Fluctuation-induced quantum phases and fermionic quantum order by disorder

Orateur : Andrew G. GREEN (London Centre for Nanotechnology, UK)

I will discuss how the idea of quantum order by disorder, first developed by Villain, may be used to describe instabilities of quantum critical me tals. The central idea is that the introduction of some order parameter - for example magnetic order (possibly spatially modulated) or superconducting order - modifies the mean field electron dispersion. This in turn modifies the spectrum of low lying particle-hole excitations. In regions of the phase diagram where quantum fluctuations are large, the groundstate of the system may be determined by minimising the zero-point fluctuations of these low lying modes. I will show how this simple idea can be turned into explicit calculations for metallic ferromagnets and review several puzzling experimental results that are captured by these calculations. Following this, I will discuss how these calculations might be extended to other systems and potentially incorporated into numerical/ab initio calculations.

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