Evolution of the charge carrier properties and electronic correlations in layered organic metals near the Mott metal-insulator transition
Final Report Abstract
In this project we have studied several κ-type organic charge transfer salts as model systems for tracking the evolution of the charge carrier properties in a metal with a half-filled conduction band approaching the Mott metal-insulator transition (MIT). The main objective of the project was the disentanglement of contributions of the charge, spin, and lattice degrees of freedom to the insulating instability of the metallic ground state regulated by applying pressure, “chemical pressure” or varying disorder potential. The main results of the study can be summarized as follows. 1. Magnetic quantum oscillations and anisotropic magnetoresistance experiments on high-quality κ-Cl crystals, fine-tuned with respect to the MIT by precisely controlled pressure, have revealed fully coherent charge carriers with a large Fermi surface persisting till the very edge of stability of the metallic ground state. No signatures of a pseudogap or a noticeable reduction of the quasiparticle lifetime induced by the Mott insulating instability have been detected. 2. The quantum oscillation technique has proved extremely useful for evaluation of the key parameters governing the MIT in our materials, – the electronic correlation strength ratio U/t and spin frustration ratio t ′ /t. While the correlation strength is known to be sensitive to pressure, our quantitative analysis reveals an unexpectedly high sensitivity, exceeding that inferred from first-principle band-structure calculations by an order of magnitude. This stark discrepancy between the theory and experiment seems to have a universal character and thus challenges our present understanding of the correlation effects on the band structure near the bandwidth-controlled MIT. Further on, the spin frustration, determined by the in-plane electronic anisotropy, is also found to modify even under hydrostatic pressure. 3. We have investigated the ”chemical pressure” effect in κ-(BEDT-TTF)2 X induced by (i) isoelectronic anion substitution and (ii) replacement of all the hydrogen atoms in terminal ethylene groups (TEG) of BEDT- TTF by deuterium. In the former case, neither of the substitutions works as an analog of hydrostatic pressure. In the row of the κ-Cl, -Br, and -NCS salts only the spin frustration degree varies, whereas the electronic correlations remain essentially the same. Going to κ-CN results in a significant change of both factors; however, the signs of the changes are different from those obtained under pressure. On the other hand, deuteration is found to enhance the correlation strength to the extent consistent with the enhancement of the insulating instability and thus can be considered as an analog of negative pressure. 4. A potential impact of the conformational (dis)order of the TEG on the electronic correlations in κ-Br has been checked by measuring the influence of thermal history on the effective mass. Despite a very strong effect of thermal history on the low-temperature transport properties, the effective mass and hence, the correlation strength remain practically unchanged. Thus, the mechanism controlling the insulating instability in this case is most likely associated with a scattering potential. 5. Significant advance in understanding the electronic states on both sides from the MIT in the relatively new member of the Mott-insulating κ-salt family,κ-(BETS)2 Mn[N(CN)2 ]3 , has been achieved. The analysis of the magnetic properties in the insulating state give strong arguments for a chiral magnetic ordering of the spin-vortex crystal type, a parent state of a chiral quantum spin-liquid in the π-electron system, whereas fluctuation and dielectric spectroscopy experiments indicate a relaxor-type electronic ferroelectricity setting in at the MIT. Thus, the compound can be regarded as a new multiferroic candidate. The behaviour of the electronic correlations under pressure, revealed by quantum oscillation measurements is essentially the same as that in the κ-(BEDT-TTF)2 X salts, thus confirming the Mott instability as a driving mechanism of the MIT at low temperatures.
Publications
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Spin Vortex Crystal Order in Organic Triangular Lattice Compound. Physical Review Letters, 127(14).
Riedl, Kira; Gati, Elena; Zielke, David; Hartmann, Steffi; Vyaselev, Oleg M.; Kushch, Nataliya D.; Jeschke, Harald O.; Lang, Michael; Valentí, Roser; Kartsovnik, Mark V. & Winter, Stephen M.
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Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition. Physical Review B, 107(7).
Oberbauer, S.; Erkenov, S.; Biberacher, W.; Kushch, N. D.; Gross, R. & Kartsovnik, M. V.
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Electronic correlations and spin frustration in the molecular conductors κ-(BEDT-TTF) ₂X probed by magnetic quantum oscillations. Physical Review B, 110(20).
Erkenov, S.; Fust, S.; Oberbauer, S.; Biberacher, W.; Kushch, N. D.; Müller, H.; Pratt, F. L.; Gross, R. & Kartsovnik, M. V.
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Slow and non-equilibrium dynamics due to electronic ferroelectricity in a strongly-correlated molecular conductor. npj Spintronics, 2(1).
Thomas, Tatjana; Agarmani, Yassine; Hartmann, Steffi; Kartsovnik, Mark; Kushch, Natalia; Winter, Stephen M.; Schmid, Sebastian; Lunkenheimer, Peter; Lang, Michael & Müller, Jens
