Hatsumi Mori

Growth, Structure Analysis and Anisotropic Superconducting Properties of MgB2 Single Crystals

Here we report the growth of sub-millimeter MgB 2 single crystals of various shapes under high pressure in Mg–B–N system. Structure refinement using a single-crystal X-ray diffraction analysis gives lattice parameters a =3.0851(5) A and c =3.5201(5) A with small reliability factors ( R w =0.025, R =0.018), which enables us to analyze the Fourier and Fourier difference maps. The maps clearly show the B sp 2 orbitals and covalency of the B–B bonds. The sharp superconducting transitions at T c =38.1–38.3 K were obtained in both magnetization (Δ T c =0.6 K) and resistivity (Δ T c <0.3 K) measurements. Resistivity measurements with magnetic fields applied parallel and perpendicular to the Mg and B sheets reveal the anisotropic nature of this compound, with upper critical field anisotropy ratio of about 2.7.

An organic thyristor

Thyristors are a class of nonlinear electronic device that exhibit bistable resistance—that is, they can be switched between two different conductance states. Thyristors are widely used as inverters (direct to alternating current converters) and for the smooth control of power in a variety of applications such as motors and refrigerators. Materials and structures that exhibit nonlinear resistance of this sort are not only useful for practical applications: they also provide systems for exploring fundamental aspects of solid-state and statistical physics. Here we report the discovery of a giant nonlinear resistance effect in the conducting organic salt θ-(BEDT-TTF)2CsCo(SCN)4, the voltage-current characteristics of which are essentially the same as those of a conventional thyristor. This intrinsic organic thyristor works as an inverter, generating an alternating current when a static direct-current voltage is applied. Whereas conventional thyristors consist of a series of diodes (their nonlinearity comes from interface effects at the p-n junctions), the present salt exhibits giant nonlinear resistance as a bulk phenomenon. We attribute the origin of this effect to the current-induced melting of insulating charge-order domains, an intrinsically non-equilibrium phenomenon in the sense that ordered domains are melted by a steady flow.

Evidence of the Chemical Uniaxial Strain Effect on Electrical Conductivity in the Spin-Crossover Conducting Molecular System: [FeIII(qnal)2][Pd(dmit)2]5·Acetone

A novel spin-crossover molecular conductor, [Fe(qnal)2][Pd(dmit)2]5.acetone, was prepared and characterized. The crystal structural analyses of both the low- and high-temperature phases revealed that the supramolecular pi-pi interactions between the spin-crossover Fe(qnal)2 cations as well as the cation contraction play an important role in the uniaxial lattice deformation which will modulate the electrical conductivity of the conducting Pd(dmit)2 layer.

Materials Viewpoint of Organic Superconductors

Since the first discovery of an organic superconductor, TMTSF 2 PF 6 (TMTSF = tetramethyltetraselenafulvalene) with the superconducting transition temperature ( T c )=0.9 K under 12 kbar, by the su...

Transfer Integrals and the Spatial Pattern of Charge Ordering in θ-(BEDT-TTF)2RbZn(SCN)4 at 90 K

θ-(BEDT-TTF) 2 RbZn(SCN) 4 is a two-dimensional organic conductor exhibiting a metal–insulator transition at T MI =200 K. This insulating charge ordered state below T MI was investigated by a single crystal structural analysis taking account of the superlattice formation below T MI . A large displacement (about 0.2 A) of BEDT-TTF molecules associated with the transition was found. Transfer integrals between BEDT-TTF molecules were also calculated from the structural data. The two-dimensional net of transfer integrals above T MI was modulated into the quasi one-dimensional transfer integral chain by the superlattice formation below T MI . The ionicities of BEDT-TTF molecules are estimated from the intramolecular bond length distribution. As a result, charge disproportionation between BEDT-TTF molecules was found, and the ionicity was 0 to +0.2 and +0.8 to +1.0 respectively. The hole-rich sites are laid on the quasi one-dimensional transfer integral chain. We conclude that the spatial pattern of charge orde...

Electrical properties and crystal structures of mercury (II) thiocyanate salts based upon BEDTTTF with Li+, K+, NH+4, Rb+, and Cs+

Abstract A new series of BEDTTTF salts with a cation (Li + , K + , NH + 4 , Rb + , or Cs + ) as well as an anion [Hg(SCN) 4 ] 2- , was prepared. The crystal structures and the electrical properties were investigated with changing the cation from Li + (0.73 A) to Cs + (1.70 A). The superconductivity of (BEDTTTF) 2 [NH 4 Hg(SCN) 4 ] was confirmed at 0.8 K by the electrical transport measurements.

Hydrogen bond-promoted metallic state in a purely organic single-component conductor under pressure

Purely organic materials are generally insulating. Some charge-carrier generation, however, can provide them with electrical conductivity. In multi-component organic systems, carrier generation by intermolecular charge transfer has given many molecular metals. By contrast, in purely organic single-component systems, metallic states have rarely been realized although some neutral-radical semiconductors have been reported. Here we uncover a new type of purely organic single-component molecular conductor by utilizing strong hydrogen-bonding interactions between tetrathiafulvalene-based electron-donor molecules. These conductors are composed of highly symmetric molecular units constructed by the strong intra-unit hydrogen bond. Moreover, we demonstrate that, in this system, charge carriers are produced by the partial oxidation of the donor molecules and delocalized through the formation of the symmetric intra-unit hydrogen bonds. As a result, our conductors show the highest room-temperature electrical conductivity and the metallic state under the lowest physical pressure among the purely organic single-component systems, to our knowledge.

Large Dielectric Constant and Giant Nonlinear Conduction in the Organic Conductor θ-(BEDT-TTF)2CsZn(SCN)4

The dielectric constant and ac conductivity have been measured for the layered organic conductor θ-(BEDT-TTF) 2 CsZn(SCN) 4 along the out-of-plane direction, which show a relaxation behavior similar to those in the charge-density-wave conductor. Most unexpectedly, they exhibit a large bias dependence with a hysteresis, and changes in magnitude by 100–1000 times at a threshold. These findings are very similar to the collective excitation of the charge density wave. θ-(BEDT-TTF) 2 CsZn(SCN) 4 has collective excitations associated with charge ordering, though it shows no clear indication of long range order.

Novel Pressure-Induced 2k F CDW State in Organic Low-Dimensional Compound θ-(BEDT-TTF) 2CsCo(SCN) 4

A band structure calculation based on the X-ray structure analysis reveals that pressure raises quasi one-dimensionality of the Fermi surface of the title compound. A 2 k F lattice modulation is ob...

A new ambient-pressure organic superonductor, κ-(BEDT-TTF)2Ag(CN)2H2O (TC=5.0 K)

Abstract Superconductivity of κ-(BEDT-TTF)2Ag(CN)2H2O is observed by zero electrical resistivity and the Meissner effect. The critical temperature is determined to be 5.0 K at the midpoint of the resistance drop under an ambient pressue. The Meissner signal is observed below 5.0 K under the magnetic field of 5 Oe.