Naoki Toyota

Control of Charge Transfer in a Series of Ru2II,II/TCNQ Two-Dimensional Networks by Tuning the Electron Affinity of TCNQ Units: A Route to Synergistic Magnetic/Conducting Materials

The isostructural series of two-dimensional (2-D) fishnet-type network compounds, [{Ru(2)(O(2)CCF(3))(4)}(2)(TCNQR(x))] x n(solv) (R(x) = H(4), 1; Br(2), 2; Cl(2), 3; F(2), 4; F(4), 5), has been synthesized from the reactions of a paddlewheel diruthenium(II, II) complex, [Ru(2)(II,II)(O(2)CCF(3))(4)], and neutral TCNQ derivatives (TCNQR(x) = 2,3,5,6- or 2,5-halogen-substituted 7,7,8,8-tetracyanoquinodimethane) under anaerobic conditions. Corresponding Rh compounds 1-Rh-5-Rh, which are diamagnetic and redox-inactive, were also synthesized for the purpose of comparison with 1-5. According to the electron affinity of TCNQR(x), which is related to its first reduction potential, the Ru(2) series (1-5) has the requisite driving force for charge transfer from [Ru(2)(II,II)(O(2)CCF(3))(4)] to TCNQR(x), which can lead to a mixed-valence state of [{Ru(2)(4.5+)}-(TCNQR(x)(*-))-{Ru(2)(4.5+)}] for the 2-D network. Such a charge (or electron) transfer results in magnetic exchange interactions between [Ru(2)] units (S = 1 for [Ru(2)(II,II)] and S = 3/2 for [Ru(2)(II,III)](+)) via TCNQR(x)(*-) S = 1/2 radicals that lead to long-range magnetic ordering in the layer. In the present series, only 5 demonstrated the full electron transfer (1-e(-) transfer) to the mixed-valence state, whereas other members are essentially in the state [{Ru(2)(4+)}-(TCNQR(x)(0))-{Ru(2)(4+)}]. Whereas 1-4 are paramagnetic, 5 is a metamagnet undergoing 3-D long-range antiferromagnetic ordering at 95 K (= T(N)) and reverts to a magnetic-field-induced ferromagnetic state exhibiting coercivity up to 60 K. This result is consistent with the fact that TCNQF(4) has the strongest electron affinity among the TCNQR(x) molecules. Even in neutral forms, however, 1-4 can be observed to undergo thermally and/or field-activated charge transfers from [Ru(2)(II,II)] to TCNQR(x) to give semiconductors with an activation energy of 200-300 meV, which is a driving force to transport electrons over the lattice. As determined by their conducting properties, the ease of thermally and/or field-activated charge transfers is on the order of 1 < 4 < 2 approximately = 3 << 5, which is in agreement with the order of electron affinity of TCNQR(x). Indeed, a magnetic anomaly with short-range order associated with the localization of charge-transferred electrons was revealed in the low-temperature susceptibility data for 2 and 3. Finally, 5 was subjected to terahertz time-domain spectroscopy, the data from which revealed that transport hopping electrons scattered at high temperatures interact with magnetically ordered spins with the scattering being suppressed at T(N), at which temperature the real part of the complex electronic conductivity (sigma(1)) and dielectric permeability (epsilon(1)) are dramatically altered. From these collective data, we conclude that molecular design based on an interunit charge transfer in a paramagnetic lattice is an efficient route to the design of materials with synergism between magnetic and conducting properties.

Magnetic breakdown effect in organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract The Shubnikov-de Haas oscillations with a high frequency (3800T), superimposed on the well known, low frequency (625T) oscillations, have been observed above 22T perpendicular to the conducting plane in an organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . The cross-sectional area of the high frequency orbit amounts to just 100% of the first Brillouin zone. This observation is in agreement with the band structure calculations predicting the hole-like closed and the electron-like open Fermi surfaces, which are separated each other by the small energy gap along the Z-M line. The high frequency oscillations are concluded due to the magnetic breakdown effect between these two Fermi surfaces. The cyclotron effective mass of the breakdown orbit is evaluated to be (6.9±0.8) m o , which is about two times heavier than that of 3.5 m o for the hole-like Fermi surface.

Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe2

The temperature and angular dependences of upper critical fieldsHc2have been measured for several 2H-NbSe2 single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperatureTc, the value ofHc2‖(parallel to the layer planes) decreases with a positive curvature, while the value ofHc2⊥(perpendicular to the layer planes) decreases almost linearly. The ratio ofHc2‖toHc2⊥increases monotonically from 2.4 nearTcwith decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior ofHc2can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲ε12≲0.25 and −0.6≲ε2≲−0.3, where ε1 is the mass anisotropy parameter and ε2 the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.

Evidence of many-body renormalizations in some organic conductors

Abstract Magnetic quantum oscillations in (BEDT-TTF)-based salts have shown a correlation between the cyclotron mass m c and Dingle temperature T D of electrons in the basal planes. “Heavy-electron” systems like k -(BEDT-TTF) 2 Cu(NCS) 2 with m c = (2–5) m 0 have a low T D less than 1 K, while “light-electron” systems like κ L -(BEDT-TTF) 2 I 3 with m c = (0.3–0.5) m 0 have a much higher T D of several to 10 K. Both systems however have the same magnitude of the mean free path (∼ 1000 A) and of the band width (∼0.5 eV). This is an evidence of many-body renormalizations of m c and τ. An importance of the on-site and/or nearest neighbor Coulomb correlations in heavy-electron organic conductors is discussed.

Magnetic phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

Abstract The magnetic field-vs.-temperature phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4 is presented. There appears around TA=10K at H=0 a second-order phase transition from a normal metallic state to another metallic state associated with an antiferromagnetic-like magnetic-order. When the magnetic fields are applied normal to the conducting plane, TA shifts to lower temperatures, and goes to zero at HA(T=0)=23T. The inside region of the phase boundary encompassed by TA and HA is divided into two states by HB(=7T around 1K; the reversible and irreversible states of both the magnetoresistance and magnetization for H HB(T), respectively. The third characteristic field HC(T)(=10T at 1K) defined as the magnetic field where the magnetoresistance takes a maximum is located slightly above HB(T). Discussion are made on this phase diagram.

Cyclotron Mass and Dingle Temperature of Conduction Electrons Moving in Layered Planes of Organic Superconductors: β-(BEDT-TTF)2IBr2, β-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu(NCS)2

The cyclotron mass m c and the Dingle temperature T D in the layered planes of the organic superconductors are determined by the Shubnikov-de Haas effect at ambient pressure; m c =0.4 m 0 and T D =7–8 K, for β-(BEDT-TTF) 2 IBr 2 ( T c =2 K), and (0.4–0.5) m 0 and 11 K for β-(BEDT-TTF) 2 I 3 (=1 K), and 3.5 m 0 and (0.4±0.1) K for κ-(BEDT-TTF) 2 Cu(NCS) 2 ( T c =11 K). A systematic difference is clarified in m c and T D between these salts, suggestive of the origin of different T c s among these salts.

Growth of Single Crystals in the Bi-Sr-Ca-Cu-O System Using KCl as a Flux

Single crystals of Bi2Sr2CaCu2Ox having good crystallinity were successfully obtained. Nitrates of Bi, Sr, Ca and Cu were used as raw materials. Specially prepared solute calcined at low temperatures (below 500°C) were used with a KCl flux. The solvation of this solute to the KCl flux proceeded very smoothly. Single crystals having smooth surface were extracted from the KCl flux. Fundamental electrical and magnetic properties of the crystals have been investigated.

Crystallization characteristics of an amorphous Nb81Si19 alloy under high pressure and formation of the A15 phase

Amorphous Nb-19 at% Si alloy, prepared by rapid quenching from the molten state, was annealed while being subjected to a pressure of 10 GPA. X-ray diffraction investigations on the alloy specimens quenched to ambient conditions have shown that pressure greatly alters the crystallization characteristics and the cubic A15 (Nb3Si)-phase forms in preference to the tetragonal Nb3Si-phase at temperatures in the range from 710° C to 800° C. Up to 680° C, the component atoms do not show any tendency towards ordering upon crystallization and the body-centred tetragonal solid solution forms; while, at 830° C, niobium atoms diffuse to form the body-centred cubic Nb precipitates. Superconducting properties have been measured for the single-phase A15 structure with the lattice parametera=0.5155 nm with the results that the transition temperature,TC, is 3.4 K and the temperature coefficient of the upper critical field,HC2, is 1.2 MA m−1 K−1 (15 kOe K−1).

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...

1:2 TCNQ/TCNQ−. mixed salts exhibiting ferromagnetic behavior at room temperature

Abstract Ferromagnetic behavior at room temperature was observed in the tetramethylammonium (NMe + 4 ) and cesium (Cs + ) salts of tetracyanoquinodimethane (TCNQ) and its radical anion (TCNQ −. ) in a molecular ratio of 1:2. The saturation magnetizations and coercive forces are 0.79 emu/mol and ≈300 Oe for ( NMe + 4 · TCNQ −. ) · 1 2 TCNQ , and 1.46 emu/mol and ≈100 Oe for ( Cs +. · TCNQ −. ) · 1 2 TCNQ , respectively. In contrast, the 1:1 TCNQ/TCNQ −. mixed tetraethylammonium (NEt 4 + salt, (NEt 4 + ·TCNQ − ·)·TCNQ, exhibited no ferromagnetic behavior at room temperature nor at lower temperatures.