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(Color online) Schematic drawing showing how the development of a positive π charge in the central core of the donor and a positive σ charge in the hydrogen making an hydrogen bond with the donor are interrelated through a negative σ charge shift. Note that the hydrogen bonding on the left and right sides of BEDT-TTF are not necessarily identical.
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α-(BEDT-TTF)2I3 exhibits a metal to insulator transition around 135Kthat has been ascribed to charge ordering in the donor layers containing three different donors (A-A dimers, B and C). First-principles density-functional theory (DFT) calculations provide a description of the electronic structure of this system in agreement with the presently avai...
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... it may seem unnatural that the charge in the π electron cloud of the inner TTF part of the donor may be affected by the strength of the hydrogen bond at the outer part of the donor. The reason is that the electron density associated with the hydrogen bonding is part of the σ skeleton of the molecule. However, as schematically illustrated in Fig. 7, an increase of the positive charge in the hydrogen atom involved in an hydrogen bond with the anion is coupled with an increase of the positive charge in the inner π system via the polarization induced in the σ electron density. Thus an anion shift toward the donor, which increases the strength of the hydrogen bonding, makes the ...
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... of the hydrogen bonding for the different donors and, consequently, should lead to a modification of the charges. Note that donor A is associated with the shorter hydrogen bonds of each type (2.807 versus 2.823Å823˚823Å and 2.895 versus 2.908Å908˚908Å) whereas the opposite is true for donor A . According to the mechanism outlined before (see Fig. 7), this means that the hydrogen bonding associated with donor A will increase in a very sizable way so that the positive charge associated with the π system (and consequently, the total charge) of the molecule will increase. This is indeed what is found in the calculations since the charge of donor A changes from + 0.521 at room ...
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... is neatly smaller than it was for donor A. Consequently, a considerably smaller charge increase should be expected. Again, this is also clearly reflected in the calculations that indicate a change from + 0.546 at room temperature to + 0.577 below the transition. Thus the structural analysis and the mechanism of charge redistribution outlined in Fig. 7 provide a simple yet detailed understanding of the redistribution of holes as a result of the ...
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... hydrogen bonds with donors A, finds the way to compensate for the unfavorable "abnormal" contraction by destroying the inversion centers. The equivalence of the two donors A is thus lost so as to globally increase the net hydrogen bonding between the zigzag I − 3 chains 1 with these two donors. This results, through the polarization mechanism of Fig. 7, with a redistribution of holes within the A-A dimers so that whereas donor A, which is associated with the stronger hydrogen bonds, becomes more positively charged, donor A , associated with weaker hydrogen bonds, becomes less positively charged. This leads to a charge ordering with an horizontal stripe ...
Similar publications
A first-principles density functional theory (DFT) study of theta -(BEDT-TTF)2X molecular conductors with X = I3, CsCo(SCN)4 (ambient pressure, 7.5 kbar and 10 kbar), CsZn(SCN)4,TlCo(SCN)4, RbCo(SCN)4 and RbZn(SCN)4 (220 K and 90 K) is reported. It is shown that these salts exhibit three different types of band structure each of them associated wit...
Citations
... In the realm of quasi-two-dimensional organic conductors [2,3], the presence of a two dimensional Dirac fermion system has been unveiled in α-(BEDT-TTF) 2 I 3 through tight-binding model analysis [4] and the first-principles band calculations [5,6], and the existence of the Dirac point at the Fermi energy (ε F ) has been also experimentally confirmed through the analysis of transport quantities [7][8][9] and thermodynamic quantities such as electronic specific heat [10]. Similarly, for α-(BETS) 2 I 3 it has been recently revealed via firstprinciple calculations [11][12][13] and through magnetoresistance measurements [14]. ...
Dirac points in two-dimensional massless Dirac fermions are topologically protected. Although single Dirac point cannot disappear solely, a pair of two Dirac points annihilates after merging at a time-reversal invariant momentum (TRIM). This process triggers a topological phase transition. In this paper, we numerically calculate the electronic specific heat ($C$) of the systems with a honeycomb lattice and $\alpha$-(BEDT-TTF)$_2$I$_3$ in the case that two Dirac points are moving and merging by changing the ratio of the magnitude between the transfer integrals, which can be controlled by uniaxial pressure for example. When two Dirac points are close to but not at TRIM, the temperature dependence exhibits a crossover from $C\propto T^{2.0}$ (expected for separated Dirac points) at low temperatures ($T\leq T_{\rm co}$) to $C\propto T^{1.5}$ (expected in the case of the merged Dirac points) at high temperatures ($T\geq T_{\rm co}$). Here, $T_{\rm co}$ denotes the crossover temperature, which is determined by the potential barrier magnitude between two Dirac points. Our findings demonstrate that the precursor of the topological phase transition is observed through the temperature-dependence of the electronic specific heat.
... Particularly in κ-(ET) 2 Cu[N(CN) 2 ]I, the conformation of the ethylene groups changes the ground state [15,16]. Moreover, hydrogen bonding between protons of the ethylene groups and the anion has been discussed [17,18]; therefore, the relationships between the dynamics of ethylene groups and the interlayer interactions should be addressed. ...
We present the results of $^{69,71}$Ga-NMR measurements on an organic antiferromagnet $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$ [BEDSe-TTF=bis(ethylenediseleno)tetrathiafulvalene], with comparison to reports on $\lambda$-(BETS)$_2$GaCl$_4$ [BETS=bis(ethylenedithio)tetraselenafulvalene] [T. Kobayashi et al., Phys. Rev. B 102, 235131 (2020)]. We found that the dynamics of two crystallographically independent ethylene groups induce two types of quadrupolar relaxation in the high-temperature region. As the ethylene motion freezes, hyperfine (HF) interactions develop between $\pi$ spin and Ga nuclear spin below 100 K, and thereby magnetic fluctuations of the $\pi$-spin system are detected even from the Ga site. The HF interaction in $\lambda$-(BETS)$_2$GaCl$_4$ was more than twice as large as in $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$, implying that the short contacts between Cl atoms and the chalcogens of fulvalene part are essential for the transferred HF interaction. We propose that NMR using nuclei in anion layers is useful for studying interlayer interactions in organic conductors, which have not been studied experimentally. In addition, because the mechanism of the transferred HF interaction is considered to be the same as $\pi$-$d$ interaction in isostructural Fe-containing $\lambda$-type salts, our findings aid in the understanding of their physical properties.
... The transition temperature T CO is, nevertheless, little affected, showing the cohesive properties in the conduction layers are substantially retained. Such a disorder effect, however, should affect on the transport properties seriously [30]. In addition, the high-temperature phase of α-(ET) 2 I 3 above T CO is semimetallic, consisting of small number of electrons and holes (n e ≈ n h ≈ 10 18 cm −3 ) with high mobility (µ e ≈ µ h ≈ 10 2 cm 2 V −1 s −1 ), where n e(h) and µ e(h) are the carrier density and the mobility of the electrons (holes), respectively [18]. ...
... Although the conventional band picture is not applicable to the present charge order system with strong electron correlation, we speculate that the edge of the DOS, where the carriers are thermally excited, may show a steep change as a function of energy owing to the two dimensionality, leading to the enhanced thermopower. This effect can be enhanced by the in-plane anisotropy, or pseudo-1D character at the bottom of the upper band (electron carriers), as pointed out by the calculations [30]. ...
Confined electrons in low dimensions host desirable material functions for downscaled electronics as well as advanced energy technologies. Thermoelectricity is a most fascinating example, since the dimensionality modifies the electron density of states dramatically, leading to enhanced thermopower as experimentally examined in artificial two-dimensional (2D) structures. However, it is still an open question whether such an enhanced thermopower in low dimensions is realized in layered materials with strong 2D characters such as cuprates. Here, we report unusual enhancement of the thermopower in the layered organic compound $\alpha$-(BEDT-TTF)$_2$I$_3$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the slope in the Jonker plot (thermopower $S$ vs. logarithm of electrical conductivity $\log\sigma$) for $\alpha$-(BEDT-TTF)$_2$I$_3$ is significantly larger than that of conventional semiconductors. Moreover, the large slope is also seen in the related layered salt, demonstrating the impact of the 2D-confined carriers in the layered organics on thermoelectricity.
... While these theories consider only interactions within the intra-ET layers, some approaches actively incorporate the role of the anion layer through hydrogen bonding. Reference [33] pointed out the significant role of the H-I −1/2 σ-bond between the anion and edge protons in the ET molecule. Since the energy difference between the σ-bond and π-electrons of donor molecules are far apart, the σ hybridization is not expected to directly cause CO. ...
... This argument is consistent with our observations. Reference [33] argues that the H-I −1/2 hybridization between the I −1 3 anions and edge protons in ET molecules is the crucial source of the CO. As shown in Fig. 3(b), potential protons that hybridize with both I1 and I3 are equally the edge protons of B molecules. ...
We present $^{127}$I nuclear quadrupole resonance spectra and nuclear relaxation of $\alpha$-(BEDT-TTF)$_2$I$_3$ that undergoes a charge ordering transition. Only one of the two I$_3$ anion sites shows a significant differentiation in the electric field gradients across the first-order transition, which shows that the anion-donor hybridization through hydrogen bonding is a secondary contribution to the charge ordering. The dominating source for the nuclear relaxation is the local libration of the I$_3$ anions, but an anomalous peak is detected just below the transition, as observed by $^{13}$C NMR.
... At ambient pressure, charge-ordering develops below the metal-insulator phase transition T CO = 135 K with inversion symmetry broken and charge disproportionated between neighboring sites. Increasing pressure suppresses charge order and a massless Dirac fermions phase emerges, thus making α-(BEDT-TTF) 2 I 3 the first bulk material, in which the impact of electron correlations on the Dirac-point conductance can be studied [132,138,139] (see Section 3.3.2). ...
... According to the density functional calculations [138], as well as extended Hückel molecular-orbital calculations [48], the system is a semi-metal with very small electron and hole pockets; this results in an experimentally observed weakly metal-like conductivity within the molecular plane [52,53,133]. The direct experimental proof for semimetallicity is provided by recent Hall effect and magnetoresistance measurements, which show that dc transport is governed by the high mobility of electrons and holes resulting in an almost temperature-independent conductivity. ...
... Resolving the associated splitting indicates that charge imbalance is present only within stack II and is rather small 2δ ρ < 0.2e ( Figure 15); a similar result is deduced from x-ray scattering measurements [115,154]. The values of charge density at different molecular sites A, A , B and C agree well with results obtained by density functional theory (DFT) calculations: +0.52e (A, A ), +0.55e (B) and +0.38e (C) [138,164]. ...
This review provides a perspective on recent developments and their implications for our understanding of novel quantum phenomena in the physics of two-dimensional organic solids. We concentrate on the phase transitions and collective response in the charge sector, the importance of coupling of electronic and lattice degrees of freedom and stress an intriguing role of disorder. After a brief introduction to low-dimensional organic solids and their crystallographic structures, we focus on the dimensionality and interactions and emergent quantum phenomena. Important topics of current research in organic matter with sizeable electronic correlations are Mott metal-insulator phase transitions, charge order and ferroelectricity. Highly frustrated two-dimensional systems are established model compounds for studying the quantum spin liquid state and the competition with magnetic long-range order. There are also unique examples of quantum disordered state of magnetic and electric dipoles. Representative experimental results are complemented by current theoretical approaches.
... 4 α-(BEDT-TTF) 2 I 3 is the first realization of a two-dimensional multilayer massless Dirac fermion bulk system. 5 The Dirac-fermion state has been proposed via different theoretical approaches [5][6][7][8][9] and hints of the Dirac state were given by various experimental methods, such as dc transport, 10,11 interlayer magnetoresistance, 12,13 quantum Hall effect, 14 optical, 15,16 and nuclear magnetic resonance (NMR) spectroscopy. 17,18 The exotic behavior of the Dirac electrons is distinct from two-dimensional graphene in many aspects and a complete understanding is still lacking. ...
... 39 For our estimation the dielectric constant ϵ 1 % 4 was directly extracted from the highfrequency limit (See Supplementary Section 3); the Fermi velocity v F ≈ 2.4-10 × 10 4 ms −1 was taken from refs. 8,10,17 . With the assumption of N = 2, we obtain Δ ≈ 300 cm −1 , in good agreement with our optical data. ...
Dirac fermions attract considerable interest for several years and tremendous efforts are devoted to unveil the Dirac/Weyl semimetallic state in real crystalline systems. The behavior of Dirac fermions under strong correlations and in the proximity of other ordered states is under particular scrutiny as robust experimental signatures are scarce. α-(BEDT-TTF)2I3 constitutes a superior model in this regard because the Dirac state occurs next to an electronically ordered ground state enabling us to investigate and deliberately vary the exotic properties in correlated Dirac fermions. The charge-ordered insulator gradually evolves to a metal when pressure is applied, and at low temperatures the electronic bands form tilted Dirac-like cones. Here, we present systematic low-temperature infrared experiments on α-(BEDT-TTF)2I3 in an extended pressure range. A metallic state with a frequency-independent optical conductivity indicates the coexistence of the trivial and massless Dirac electrons. We discover the opening of an energy gap due to correlated Dirac fermions at the boundary to the insulating state; it is gradually suppressed when pressure increases. The unique possibility of tuning the correlated Dirac state provides unprecedented insight into this novel electronic state and yields information relevant for Dirac electron systems in general.
... These three kappa-BEDT-TTF materials, as well as other BEDT-TTF compounds are not simple molecular crystals, rather they should be classified as complex chemically bound charge transfer or cation-anion bulk solids. The previous DFT studies on different organic charge-transfer salts have neither included nor discussed the van der Waals (vdW) interactions at all [64][65][66][67][68][69][70]. The only exception is found in our work [23,24,39]. ...
Organic layered charge-transfer salts κ -(BEDT-TTF) 2 X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts κ -(BEDT-TTF) 2 Cu 2 (CN) 3 and κ -(BEDT-TTF) 2 Ag 2 (CN) 3 have been considered as prime candidates for a quantum spin liquid, while κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl has been suggested as a prototypical charge-order-driven antiferromagnet. In this paper, we summarize and discuss several key results, including some not reported previously, obtained in search to clarify the competition of these two ground states. The origin of anomalous dielectric response found at low temperatures in all three salts is also discussed. We conclude by pointing out the relevant new insights into the role of frustration and random disorder in the suppression of magnetic ordering and formation of the spin liquid state.
... Concerning the hydrogen bonds between the electropositive H atoms from the ethylene groups and the electronegative atoms of the anionic stack, their presence in organic systems is very frequent. It is an important structural effect that controls the packing of the BEDT-TTF layer and which is essential to stabilize various instabilities such as the CO ground state [57,58]. In the case of κ-ET-Cu, hydrogen bonds between the electropositive H atoms of the ethylene groups and the electronegative N or C atoms of the anionic stack are shown in Figure 3a. ...
... In order to further check the possibility of a sizeable charge disproportionation between dimers, we studied the hydrogen bond network between the anionic and donor layers. In previous studies of α-(BEDT-TTF) 2 I 3 [58] and θ-(BEDT-TTF) 2 MM'(SCN) 4 [60], it was shown that the hydrogen bond distance between the terminal C 2 H 4 groups of BEDT-TTF and the anion reflects the degree of charge of the donor as a result of a subtle polarization mechanism detailed in [58]. Here, as discussed in detail above, the BEDT-TTF of Dimer 1 is related to the anionic plane by three short hydrogen bonds, smaller than 2.73 Å as well as the BEDT-TTF of Dimer 2, which is also connected by three hydrogen bonds shorter than 2.73 Å. ...
... In order to further check the possibility of a sizeable charge disproportionation between dimers, we studied the hydrogen bond network between the anionic and donor layers. In previous studies of α-(BEDT-TTF) 2 I 3 [58] and θ-(BEDT-TTF) 2 MM'(SCN) 4 [60], it was shown that the hydrogen bond distance between the terminal C 2 H 4 groups of BEDT-TTF and the anion reflects the degree of charge of the donor as a result of a subtle polarization mechanism detailed in [58]. Here, as discussed in detail above, the BEDT-TTF of Dimer 1 is related to the anionic plane by three short hydrogen bonds, smaller than 2.73 Å as well as the BEDT-TTF of Dimer 2, which is also connected by three hydrogen bonds shorter than 2.73 Å. ...
We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)2Cu2(CN)3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symmetry with two non-equivalent dimers in the unit cell, which implies a charge disproportionation between the dimers. However, structural refinement shows that the charge disproportionation is quite weak at room temperature.
... The relationship between the charge order and anion has been investigated in α-(BEDT-TTF) 2 I 3 [21]. The hydrogen bonds of the hole-rich donors are shorter than those of the hole-poor donors in α-(BEDT-TTF) 2 I 3 ; the hydrogen bonds between the donors and anions affect the charge ordering transition. ...
Structural, transport, and magnetic properties of new organic conductors composed of (BEDT-TTF) 2 TaF 6 , where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, have been investigated. Two δ -type polymorphs, monoclinic and orthorhombic phases are obtained by the electrocrystallization. Both phases show a semiconductor-insulator phase transition at 276 K and 300 K for the monoclinic and orthorhombic phases, respectively; the ground state of both salts is a nonmagnetic insulating state. The low-temperature X-ray diffraction measurements show two-fold superlattice reflections in the intercolumnar direction. The low-temperature crystal structures show a clear charge ordered state, which is demonstrated by the molecular shape and intramolecular bond lengths. The observed checkerboard charge ordered state is in agreement with the charge ordering in a dimer Mott insulator. If we distinguish between the monoclinic and orthorhombic phases, the transition temperature of the δ -type (BEDT-TTF) 2 M F 6 conductors ( M = P, As, Sb, and Ta) increases continuously with increasing the anion volume.
... It is interesting to note that this behavior is in strong contrast to the behavior of the TMTTF-salts [48] where the transition of the charge ordered state evolves continuously indicated by the charge imbalance. In this way, it resembles the behavior observed at the charge-order phase transition of α-(BEDT-TTF) 2 I 3 where a strong involvement of the lattice was shown recently [61][62][63][64]. Moreover, several features reveal a positive shift instead of a negative one. ...
The neutral-ionic phase transition in TTF-CA was investigated by steady-state and time-resolved infrared spectroscopy. We describe the growth of high-quality single crystals and their characterization. Extended theoretical calculations were performed in order to obtain the band structure, the molecular vibrational modes and the optical spectra along all crystallographic axes. The theoretical results are compared to polarization-dependent infrared reflection experiments. The temperature-dependent optical conductivity is discussed in detail. We study the photo-induced phase transition in the vicinity of thermally-induced neutral-ionic transition. The observed temporal dynamics of the photo-induced states is attributed to the random-walk of neutral-ionic domain walls. We simulate the random-walk annihilation process of domain walls on a one-dimensional chain.
