Wednesday, September 23, 2009

More on Spin-Charge Separation

It appears that a lot of new results are coming out on this lately. A while back I reported on the possibility of a clearer observation of spin-charge separation in a 1D system via the tunneling phenomenon.

This time, the observation of spin-charge separation comes from photoemission spectroscopy. The link in the article also gives you free access to the publication. What is interesting here is that they may have found something that isn't consistent with the Luttinger Liquid theory that describes such 1-D system and spin-charge separation.

These findings are surprising, given the generality of the previous argument and the robustness of Luttinger-liquid physics. If the relation between η and ν would hold, the positive value of η would imply a very fast decay of the single-particle correlation function (i.e., a much larger exponent ν) than anticipated, or indeed directly measured. There could be several ways out of this predicament. The simplest one would be some experimental artifact or surface problem, but that hardly seems compatible with the good quality of the data, the observation of the momentum dependence, the observed scaling, and the agreement between the ARPES and STM measurements. Salvation could come from the theory side: the fact that the material is not a system that can be directly mapped to a single-chain one-dimensional system, but rather to a double-chain one—a ladder system. Those systems are known to develop gaps in their excitation spectrum, in contrast to single-chain ones. Such gaps would be compatible with rapid decay of the single-particle correlations. Of course this would not explain the measured value of ν, or the more severe catch: such gaps should normally be seen in both STM and ARPES, and none have been observed at the relevant energy scales here. Other routes, such as disorder, can be explored but, as of today, the question remains.

I love surprises like this. It means that there's a lot more physics to be done and studied.


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