Two-dimensional Organic Conductor Research Articles

Role of the Anion Layer’s Polarity in Organic Conductors β″-(BEDT-TTF)2XC2H4SO3 (X = Cl and Br)

A two-dimensional (2D) organic conductor β″-(BEDT-TTF)2ClC2H4SO3 (1) crystallized in the P21/m and has a polar anion located on the mirror plane, parallel to the 2D BEDT-TTF conducting layer. A temperature-induced phase transition tilts the anion such that a component of its electric dipole becomes perpendicular to the conducting plane. This low-temperature phase β″-β′′-(BEDT-TTF)2ClC2H4SO3 (1L) has two crystallographically independent donor layers, A and B, each of which is bordered by the positive or negative side of the anion’s dipole (← B → A ← B → A ←). This exposes each donor layer to different effective electric fields and leads to layers of A and B with dissimilar oxidation states. Consequently, the transition can be called the temperature-induced non-doped-to-doped transition. The low-temperature phase (1L) is isomorphous with β″-β′′-(BEDT-TTF)2BrC2H4SO3 (2) from room temperature to at least 100 K, suggesting that 2 is also doped and it shows a very broad MI transition at 70 K. Applying only 2 kbar of static pressure sharpens the MI transition, indicating that the tilted anion straightens, and therefore, we suggest that it can be termed a pressure-induced doped-to-non-doped transition.

A Database for Crystalline Organic Conductors and Superconductors

We present a prototype database for quasi two-dimensional crystalline organic conductors and superconductors based on molecules related to bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, ET). The database includes crystal structures, calculated electronic structures, and experimentally measured properties such as the superconducting transition temperature and critical magnetic fields. We obtained crystal structures from the Cambridge Structural Database and created a crystal structure analysis algorithm to identify cation molecules and execute tight binding electronic structure calculations. We used manual data entry to encode experimentally measured properties reported in publications. Crystalline organic conductors and superconductors exhibit a wide variety of electronic ground states, particularly those with correlations. We hope that this database will ultimately lead to a better understanding of the fundamental mechanisms of such states.

Mechanism of Superconductivity in Two-Dimensional Organic Conductors

Mechanism of Superconductivity in Two-Dimensional Organic Conductors

Three-quarter Dirac points, Landau levels, and magnetization in α−(BEDT−TTF)2I3

The energies as a function of the magnetic field ($H$) and the pressure are studied theoretically in the tight-binding model for the two-dimensional organic conductor, $\alpha$-(BEDT-TTF)$_2$I$_3$, in which massless Dirac fermions are realized. The effects of the uniaxial pressure ($P$) are studied by using the pressure-dependent hopping parameters. The system is semi-metallic with the same area of an electron pocket and a hole pocket at $P < 3.0$~kbar, where the energies $(\varepsilon_{\rm D}^0$) at the Dirac points locate below the Fermi energy $(\varepsilon_{\rm F}^0$) when $H=0$. We find that at $P=2.3$~kbar the Dirac cones are critically tilted. In that case a new type of band crossing occurs at "three-quarter"-Dirac points, i.e., the dispersion is quadratic in one direction and linear in the other three directions. We obtain new magnetic-field-dependences of the Landau levels $(\varepsilon_n)$; $\varepsilon_n-\varepsilon^0_{\textrm{D}} \propto (n H)^{4/5}$ at $P=2.3$~kbar ("three-quarter"-Dirac points) and $|\varepsilon_n-\varepsilon_{\rm F}^0| \propto (n H)^2$ at $P=3.0$~kbar (the critical pressure for the semi-metallic state). We also study the magnetization as a function of the inverse magnetic field. We obtain two types of quantum oscillations. One is the usual de Haas van Alphen (dHvA) oscillation, and the other is the unusual dHvA-like oscillation which is seen even in the system without the Fermi surface.

Pressure dependence of the metal-insulator transition in κ-(BEDT-TTF)2Hg(SCN)2Cl: optical and transport studies

The two-dimensional organic conductor κ-(BEDT-TTF)2-Hg(SCN)2Cl exhibits a pronounced metal-insulator transition at K. From the splitting of the molecular vibrations, the phase transition can be unambiguously assigned to charge-ordering with . We have investigated the pressure evolution of this behavior by temperature-dependent electrical transport measurements and optical investigations applying hydrostatic pressure up to 12 kbar. The data reveal a mean-field like down-shift of with a critical pressure of kbar and a metallic state above the suppression of the charge-ordered state; no traces of superconductivity could be identified down to T = 1.5 K. As the charge order sets in abruptly with decreasing temperature, its size remains unaffected by pressure. However, the fraction of charge imbalanced molecules decreases until it is completely absent above 1.6 kbar.

Electrically induced phase transition inα−(BEDT-TTF)2I3: Indications for Dirac-like hot charge carriers

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.

Piston pressure cell for low-temperature infrared investigations.

The design of a piston pressure cell ranging up to approximately 11 kilobars is reported, which allows for optical reflection measurements in the infrared spectral range from 100 to 8000 cm(-1) down to temperatures as low as 6 K. The mechanical alignment and vacuum considerations are discussed before details of the sample preparation are given, with particular emphasis on small and fragile single crystals, mosaics, and pressed powder. A few examples of one- and two-dimensional organic conductors illustrate the performance.

Diamagnetic phase transitions in two-dimensional conductors

A theory describing the susceptibility amplitude and the magnetic induction bifurcation near the dHvA driven diamagnetic phase transitions in quasi two-dimensional (2D) organic conductors of the (ET)2X with X=Cu(NCS)2,KHg(SCN)4,I3,AuBr2,IBr2, etc. is presented. We show that there is a drastic increase in the temperature and magnetic field dependence of the susceptibility amplitude on approaching the diamagnetic phase transition point. Near the phase transition point the temperature and magnetic field dependences are fitted by the ones typical of the mean-field phase transition theory. These dependences confirm the long-range character of the magnetic interactions among the conduction electrons leading to diamagnetic phase transitions. We demonstrate that the magnetic induction splitting of nuclear magnetic resonance (NMR) and muon spin-rotation spectroscopy (μSR) lines due to two Condon domains decreases tending to zero on approaching the diamagnetic phase transition. This decrease is fitted by the temperature and magnetic field dependence of the susceptibility characteristic of the mean-field theory of phase transitions. Performing new susceptibility, NMR and μSR experiments will enable to detect diamagnetic phase transitions and Condon domains in quasi 2D metals.

Magnetic and Transport Properties of π–d System κ-(BDH-TTP)2FeCl4

Resistance, magnetic susceptibility, magnetic torque, and ESR measurements have been performed to investigate the electronic state of the two-dimensional organic conductor κ-(BDH-TTP)2FeCl4. The temperature dependence of the resistance shows monotonic metallic behavior down to 30 mK, and the magnetic susceptibility, arising mainly from the Fe \(3d\) spins, follows the Curie–Weiss law down to 2 K. At low temperatures below 0.4 K, the magnetic torque steeply changes at low magnetic fields, associated with a sign change. The systematic measurements elucidate that the \(3d\) spins undergo an antiferromagnetic order at about 0.4 K and that the torque sign change is caused by a spin-flop transition. A rapid decrease of the magnetoresistance at the spin-flop field provides clear evidence of a finite π–\(d\) interaction. Characteristic temperature dependences of the magnetic susceptibility and ESR \(g\)-value are found in the paramagnetic phase, which are explained in terms of the single-ion anisotropy effect.

Collective response to alternating current in the organic conductor α-(bis(ethylenedithio)tetrathiafulvalene)2I3

Interference phenomena between dc fields and ac currents are investigated in the two-dimensional charge-ordered organic conductor α-(bis(ethylenedithio)tetrathiafulvalene)2I3. When alternating current is applied together with a dc electric field, steps analogous to Shapiro steps in the ac Josephson effect appear in the voltage-current characteristics at the internal frequency, where the step height follows the first-order Bessel function of the ac field. These results are discussed through an analogy with sliding charge-density waves.

Kosterlitz-Thouless-Type Transition in a Charge Ordered State of the Layered Organic Conductorα−(BEDT−TTF)2I3

The current-voltage characteristics in the charge order state of the two-dimensional organic conductor α-(BEDT-TTF)(2)I(3) exhibit power law behavior at low temperatures. The power law is understood in terms of the electric-field-dependent potential between electrons and holes, which are thermally excited from the charge order state. The power law exponent steeply changes from 1 to 3 in the range from 30 to 45 K with decreasing temperature, thereby suggesting the occurrence of a Kosterlitz-Thouless-type transition; many (few) unbound electron-hole pairs are thermally excited above (below) the transition. The effects of the finite size and interlayer coupling on the power law behavior are discussed.

The Mott State and Superconductivity in Face-Centred Cubic Structured Cs3C60: A133Cs-Nuclear Magnetic Resonance Study under Pressure

Over the past 20 years, fullerides have been studied as the source of high-transition-temperature (Tc) superconductivity except for copper oxides. The recent finding of the Mott insulating state right beside superconductivity in Cs3C60 has suggested that magnetism helps raise Tc even in fullerides as in heavy-fermion compounds, high-Tc copper oxides, two-dimensional organic conductors, and iron pnictides. Namely, one tends to think that the link between Mott insulator and superconductivity takes place in fullerides, which can give rise to the mechanism beyond the Bardeen-Cooper-Schrieffer framework. However, the relationship between the Mott state and the superconductivity in Cs3C60 is still under debate. By nuclear magnetic resonance measurements under pressure, we find that the magnetism and superconductivity in Cs3C60 are competing orders. Different from previous reports, the phase separation of Cs3C60 crystals into the Mott and metallic states allows us to systematically study the evolution of the ground state under pressure. Our careful experiments have found that the prevention of a magnetic order is rather essential for the superconductivity in face-centred cubic Cs3C60, which presents a basic strategy for finding still higher Tc in this system.

Rapid voltage oscillations and ac-dc interference phenomena in the two-dimensional charge-ordered organic conductor β″-(bis(ethylenedithio)tetrathiafulvalene)3(ClO4)2

Giant nonlinear conductivity and rapid voltage oscillations are investigated in a two-dimensional organic conductor β″-(BEDT-TTF)3(ClO4)2, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. Below the metal-insulator transition temperature at TMI=170 K, remarkable negative differential resistance appears, where voltage oscillation with a frequency of 20–60 kHz is observed, and the frequency increases in proportion to the applied current. This phenomenon is reminiscent of narrow band noise originating from the sliding charge density waves in one-dimensional conductors, but the oscillation appears in two directions of the two-dimensional non-stripe charge order. The ac-dc interference phenomena are investigated; when alternating current is superposed, an anomalous jump appears in the current-voltage characteristics at the position where the applied frequency coincides with the internal frequency corresponding to the applied dc current. The jump is interpreted in analogy with the Shapiro steps in the ac Josephson effect, in which the step height changes following the first-order Bessel function of the applied ac current.

Charge Transport in Charge-Ordered States of Two-Dimensional Organic Conductors, α-(BEDT-TTF)2I3 and α'-(BEDT-TTF)2IBr2

Electric conductance and dielectric constant have been measured in the charge-ordered states of the quasi two-dimensional organic conductors α-(BEDT-TTF) 2 I 3 and α'-(BEDT-TTF) 2 IBr 2 to understand the charge transport mechanism. The current–voltage ( I – V ) characteristics show a strong nonlinear behavior, which is qualitatively explained by the transport of individual carriers thermally excited from the charge-ordered states in BEDT-TTF layers. The discrepancy with the simulation based on the thermal activation model at low temperatures is likely due to quantum tunneling through the potential barrier. The dielectric constants show a large difference between the in-plane and out-of-plane directions of the samples, showing a highly two-dimensional nature of Coulomb potential. The temperature dependence of dielectric constant is ascribed to the polarization effect of the thermally excited electron–hole pairs, consistent with the model of nonlinear I – V characteristics. All these features are qualitativ...

Electronic and magnetic studies of [formula omitted

DC resistivity and electron spin resonance (ESR) measurements have been performed on the two-dimensional organic conductor κ-(BEDT-TTF)2Hg(SCN)2Cl. Due to electronic correlations, the compound undergoes a transition from a metallic to an insulating state at TMI=34K. The ESR parameters exhibit a drastic change below Tafm=27K that evidences a magnetic phase transition. In this low-temperature state, we observe the characteristics of antiferromagnetic resonances in the angular dependence of the g-value.

Charge and spin criticality for the continuous Mott transition in a two-dimensional organic conductor

We study the continuous bandwidth-controlled Mott transition in the two-dimensional single-band Hubbard model with a focus on the critical scaling behavior of charge and spin degrees of freedom. Using plaquette cluster dynamical mean-field theory, we find charge and spin criticality consistent with experimental results for organic conductors. In particular, the charge degree of freedom measured via the local density of states at the Fermi level shows a smoother transition than expected for the Ising universality class and in single-site dynamical mean-field theory, revealing the importance of short-ranged nonlocal correlations in two spatial dimensions. The spin criticality measured via the local spin susceptibility agrees quantitatively with nuclear magnetic resonance measurements of the spin-lattice relaxation rate.

Coherent particle response of two-dimensional organic conductors upon crossing the Mott transition

The two-dimensional organic charge-transfer salt κ-(BEDT-TTF)2Cu[N(CN)2]-BrxCl1–x is tuned across the metal-insulator transition by varying the Br content. Temperature dependent measurements of the optical conductivity yield information on the electrodynamics of the half-filled correlated electron system. At room temperature a broad incoherent response is observed with no clear Drude peak. As the temperature drops below 50 K, an energy gap develops in the Cl-rich samples which grows to approximately 1000 cm−1 for T → 0. With increasing Br concentration spectral weight shifts into the gap region and eventually fills it up completely. A Drude-like component develops due to the coherent quasiparticles.

Thermoelectric figure of merit of [formula omitted] conductors of several donors

Dimensionless thermoelectric figure of merit ZT is investigated for two-dimensional organic conductors τ ‐ ( EDO- S , S -DMEDT-TTF ) 2 ( AuI 2 ) 1 + y , τ ‐ ( EDT- S , S -DMEDT-TTF ) 2 ( AuI 2 ) 1 + y and τ ‐ ( P- S , S -DMEDT -TTF ) 2 ( AuI 2 ) 1 + y ( y ≤ 0.875 ) , respectively. The ZT values were estimated by measuring electrical resistivity, thermopower and thermal conductivity simultaneously. The largest ZT is 2.7 × 10 − 2 at 155 K for τ ‐ ( EDT- S , S -DMEDT-TTF ) 2 ( AuI 2 ) 1 + y , 1.5 × 10 − 2 at 180 K for τ ‐ ( EDO- S , S -DMEDT -TTF ) 2 ( AuI 2 ) 1 + y and 5.4 × 10 − 3 at 78 K for τ ‐ ( P- S , S -DMEDT-TTF ) 2 ( AuI 2 ) 1 + y , respectively. Substitution of the donor molecules fixing the counter anion revealed EDT- S,S-DMEDT-TTF is the best of the three donors to obtain larger ZT.

The problem with spins

The metal–insulator Mott transition, which has been extensively studied by means of charge transport, is now detected through the electron spins in a two-dimensional organic conductor.

13C-NMR Study of Charge Ordering State in the Organic Conductor, α-(BEDT-TTF)2I3

The pattern of charge ordering of the quasi two-dimensional organic conductor, α-(BEDT-TTF) 2 I 3 [BEDT-TTF: bis(ethylenedithio)tetrathiafulevalene], provides important information about the usual transport properties of this salt. Experiments on this salt have yielded conflicting results. Especially, the results of nuclear magnetic resonance (NMR) measurement and X-ray Diffraction (XRD) study have suggested contradictory charge patterns. To clarify these discrepancies, we performed 13 C-NMR measurements using a single side 13 C enriched molecule. Above the metal–insulator transition temperature, the spin susceptibility at the B site decreased and that at the C site increased with decreasing temperature. Below the metal–insulator (M–I) transition temperature, however, the angular dependence of the NMR shift suggested that the charge order pattern was a horizontal stripe, in which the A(A') and B molecules are charge rich sites and the A'(A) and C molecules are charge poor.