nLab abelian Chern-Simons theory

Context

\infty-Chern-Simons theory

∞-Chern-Weil theory

∞-Chern-Simons theory

∞-Wess-Zumino-Witten theory

Ingredients

Definition

Examples

Quantum field theory

Topological physics

Contents

Idea

By abelian Chern-Simons theory one means Chern-Simons theory with abelian gauge group (typically the circle group or a torus-product of copies of these).

One major application of abelian Chern-Simons theory is as an effective field theory of the fractional quantum Hall effect.

Properties

Space of quantum states

For abelian Chern-Simons theory with NN gauge fields (a (i)) i=1 N(a^{(i)})_{i = 1}^N and Lagrangian density of the form (using Einstein summation convention) K ija (i)da (j)K_{i j} a^{(i)} \wedge \mathrm{d} a^{(j)} (10) , for KK an N×NN \times N even integer symmetric matrix (the diagonal entries are even numbers), the dimension of the Hilbert space of quantum states g\mathscr{H}_g (obtained by geometric quantization, cf. quantization of D=3 Chern-Simons theory) over an orientable surface of genus gg is the absolute value of the determinant of KK raised to the ggth power:

dim( g)=|det(K)| g. dim(\mathscr{H}_g) \;=\; \left\vert det(K)\right\vert^g \,.

(for g=1g = 1 this is Wesolowski, Hosotani & Ho 1994 (3.25), for N=1N=1 see Manoliu 1998a p 40, for general NN this goes back to Wen & Zee 1992 (1.2), recalled in Fradkin 2013 (14.23), Belov & Moore 2005 p 26).

For the modular group-action on these state spaces see at integer Heisenberg group the section Modular automorphisms (there for g=1g=1).

For a non-orientable surface with kk crosscaps, it is

dim( k)=|det(K)| k1. dim(\mathscr{H}_k) \;=\; \left\vert det(K)\right\vert^{k-1} \,.

(e.g. arXiv:1509.03920 (73))


Abelian Chern-Simons as effective QFT for FQH systems

The following is a streamlined digest of the traditional argument and Ansatz [originally culminating in Zee 1995, Wen 1995, more recently highlighted by Witten 2016 pp 30, Tong 2016 §5] for abelian Chern-Simons theory at level kk \in \mathbb{N} as an effective field theory for fractional quantum Hall systems at filling fraction ν=1/k\nu = 1/k.


Preliminaries

Consider a spacetime Σ\Sigma of dimension 1+21 + 2, to be thought of as the worldvolume of the conducting sheet that hosts the fractional quantum Hall system.

On this spacetime, the electric current density is a differential 2-form JJ.

Note that the corresponding electric current vector field J\vec J is characterized by its contraction into the given volume form dvoldvol being equal to the density 2-form

J=dvol(J,), J \;=\; dvol\big(\, \vec J, \cdots \big) \,,

and that the conservation law for J\vec J, hence the vanishing of its divergence is equivalent to the density 2-form being closed differential form:

(1)divJ=0dJ=0. div \vec J \;=\; 0 \;\;\;\;\;\; \Leftrightarrow \;\;\;\;\;\; \mathrm{d} J \;=\; 0 \,.

We write AA for the vector potential 1-form on Σ\Sigma which encodes the background electromagnetic field, and F=dAF \,=\, \mathrm{d}A for its field strength, the Faraday tensor. Since we regard this as a fixed background field, we do not (need to) consider the Maxwell Lagrangian density L YMFFL_{YM} \,\coloneqq\, F \wedge \star F.

But we do (need to) consider the interaction term between the electromagnetic field and the electric current density, which is

(2)L intAJ. L_{int} \,\coloneqq\, A \wedge J \,.

Here we assume that AA is globally defined, in fact we shall assume that Σ\Sigma has vanishing de Rham cohomology in degree=2,

(3)H dR 2(Σ)=0. H^2_{dR}(\Sigma) \;=\; 0 \,.

This means we are focused on the local nature of fields, not on their global properties, as (for better or worse) usual for Lagrangian field theory.


The Ansatz

The auxiliary gauge potential. With the assumption (3) also the conserved current density (1) admits a coboundary, a differential 1-form to be denoted aa:

(4)J=da. J \,=\, \mathrm{d}a \,.

The central Ansatz of the approach is to think of this 1-form as an effective dynamical gauge field for the FQH dynamics.


The Lagrangian density. This in turn suggests that the effective Lagrangian density should be the sum of

  1. the interaction term (2) already mentioned, which thus specializes to

    L int=AJ=Ada, L_{int} \;=\; A \wedge J \;=\; A \wedge \mathrm{d}a \,,
  2. a further interaction term for aa, now regarded as a gauge potential, to its own current density jj, to be thought of as the current of FQH quasi-particles (or quasi-holes)

    (5)L quas=aj L_{quas} \;=\; a \wedge j
  3. a kinetic term for aa itself, where the only sensible choice is the Chern-Simons form at level k=K/2/2k = K/2 \in \mathbb{Z}/2

    L CS=K12ada. L_{CS} \;=\; K \tfrac{1}{2} a \wedge \mathrm{d}a \,.

In total, the traditional Ansatz for the effective Lagrangian density for “single layer” FQH systems at filling fraction 1/K1/K is hence:

(6)LK12adaAdaJaj. L \,\coloneqq\, K \tfrac{1}{2}\, a\wedge \mathrm{d}a \,-\, A \wedge \underset{J}{ \underbrace{ \mathrm{d}a } } \,-\, a \wedge j \,.

[Wen 1995 (2.11), reviewed in Wen 2007 (7.3.10)]


The equation of motion. The Euler-Lagrange equation of motion for the effective gauge field aa induced by (6) is simply

(7)δLδA=0J=1k(F+j). \frac{\delta L}{\delta A} \;=\; 0 \;\;\;\;\;\;\;\;\;\; \Leftrightarrow \;\;\;\;\;\;\;\;\;\; J \,=\, \tfrac{1}{k}\Big( F + j \Big) \,.

(This means in particular that if, on topologically non-trivial Σ\Sigma, one were to insist (which is not a logical necessity) to subject the effective gauge field aa to Dirac charge quantization (as AA is), then the ordinary electromagnetic field would have to be assumed to carry magnetic charge being a multiple of kk.)


Reproducing FQH phenomena

Recovering the Hall conductivity. The first plausibility check on the effective model is to observe that its equation of motion (7) implies the correct Hall conductivity relation for a fractional quantum Hall system at filling fraction 1/k1/k:

Namely assuming that the electric field is oriented in the yy-direction, so that the Faraday tensor is of the form (cf there):

F=E ydtdy+Bdxdy F \;=\; E_y \mathrm{d}t \wedge \mathrm{d}y + B \mathrm{d}x\wedge \mathrm{d}y

and assuming that the quasi-particles are stationary, so that

jdvol( 0)dxdy, j \;\propto\; dvol(\partial_0) \;\propto\; \mathrm{d}x \wedge \mathrm{d}y \mathrlap{\,,}

the temporal component of the equation of motion (7) becomes

J x=1kE y J_x \;=\; \tfrac{1}{k} E_y

which is the correct form of the Hall field E yE_y for given longitudinal current J XJ_X at filling fraction ν=1/k\nu = 1/k – see there.

(Following Zee 1995 (4.5), later authors [Witten 2016 §2.3, Tong 2016 p 161] deduce this in a more roundabout way, by first inserting the equation of motion back into the Lagrangian density and then varying that with respect to AA. It seems that this unnecessary step is brought about by first tacitly renaming “JJ” to “ff” and then forgetting that this was just a renaming.)


Recovering the fractional quasi-particles. Second, the spatial component of the equation of motion (7) says that (1.) the magnetic flux quanta and (2.) the quasi-particles contribute their kkth fraction to the total charge density:

J 0=1K(B+j 0). J_0 \;=\; \tfrac{1}{K}(B + j_0) \,.

Here

  • statement (1) reflects exactly the composite fermion model, where at kkth filling fraction each electron is bound to kk quanta of magnetic flux

  • statement (2) is the desired property of quasi-particles to have kkth fractional charge.


This largely concludes the traditional justification for the effective abelian Chern-Simons theory (6).


Hierarchical K-matrix formalism

Following the hierarchical picture of FQH systems at non-unit filling fraction (due to Haldane 1983 and Halperin 1984), where quasiparticles at unit filling fractions are imagined to support themselves a secondary FQH effect with secondary quasi-particles, and so on (a picture that faces various experimental challenges, cf. Jain 2007 §12.1, Jain 2014, esp. p 8 and Rem. below), authors consider abelian Chern Simons theory of multiple gauge fields that are hierarchically coupled to each other, called the “K-matrix formalism” (Blok & Wen 1990, Wen & Zee 1992, early review in Wen 1995 §2.1, textbook account in Wen 2007 §7.3.3).

The basic idea is to observe that the above step from (i) introducing a gauge potential (4) for the electron current density JJ to (ii) the effective Chern-Simons action (6) with quasi-particle current density jj may be repeated by next introducing also a gauge potential for the quasi-particle current (cf. Wen 2007 (7.3.13)), and so on.

Concretely, postulating that the quasi-particle current jj (5) is itself the field strength of a secondary auxiliary gauge field a (2)a^{(2)}

j=da (2) j \,=\, \mathrm{d} a^{(2)}

(and renaming the previous auxiliary gauge field to a (1)aa^{(1)} \equiv a)

and then adjoining for that secondary gauge field another Chern-Simons kinetic term, turns the effective Lagrangian density (6) into

(8)L 1,2K 112a (1)da (1)Ada (1)Ja (1)da (2)j+K 212a (2)da (2). L_{1,2} \,\coloneqq\, K_1 \tfrac{1}{2}\, a^{(1)} \wedge \mathrm{d} a^{(1)} \,-\, A \wedge \underset{J}{ \underbrace{ \mathrm{d} a^{(1)} } } \,-\, a^{(1)} \wedge \underset{j}{ \underbrace{ \mathrm{d} a^{(2)} } } \,+\, K_2 \tfrac{1}{2}\, a^{(2)} \wedge \mathrm{d} a^{(2)} \,.

(cf. Wen 1995 (2.15), Wen 2007 (7.3.13))

meant to be an effective theory at filling fraction

(9)ν=1K 11K 2. \nu \;=\; \tfrac { 1 } { K_1 - \tfrac{1}{K_2} } \,.

(cf. Wen 1995 (2.18), Wen 2007 (7.3.15))

Now this secondary effective Lagrangian (8) may be rewritten equivalently as (using Einstein summation convention)

(10)L 1,2=K ij12a (i)da (j)Q iAda (i) L_{1,2} \;=\; K_{i j} \tfrac{1}{2} a^{(i)} \wedge \mathrm{d} a^{(j)} - Q_i \, A \wedge \mathrm{d} a^{(i)}

with the K-matrix

K[K 1 1 1 K 2] K \;\coloneqq\; \left[ \begin{matrix} K_1 & -1 \\ -1 & K_2 \end{matrix} \right]

and the charge vector

Q[1 0]. Q \;\coloneqq\; \left[ \begin{matrix} 1 \\ 0 \end{matrix} \right] \,.

(cf. Wen 1995 (2.19), Wen 2007 (7.3.15)).

Observing that the inverse K-matrix is

(11)K 1=1K 1K 21[K 2 1 1 K 1] K^{-1} \,=\, \tfrac{1}{K_1 K_2 - 1} \left[ \begin{matrix} K_2 & 1 \\ 1 & K_1 \end{matrix} \right]

the second-layer filling fraction (9) is recognized as the top left entry of (11) and hence re-expressed in terms of these matrix quantities as

ν=Q tK 1Q. \nu \,=\, Q^t \cdot K^{-1} \cdot Q \,.

(cf. Wen 2007 (7.3.19))

Remark

The phenomenological (experimental) validity already of the HH-hierarchy underlying this K-matrix formulation, when applied to ordinary single-component FQH systems, has been called into question (Jain 2007 §12.1 and Jain 2014, esp. p 8): The required assumptions on quasi-particle densities necessary for the hierarchy to be physically plausible seem to be drastically violated, and in any case the experimentally observed filling fractions do not reflect the predicted hierarchy.

On the other hand, the K-matrix formalism may be the natural description of multi-component FQH systems, where each column/row of the K-matrix corresponds to one “component” of possible electron modes (e.g. different spin polarizations or different layers in multi-layer materials), cf. Barkeshli & Wen 2017, Sodemann et al 2017, Zeng 2021 Hu, Liu & Zhu 2023, Zeng 2024.


References

General

Many general reviews of Chern-Simons theory have a section focused on the abelian case, for instance:

  • Gregory Moore, §2 in: Introduction to Chern-Simons Theories, TASI lecture notes (2019) [pdf, pdf]

  • David Grabovsky, §1 in: Chern–Simons Theory in a Knotshell (2022) [pdf, pdf]

On the light-cone quantization of abelian Chern-Simons theory:

Via the Reshetikhin-Turaev construction:

  • Enore Guadagnini, Francesco Mancarella: Abelian link invariants and homology, J. Math. Phys. 51 (2010) 062301 [arXiv:1004.5211]

  • Philippe Mathieu, Frank Thuillier: Abelian Turaev-Virelizier theorem and U(1)U(1) BF surgery formulas [arXiv:1706.01845]

On abelian Chern-Simons theory via theta functions and the representation theory of the integer Heisenberg group:

On boundary conditions and line-defects in abelian Chern-Simons theory and on the corresponding ground state degeneracy (topological order) on surfaces with (“gapped”) boundaries:

and with fermionic boundary 2d CFT:

  • Kohki Kawabata, Tatsuma Nishioka, Takuya Okuda, Shinichiro Yahagi: Fermionic CFTs from topological boundaries in abelian Chern-Simons theories [arXiv:2502.08084]

Discussion via locally covariant algebraic quantum field theory:

On lattice formulation of abelian Chern-Simons theory:

and its canonical quantization:

In relation to the dilogarithm:

On unoriented manifolds:

  • Ippo Orii: On dimensions of (2+1)D(2+1)D abelian bosonic topological systems on unoriented manifolds [arXiv:2502.13532]

Coupled to a Higgs field:

  • Yoonbai Kim, O-Kab Kwon, Hanwool Song, Chanju Kim: Inhomogeneous Abelian Chern-Simons Higgs Model with New Inhomogeneous BPS Vacuum and Solitons [arXiv:2409.11978]

Abelian Chern-Simons for fractional quantum Hall effect

The idea of abelian Chern-Simons theory as an effective field theory exhibiting the fractional quantum Hall effect (abelian topological order) goes back to

and was made more explicit in:

Early review:

Further review and exposition:

For discussion of the fractional quantum Hall effect via abelian but noncommutative (matrix model-)Chern-Simons theory

More on edge modes:

Amplification of the K-matrix formalismheory#HierarchicalKMatrixFormalism) as being about (not the usual single-component but) multi-component FQH systems:

Further developments:

Abelian Maxwell-Chern-Simons theory for superconductivity

Coupling the Chern-Simons term to 1+2D Maxwell theory for an effective description of superconductivity:

(…)

For baryons in N f=1N_f=1 QCD

On baryons in N f = 1 N_f = 1 QCD as quantum Hall droplets effectively described by abelian Chern-Simons theory:

Last revised on May 26, 2025 at 16:05:18. See the history of this page for a list of all contributions to it.