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We have measured the specific heat on λ-(BEDT-TSF)2FeCl4 as a function of temperature by a thermal relaxation method to study the metal-to-insulator (MI) transition around 8 K and anomalous ferroelectric-like state revealed by microwave measurements for the metallic state. The specific heat exhibits a sharp peak due to the antiferromagnetic ordering at TMI=8.3 K. We have found a remarkable jump at 70 K labeled as TFM. Together with the ferroelectric-like response emerging around TFM, this result confirms thermodynamically a ferroelectric-like transition at TFM in the metallic states. Furthermore, the thermal conductance of the sample may exhibit an anomaly at TFM. [Copyright &y& Elsevier]

Using a capacitance bridge with a phase sensitive detection at frequencies f (=ω/2π)=101 to 105 Hz, we have measured the complex dielectric constant ϵ(ω)=ϵ1(ω)+iϵ2(ω) along the c-axis in the antiferromagnetic insulating state of λ-(BEDT-TSF)2FeCl4 below TMI=8.3 K. The magnitude of ϵ1 is quite large in the order of 103 to 104 in contrast to 45 at microwave frequencies. Both ϵ1 and ϵ2 increase very drastically on approaching to TMI, suggesting some divergent behavior near TMI. [Copyright &y& Elsevier]

We have measured the specific heat of λ-(BEDT-TSF)2GaCl4 by a thermal relaxation method below 7 K to study the superconducting state. The specific heat jump due to the superconducting transition at Tc=4.8 K is observed. For the normal state, we have determined the electronic specific heat coefficient γ=14.4±3.4 mJ/mol K2 and the Debye temperature of ΘD=194.0±0.4 K. The discontinuity of the specific heat is evaluated to be ΔC/γTc=1.37±0.32, which coincides with a Bardeen–Cooper–Schrieffer (BCS) relation ΔC/γTc=1.43 for a weak-coupling limit. The zero-crossing (ΔC=0) occurs at 0.67Tc, which has been similarly observed in BEDT-TTF-based superconductors. [Copyright &y& Elsevier]

Abstract: We have investigated the microwave response at 45GHz in an organic superconductor λ-(BEDT-TSF)2GaCl4 with T c =4.8K. We determine the μ 0 H c2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξ GL ∼10). Although the real part of the complex conductivity (=σ 1 +iσ 2) does not show a coherence peak just below T c, the London penetration depth completely saturates at low temperatures down to T/T c =0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50K, (parallel to the c-axis) deviates downward from , while σ 2, which should be zero in a conventional metal, increases exponentially toward T c. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ 2. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ 2 may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl4 salt with the anomalous metallic states, which shows a negative σ 2 in contrast to the GaCl4 salt. Just the opposite of ground states in between the GaCl4 and FeCl4 salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ 2. [Copyright &y& Elsevier]

The microwave conductivity σ1c and dielectric constant ϵ1c along the c-axis in λ-(BEDT-TSF)2FeCl4 have been obtained by a cavity perturbation technique with TE0 1 1 resonant mode at 16.3 GHz. Even though the d.c. resistivity indicates a highly metallic state, we have found a novel dielectric response in ϵ1c amounting to the order of 103 from the metal-to-insulator transition temperature TMI=8.3 K to TFM=70 K, at which the noticeable specific-heat jump and 1H NMR spectral splitting have been recently observed. The dielectric anomaly is attributed to the dynamical response of π-electron’s charge degrees of freedom and could be interrelated to the π–d interaction. [Copyright &y& Elsevier]

We have measured the complex conductivity (σ*c=σ1c+iσ2c) along the c-axis in λ-(BEDT-TSF)2FeCl4 below the metal-to-insulator (MI) transition at TMI=8.3 K. Microwave experiments have been carried out at 16.3 and 44.5 GHz employing a cavity perturbation method. The temperature dependence of σ1c below TMI shows a large frequency dispersion, which could be attributed to a low energy excitation inside the gap such as a collective mode. With decreasing temperature, the real part of the dielectric constant ϵ1c indicates a remarkable decrease at TMI. [Copyright &y& Elsevier]

Zero field (ZF) muon spin relaxation measurements on the organic metal κ-BETS2FeCl4 clearly show the formation of an antiferromagnetically ordered state below TN=0.45 K. The magnetic order remains unperturbed on cooling through the superconducting transition Tc ∼0.17 K, providing unambiguous evidence for the coexistence of antiferromagnetic order and superconductivity in this system. Detailed studies of the internal field distribution show contrasting behavior for different muon sites and provide experimental evidence for an ordered spin structure in the BETS layers in addition to that taking place in the Fe layers. [Copyright &y& Elsevier]

We report here on novel dielectric states and related properties revealed in low-frequency and microwave dynamical conductivity, specific heat, and proton nuclear magnetic resonance measurements on the highly metallic states and low-temperature antiferromagnetic insulating states in a π–d coupled λ-(BEDT-TSF)2FeCl4. [Copyright &y& Elsevier]

The influence of magnetic field on the crystallization process has been examined in Cs2TCNQ3 (TCNQ=tetracyanoquinodimethane), a strongly Coulomb-correlated organic semiconductor. The crystal structure is unaffected by the magnetic field, while the electric, magnetic and optical properties change markedly if the magnetic field higher than a threshold (∼4 T) is applied during the crystal growth. The high-field crystals exhibit a very weak but distinct spontaneous magnetization with the exceedingly high Curie temperature of ∼420 K. The infrared and visible spectroscopy data show that this novel ferromagnetism concurs with the renormalization of the π* state of TCNQ radical anions, TCNQ−. [Copyright &y& Elsevier]

We have performed 1H and 77Se NMR measurements on a π–d organic metal λ-(BEDT-TSF)2FeCl4 containing paramagnetic ions Fe3+. It is found from 1H NMR measurements that the resonance line starts to split around TFM=70K, just a temperature where a ferroelectric-like transition occurs, and with further decreasing temperature, the spectrum becomes significantly broad near the metal–insulator transition TMI=8.3K. From 77Se NMR measurements, near the phase boundary between the forced-ferromagnetic metallic state and the canted antiferromagnetic insulating state at 1.6K and 11.67T, a single peak becomes split with two new peaks emerging at the both sides of the zero-shift. [Copyright &y& Elsevier]