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percolation [2021/04/11 20:35]
Scott Lawrence [Computational methods] 		percolation [2021/05/05 19:06] (current)
Scott Lawrence [Bethe lattice]
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 Percolation is a [[tag:logarithmic|logarithmic CFT]]. Among other things, this means it is not unitary. Percolation is a [[tag:logarithmic|logarithmic CFT]]. Among other things, this means it is not unitary.
  
-This page focuses on "standard percolation" --- see [[#Related theories]] below for alternative definitions that yield different CFTs.+This page focuses on "standard percolation" --- see [[#related theories]] below for alternative definitions that yield different CFTs.
  
-===== Lattice theory =====+===== Lattice theories =====
  
-Percolation is+Percolation can be defined as either bond or site percolation, and on any geometry of lattice. In both cases, by "lattice" we mean an (infinite) undirected graph.
  
-Because percolation is not a unitary CFT, there is no Hamiltonian lattice theory that flows to percolation in the IR.+Because percolation is not a unitary CFT, there is no Hamiltonian lattice theory that flows to percolation in the IR. All lattices in this section are spacetime lattices. 
 + 
 +Usually percolation is defined on a square lattice. Many other possibilities exist, with the same critical behavior. 
 + 
 +==== Site percolation ==== 
 + 
 +==== Bond percolation ====
  
  
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 ==== Lattice ==== ==== Lattice ====
 +
 +The most straightforward computational method is to instantiate a random lattice of some finite size and compute cluster sizes. This gives a single sample, and the procedure can be repeated as much as desired to improve the statistics. Note that although this is a Monte Carlo method, there is no (non-trivial) Markov chain. In other words, there is no "thermalization" or "mixing time" needed in the simulation.
  
 ==== Epsilon expansion ==== ==== Epsilon expansion ====
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 ===== Two dimensions ===== ===== Two dimensions =====
  
 +For bond percolation on a 2-dimensional square lattice, $p_c = 0.5$ is known exactly.
 ===== Six or more dimensions ===== ===== Six or more dimensions =====
  
 In six or more dimensions critical percolation is described by [[mean-field theory|mean-field theory]]. In six or more dimensions critical percolation is described by [[mean-field theory|mean-field theory]].
 +
 +===== Bethe lattice =====
 +
 +The Bethe lattice can be thought of as the $d\rightarrow\infty$ lattice. It is the infinite tree in which each vertex has the same number ($z$ --- called the coordination number) of neighbors. Note that $z=1$ is invalid, and $z=2$ is just a one-dimensional lattice.
 +
 +As with many statistical models, percolation can be exactly solved on a Bethe lattice: see for instance [[https://web.mit.edu/ceder/publications/Percolation.pdf|these notes]]. The critical exponent is $\beta = 1$; the percolation threshold is $p_c = (z-1)^{-1}$.
 ===== Related theories ===== ===== Related theories =====
  
   * [[Directed percolation]]   * [[Directed percolation]]
  
 +
 +===== External links =====
 +  * [[https://en.wikipedia.org/wiki/Percolation_theory|Wikipedia]]
Last modified: 2021/04/11 20:35
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