Tokutaro Komatsu

The Role of a Three Dimensionally Ordered Defect Sublattice on the Acidity of a Sulfonated Metal–Organic Framework

Understanding the role that crystal imperfections or defects play on the physical properties of a solid material is important for any application. In this report, the highly unique crystal structure of the metal-organic framework (MOF) zirconium 2-sulfoterephthalate is presented. This MOF contains a large number of partially occupied ligand and metal cluster sites which directly affect the physical properties of the material. The partially occupied ligand positions give rise to a continuum of pore sizes within this highly porous MOF, supported by N2 gas sorption and micropore analysis. Furthermore, this MOF is lined with sulfonic acid groups, implying a high proton concentration in the pore, but defective zirconium clusters are found to be effective proton trapping sites, which was investigated by a combination of AC impedance analysis to measure the proton conductivity and DFT calculations to determine the solvation energies of the protons in the pore. Based on the calculations, methods to control the pKa of the clusters and improve the conductivity by saturating the zirconium clusters with strong acids were utilized, and a 5-fold increase in proton conductivity was achieved using these methods. High proton conductivity of 5.62 × 10(-3) S cm(-1) at 95% relative humidity and 65 °C could be achieved, with little change down to 40% relative humidity at room temperature.

New ambient-pressure organic superconductors based on BEDT-TTF, Cu, N(CN)2 and CN with Tc=10.7K and 3.8K

Abstract Syntheses and superconductivities of two new ambient-pressure organic superconductors of BEDT-TTF with Cu, N(CN)2 and CN are described. The resistivity measurements showed Tc (midpoint of resistive transition) of 10.7K (high Tc salt) and 3.8K (low Tc salt) while the SQUID measurements showed the onset of superconductivity at 10.5–11.0K and >3.5K, respectively. They have almost the same lattice parameters but slightly different diffraction patterns and crystal densities.

Magnetic Determination of Ginzburg-Landau Coherence Lengthfor Organic Superconductor κ-(BEDT-TTF)2X (X=Cu(NCS)2,Cu[N(CN)2]Br): Effect of Isotope Substitution

The Ginzburg-Landau coherence lengths are determined from the temperature dependence of the magnetization by fitting measured results with the renormalization theory of the fluctuation developed for layered superconductors. The derived interlayer coherence lengths are 6±2 A and 3.2±0.5 A for κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br and κ-(BEDT-TTF) 2 Cu(NCS) 2 , respectively, which are remarkably shorter than the interlayer spacing. It is also found that these coherence lengths are virtually unaffected by isotope substitution in BEDT-TTF molecules.

Recent progress in organic superconductors

Abstract This paper describes part of the present status of the research in Japan on the crystal structures and physical properties of organic superconductors κ-(BEDT-TTF) 2 [Cu(NCS) 2 ], κ-(BETD-TTF) 2 Cu[N(CN) 2 ]Br, α-(BEDT-TTF) 2 [NH 4 Hg(SCN) 4 ] and other related materials. The Cu(NCS) 2 salt shows an inverse isotope effect. The Cu[N(CN) 2 ]Br salt shows T c of 11.7 K. H c2 value and lattice parameters of these κ-type salts are compared.

Overview of organic superconductor κ-(BEDT-TTF)2[Cu(NCS)2] and its related materials

Abstract Crystal structures and physical properties of organic superconductors κ-(BEDT-TTF) 2 Cu(NCS) 2 , κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br and α-(BEDT-TTF) 2 [NH 4 Hg(SCN) 4 ] are discussed. The Cu(NCS) 2 salt shows an inverse isotope effect. The Cu[N(CN) 2 ]Br salt shows Tc of 11.7–11.8K. Conductivities, Hc 2 values and lattice parameters of these κ-type salts are compared.

Anomalous resistive transition characteristics in the mixed state of high-Tc κ-type BEDT-TTF superconductors

Abstract The resistive transition of organic superconductors such as κ-(BEDT-TTF) 2 and κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br in a high magnetic field normal to the conducting plane exhibits a variety of behaviors. We propose that this is ascribed in part to the effect of superconducting weak links appearing at the boundaries between superconducting regions in inhomogeneous samples.

Physical Properties and Dimensionality of κ-(BEDT-TTF)2Cu(CN)[N(CN)2]

The electrical resistivity measurements for the novel organic superconductor κ-(BEDT-TTF) 2 Cu(CN)[N(CN) 2 ] ( T c =11.2 K) [BEDT-TTF represents bis (ethylenedithiolo)tetrathiafulvalene] under pressure or in high magnetic field were studied. Several physical parameters, such as the superconducting transition temperature ( T c ), the depressibility of T c with pressure (- d T c / d P ), the residual resistance ratio ( R R R ), the upper critical field ( H C2 ), and the effective Ginzburg-Landau mass anisotropy between perpendicular and parallel to the conducting plane ( m // / m ⊥ ), are determined. They are discussed in comparison with those of other κ-type organic superconductors with T c above 10 K; κ-(BEDT-TTF) 2 Cu(NCS) 2 and κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br. A simple relation was observed between these parameters ( T c , - d T c / d P and R R R ) and the dimensionality of the electronic structures of these superconductors.

Isotope Effect on Physical Properties of BEDT-TTF Based Organic Superconductors

Abstract The isotope shifts of the superconducting transition temperature for K-(BEDT-TTF)2Cu[N(CN)2]Br were evaluated from the resistivity and dc magnetization measurements. The deuteration of the terminal ethylene groups of the donor molecule lowered Tc by 0.4 to 0.5K, whereas the 13C substitution on the same fragments gave no isotope shifts within the experimental accuracy.

On superconductivity of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

Abstract The resistivity decrease in α-(BEDT-TTF)2KHg(SCN)4 appearing in the temperature region below 300 mK was found to be suppressed by a magnetic fields of less than 0.1 T. We argue that the decrease in resistivity is due to the superconductivity occurring in a fibril-like structure. The observed behavior is in part due to the inhomogeneous molecular arrangement, but is also related to the subtle interplay between magnetism and superconductivity in this system.

Resistive superconducting transition of [kappa]-type BEDT-TTF organic superconductors in a magnetic field

The superconducting transition of the organic compoundsκ-(BEDT-TTF)2X is studied by resistive measurement in a magnetic field up to 10 T applied normal to the conducting plane. For the salts withX=Cu[N(CN)2]Br andX=CuCN[N(CN)2] the transition shows fanshaped broadening caused by superconductivity fluctuation. For theX=Cu(NCS)2 salt the resistivity shows a peak in the transition region in a magnetic field below 4 T.This phenomenon is suppressed in defect-reduced samples for intralayer conduction. We discuss this peak in relation to the thermal fluctuation on the Josephson junction structures in this salt.