications

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  • Good experimental agreement with phase diagram.

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Extra Slides

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Casimirs and the SU(4) Hamiltonian

Casimir operators

Model Hamiltonian:

Effect of parameter (p) :

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High TC Superconductivity - SU(4) lie algebra

  • Physical intuition and experimental clues:Mechanism: D-wave pairing Ground states:Antiferromagnetic insulators

  • So, relevant operators must create singlet and triplet d-wave pairs

  • So, we form a (truncated) space ~ ‘collective subspace’ whose basis states are various combinations of such pairs -

  • We then identify 16 operators that are physically relevant:

  • 16 operators ~ U(4) group [# generators of SU(N) = N2 ]

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Noether’s Theorem

  • If is the Hamiltonian for a system and is invariant under the action of a group  U U -1= 

  • Operating on the right with U, U U -1 U=  U

  • i.e. Commutator is zero U -  U = 0 = [ U ,  ]

  • Quantum Mechanical equation of motion :

  • So, if , then U is a constant of the motion

  • Continuous compact groups can be represented by Unitary matrices.

  • U can be expressed as (i.e. a Taylor expansion)

  • Since U is unitary, we can prove that A is Hermitian

  • So, A corresponds to an observable and U constant  A constant

  • So, eigenvalues of A are constant ‘Quantum numbers’  conserved

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Nature of U and A

  • For any finite or (compact) infinite group, we can find Unitary matricesthat represent the group elements

  • U = eiA = exp(iA)(A - generator,  - parameter)

  • U = unitary  UU = 1 (U - Hermitian conjugate)

  • exp(-iA) exp(iA) = 1

  • exp ( i(A - A)) = 1

  • (A - A) = 0  A = A

  • So, A is Hermitian and it therefore corresponds to an observable

  • ex. A can be Px- the generator of 1D translations

  • ex. A can be Lz- the generator of rotations around one axis

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Angular momentum theory

1.System is in state with angular momentum ~ | ~ state is invariant under 3D rotations of the system.

2.So, system obeys lie algebra defined by generators of rotation group ~ su(2) algebra ~ SU(2) group [simpler to use]

3.Commutation rule:[Lx,Ly]= i  Lz , etc.

4.Maximally commuting subset of generators ~ only one generator

5.Cartan subalgebra ~ LzStepping operators ~ L+ = Lx + i Ly L- = Lx - i LyCasimir operator ~ C = L2 = Lx2 + Ly2 + Lz2

6.C commutes with all group elements ~ CU = UC ~ UCU-1 = CC is invariant under the action of the group

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