Mermin-Wagner Theorem – Sound in Low Dimensional Solids and Spontaneous Symmetry Breaking

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The Mermin-Wagner theorem states that there cannot be any spontaneous symmetry breaking happening in systems with short range interactions below dimension 3. Moreover, we know that Goldstone boson, such as sound in solids, occurs due to the spontaneous breakdown of rotational and translational symmetries. Since in 2D we can't break the translational invariance, do phonons exist in 2D solids?

My thought:

  1. The rotational symmetry is broken in 2D solids due to an effective gapping of the orientational modes (see this link for example). This might allows us to obtain phonons.

  2. We also see phonons in liquid where no symmetry is broken. One might not called these sound waves phonons as in this PSE post. However, it is shown in the book Boulevard of broken symmetries that sound in liquids can be associated to the Goldstone boson linked to the breakdown of Galilean symmetry when one linearize the Navier-Stokes equations (which are Galilean invariant) to obtain the wave equation (not Galilean invariant).

  3. We can very well write down an Hamiltonian in 1D or 2D: $H = \sum |k|u^2$ and find gapless excitations. Obviously, this is wrong but I can't figure out why?

There are some related posts. For example these two:
Are there Goldstone bosons in 1D or 2D?
Spontaneous symmetry breaking in fluids

Best Answer

You do have sound in $1+1d$. Mermin-Wagner do not claim that there are no massless particles in low dimensions -- indeed, some of the best understood QFTs are two-dimensional gapless theories, namely the wonderful world of $2d$ CFTs.

What Mermin-Wagner does, is proving that there are no order parameters for symmetry breaking in $2d$. This is not the same as saying there are no Goldstones. There are such particles, just no interpolating field for them.

See ref.1 for a nice recent review.

References:

  1. arXiv:2306.00085, ยง2.
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