Energy scales for electronic noise processes in the quasi-two-dimensional organic Mott system $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl

TL;DR

This study uses resistance noise spectroscopy on a quasi-two-dimensional organic Mott insulator to reveal inhomogeneous electronic fluctuations and their temperature dependence, highlighting intrinsic inhomogeneity and molecular conformational effects.

Contribution

It provides the first detailed analysis of electronic noise scales and their relation to molecular conformations in $$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl under varying pressure conditions.

Findings

Electronic noise is significantly enhanced, indicating intrinsic inhomogeneity.

The noise spectra follow a $1/f^\u03b1$ form across temperatures.

A peak at 100 K relates to molecular conformational degrees of freedom.

Abstract

Resistance noise spectroscopy is applied to bulk single crystals of the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl both under moderate pressure and at ambient-pressure conditions. When pressurized, the system can be shifted to the inhomogeneous coexistence region of antiferromagnetic insulating and superconducting phases, where percolation effects dominate the electronic fluctuations [J. M\"uller {\it et al.}, Phys. Rev. Lett. {\bf 102}, 047004 (2009)]. Independent of the pressure conditions, at higher temperatures we observe generic $1/f^\alpha$-type spectra. The magnitude of the electronic noise is extremely enhanced compared to typical values of homogeneous semiconductors or metals. This indicates that a highly inhomogeneous current distribution may be an intrinsic property of organic charge-transfer salts. The temperature dependence of the nearly $1/f$ spectra can be very well described by a generalized random fluctuation model [Dutta, Dimon, and Horn, Phys. Rev.\ Lett. {\bf 43}, 646 (1979)]. We find that the number of fluctuators and/or their coupling to the electrical resistance depends on the temperature, which possibly relates to the electronic scattering mechanisms determining the electrical resistance. The phenomenological model explains a pronounced peak structure in the low-frequency noise at around 100 K, which is not observed in the resistivity itself, in terms of the thermally-activated conformational degrees of freedom of the ET molecules' ethylene endgroups.