The organic conductor alpha-(BEDT-TTF)2I3 provides the prime example of a charge-order-driven metalinsulator transition. Restricted chemical substitution of S atoms by Se in the constituent molecules allows us to modify the electronic properties. This not only decreases the transition temperature but, in addition, alters the phase transition mechanism, resulting in the ground state deviating from the charge-ordered insulator state of the parent compound. Employing infrared optical spectroscopy, we investigate changes in the charge dynamics. Furthermore, we demonstrate the absence of charge ordering in the Se-substituted materials and suggest that the phase transition is instead driven by the localization of the itinerant charge carriers due to strong electron-phonon interactions.

Correlation-driven metal-insulator phase transitions are supposed to be of second order. Many experimental observations, however, appear to disagree with this classification. Using generalized ellipsometry, we have studied prominent examples of two-dimensional charge-transfer salts with a pronounced charge-order transition: alpha-(BEDT-TTF)2I3 and its selenium-substituted analogs, where effective correlations are reduced. Mueller matrix spectroscopic and temperature-dependent ellipsometry allows us to determine the anisotropic dielectric functions of these molecular materials across their metal-insulator transitions. Applying an effective-medium approximation, the nature of the metal-insulator phase transition of alpha-(BEDT-TTF)2I3 and its selenium-substituted salts is uncovered. Our findings confirm phase coexistence near the transition in alpha-(BEDT-TTF)2I3, establishing its first-order nature, while the substituted salts exhibit a more gradual evolution, indicative of a continuous phase transition. Through this comprehensive approach, generalized ellipsometry unravels the nature of the correlation-driven metal-insulator transition.

Determining the optical and electronic properties of strongly anisotropic materials with symmetries below orthorhombic remains challenging; generalized ellipsometry is a powerful technique in this regard. Here, we employ Mueller matrix spectroscopic and temperature-dependent ellipsometry to determine the frequency dependence of six components of the dielectric-function tensor of the two-dimensional charge-transfer salt α-(BEDT-TTF)2I3 across its metal–insulator transition. Our results offer valuable insights into temperature-dependent changes of the components of the spectroscopic dielectric-function tensor across the metal–insulator transition. This advanced method allows extension to other electronic transitions.

Tomislav Ivek, Matija Čulo, ∗ Marko Kuveždić, Eduard Tutǐs, Mario Basletić, Branimir Mihaljević, Emil Tafra, Silvia Tomić, Anja Löhle, Martin Dressel, Dieter Schweitzer, and Bojana Korin-Hamzić Institut za fiziku, P. O. Box 304, HR-10001 Zagreb, Croatia Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany (Dated: April 30, 2019)

alpha-(BEDT-TTF)(2)I-3 is a prominent example of charge ordering among organic conductors. In this work, we explore the details of transport within the charge-ordered as well as semimetallic phase at ambient pressure. In the high-temperature semimetallic phase, the mobilities and concentrations of both electrons and holes conspire in such a way to create an almost temperature-independent conductivity as well as a low Hall effect. We explain these phenomena as a consequence of a predominantly interpocket scattering which equalizes mobilities of the two types of charge carriers. At low temperatures, within the insulating charge-ordered phase two channels of conduction can be discerned: a temperature-dependent activation, which follows the mean-field behavior, and a nearest-neighbor-hopping contribution. Together with negative magnetoresistance, the latter relies on the presence of disorder. The charge-ordered phase also features a prominent dielectric peak which bears a similarity to relaxor ferroelectrics. Its dispersion is determined by free-electron screening and pushed by disorder well below the transition temperature. The source of this disorder can be found in the anion layers which randomly perturb BEDT-TTF molecules through hydrogen bonds.

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-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-CO by 8-9 K/kbar. The charge disproportionation Delta rho diminishes by 0.017e/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 electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

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.

A detailed investigation of the out-of-plane electrical properties of charge-ordered $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{I}}_{3}$ provides clear evidence for ferroelectricity. Similar to multiferroic $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}[\mathrm{N}(\mathrm{CN})}_{2}]\mathrm{Cl}$, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Dielectric response typical for relaxor ferroelectricity is found deep in the charge-ordered state. We suggest an explanation in terms of the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.

A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)(2)I-3 provides clear evidence for ferroelectricity. Similar to multiferroic kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Dielectric response typical for relaxor ferroelectricity is found deep in the charge-ordered state. We suggest an explanation in terms of the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.

A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)2I3 provides clear evidence for ferroelectricity. Similar to multiferroic alpha-(BEDT-TTF)2Cu[N(CN)2]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Dielectric response typical for relaxor ferroelectricity is found deep in the charge-ordered state. We suggest an explanation in terms of the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.

The Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl consists of molecular dimers arranged on an anisotropic triangular lattice. At low temperatures a pronounced dielectric anomaly is observed, and eventually a canted antiferromagnetic ground state forms. Optical spectroscopy clearly rules out charge imbalance and the existence of quantum electric dipoles with a dipolar-spin coupling. Here we suggest a novel form of spin–charge coupling where the prominent in-plane dielectric response in κ-(BEDT-TTF)2Cu[N(CN)2]Cl is explained by short-range discommensurations of the antiferromagnetic phase in the temperature range 30 K < T < 50 K, and by relaxation of charged domain walls in the ferromagnetic structure at lower temperatures.

We present ultra-high-resolution dilatometric studies in magnetic fields on a quasi-two-dimensional organic conductor $\kappa$-(D8-BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, which is located close to the Mott metal-insulator (MI) transition. The obtained thermal expansion coefficient, $\alpha(T)$, reveals two remarkable features: (i) the Mott MI transition temperature $T_{MI}$ = (13.6 $\pm$ 0.6)\,K is insensitive to fields up to 10\,T, the highest applied field; (ii) for fields along the interlayer \emph{b}-axis, a magnetic-field-induced (FI) phase transition at $T_{FI}$ = (9.5 $\pm$ 0.5)\,K is observed above a threshold field $H_c \sim$ 1 T, indicative of a spin reorientation with strong magneto-elastic coupling.

We study the low‐frequency dynamical properties of correlated charge carriers in various of the quasi‐two‐dimensional organic charge‐transfer salts κ‐(BEDT‐TTF) 2 X by means of fluctuation (noise) spectroscopy. Close to the critical endpoint of the Mott metal‐insulator transition, a pronounced increase of the 1/ f ‐noise level accompanied by a substantial shift of spectral weight to low frequencies indicates a sudden increase of the time scale of the charge fluctuations. For the less correlated, more metallic materials, we find a crossover/transition from hopping transport of more‐or‐less localized carriers at elevated temperatures to a low‐temperature regime, where a metallic coupling of the layers allows for coherent interlayer transport of delocalized electrons.

Electric-field-dependent pulse measurements are reported in the charge-ordered state of alpha-(BEDT-TTF)2I3. At low electric fields up to about 50 V/cm only negligible deviations from Ohmic behavior can be identified with no threshold field. At larger electric fields and up to about 100 V/cm a reproducible negative differential resistance is observed with a significant change in shape of the measured resistivity in time. These changes critically depend whether constant voltage or constant current is applied to the single crystal. At high enough electric fields the resistance displays a dramatic drop down to metallic values and relaxes subsequently in a single-exponential manner to its low-field steady-state value. We argue that such an electric-field induced negative differential resistance and switching to transient states are fingerprints of cooperative domain-wall dynamics inherent to two-dimensional bond-charge density wave with ferroelectric-like nature.

The Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl consists of molecular dimers arranged on an anisotropic triangular lattice and develops a canted antiferromagnetic ground state. It has recently been suggested that this system features purely electronic ferroelectricity which requires an electric dipole moment. Optical spectroscopy clearly rules out charge imbalance in this system, which excludes the existence of quantum electric dipoles on the dimers and subsequently a dipolar spin coupling. We suggest that the prominent in-plane dielectric response in κ-(BEDT-TTF)2Cu[N(CN)2]Cl is due to short-range discommensurations of the antiferromagnetic phase in the temperature range 30 < T < 50 K, and domain wall relaxations at lower temperatures.

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate strength ( Δε ≈ 3 × 10 3 ) emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively associate this phase, whose tiny contribution was sample dependent, with a Cu 2+ magnetic subsystem.

Crystals of kappa-[(BEDT-TTF)(2)][Cu(N(CN)(2))Br] were available with good quality and high reproducibility from the BEDT-TTF electro-oxidation with an electrolyte molar ratio [Ph4P][N(CN)(2)]:CuBr of 1:1 in a mixture of THF and ethylene glycol. Surprisingly, by varying the electrolyte molar ratio from 1:1 to 1:8 a novel type of BEDT-TTF salt, [BEDT-TTF](2)[Cu2Br3], was formed. The solid-state structure of [BEDT-TTF](2)[Cu2Br3] consists of stacks of non-coordinated BEDT-TTF molecules and stacks of the contact ions [BEDT-TTF][Cu2Br3] (triclinic space group, P1). The two crystallographically independent [BEDT-TTF][Cu2Br3] contact ions in the second stack are related by a pseudo-inversion center. Resistivity measurements on single crystals reveal that [BEDT-TTF](2)[Cu2Br3] possesses a semiconductor-like behavior under normal pressure from room temperature down to at least 10 K.

The anisotropy of the electrical transport properties at room temperature of the α- and β-phase of (BEDT-TTF)2I3 was determined from the polarized reflectance spectra.

We report on the anisotropic response, the charge and lattice dynamics of normal and charge ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(BEDT-TTF)(2)I-3 determined by dc resistivity and dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature-dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations nu(27)(B-u) evidences the abrupt onset of static charge order below T-CO = 136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens at approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength Delta epsilon(LD) approximate to 5000 and Delta epsilon(SD) approximate to 400, centered at 1 kHz < nu(LD) < 100 MHz and nu(SD) approximate to 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phasonlike excitation is best described as a long-wavelength excitation of a 2k(F) bond charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectriclike nature of the charge ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.

We investigate the dynamics of correlated charge carriers in the vicinity of the Mott metal-to-insulator transition in various of the title quasi-two-dimensional organic charge-transfer salts by means of fluctuation (noise) spectroscopy. The observed $1/f$-type fluctuations are quantitatively very well described by a phenomenological model based on the concept of non-exponetial kinetics. The main result is a correlation-induced enhancement of the fluctuations accompanied by a substantial shift of spectral weight to low frequencies close to the Mott critical endpoint. This sudden slowing down of the electron dynamics may be a universal feature of metal-to-insulator transitons. For the less correlated metallic materials, we find a crossover/transition from hopping transport of more-or-less localized carriers at elevated temperatures to a low-temperature regime, where a metallic coupling of the layers allows for coherent interlayer transport of delocalized electrons.