Next state of quantum matter

It would be impossible to deny that in the last year there has been a considerable amount of interest in topological insulators, and it is not hard to see why. These bizarre materials act like insulators within their bulk, while a current is able to flow across the surface. These materials therefore represent a new state of quantum matter. Bi₂Se₃ is one such topological insulator, and one of the most studied. However, it was recently discovered that by adding copper, a superconducting state can be induced within the material.

 

Now, a study headed by researchers at Princeton University and the Advanced Light Source (Berkeley) has resulted in the observation of the topological nature of this exciting material for the first time [Wray et al., (2010) Nat Phys, 6, 855]. According to Dr Andrew Wray, “There has been a global search for a topological insulator that can superconduct and drop the surface resistance to zero. Our measurements show that this has been achieved; that superconductivity can be introduced in the compound Cu_xBi₂Se₃ without destroying the critical topological insulator surface properties”.

 

The groups used angle-resolved photoemission spectroscopy (ARPES) to study the electronic structure of the material. Their results indicate that the copper can add both electrons and holes, depending on where the particular copper atom is situated in the structure. One effect of the copper is that it seems to reduce the velocity of electrons at the surface.

 

Perhaps the most important finding of this new study is that, contrary to previous calculations, the surface of the copper doped material can support superconductor vortices. Such a phenomena is critical if there is to be any hope of detecting Majorana fermions. These particles could exist in superconductors as quasiparticles, and are of value because they are electrically neutral. Their movement is therefore much more predictable than that of charged particles, and so they could eventually form the basis of a topological quantum computer.

 

The exciting results do not stop there, as Wray reveals, “the form of superconductivity in the topologically ordered electronic environment we observed is expected to be highly unusual, and may give rise to a state of matter that has never been seen before, known in theoretical literature as a topological superconductor. This is a model-dependent interpretation, but so far Cu_xBi₂Se₃ is the best candidate material”.

 

Having observed superconductivity, Wray and coworkers are now keen to “explore many other perturbations on the surface, such as magnetism, electric fields, nanoscale interfaces and different quantum-interacting experimental configurations”.