Tuesday, April 10, 2012

Are you impressed or depressed?

Previously I posted about just how hard it is to predict new phases of matter, particularly in a specific material. I more or less claimed this has never be done. I was incorrect. Ben Powell pointed out to me two significant counter examples: Bose Einstein Condensates (BECs) and Topological Insulators. Both represent monumental and profound achievements. But, how impressed (or smug) should we be?

After all, both these examples involve non-interacting particles, or at least particles just interacting at the mean-field level. Hence, this just further underscores to me just how hard it is to actually predict truly emergent phenomena, involving "non-trivial" quantum many-body physics.


  1. I made a prediction about the bismuth iodide molecule as it relates to the extended molecular strongly interacting state.

    It was less than a year later that Alexsey Alexseyev and Robert Buenker of the Buenker group performed and published ab initio determinations of the low lying excited electronic states of this molecule, which more or less confirmed I was in the ballpark. Now I recently read that the breaking the metal metal bonds in IrTe can initiate superconductivity with fluctuating structural order : http://arxiv.org/abs/1204.1421

    I remain optimistic. This is a great result.

  2. A possible counter example: how about the Haldane phase which is an old example of topological order in interacting spin chains for integer spin? As far as I know, it was discovered in the 80's and realized experimentally afterwards, furthermore it (I think) is considered to be a relatively important discovery.

  3. Hi HM,
    You are correct. That is a great counter-example!

  4. Type-II superconductivity was predicted by Abrikosov and only later found to provide the right explanation for a host of previous experiments on hard superconductors. In that sense, too, I'd consider it a theoretical prediction in an experimental vacuum.

  5. Haldane phase is a nice example indeed. Few other examples:

    1. Luttinger liquid physics was verified in experiments long after theoretical predictions. Both Haldane phase and this example point to the fact that interactions are understood in 1-d much better than higher dimensions.

    2. The theory for much of quantum criticality was developed prior to experimental observations. Even the simplest example of 1-d transverse field Ising was seen only lately in experiments with some of the striking predictions like emergent E_8 symmetry verified.

    3. Efimov effect for interacting few body systems was predicted in 70's and verified in cold atoms only in recent years. Indeed, how often one gets to verify the value of e^pi in experiments :) ? True, this is more of "few-body physics" but it's a non-trivial universal result of interactions.

    4. Prediction and observation of fractional charge in fractional quantum Hall systems (hopefully, fractional statistics would also be measured someday soon).