For a quantum system with time-reversal symmetry, other than the absence of a magnetic field, can we infer anything else about the system?
$\begingroup$
$\endgroup$
3
-
3$\begingroup$ The symmetry itself tells you that eigenfunctions are real--- there are constraints on the random matrix theory, what exactly are you interested in? "anything else" is too vague. $\endgroup$– Ron MaimonCommented Mar 30, 2012 at 7:18
-
$\begingroup$ Perhaps a better way to phrase the question: other than the presence of a magnetic field, what other conditions would break time-reversal symmetry? $\endgroup$– leongzCommented Apr 2, 2012 at 23:06
-
$\begingroup$ That's a nice one! There is the idea that you can have a spontaneous PT symmetry breaking in High Tc superconductors, due to Zee and collaborators ca 1989. Other than that, I don't know. That would be a better question, I'd +1 it in a heartbeat--- perhaps you can ask that separately. $\endgroup$– Ron MaimonCommented Apr 2, 2012 at 23:52
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
We can infer some general topological properties of the system for special cases. This has been used in the recent "trending topic" of condensed matter physics, topological insulators. For simplicity, I will restrict myself in the following to $2\text{D}$ systems although one can generalize everything to $3\text{D}$.
The time-reversal operator is an antiunitary operator that admits the following representation:
$\hat{\Theta} = \exp \left(i\pi\hat{S}_y/\hbar \right)K$
where $K$ means complex conjugation and $\hat{S}_y$ is the spin operator along the $\hat{y}$ axis. Consider a fermionic Hamiltonian for spin $s=1/2$ electrons. Then
$\hat{\Theta} =-\hat{1}$ [*]
In this case, Kramers theorem applies:
Let $\hat{\mathcal{H}}$ be a $T$-invariant (fermionic) Hamiltonian. Then, all the eigenstates of the Hamiltonian are twofold degenerate.
The proof of this statement is simple once you have understood [*]. As a consequence, $T$-invariant fermionic systems must have topologically protected twofold degenerate states. The $T$-invariant Hamiltonian satisfies
$ \hat{\Theta}\hat{\mathcal{H}} (\mathbf{k}) =\hat{\mathcal{H}} (-\mathbf{k}) \hat{\Theta}$
and can be classified by a new topological index, called the $\mathbb{Z}_2$ index. The $\mathbb{Z}_2$ index, $\nu$, is an integer given by the number of edge states modulo $2$ and distinguishes the $\nu = 0$ or insulating phase from the $\nu = 1$, the topological insulator. Thus, the equivalence classes of $T$-invariant Hamiltonians for insulators can be classified by its $n = 0$ Thouless-Kohmoto-Nightingaleden Nijs invariant [i.e. its $C=0$, first Chern index] and the additional index $\nu$. This gives a $\mathbb{Z} \times \mathbb{Z}_2$ symmetry for the $2\text{D}$ band structures.
After all, what can we infer from the Hamiltonian? For example, that we have time reversal invariant electronic states with a bulk electronic band gap that supports the transport of charge and spin in gapless edge states. This is exactly what we get in the so-called quantum spin Hall phase.
