Signatures of an exotic Mott insulator

There is a really nice preprint Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound κ-(BEDT-TTF)2Cu2(CN)3
by Sebastian Elsasser, Dan Wu, Martin Dressel, John A. Schlueter

The simplest possible picture of charge excitations in a Mott insulator is that it is similar to a band insulator but the origin of the gap is from correlations. Then one expects that

1. There is a non-zero charge gap.
2. The same energy gap occurs in the dc conductivity (activation energy) and the optical conductivity.
3. At finite temperature the subgap absorption in the optical conductivity arises due to thermal excitations across the energy gap. Hence, the subgap absorption decreases with decreasing temperature.

Indeed such behaviour is observed in the organic Mott insulator kappa-(BEDT-TTF)2Cu(NCN)2Cl which has an antiferromagnetic ground state [denoted kappa-Cl below].

However, this preprint reports qualitatively different behaviour in the title compound [denoted kappa-CN below]. Specifically,
A. There is activated behaviour in the dc conductivity.
B. Power law behaviour occurs in the optical conductivity, from about 20 to 1000 cm-1.
C. This "subgap" absorption increases in intensity with decreasing temperature.

Motivated by earlier, but less detailed, measurements showing similar results, Tai-Kai Ng and Patrick Lee, developed a theory which captures some of these properties. The key physics is the following. In the spin liquid state the elementary excitations are spinons which are charge neutral [and have a Fermi surface] and U(1) internal gauge fields associated with spin chirality fluctuations. An external electromagnetic field couples to the gapless spinon excitations indirectly via the internal gauge field. This produces the low frequency absorption.

A couple of cautionary caveats:

a. the experiment does require subtracting off a background signal associated with molecular vibrations, but that does appear robust to me.

b. the observed power law exponent is smaller than predicted by Ng and Lee.

However, doing reliable theory in this regime is so challenging, requiring uncontrolled approximations, I think it is unrealistic to expect this theory to get all the details right.

I thank Martin Dressel for bringing this work to my attention.