Low-Frequency Dynamics of Strongly Correlated Electrons in (BEDT-TTF)2X Studied by Fluctuation Spectroscopy

Abstract

Fluctuation spectroscopy measurements of quasi-two-dimensional organic charge-transfer salts (BEDT-TTF) 2 X are reviewed. In the past decade, the method has served as a new approach for studying the low-frequency dynamics of strongly correlated charge carriers in these materials. We review some basic aspects of electronic fluctuations in solids, and give an overview of selected problems where the analysis of 1/ f-type fluctuations and the corresponding slow dynamics provide a better understanding of the underlying physics. These examples are related to (1) an inhomogeneous current distribution due to phase separation and/or a percolative transition; (2) slow dynamics due to a glassy freezing either of structural degrees of freedom coupling to the electronic properties or (3) of the electrons themselves, e.g., when residing on a highly-frustrated crystal lattice, where slow and heterogeneous dynamics are key experimental properties for the vitrification process of a supercooled charge-liquid. Another example is (4), the near divergence and critical slowing down of charge carrier fluctuations at the finite-temperature critical endpoint of the Mott metal-insulator transition. Here also indications for a glassy freezing and temporal and spatial correlated dynamics are found. Mapping out the region of ergodicity breaking and understanding the influence of disorder on the temporal and spatial correlated fluctuations will be an important realm of future studies, as well as the fluctuation properties deep in the Mott or charge-ordered insulating states providing a connection to relaxor or ordered ferroelectric states studied by dielectric spectroscopy.

Evolution of the structural, magnetic, and electronic properties of the triple perovskite Ba3CoIr2O9

“…In contrast to resistance measurements, fluctuation (noise) spectroscopy is much more sensitive to electronic inhomogeneities and the microgeometry of a material. As pointed out in [63,64] and references therein, an inhomogeneous current distribution, percolation, disorder, or phase separation may lead to strong features in the 1/ f noise, while the resistance remains nearly unaffected. We believe this is the case for Ba 3 CoIr 2 O 9 , where the overall resistivity dynamics does not change as a function of temperature, but the fluctuations increase due to changing phase ratios and increasing lattice distortions.…”

“…These coexisting phases can be assumed to differ in their individual electronic properties. It is well known for other materials with coexisting electronic phases that an inhomogeneous conduction can lead to strong modifications of the measured noise magnitude [63,64]. In contrast to resistance measurements, fluctuation (noise) spectroscopy is much more sensitive to electronic inhomogeneities and the microgeometry of a material.…”

Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr2Se4 with muon-spin spectroscopy and resistance-fluctuation measurements

“…Single crystals of HgCr 2 Se 4 were measured in a continuous heliumflow cryostat with variable temperature insert equipped with a superconducting magnet, using a four-terminal AC technique and a lock-in amplifier. For the resistance noise measurements, after preamplification of the voltage drop across the sample, the signal is demodulated by the lock-in amplifier and the voltage noise power spectral density (PSD), S V ( f , T ), of the fluctuations is processed and recorded by a spectrum analyzer [27,28]. As required, the voltage noise PSD scales as S V ∝ I 2 and care has been taken to rule out spurious noise sources contributing to the measured signal, e.g., from the preamplifier or the electrical contacts.…”

Advances in Organic Conductors and Superconductors

“…The low-temperature anomaly observed in the quantum spin liquid compound κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , however, remains an open issue [6]. In any case, the electrodynamic properties of these organic dimer Mott insulators appear to be fairly well understood by now [7,8]. The Mott transition observed in the two-dimensional organic κ-phase BEDT-TTF salts is the focus of the detailed pressure-dependent measurements of the heat capacity [9] and thermal expansion [10].…”