Eiichi Negishi

Anomalous Dielectric Response in the π–d Correlated Metallic State of λ-(BEDT-TSF)2FeCl4

The temperature dependence of the microwave conductivity σ 1 and dielectric constant e 1 has been measured on λ-(BEDT-TSF) 2 FeCl 4 by a cavity perturbation method at 16.3 GHz, employing the TE 011 resonant mode. In spite of the highly metallic state above the metal-to-insulator (MI) transition at T MI = 8.3 K, a novel ferroelectric-like response is observed in e 1 c along c at high temperatures from T MI to T FM = 70 K, which is enhanced to the order of 10 3 just above T MI . Then a transition from a ferroelectric-like to paraelectric-like state with e 1 c ∼ 45 takes part in the MI transition. In contrast to the dc conductivity, σ 1 c becomes much more conductive as considered by a collective mode. The ferroelectric-like response is associated with the dynamical response of the charge degrees of freedom in the π electronic system. An absence of these dielectric and thermodynamical anomalies in the nonmagnetic Ga analogue indicates the importance of the π– d interaction for the ferroelectric-like state as...

Anomalous Splitting of 1H-NMR Spectra in λ-(BEDT-TSF)2FeCl4

For an organic π–d metal λ-(BEDT-TSF) 2 FeCl 4 , where BEDT-TSF is an abbreviation of bis(ethylenedithio)tetraselenafulvalene, we have measured 1 H-NMR to clarify the quite anomalous ferroelectric-...

Relaxor-like Behavior in λ-(BETS)2FeCl4 Studied by SR X-ray Diffraction

We performed precise X-ray diffraction measurements on an organic conductor, λ-(BETS) 2 FeCl 4 , with synchrotron radiation, where BETS is bis(ethylendithio)tetraselenafulvalene. Around 70 K (= T FM ), the width of the Bragg reflection increased, and its peak was split into two with decreasing temperature. The direction of splitting was 0.017 a * - 0.010 b * + 0.002 c * in the reciprocal space. At 8.3 K (= T MI ), the discontinuous change in c * was observed, and the peak of the Bragg reflection became single again, but remained broad. In the temperature region of T FM > T > T MI , it is considered that the system has a heterogeneous structure, and the coexistence of the high- and low-temperature phases below T FM is presumably related to the anomalous behavior of dielectric constant in the metallic phase. The size of regions/domains was estimated to be about 0.4 µm.

77Se NMR Evidence for the Development of Antiferromagnetic Spin Fluctuations of π-Electrons in λ-(BETS)2GaCl4

The microscopic magnetic properties of superconducting (\(T_{\text{C}}\cong 6\) K) λ-(BETS) 2 GaCl 4 (BETS = bis(ethylenedithio)tetraselenafulvalene) have been investigated by 77 Se NMR in the normal state between 7.1 and 100 K on randomly oriented single crystals with κ-(BETS) 2 GaCl 4 as a reference compound. From the T -dependences of the Knight shift and the linewidth, the intrinsic uniform susceptibility χ q =0 ( T ) has been found to decrease with decreasing T below 30 K for λ-(BETS) 2 GaCl 4 . Nevertheless, 1/( T 1 T ) does not show any corresponding decrease but is constant down to the lowest T , which indicates increasing contributions with decreasing T from antiferromagnetic spin fluctuations. The Knight shift and 1/( T 1 T ) have been analyzed quantitatively and the enhancement factor K (α)>1 is deduced in the low- T limit. On the basis of these results, the development of antiferromagnetic spin fluctuations of π-electrons at low T is concluded for λ-(BETS) 2 GaCl 4 .

Magnetic field effects on nonlinear electrical transport in λ-(BEDT-TSF)2FeCl4, where BEDT-TSF is bis(ethylenedithio)tetraselenafulvalene

We have studied magnetic field effects on the nonlinear electrical transport associated with a negative resistance and switching phenomena in an antiferromagnetic insulating (AFI) ground state of the title π– d coupled conductor below the metal-to-insulator transition temperature T MI =8.3 K. The overall characteristics of current density ( J ) versus electric field ( E ) are well reproduced by an empirical formula with a current-density-dependent conductivity proportional to J n ; if n >1, a negative resistance effect (d J /d E 0) appears. With increasing magnetic field, there appears a crossover from n >1 to n <1 around H * ≃7 T, followed by a conventional ohmic transport at fields higher than the critical magnetic field H C =10.6 T at 4.2 K for the reentrant metallic transition. A switching phenomenon, which always occurs at J < J T in the well-ordered AFI states, triggers a discontinuous transition from low-c...

Crystal structures and conducting/magnetic properties in two different crystals of (ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide)2·MBr4 (M = Fe, Ga)☆

Abstract The needle and plate crystals were obtained for each of (ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide) 2 ·MBr 4 (M=Fe, Ga) charge-transfer salts. Their crystal structures, and electrical conducting and magnetic properties were investigated.

Enhancement of the 1H NMR Relaxation Rate at Low Temperatures and Development of Antiferromagnetic Spin Fluctuations of π-Electrons in Superconducting λ-(BETS)2GaCl4

Extending previous 77 Se NMR studies [J. Phys. Soc. Jpn. 72 (2003) 483], the microscopic magnetic properties of superconducting (\(T_{\text{C}}\cong 5{\mbox{--}}6\) K) λ-(BETS) 2 GaCl 4 [BETS = bis(ethylenedithio)tetraselenafulvalene] have been investigated by 1 H NMR in the normal state between 7.1 and 140 K on randomly oriented single crystals. Contrary to almost T -independent 1/( 77 T 1 T ) of the 77 Se NMR, 1/( 1 T 1 T ) of the 1 H NMR shows a significant increase below about 25 K. This quite contrasting low- T behavior in 1/( T 1 T ) between 77 Se and 1 H is shown to be explained consistently in terms of the development of antiferromagnetic spin fluctuations of π-electrons, which has already been indicated by the 77 Se NMR as a significant low- T decrease in the intrinsic uniform susceptibility χ q =0 ( T ) and no concomitant decrease in 1/( 77 T 1 T ). In the explanation, it is pointed out that the inter-atomic dipolar interaction with the extended spin polarization of the HOMO plays an important r...

Dielectric Slowing Down toward the Magnetically Ordered State in a Polar Conductor (EDT-TTFVO)2FeBr4

We have measured the dielectric response as a function of both frequency ( f = 0.3 Hz–100 kHz) and electric bias field for a quasi-one-dimensional, polar conductor (EDT-TTFVO) 2 FeBr 4 undergoing a ferrimagnetic-like transition at T C ∼0.9 K. Around T * = 15 K, the dielectric constant at low f exhibits a peak that shifts to higher T at higher f , being reminiscent of relaxor ferroelectrics with glassy polarizations. Since this dielectric anomaly is also present in the nonmagnetic isostructural GaBr 4 salt, the polarizations may be ascribed to both π and σ electrons localized on the polar donor. At T C < T < 7 K, the complex dielectric constant clearly exhibits a Debye relaxation, and the relaxation time follows a critical behavior as ( T - T 0 ) - n τ ( n τ = 6.5) toward T 0 = 0.75±0.15 K just very close to T C . This phenomenon of the dielectric-slowing-down toward the magnetically ordered state is discussed as a novel type of multifunctional coupling in the short-range ordered state between the possible...

NMR evidence for antiferromagnetic spin fluctuations in superconducting λ-(BETS)2GaCl4

The microscopic magnetic properties of superconducting λ-(BETS) 2 GaCl 4 (T C ≅6 K) have been investigated by both 77 Se and 1 H NMR in the normal state. Both the Knight shifts and the linewidths of 77 Se NMR decrease significantly below 30 K, whereas 1/( 77 T 1 T) does not show any corresponding decrease down to 7.1 K. 1/( 1 T 1 T) of 1 H NMR, on the other hand, increases remarkably below 25 K. These results are explained consistently in terms of the development of antiferromagnetic spin fluctuations in π-electrons at low T.

π–d Correlation in λ-(BEDT-TSF)2Fe1-xGaxCl4 by ESR Measurements

Electron spin resonance experiments with 15.4 GHz microwave have been performed on λ-(BEDT-TSF) 2 Fe 1- x Ga x Cl 4 ( x =0.0, 0.4, and 1.0) single crystals at various temperatures down to 1.5 K, wh...