The order parameter is normally a quantity which is zero in one phase (usually above the critical point), and non-zero in the other. It characterises the onset of order at the phase transition. The order parameter susceptibility will usually diverge approaching the critical point. For a ferromagnetic system undergoing a phase transition, the order parameter is the net magnetization. For liquid/gas transitions, the order parameter is the difference of the densities.
When symmetry is broken, one needs to introduce one or more extra variables to describe the state of the system. For example, in the ferromagnetic phase, one must provide the net magnetization, whose direction was spontaneously chosen when the system cooled below the Curie point. Such variables are examples of order parameters. An order parameter is a measure of the degree of order in a system; it ranges between zero for total disorder and the saturation value for complete order. For example, an order parameter can indicate the degree of order in a liquid crystal. However, note that order parameters can also be defined for non-symmetry-breaking transitions. Some phase transitions, such as superconducting and ferromagnetic, can have order parameters for more than one degree of freedom. In such phases, the order parameter may take the form of a complex number, a vector, or even a tensor, the magnitude of which goes to zero at the phase transition.
There also exist dual descriptions of phase transitions in terms of disorder parameters. These indicate the presence of line-like excitations such as vortex- or defect lines.
Other articles related to "order, order parameters, order parameter":
... In contrast to materials with a single ferroic order, domains in multiferroics have additional properties and functionalities ... For instance, they are characterized by an assembly of at least two order parameters ... The order parameters may be independent (typical yet not mandatory for a split-order-parameter multiferroic) or coupled (mandatory for a joint-order-parameter multiferroic) ...
... formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously (i.e ... The four basic primary ferroic order parameters are ferromagnetism ferroelectricity ferroelasticity ferrotoroidicity (?) the latter still being under debate as there has been no proof for ... in the field consider materials as multiferroics only if they exhibit coupling between the order parameters ...
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