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A (pre-)quantum field theory with defects is, roughly a field theory that assigns data not just to plain manifolds/cobordisms, but to spaces that may carry certain singularities and/or colorings. At the locus of such a singularity the bulk field theory may then undergo transitions.
Such defects are known by many names. In codimension 1 they are often called domain walls. If they are boundaries they are often called branes, the corresponding domain walls are then sometimes called bi-branes. Examples of Dimension-1 defects are Wilson lines and cosmic strings (at least in gauge theory) and dimension-0 defects are often called monopoles.
A plain $n$-dimensional local FQFT (a bulk field theory) is a symmetric monoidal (∞,n)-functor from the (∞,n)-category of cobordisms.
If one replaces plain cobordisms here with cobordisms $Bord_n^{Def}$ “with singularities” including boundaries and corners but also partitions labeled in a certain index set $Def$, one calls a morphism
a TQFT with defects. A general formalization is in (Lurie, section 4.3), see at Cobordism theorem – For cobordisms with singuarities (boundaries/branes and defects/domain walls). See also (Davydov-Runkel-Kong and Carqueville-Runkel-Schaumann).
Such a morphism carries data as follows:
for each label in $Def$ of codimension 0 there is an ordinary bulk field theory;
for each label in $Def$ of codimension 1 data on how to “connect” the two TQFTs on both sides
etc.
So one may think of the codimension $k$ colors as defects where the TQFT that one is looking at changes its nature.
In particular, when the QFT on one side of the defect is trivial, then the defect behaves like a boundary condition for the remaining QFT. Since at least for $n=2$ QFT such boundary conditions are also called branes, defects are also called bi-branes.
The statement of the cobordism theorem with singularities (Lurie, theorem 4.3.11) is essentially the following:
Given a symmetric monoidal (∞,n)-category $\mathcal{C}^\otimes$, then for every choice of pasting diagram of k-morphisms for all $k$, there is a type of manifolds with singularity $Def$, such that $Bord_n^Def$ is the free symmetric monoidal $(\infty,n)$-category on this data, hence such that TFTs with defects $(Bord_n^Def)^\otimes \to \mathcal{C}^\otimes$ are equivalently given by realizing such a pasting diagram in $\mathcal{C}$, where each of the given k-morphism appears as the value of a codimension $(n-k)$-defect. (See also Lurie, remark 4.3.14).
under construction
An old notion of defects in field theory – well preceeding the above general notion in the context of FQFT – is that of topological defects in the vacuum structure of gauge theories that exhibit spontaneous symmetry breaking (such as a Higgs mechanism).
A comprehensive review in in (Vilenkin-Shellard 94. Steps towards conceptually systematizing these broken-symmetry defects and their interaction are made in Preskill-Vilenkin 92. We now discuss this may be translated to and formalized in the general FQFT definition above along the lines of (Fiorenza-Valentino, FSS).
Let $G$ be a Lie group, to be thought of as the (local or global) gauge group of some gauge theory. Let $H \hookrightarrow G$ a subgroup, to be thought of as the subgroup of global symmetries preserved by some vacuum configuration (which “spontaneously breaks” the symmetry from $G$ to $H$, the archetypical example is the Higgs mechanism).
Then the space of such vacuum configurations is the coset space $G/H$. So given a manifold $X$ (spacetime), vacuum configuratons are given by functions $X \to G/H$. Hence $G/H_1$ is the “moduli space of vacua” or “vacuum space” in this context.
The functions $X \to G/H$ are to be smooth functions in the bulk of spacetime. If they are allowed to be non-smooth or even non-continuous along given strata of $X$, then these are called defects in the sense of broken gauge symmetry.
In particular (with counting adapted to $dim X = 4$)
if $X \to G/H$ is not smooth but is smooth on the pre-image of each element of $\pi_0(G/H)$ and becomes a smooth function on $X-S_1$ where $S_1 \hookrightarrow X$ is a codimension-1 submanifold, then $S_1$ is said to be a domain wall for vacuum configurations.
if $X \to G/H$ is not smooth but becomes smooth on $X-S_2$, where $S_2$ is a codimension-2 submanifold, then $S_2$ is calld a cosmic string-defect of the vacuum configurations;
if $X \to G/H$ is not smooth but becomes smooth on $X-S_3$, where $S_3$ is a codimension-3 submanifold, then $S_3$ is calld a monopole-defect of the vacuum configurations.
hm, need to fine-tune the technical conditions here, to make the following statement come out right…
So
domain walls can appear when $\pi_0(G/H)$ is non-trivial;
cosmic strings can appear when $\pi_1(G/H)$ is non-trivial;
monopoles can appear when $\pi_2(G/H)$ is non-trivial.
Next consider a sequence of subgroups
to be thought of as coming from two consecutive steps of spontaneous symmetry breaking, the first one down to $H_1$ at some energy-scale $E_1$, and the second at some lower energy scale $E_2 \lt E_1$.
Then we say that vacuum defects at energy $E_2$ of codimension-$k$ which wind around an element $\pi_k(H_1/H_2)$ are metastable if they become unstabe at energy $E_1$, hence if their image in $\pi_k(H_0/H_2)$ is trivial.
So if we add to the singular cobordism category the $k$-morphism which is the $k$-dimenional unit cube with an open $k$-ball removed, then the boundary field data for metastable codimension $n-k$-defects is
we have a homotopy fiber sequence
This induces a long exact sequence of homotopy groups
So for every metastable defect of codimension $n-k$ given by $c \in ker(\pi_k(H_1/H_2) \to \pi_k(H_0/H_2))$ there is an element in $\pi_{k+1}(H_0/H_1)$ of one codimension higher. One says (Preskill-Vilenkin 92) that the codimenion $(n-k)$-defect may end on that codimension $(n-k-1)$-defect.
(…)
In order to formalize this we introduce, following the cobordism theorem with singularities, cells in $Span_n(\mathbf{H})$ which label spontaneous-symmetriy-breaking defects as well as their defects-of-defects which exhibit their decay by higher codimension defects.
Consider a span of the form
Comparing this to the span that comes from the “cap” $S^{k-1} \to D^{k} \leftarrow \emptyset$ in the theory at energy level $E_2$, which is just
shows that the former models a $k$-disk which is not filled with spacetime, but nevertheless closes the disk. This is the defect given as a removal of a piece of spacetime.
In order to formalize how these defects may decay at higher energy, consider next a span of field configurations of the form
Comparing again to the span that comes from the “cap” $S^{k-1} \to D^{k} \leftarrow \emptyset$ in the theory at energy level $E_2$, which is just
shows that the former models a $k$-disk whose center point carries a singularity: the fields at the bounding $S^{k-1}$ take values in the moduli space of the ambient theory $\Pi(H_1/H_2)$, but then at the tip of the “cap” there is a “field insertion” of a field with values in $\Pi(H_0/H_2)$. Hence this labels a defect of codimension $k$.
To construct such a decay-process span that captures the above story from Preskill-Vilenkin 92, consider the following diagram:
This may be read as follows:
on the far left $[\Pi(S^k), \Pi(H_1/H_2)]$ is the space of fields of the ambient theory at energy scale $E_2$ around the defect;
the bottom left map is the “fluctuation” map that sends these fields to fields at the higher energy scale $E_1$;
the bottom right map exhibt the possible “decays”: a lift through this map takes a field configuration that winds around a $k$-ball and contracts it through that $k$-ball, hence going forth and back through the bottom two maps corresponds to carrying a defect over the energy barrier from $E_2$ to $E_1$ and there having it decay away.
these decaying configurations are therefore given by the homotopy fiber product of the bottom two functions, which is $[\Pi(S^{k-1}), \Omega\Pi(H_0/H_1)]$, as indicated. But this space is really given by field configurations at energy scale $E_1$ that wind around a $(k+1)$-ball, as shown at the very top.
Hence the top part of this diagram is a span that exhibits a defect-of-defects which tells just the story that (Preskill-Vilenkin 92) is telling: a codimension-$k$ defect of the low energy theory decays at higher energy, and the decay is witnessed by the appearance of a codimension $(k+1)$-defect of the high energy theory.
(…)
singularity | field theory with singularities |
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boundary condition/brane | boundary field theory |
domain wall/bi-brane | QFT with defects |
A general formulation via an (∞,n)-category of cobordisms with defects is in section 4.3 of
Defect TQFTs as 1-functors on stratified decorated bordisms are discussed in
Alexei Davydov, Ingo Runkel, Liang Kong, Field theories with defects and the centre functor in Hisham Sati, Urs Schreiber (eds.) Mathematical Foundations of Quantum Field and Perturbative String Theory AMS, 2011
Nils Carqueville, Ingo Runkel, Gregor Schaumann, Orbifolds of n-dimensional defect TQFTs, (arXiv:1705.06085)
Details in dimension 2 and 3 are discussed in
Discussion of defects in prequantum field theory, hence for coefficients in an (∞,n)-category of spans is in
Examples in physics of interaction of defects of various dimension is discussed in
Defects in 2-dimensional conformal field theory have a long history in real-world application, for instance in Kramers-Wannier duality
Jürg Fröhlich, Jürgen Fuchs, Ingo Runkel, Christoph Schweigert, Kramers-Wannier duality from conformal defects (arXiv:cond-mat/0404051)
Fröhlich, Fuchs, Runkel, Schweigert, Duality and defects in rational conformal field theory (arXiv)
Defects in 2-dimension TFT have been studied a lot in the context of genus-0 TFT, where they are described using the language of planar algebras.
Lecture notes include
Defects in Chern-Simons theory and related systems are discussed in
Davide Gaiotto, Edward Witten, S-Duality of Boundary Conditions In N=4 Super Yang-Mills Theory (arXiv:0807.3720)
Anton Kapustin, Mikhail Tikhonov, Abelian duality, walls and boundary conditions in diverse dimensions, JHEP 0911:006,2009 (arXiv:0904.0840)
Anton Kapustin, Natalia Saulina, Surface operators in 3d TFT and 2d Rational CFT in Hisham Sati, Urs Schreiber (eds.) Mathematical Foundations of Quantum Field and Perturbative String Theory AMS, 2011
Defects in higher dimensional Chern-Simons theory on manifolds with corners are discussed in
The following references discuss the traditional notion of topological defects in the vacuum structure of gauge theory with spontaneous symmetry breaking such as domain walls, cosmic strings and monopoles.
Discussion of “topological defects in gauge theory” in higher codimension is in
Defects field theory motivated from solid state physics is discussed in