Is there a crystal angular momentum?

In summary, when considering electrons as Bloch waves in a crystal, their momentum is not well-defined due to the inability for both the Hamiltonian and momentum to be diagonalized in the same basis. Instead, the crystal momentum is conserved, which is not derived from a Noether-like theorem due to the broken translational symmetry in a crystal. The crystal momentum can also have an angular component, but it is not commonly mentioned in solid state physics books. Additionally, the conservation of crystal angular momentum leads to a selection rule, which is determined by the group theory behind it. This concept is not directly related to ordinary momentum and is a consequence of the nuclear permutation group. However, it is not often discussed in literature.
  • #1
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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  • #2
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.
 

1. What is crystal angular momentum?

Crystal angular momentum refers to the angular momentum of the electrons in a crystal lattice. It is a quantum mechanical property that describes the rotational motion of an electron around the crystal's center of mass.

2. How is crystal angular momentum related to crystal structure?

The crystal structure of a material determines its symmetry and the arrangement of its atoms. This, in turn, affects the distribution of angular momentum in the crystal. Therefore, crystal angular momentum is closely related to the crystal structure of a material.

3. Can crystal angular momentum be measured?

Yes, crystal angular momentum can be measured using techniques such as spectroscopy and magnetic resonance. These methods allow scientists to observe the behavior of electrons in a crystal and determine their angular momentum.

4. What factors affect the magnitude of crystal angular momentum?

The magnitude of crystal angular momentum is affected by the size and shape of the crystal, the number and type of atoms present, and the strength of the crystal's bonds. The crystal's temperature and external magnetic or electric fields can also influence the magnitude of crystal angular momentum.

5. How is crystal angular momentum used in materials science?

Crystal angular momentum is a crucial concept in materials science and is used to understand the properties and behavior of materials. It plays a significant role in determining the magnetic, optical, and electronic properties of materials and is essential for developing new technologies and materials with specific desired properties.

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