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Multicritical point
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Multicritical point : ウィキペディア英語版
Multicritical point
Multicritical points are special points in the parameter space of thermodynamic or
other systems with a continuous phase transition. At least two thermodynamic or other
parameters must be adjusted to reach a multicritical point. At a multicritical point the
system belongs to a universality class different from the "normal" universality class.
A more detailed definition requires concepts from the theory of critical phenomena,
a branch of physics that reached a very satisfying state in the 1970s.

== Definition ==
The union of all the points of the parameter space for which the system is critical is
called a critical manifold.
As an example consider a substance ferromagnetic below a
transition temperature T_, and paramagnetic above T_c. The parameter space here is
the temperature axis, and the critical manifold consists of the point T_c. Now add
hydrostatic pressure P to the parameter space. Under hydrostatic pressure the substance
normally still becomes ferromagnetic below a temperature T_(P).
This leads to a
critical curve in the (T,P) plane - a 1-dimensional critical manifold. Also taking into account
shear stress K as a thermodynamic parameter leads to a critical surface T_c(P,K) in the
(T,P,K) parameter space - a 2-dimensional critical manifold.
Critical manifolds of dimension d > 1 and d > 2 may have physically reachable borders of dimension
d-1 which in turn may have borders of dimension d-2. The system still is critical at
these borders. However, criticality terminates for good reason, and the points on the
borders normally belong to another universality class than the universality class realized
within the critical manifold. All the points on the border of a critical manifold are
multicritical points.
Instead of terminating somewhere critical manifolds also may branch or intersect.
The points on the intersections or branch lines also are multicritical points.
At least two parameters must be adjusted to reach a multicritical point.
A 2-dimensional critical manifold may have two 1-dimensional borders intersecting at a point. Two parameters must be adjusted to reach such a border, three parameters must be adjusted to reach the intersection of the two borders. A system of this type represents up to four universality classes: one within the critical manifold, two on the borders and one on the intersection of the borders.
The gas-liquid critical point is not multicritical, because the phase transition at
the vapour pressure curve P(T) is discontinuous and the critical manifold thus consists of a single point.
== Examples ==

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Multicritical point」の詳細全文を読む



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