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λ-type molecular π–d systems are coupled systems consisting of π electron systems with strong magnetic interactions and 3d electron systems with weak magnetic interactions. They exhibit an unconven...

We performed heat capacity measurements on the molecular π–d system λ-(BEDT-STF)2FeCl4 to discuss the unconventional paramagnetic-like behavior. We observed a Schottky-type anomaly below TN of 16 K...

Angle-resolved heat capacity (ARHC) measurements with multi-directional magnetic fields for an organic superconductor λ-(BETS)2GaCl4 are performed to investigate the out-of-plane (polar angle) and ...

We have investigated the magnetic properties of λ-(BEDT-STF)2FeCl4 by Mossbauer measurement to reveal the origin of the paramagnetic-like behavior of the 3d spin system observed in λ-(BETS)2FeCl4 u...

The ground state of $\lambda$-(BEDT-TTF)$_2$GaCl$_4$, which has the same structure as the organic superconductor $\lambda$-(BETS)$_2$GaCl$_4$, was investigated by magnetic susceptibility and $^{13}$C NMR measurements. The temperature dependence of the magnetic susceptibility revealed an antiferromagnetic (AF) correlation with $J/k_{\rm B} \simeq$ 98 K. NMR spectrum splitting and the divergence of $1/T_1$ were observed at approximately 13 K, which is associated with the AF transition. We found that the AF structure is commensurate according to discrete NMR peak splitting, suggesting that the ground state of $\lambda$-(BEDT-TTF)$_2$GaCl$_4$ is an AF dimer-Mott insulating state. Our results suggest that the superconducting phase of $\lambda$-type salts would be located near the AF insulating phase., Comment: 5 pages, 4 figures

We present the optical conductivity and Raman spectra of θ-(BEDT-TTF) 2 CsZn(SCN) 4 . The vibrational and vibronic bands in the 1200–1600 cm -1 region was assigned with the aid of the 13 C-substituted compound. The nearly uniform charge distribution was found in a whole temperature range from 300 to 6 K. However, the space group symmetry I 222 is locally broken already at room temperature. The broken symmetry suggests the resemblance to the high-temperature phase and frozen state of θ-(BEDT-TTF) 2 RbZn(SCN) 4 , which consists of short-range ordered charge-ordering domains. The amplitude in the charge-ordering domain of θ-(BEDT-TTF) 2 CsZn(SCN) 4 is extremely small in a whole temperature range. At room temperature, hydrostatic pressure narrows the bandwidth of θ-(BEDT-TTF) 2 CsZn(SCN) 4 to enlarge the amplitude in short-ranged ordered domains. The long-range ordered charge ordering in θ-(BEDT-TTF) 2 CsZn(SCN) 4 was found at 10 K under the hydrostatic pressure of 2.5 GPa.

Low-temperature heat capacity of the organic superconductor λ-(BETS)2GaCl4 in a temperature range between 0.6 and 10 K was measured for a single crystal sample by relaxation calorimetry technique. The contribution of electronic heat capacity in the superconducting state shows T2 dependence at lower temperature that indicates the existence of quasi-particle excitations from the line-nodal gap structure of d-wave pairing. The recovery of the electronic heat capacity coefficient, γ by magnetic field shows H1/2 dependence that is consistent with the picture of line-nodal superconductor. In spite of the different packing from typical organic superconductors κ-(BEDT-TTF)2X, the λ-type BETS salt also shows a feature of d-wave symmetry pairing.

We present experimental evidence for the charge fluctuation in the spin-liquid candidate κ-(BEDT-TTF)2Cu2(CN)3. The temperature and pressure dependence of the Raman spectrum in the region of 1300–1600 cm−1 were measured. The charge-sensitive ν2 mode at 1500 cm−1 exhibited a linewidth broadening below 60 K when the sample was slowly cooled. This broadening is induced by a structural change (increase in interdimer transfer integrals). The hydrostatic pressure significantly suppressed the broadening in the metallic phase. This result indicates that the linewidth broadening originates from the valence (charge) fluctuation within the BEDT-TTF dimer, because the screening effect suppresses the interdimer Coulomb interaction that causes the charge disproportionation. The line shape was analyzed using the fast fluctuating model and slow or static inhomogeneous model.

Electrical resistivity measurements under pressure were conducted on the organic conductor λ-(BEDT-STF)2GaCl4, which is situated between λ-(ET)2GaCl4 and λ-(BETS)2GaCl4. Magnetic susceptibility was also measured at ambient pressure. A drop in resistivity, which is associated with the superconducting transition, was observed at Tc ≈ 5 K above 1.22 GPa. The superconducting phase was confirmed by pair-breaking under a magnetic field. This is the first observation of superconductivity in λ-(BEDT-STF)2GaCl4. The temperature dependence of spin susceptibility suggests a two-dimensional antiferromagnetic spin system without any magnetic ordering. The phase adjacent to the superconducting phase in λ-type systems is not the antiferromagnetic phase, unlike the case for κ-(ET)2X.

We performed tunneling spectroscopy (STS) measurement in an organic superconductor, partially deuterated κ-(BEDT-TTF-d[3,3])2Cu[N(CN)2]Br. We carried out angle-resolved STS on cut lateral surfaces by the focused ion beam (FIB) method as well as as-grown single-crystal surfaces. It was found that the d[3,3]-Br salt is a strong-coupling d-wave superconductor and that the nodal direction is at an angle of π/4 from the a*-axis, corresponding to the \(d_{x^{2} - y^{2}}\) symmetry in the bulk superconducting phase. It is understood that the electron correlation in the d[3,3]-Br salt is not strong enough on the basis of the spin fluctuation mechanism. On the other hand, we also observed two types of superconducting gap. This suggests the coexistence of the \(d_{x^{2} - y^{2}}\) and dxy symmetries. This indicates a change in the symmetry from \(d_{x^{2} - y^{2}}\) in the bulk superconducting phase to dxy around the insulating region with increasing electron correlation.

κ-(BEDT-TTF) 2 Cu(NCS) 2 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] has semiconductive properties at high temperatures, but these properties are suppressed by X-ray irradiation. Utilizing 13 C-NMR, we assessed the effects of X-ray irradiation. When we compared the spectrum and spin–spin relaxation times, T 2 , of irradiated and nonirradiated κ-(BEDT-TTF) 2 Cu(NCS) 2 , we found that defects caused by irradiation reduced the connection between ethylene motion and conduction electrons. Measurements of Knight shift and ( T 1 T ) -1 confirmed that irradiation had no effect on carrier density.

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 ...

We present the anomalies at ∼200 K in electrical resistivity, transmittance and reflectance in the far-infrared region, and the magnetic susceptibility of α'-(BEDT-TTF) 2 IBr 2 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. This phase transition is investigated by infrared, Raman, and far-infrared spectroscopies. Through the spectroscopic investigation, we found that the positive charges on BEDT-TTF molecules are strongly localized owing to a strong Coulomb interaction in both low-temperature (LT) and high-temperature (HT) phases. We show the spectroscopic data that suggest the charge order in the LT phase and the charge disorder in the HT phase. The phase transition at ∼200 K in α'-(BEDT-TTF) 2 IBr 2 is the charge order–disorder phase transition not accompanied by structural changes. On the basis of the localized picture, we qualitatively interpreted the experimental results above and below the phase transition. The effect of hydrostatic pressure was investigated by electrical resistivity measuremen...