Topological defect

In mathematics and physics, solitons, topological solitons and topological defects are three closely related ideas, all of which signify structures in a physical system that are stable against perturbations. Solitons won't decay, dissipate, disperse or evaporate in the way that ordinary waves (or solutions or structures) might. The stability arises from an obstruction to the decay, which is explained by having the soliton belong to a different topological homotopy class or cohomology class than the base physical system. The mathematics behind topological stability is both deep and broad, and a vast variety of systems possessing topological stability have been described. This makes categorization somewhat difficult.

The original soliton was observed in the 19th century, as a solitary water wave in a barge canal. It was eventually explained by noting that the Korteweg-De Vries (KdV) equation, describing waves in water, has homotopically distinct solutions. The mechanism of Lax pairs provided the needed topological understanding.

The general characteristic needed for a topological soliton to arise is that there should be some partial differential equation having distinct classes of solutions, with each solution class belonging to a distinct homotopy class. In many cases, this arises because the base space -- 3D space, or 4D spacetime, can be thought of as having the topology of a sphere, obtained by one-point compactification: adding a point at infinity (this is reasonable, as one is generally interested in solutions that vanish at infinity, and so are single-valued at that point.) The variables in the differential equation can typically also be viewed as living in some a compact topological space. As a result, the mapping from space(time) to the variables in the PDE are describable by means of the homotopy groups of spheres.

To restate more plainly: one solution of the PDE cannot be continuously transformed into another; to get from one to the other would require "cutting" (as with scissors), but "cutting" is not a defined operation for solving PDE's. The cutting analogy arises because some solitons are described as mappings $U(1)\to U(1)$ , where $U(1)\simeq S^{1}$ is the circle; the mappings arise in the circle bundle. Such maps can be thought of as winding a string around a stick: the string cannot be removed without cutting it. The most common extension of the analogy is to maps $S^{3}\to SU(2)$ , where the three-sphere $S^{3}$ stands for compactified 3D space, and $SU(2)$ is the double covering of the rotation group.

Historically, many prototypical examples arose in quantum field theory; for example, the Skyrmion, proposed in the 1960's as a model of the nucleon (neutron or proton) owed its stability to the mapping $S^{3}\to SU(2)$ . In the 1980's, the instanton, occurring as a solution in Wess–Zumino–Witten models, rose to considerable popularity because it offered a non-perturbative interpretation in a field that was otherwise dominated by perturbative calculations done with Feynmann diagrams. It provided the impetus for physicists to study the concepts of homotopy and cohomology, which were previously the exclusive domain of mathematics. Further development identified the pervasiveness of the idea: for example, the Schwarzschild solution and Kerr solution to the Einstein field equations (black holes) can be recognized as a topological soliton: this is the Belinski–Zakharov transform.

A topological defect is

obstruction theory

Overview[edit]

The existence of a topological defect can be demonstrated whenever the boundary conditions entail the existence of homotopically distinct solutions. Typically, this occurs because the boundary on which the conditions are specified has a non-trivial homotopy group which is preserved in differential equations; the solutions to the differential equations are then topologically distinct, and are classified by their homotopy class. Topological defects are not only stable against small perturbations, but cannot decay or be undone or be de-tangled, precisely because there is no continuous transformation that will map them (homotopically) to a uniform or "trivial" solution.

Formal classification[edit]

An ordered medium is defined as a region of space described by a function f(r) that assigns to every point in the region an order parameter, and the possible values of the order parameter space constitute an order parameter space. The homotopy theory of defects uses the fundamental group of the order parameter space of a medium to discuss the existence, stability and classifications of topological defects in that medium.^[1]

Suppose R is the order parameter space for a medium, and let G be a Lie group of transformations on R. Let H be the symmetry subgroup of G for the medium. Then, the order parameter space can be written as the Lie group quotient^[2] R = G/H.

If G is a universal cover for G/H then, it can be shown^[2] that π_n(G/H) = π_n−1(H), where π_i denotes the i-th homotopy group.

Various types of defects in the medium can be characterized by elements of various homotopy groups of the order parameter space. For example, (in three dimensions), line defects correspond to elements of π₁(R), point defects correspond to elements of π₂(R), textures correspond to elements of π₃(R). However, defects which belong to the same conjugacy class of π₁(R) can be deformed continuously to each other,^[1] and hence, distinct defects correspond to distinct conjugacy classes.

Poénaru and Toulouse showed that^[3] crossing defects get entangled if and only if they are members of separate conjugacy classes of π₁(R).

Examples[edit]

Topological defects occur in partial differential equations and are believed^{[according to whom?]} to drive^[how?] phase transitions in condensed matter physics.

The authenticity^{[further explanation needed]} of a topological defect depends on the nature of the vacuum in which the system will tend towards if infinite time elapses; false and true topological defects can be distinguished if the defect is in a false vacuum and a true vacuum, respectively.^{[clarification needed]}

Solitary wave PDEs[edit]

Examples include the soliton or solitary wave which occurs in exactly solvable models, such as

screw dislocations in crystalline materials,
skyrmion in quantum field theory, and
topological defects^{[clarification needed]} of the Wess–Zumino–Witten model.

Lambda transitions[edit]

Topological defects in lambda transition universality class^{[clarification needed]} systems including:

screw/edge-dislocations in liquid crystals,
magnetic flux "tubes" known as fluxons in superconductors, and
vortices in superfluids.

Cosmological defects[edit]

Topological defects, of the cosmological type, are extremely high-energy^{[clarification needed]} phenomena which are deemed impractical to produce^{[according to whom?]} in Earth-bound physics experiments. Topological defects created during the universe's formation could theoretically be observed without significant energy expenditure.

In the Big Bang theory, the universe cools from an initial hot, dense state triggering a series of phase transitions much like what happens in condensed-matter systems such as superconductors. Certain^[which?] grand unified theories predict the formation of stable topological defects in the early universe during these phase transitions.

Symmetry breaking[edit]

Depending on the nature of symmetry breaking, various solitons are believed to have formed in cosmological phase transitions in the early universe according to the Kibble-Zurek mechanism. The well-known topological defects are:

Cosmic strings are one-dimensional lines that form when an axial or cylindrical symmetry is broken.
Domain walls, two-dimensional membranes that form when a discrete symmetry is broken at a phase transition. These walls resemble the walls of a closed-cell foam, dividing the universe into discrete cells.
Monopoles, cube-like defects that form when a spherical symmetry is broken, are predicted to have magnetic charge,^[why?] either north or south (and so are commonly called "magnetic monopoles").
Textures form when larger, more complicated symmetry groups^[which?] are completely broken. They are not as localized as the other defects, and are unstable.^{[clarification needed]}
Skyrmions
Extra dimensions and higher dimensions.

Other more complex hybrids of these defect types are also possible.

As the universe expanded and cooled, symmetries in the laws of physics began breaking down in regions that spread at the speed of light; topological defects occur at the boundaries of adjacent regions.^[how?] The matter composing these boundaries is in an ordered phase, which persists after the phase transition to the disordered phase is completed for the surrounding regions.

Observation[edit]

Topological defects have not been identified by astronomers; however, certain types are not compatible with current observations. In particular, if domain walls and monopoles were present in the observable universe, they would result in significant deviations from what astronomers can see.

Because of these observations, the formation of defects within the observable universe is highly constrained, requiring special circumstances (see Inflation (cosmology)). On the other hand, cosmic strings have been suggested as providing the initial 'seed'-gravity around which the large-scale structure of the cosmos of matter has condensed. Textures are similarly benign.^{[clarification needed]} In late 2007, a cold spot in the cosmic microwave background provided evidence of a possible texture.^[4]

Classes of stable defects in biaxial nematics

Condensed matter[edit]

In condensed matter physics, the theory of homotopy groups provides a natural setting for description and classification of defects in ordered systems.^[1] Topological methods have been used in several problems of condensed matter theory. Poénaru and Toulouse used topological methods to obtain a condition for line (string) defects in liquid crystals that can cross each other without entanglement. It was a non-trivial application of topology that first led to the discovery of peculiar hydrodynamic behavior in the A-phase of superfluid helium-3.^[1]

Stable defects[edit]

Homotopy theory is deeply related to the stability of topological defects. In the case of line defect, if the closed path can be continuously deformed into one point, the defect is not stable, and otherwise, it is stable.

Unlike in cosmology and field theory, topological defects in condensed matter have been experimentally observed.^[5] Ferromagnetic materials have regions of magnetic alignment separated by domain walls. Nematic and bi-axial nematic liquid crystals display a variety of defects including monopoles, strings, textures etc.^[1] In crystalline solids, the most common topological defects are dislocations, which play an important role in the prediction of the mechanical properties of crystals, especially crystal plasticity.