So far in condensed matter physics, I only know anyons(abelian or nonabelian) can emerge as quasiparticles in 2D real-space.
But is there any possibility to construct anyons in momentum-space ? And what about the braiding, fusion rules in momentum-space ? I mean do anyons always live in real-space rather than in momentum-space ?
Maybe this is a trivial question, thank you very much.
The natural but rarely considered question whether anyonic braiding statistics may be realized in momentum space (as opposed to in position space, as usually discussed) seems to recently have received a decidedly positive answer, where codimension=2 loci of singular band crossings in otherwise gapped materials have been found to pick up (non-abelian) phases when braided around each other in momentum space (see the following figure):
This concerns Dirac points in 2d materials, but in particular also Weyl nodal lines in 3d materials, which means that not only ordinary point-like anyons but even their higher-dimensional analogs (see here) exist in momentum space, maybe more tangibly so than their position-space cousins, for instance as concerns plausible engineering mechanisms for their braiding (which, despite common folklore, is a serious problem in position space, see here).
The following is a list of some references that make anyonic statistics in momentum space nicely explicit, and from which a fair number of further relevant articles may be found by chasing citations.
A curious fact to beware of is that these articles tend to not use nor even mention the term "anyon" when speaking about non-abelian braiding operations in momentum space. Maybe this reflects a disconnect in the solid state community; certainly it makes search engine queries for "anyons in momentum space" come out empty-handed even in the face of sizeable relevant literature:
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A. Tiwari, T. Bzdušek,
Non-Abelian topology of nodal-line rings in PT-symmetric systems
Phys. Rev. B 101 (2020) 195130 (doi:10.1103/PhysRevB.101.195130)
a new type non-Abelian “braiding” of nodal-line rings inside the momentum space
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A. Bouhon, Q.-S. Wu, R.-J. Slager, H. Weng, O. V. Yazyev, T. Bzdušek:
Non-Abelian reciprocal braiding of Weyl points and its manifestation in ZrTe
Nature Physics 16 (2020) 1137–1143 (arXiv:1907.10611, doi:10.1038/s41567-020-0967-9)
Weyl points in three-dimensional (3D) systems with $\mathcal{C}_2\mathcal{T}$ symmetry carry non-Abelian topological charges. These charges are transformed via non-trivial phase factors that arise upon braiding the nodes inside the reciprocal momentum space.
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B. Peng, A. Bouhon, R.-J. Slager, B. Monserrat:
Multi-gap topology and non-Abelian braiding of phonons from first principles
Phys. Rev. B 105 (2022) 085115 (arXiv:2111.05872, doi:10.1103/PhysRevB.105.085115)
new opportunities for exploring non-Abelian braiding of band crossing points (nodes) in reciprocal space, providing an alternative to the real space braiding exploited by other strategies. Real space braiding is practically constrained to boundary states, which has made experimental observation and manipulation difficult; instead, reciprocal space braiding occurs in the bulk states of the band structures and we demonstrate in this work that this provides a straightforward platform for non-Abelian braiding.
(Maybe noteworthy how this quote contrasts with tparker's comment.)
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B. Peng, A. Bouhon, B. Monserrat R.-J. Slager:
Phonons as a platform for non-Abelian braiding and its manifestation in layered silicates,
Nature Communications 13 423 (2022) (doi:10.1038/s41467-022-28046-9)
multi-gap topologies with band nodes that carry non-Abelian charges, characterised by invariants that arise by the momentum space braiding of such nodes
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H. Park, W. Gao, X. Zhang, S. S. Oh,
Nodal lines in momentum space: topological invariants and recent realizations in photonic and other systems
Nanophotonics (2022) (doi:10.1515/nanoph-2021-0692)
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I asked my advisor this exact same question a couple years ago. He said that there's no sense of anyonic statistics in momentum space (or in any basis other than real space).
The reason for this is that anyons typically emerge from a microscopic Hamiltonian that is spatially local, and so strictly speaking, anyons are only well-defined when they stay far away from each other in space. If you bring two anyons close together (i.e. on the order of the correlation length or less), then they begin to lose their identity as individual particles (as is always the case in quantum mechanics with identical particles). Strictly speaking, the anyonic statistical phase factor only occurs when when they are braided at long distances - if you try to braid them close together, it becomes ambiguous which particle is which, and therefore how many times they've circled each other. But the correction becomes exponentially small at long distances: the actual phase picked up in a braiding process is
$\theta = \theta_\text{dynamical} + \theta_\text{anyonic} + o(\exp(-r/\xi)),$
where $r$ is the distance between the two anyons and $\xi$ is the correlation length. (Many people are unaware of this subtlety because their intuition is based on Kitaev's toric code, where the correlation length is zero, so anyons remain well-defined even on adjacent lattice sites.)
Anyway, if you tried to localize two anyons into small wave packets in momentum space, then in real space they would be close to plane waves and therefore have large spatial overlap, so everything would get messed up.
This unfortunate asymmetry between real and momentum space originates from the fact that anyons aren't true point particles (because you can't create just one) but are rather connected by strings, so it's very hard to directly second-quantize anyons. By contrast, with a "true" point particle, the canonical commutation relations are preserved under Fourier transforms, so everything is nice and symmetric between real and momentum space.
I was wondering something similar few month ago. Then I concluded that most of the topological staffs appear at the boundary between two different topological sector. A sector being characterised by a Chern number, or if you prefer a topological charge, one needs a boundary / an interface between two systems characterised by different topological charge.
A $k$-space (or momentum, or reciprocal, or Fourier, ...) is well defined only for periodic boundary conditions. The fact that the $x \leftrightarrow k$ is a Fourier transform imposes a periodicity in $x$ or in $k$. That's the stringent condition under which $k$ is a good quantum number. Note that we can still define some quasi-$k$ for disordered media. So we could not in principle define a $k$-space when a system has boundary. Note that infinite system are usually closed by periodic boundary condition, also called Born-von-Karman conditions.
I'm not aware so much about anyons (I'm still learning about that) but I believe they (almost all of them ? all of them ? I don't know) appear due to boundary conditions in condensed matter, for the reason I gave about the topological charge transition. So I believe it should be impossible to define anyons in $k$-space, for the simple reason that the $k$-space is not a correct description of the matter when anyons exist.
I would really appreciate comments/critics about what I said, especially if it's (partially) wrong.
