Kosterlitz–Thouless transition

Kosterlitz–Thouless transition

The Kosterlitz–Thouless transition, or Berezinsky–Kosterlitz–Thouless transition, is a special transition seen in the XY model for interacting spin systems in 2 spatial dimensions.The XY model is a 2-Dimensional vector spin model that possesses U(1) or circular symmetry. This system is not expected to possess a normal second order phase transition.This is because the expected ordered phase of the system is destroyed by transverse fluctuations, i.ethe Goldstone modes (see Goldstone boson) associated with this broken continuous symmetry, which logarithmically diverge with system size.This is a specific case of what is called the Mermin–Wagner theorem in spin systems.

The transition is named for John M. Kosterlitz and David J. Thouless

KT Transition : Disordered Phases with different correlations

In the XY model in two dimensions,a second order phase transition is not seen. However, one finds a low-temperature quasi-ordered phase with a correlation function (see statistical mechanics) that decreases with the distance like a power, depending on the temperature. The transition from the high temperature disordered phase with the exponential correlation to this low temperature quasi-ordered phase is a Kosterlitz–Thouless transition.It is a phase transition of infinite order.

Role of Vortices

In the XY model in 2 dimensions, vortices are topologically stable configurations.It is found that the high temperature disordered phase with exponential correlation is aresult of the formation of vortices.The temperature at which the KT transition occurs is in fact that at which vortex generation becomes thermodynamically favourable.At temperatures below this, the system has a power law correlation.

Informal Description

There is a very elegant thermodynamic argument for the KT transition. The energy of a single vortex is of the form kappaln(R/a), where kappa is a parameter depending upon the system the vortex is in, R is the system size, and ais the radius of the vortex core. We assume R>>a. The number of possible positions of any vortex in the system is approximately (R/a)^2. From Boltzmann's law, the entropy is S=2k_Bln(R/a), where k_B is Boltzmann's constant. Thus, the Helmholtz free energy is

:F = E - TS = (kappa - 2k_BT)ln(R/a).

When F>0, the system is unstable to having a vortex. However when F<0, the conditions are sufficient for a vortex to be in the system. We define the transition temperature for F=0. Thus,

:T_c = frac{kappa}{2k_B}.

Rigorous Analysis

We have a field φ over the plane which takes on values in S1. For convenience, we work with its universal cover R instead but identify any two values of φ(x) which differs by an integer multiple of 2π.

The energy is given by

:E=int frac{1}{2} ablaphicdot ablaphi d^2x

and the Boltzmann factor is exp(-βE).

If we take the contour integral oint_gamma dphi over any closed path γ, we would expect it to be zero if γ is contractible, which is what we would expect for a planar curve. But here is the catch. Assume the XY theory has a UV cutoff which requires some UV completion. Then, we can have punctures in the plane, holes so to speak so that if γ is a closed path which winds once around the puncture, oint_gamma dphi is only an integer multiple of 2π. These punctures are called vortices and if γ is a closed path which only winds once counterclockwise around the puncture and its winding number about any other puncture is zero, then the integer multiplicity can be attached to the vortex itself. Let's say a field configuration has n punctures at xi, i=1,...,n with multiplicities ni. Then, φ decomposes into the sum of a field configuration with no punctures, φ0 and sum_{i=1}^n n_iarg(z-z_i) where we have switched to the complex plane coordinates for convenience. The latter term has branch cuts, of course, but since φ is only defined modulo 2π they are unphysical.

Now,

:E=int frac{1}{2} ablaphicdot ablaphi d^2x+int frac{1}{2} ablasum_{i=1}^n n_iarg(z-z_i)cdot ablasum_{i=1}^n n_iarg(z-z_i) d^2x

It's easy to see that unless sum_{i=1}^n n_i=0, the second term is positive infinite, making the Boltzmann factor zero which means that we can forget all about it.

When sum_{i=1}^n n_i=0, the second term is equal to sum_{1leq i < j leq n} -2pi n_i n_j ln(|x_j-x_i|/L).

This is nothing other than a Coulomb gas, of course. The scale L contributes nothing but a constant.

Let's look at the case with only one vortex of multiplicity one and one vortex of multiplicity -1. At low temperatures, i.e. large β, because of the Boltzmann factor, the vortex-antivortex pair tends to be extremely close to one another. In fact, their separation would be around the cutoff scale. With more vortex-antivortex pairs, we have a collection of vortex-antivortex dipoles. At large temperatures, i.e. small β, the probability distribution swings the other way around and we have a plasma of vortices and antivortices. The phase transition between the two is the Kosterlitz–Thouless phase transition.

ee also

*Goldstone boson
*Ising model
*Potts model
*quantum vortex
*superfluid film
*Topological defect

References

*J. M. Kosterlitz & D. J. Thouless, " [http://dx.doi.org/10.1088/0022-3719/6/7/010 Ordering, metastability and phase transitions in two-dimensional systems] ", Journal of Physics C: Solid State Physics, Vol. 6 pages 1181-1203 (1973)
* Z. Hadzibabic et. al.: "Berezinskii–Kosterlitz–Thouless crossover in a trapped atomic gas", [http://dx.doi.org/10.1038/nature04851 Nature 441, 1118 (2006)]

Books

* H. Kleinert, "Gauge Fields in Condensed Matter", Vol. I, " SUPERFLOW AND VORTEX LINES", pp. 1--742, [http://www.worldscibooks.com/physics/0356.htm World Scientific (Singapore, 1989)] ; Paperback ISBN 9971-5-0210-0 " (also available online: [http://www.physik.fu-berlin.de/~kleinert/kleiner_reb1/contents1.html Vol. I] . Read pp. 618--688);
* H. Kleinert, "Multivalued Fields in in Condensed Matter, Electrodynamics, and Gravitation", [http://www.worldscibooks.com/physics/6742.html World Scientific (Singapore, 2008)] (also available online: [http://www.physik.fu-berlin.de/~kleinert/re.html#B9 here] )


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