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Register a new userPressure-dependent optical investigations of $alpha$-(BEDT-TTF)$_2$I$_3$: tuning charge order and narrow gap towards a Dirac semimetal
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Infrared optical investigations of $alpha$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T<T_{rm CO}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{rm CO}$ by $8-9$~K/kbar. The charge disproportionation $Deltarho$ diminishes by $0.017~e$/kbar and the optical gap $Delta$ between the bands decreases with pressure by -47~cm$^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.
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We investigate the effect of strong electronic correlation on the massless Dirac fermion system, $alpha$-(BEDT-TTF)$_2$I$_3$, under pressure. In this organic salt, one can control the electronic correlation by changing pressure and access the quantum critical point between the massless Dirac fermion phase and the charge ordering phase. We theoretically study the electronic structure of this system by applying the slave-rotor theory and find that the Fermi velocity decreases without creating a mass gap upon approaching the quantum critical point from the massless Dirac fermion phase. We show that the pressure-dependence of the Fermi velocity is in good quantitative agreement with the results of the experiment where the Fermi velocity is determined by the analysis of the Shubnikov-de Haas oscillations in the doped samples. Our result implies that the massless Dirac fermion system exhibits a quantum phase transition without creating a mass gap even in the presence of strong electronic correlations.
The organic salt $alpha$-(BEDT-TTF)$_2$I$_3$ is considered a model system for metal-insulator transition due to electronic charge ordering at $T_{rm CO}=135$~K. The optical properties obtained from polarized reflection measurements above and below $T_{rm CO}$ can be well described by calculations based on first-principle density-functional theory (DFT). We discuss the effect of electronic correlations on the metal-insulator transition.
The two-dimensional organic conductor $alpha$-(BEDT-TTF)$_2$I$_3$ undergoes a metal-insulator transition at $T_{rm CO}=135$ K due to electronic charge ordering. We have conducted time-resolved investigations of its electronic properties in order to explore the field- and temperature-dependent dynamics. At a certain threshold field, the system switches from low-conducting to a high-conducting state, accompanied by a negative differential resistance. Our time-dependent infrared investigations indicate that close to $T_{rm CO}$ the strong electric field pushes the crystal into a metallic state with optical properties similar to the one for $T>T_{rm CO}$. Well into the insulating state, however, at $T=80$ K, the spectral response evidences a completely different electronically-induced high-conducting state. Applying a two-state model of hot electrons explains the observations by excitation of charge carriers with a high mobility. They resemble the Dirac-like charge-carriers with a linear dispersion of the electronic bands found in $alpha$-(BEDT-TTF)$_2$I$_3$ at high-pressure. Extensive numerical simulations quantitatively reproduce our experimental findings in all details.
The emergence of photo-induced topological phases and their phase transitions are theoretically predicted in organic salt $alpha$-(BEDT-TTF)$_2$I$_3$, which possesses inclined Dirac cones in its band structure. By analyzing a photo-driven tight-binding model describing conduction electrons in the BEDT-TTF layer using the Floquet theorem, we demonstrate that irradiation with circularly polarized light opens a gap at the Dirac points, and the system eventually becomes a Chern insulator characterized by a quantized topological invariant. A rich phase diagram is obtained in plane of amplitude and frequency of light, which contains Chern insulator, semimetal, and normal insulator phases. We find that the photo-induced Hall conductivity provides a sensitive means to detect the predicted phase evolutions experimentally. This work contributes towards developing the optical manipulation of electronic states in matter through broadening the range of target materials that manifest photo-induced topological phase transitions.
(BEDT-TFF)$_2$I$_3$ charge transfer salts are reported to show superconductivity and pressure induced quasi two-dimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases $alpha$, $beta$ and $kappa$. edit{We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and discuss the origin of band crossings. Additionally, we discuss the chemically induced strain in $kappa$-(BEDT-TTF)$_2$I$_3$ achieved by replacing the Iodine layer with other Halogens: Fluorine, Bromine and Chlorine. In the case of $kappa$-(BEDT-TTF)$_2$F$_3$, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the non-symmorphic nature of the crystal.} The calculated electronic structures presented here are added to the organic materials database (OMDB).
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