Fine-tuning the Mott metal–insulator transition and critical charge carrier dynamics in molecular conductors

Abstract

The unique possibilities of fine-tuning their physical properties in the vicinity of the Mott metal–insulator transition make the quasi-two-dimensional organic charge-transfer salts -(BEDT-TTF)X unprecedented model systems for studying the fundamentals of electron–electron correlations and the coupling between charge, spin and lattice degrees of freedom in reduced dimensions. The critical properties and the universality class of the Mott transition, however, are controversially debated for these materials, and information on the low-frequency dynamical properties of the correlated electrons is rather limited. By introducing fluctuation (noise) spectroscopy as a powerful new tool for studying the slow dynamics of charge carriers, in the past years we have been able to extract spectroscopic information on the coupling of charge carriers to the vibrational degrees of freedom of the crystal lattice. This is related to a glassy freezing of the BEDT-TTF molecules’ ethylene end-group (EEG) rotations at elevated temperatures, which (i) results in a small amount of (intrinsic) disorder and (ii) crucially influences the ratio of bandwidth to on-site Coulomb repulsion (W / U) and therefore the samples’ position in the phase diagram, i.e. the electronic ground state. The low-frequency resistance fluctuations show a dramatic enhancement and divergent behaviour when tuning the sample close to the critical point of the Mott transition, accompanied by a strong shift of spectral weight to low frequencies and the onset of non-Gaussian behaviour. This indicates the critical slowing down of the order-parameter (doublon density) fluctuations and suggests a collective dynamics of the correlated electrons. In order to enable detailed investigations of this hypothesis in future experiments, by exploiting the structural EEG relaxation, a ‘warming cycle’ protocol can be established that allows for fine-tuning the sample across the Mott transition and therefore precisely accessing the finite-temperature critical endpoint. We ‘calibrate’ this procedure by a comparison to pressure-tuning experiments on the same sample. This method will allow to map out the region of ergodicity breaking around the critical endpoint and its dependence on disorder.

Keywords:

• Organic charge transfer salts
• Mott metal–insulator transition
• critical endpoint
• fluctuation (noise) spectroscopy
• Mott–Anderson scenario

Acknowledgements

We are grateful to M. Lang for support with the He-gas pressure experiments.

Notes

No potential conflict of interest was reported by the authors.

1 Alternatively, the fluctuation-dissipation theorem relates the noise PSD of a quantity x to the imaginary part of the complex susceptibility , where the real function is the response function of the system [45]: .

2 Note that the phase diagram shown in Figure 2(a) is schematic. The spread in the points marking the critical endpoint reflects sample-to-sample dependences. for our sample is lower than indicated.

3 For these systems, however, the divergence of slow fluctuations is observed only in the limit , whereas in the present case of -(ET)X diverging -type noise occurs at a finite-temperature critical point.

4 A 3-terminal wiring is used, hence the resistance does not reach zero.

5 The pressure experiments have been performed in a different cryogenic system. Due to somewhat different heat-flow conditions, see inset of Figure 3(a), the nominal same procedure results in a slightly different absolute value of the resistance at low temperatures.

6 Note that stretched exponential behaviuor is a characteristic of glassy relaxation processes.

Additional information

Funding

This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the collaborative research center SFB/TR 49. Part of the work was supported by JSPS KAKENHI [grant number JP16H00954], [grant number JP15K1351], [grant number JP25287080].