Topological defect

See also topological excitations and the base concepts: topology, topological manifold, differential equations, quantum mechanics and condensed matter physics.

In mathematics and physics, a topological soliton or a topological defect is a solution of a system of partial differential equations or of a quantum field theory homotopically distinct from the vacuum solution.

Overview

A topological defect can be proven to exist because 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.

Examples

Topological defects occur in partial differential equations and are believed to drive phase transitions in condensed matter physics.

The authenticity 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.

Solitary wave PDEs

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 of the Wess–Zumino–Witten model.

Lambda transitions

Topological defects in lambda transition universality class systems including:

• screw/edge-dislocations in liquid crystals,
• magnetic flux tubes in superconductors, and
• vortices in superfluids.

Cosmological defects

Topological defects, of the cosmological type, are extremely high-energy phenomena, which are deemed impractical to produce in Earth-bound physics experiments. Observation of proposed topological defects that formed during the universe's formation could theoretically be observed without significant energy expenditure, however.

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. Certain grand unified theories predict the formation of stable topological defects in the early universe, during these phase transitions.

Symmetry breakdown

Depending on the nature of symmetry breakdown, various solitons are believed to have formed 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, either north or south (and so are commonly called "magnetic monopoles").
• Textures form when larger, more complicated symmetry groups are completely broken. They are not as localized as the other defects, and are unstable.
• 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. 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.

Biochemistry

Defects have also been found in biochemistry, notably in the process of protein folding.

Formal classification

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).

Observation

Topological defects have not been observed 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). 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. In late 2007, a cold spot in the cosmic microwave background provided evidence of a possible texture.^[4]

Condensed matter

Classes of stable defects in biaxial nematics

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

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]

Images

See also

• Quantum vortex
• Dislocation
• Vector soliton
• Quantum topology
• Topological entropy in physics
• Topological order
• Topological quantum field theory
• Topological quantum number
• Topological string theory

References

External links

• Cosmic Strings & other Topological Defects
• http://demonstrations.wolfram.com/SeparationOfTopologicalSingularities/