# Simple models that exhibit topological phase transitions

I think you need to define what you mean by a "topological state of matter", since the term is used in several inequivalent ways. For example the toric code that you mention, is a very different kind of topological phase than topological insulators. Actually one might argue that all topological insulators (maybe except the Integer Quantum Hall, class A in the general classification) are only topological effects rather than true topological phases, since they are protected by discreet symmetries (time reversal, particle-hole or chiral). If these symmetries are explicitly or spontaneously broken then the system might turn into a trivial insulator.

But one of the simplest lattice models (much simpler that the toric code, but also not as rich) I know of is the following two band model (written in k-space)

$H(\mathbf k) = \mathbf d(\mathbf k)\cdot\mathbf{\sigma},$

with $\mathbf d(\mathbf k) = (\sin k_x, \sin k_y, m + \cos k_x + \cos k_y)$ and $\mathbf{\sigma} = (\sigma_x,\sigma_y,\sigma_z)$ are the Pauli matrices. This model belongs to the same topological class as the IQHE, meaning that it has no time-reversal, particle-hole or chiral symmetry. The spectrum is given by $E(\mathbf k) = \sqrt{\mathbf d(\mathbf k)\cdot\mathbf d(\mathbf k)}$ and the model is classified by the first Chern number

$C_1 = \frac 1{4\pi}\int_{T^2}d\mathbf k\;\hat{\mathbf d}\cdot\frac{\partial \hat{\mathbf d}}{\partial k_x}\times\frac{\partial \hat{\mathbf d}}{\partial k_y},$

where $T^2$ is the torus (which is the topology of the Brillouin zone) and $\hat{\mathbf d} = \frac{\mathbf d}{|\mathbf d|}$. By changing the parameter $m$ the system can go through a quantum critical point, but this can only happen if the bulk gap closes. So solving the equation $E(\mathbf k) = 0$ for $m$, one can see where there is phase transitions. One can then calculate the Chern number in the intervals between these critical points and find

$C_1 = 1$ for $0 < m < 2$, $C_1 = -1$ for $-2 < m < 0$ and $C_1 = 0$ otherwise.

Thus there are three different phases, one trivial and two non-trivial. In the non-trivial phases the system has quantized Hall response and protected chiral edge states (which can easily be seen by putting edges along one axes and diagonalizing the Hamiltonian on a computer).

If one takes the continuum limit, the model reduces to a 2+1 dimensional massive Dirac Hamiltonian and I think the same conclusions can be reached in this continuum limit but the topology enters as a parity anomaly.

More information can be found here: http://arxiv.org/abs/0802.3537 (the model is introduced in section IIB).

Hope you find this useful.

This is a very good question. Let me give a little back ground first.

For a long time, physicists thought all different phases of matter are described by symmetry breaking. As a result, all continuous phase transitions between those symmetry breaking phases involve a change of symmetry.

Now we know that there are new kind of phases of matter beyond symmetry breaking -- topological order. So we should have new continuous phase transitions between those topologically ordered phases. Those new continuous phase transitions do not change any symmetry (ie the two phases connected by the transition have the same symmetry). To have an intuition about those new kind of the phase transition, one naturally ask, what are simple models that exhibit topological phase transitions?

Heidar has given a very good and simple model. Here I will list some research papers on this topic (please feel free to add if you know more papers)

- X.-G. Wen and Y.-S. Wu, Phys. Rev. Lett. 70, 1501 (1993).
- W. Chen, M. P. A. Fisher, and Y.-S. Wu, Phys. Rev. B 48, 13749 (1993).

The above two papers describe continuous phase transitions between FQH-FQH or FQH-Mott-insultor induced by periodic potentials.

- N. Read and D. Green, Phys. Rev. B 61, 10267 (2000).

The paper describes the continuous transition between strong and weak p-wave/d-wave BCS superconductors. Heidar's example is similar to this work.

- Xiao-Gang Wen, Phys. Rev. Lett. 84, 3950 (2000). cond-mat/9908394.

The paper describes continuous transitions between double layer FQH states induced by interlayer tunneling and/or coupling

- Maissam Barkeshli, Xiao-Gang Wen, Phys.Rev.Lett.105 216804 (2010).

The paper describes a continuous transition between between an Abelain FQH state and a non-Abelian FQH state induced by an anyon condensation.

I like to know more examples of topological phase transitions.

The so-called, lyotropic liquid crystals exhibit several topological transitions. The topology of a real space changes during such transitions. The most famous of them is the transition into the so-called, sponge phase. But there are also more simply ones. For example, lipid vesicles are known to transform into a string of beads (which still are topologically equivalent to a sphere) but then they split from one another (which is already a topological change). Living cells often split out vesicles formed by a part of the membrane. The topological transition is involved in this process.

It would be much better, if you specify what phenomena you have in mind, since various things may be thought of under this general name.

For example, the transitions of the order 2.5 have been considered once to explain Mott transitions in some materials. There the Fermi surface undergoes the topological phase transformation