Is there a crystal angular momentum?

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SUMMARY

The discussion centers on the concept of crystal angular momentum in solid state physics, specifically addressing the conservation of crystal momentum in crystals and its implications. It highlights that while crystal momentum is conserved due to broken translational symmetry, the existence of angular crystal momentum remains ambiguous and is not widely discussed in literature. The conversation emphasizes the distinction between crystal momentum and ordinary momentum, noting that crystal momentum is conserved only up to a reciprocal lattice vector, while angular momentum is quantized. The complexities of group theory and selection rules related to crystal angular momentum are also mentioned.

PREREQUISITES
  • Understanding of crystal momentum and its conservation in solid state physics
  • Familiarity with Bloch waves and their relationship to momentum and energy
  • Knowledge of group theory as it applies to symmetry operations in physics
  • Basic concepts of translational symmetry and Bravais lattices
NEXT STEPS
  • Research the implications of broken translational symmetry in crystal structures
  • Study the role of group theory in determining selection rules for angular momentum
  • Explore the relationship between crystal momentum and the nuclear permutation group
  • Investigate the quantization of angular momentum in solid state systems
USEFUL FOR

Physicists, particularly those specializing in solid state physics, materials scientists, and researchers interested in the theoretical aspects of momentum conservation in crystalline structures.

fluidistic
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I've learned that in a crystal, the crystal momentum is conserved. When one considers the electrons as Bloch waves, they have a momentum that doesn't commute with the Hamiltonian and they have well definite energies, hence they cannot have a well definite momentum, because there is no basis in which both the Hamiltonian responsible for the energies and the momentum are diagonalized. Instead, the crystal momentum is what is conserved.
This conserved quantity does not come from a Noether-like theorem, because of the broken translational symmetry in a crystal. Only particular translations (by a Bravais lattice vector) yield an invariant system. However I note that some very particular rotations (only for some angles) would also keep the system invariant, while an arbitrary rotation would not, in general, leave the system invariant. This leads me to think that this property should be responsible for an angular crystal momentum. Is there such a thing? If so, why is it not commonly mentioned in solid state physics books? Why would the crystal momentum be more important than the angular crystal momentum, assuming that the latter exist?
 
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Ordinary momentum is still conserved, while crystal momentum is conserved only up to a multiple of a reciprocal lattice vector. Crystal momentum is a continuous quantity while rotations are quantized in discrete levels. The "conservation" of crystal angular momentum leads to a selection rule. The group theory behind this selection rules can get quite involved as rotations do not commute like translations and spin further complicates matters.

Edit: Personally I find crystal momentum to be quite an interesting concept. I don't think it is related to normal momentum. Rather, it is a consequence of the nuclear permutation group. However, I have never seen this discussed clearly in the literature.
 

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