Hayao Kobayashi

Magnetic-field-induced superconductivity in a two-dimensional organic conductor.

The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called ‘orbital’ effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17u2009T at a temperature of 0.1u2009K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

BETS as a source of molecular magnetic superconductors (BETS = bis(ethylenedithio)tetraselenafulvalene)

BETS-based salts (BETS = nbis(ethylenedithio)tetraselenafulvalene) incorporating Fe3+ ions nare most interesting candidate systems for the observation of interplay of nconductivity and magnetism. Indeed, in nλ-(BETS)2FeCl4, conduction electrons in the nBETS layers do interact with spins localised on the n(FeCl4)− anions. Systematic studies on n(BETS)2FexGa1−xCl n4−yBry mixed-composition ncompounds, in which the electronic properties can be finely tuned by the nchemical composition, have provided a number of interesting results, nincluding the characterisation of a molecular antiferromagnetic metal, a nconductor with an antiferromagnetic metal state at ambient pressure, and an nantiferromagnetic superconductor. Unprecedented nsuperconductor–insulator transitions and superconductor–metal ntransitions have been also observed.

Origin of ferromagnetic exchange interactions in a fullerene-organic compound.

Organic ferromagnets, which exhibit exchange interactions between unpaired electrons in π-orbitals, are rare, and the origin of ferromagnetism in these compounds has so far remained unexplained. Tetrakis(dimethylamino)ethylene–fullerene[60] (TDAE–C60) shows a transition to a ferromagnetic state with fully saturated s = 1/2 molecular spins at the relatively high Curie temperature (for organic materials) of 16u2009K (ref. 4). It has been suggested that the orientations of the C60 molecules may be important for ferromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these microscopic interactions. Here we report the results of a comparative structural study of two different magnetic forms of TDAE–C 60 crystals at low temperatures, correlating the structural properties—in particular, the intermolecular orientations—with the magnetic properties. We find that both ferromagnetism and spin-glass-likexa0ordering are possible in this material, and depend on thexa0orientational state of C60 molecules. This resolves the apparentxa0contradictions posed by different macroscopic measurements, and opens the way to a microscopic understanding of π-electron ferromagnetic exchange interactions in organic materials.

Freezing of ring-puckering molecular motion and giant dielectric anomalies in metal-organic perovskites.

Pucker up! Metal-organic perovskites containing azetidinium cations, [(CH(2))(3)NH(2)][M(HCOO)(3)] (M = Mn, Cu, Zn), all show a structural phase transition, coupled with the freezing of the ring-puckering molecular motion of azetidinium cations, and an extremely large dielectric anomaly near room temperature. Molecular dynamics simulations showed the freezing of ring-puckering motion of the four-membered-ring azetidinium cation near room temperature.

Studies on Molecular Conductors: From Organic Semiconductors to Molecular Metals and Superconductors

The field of molecular conductors was pioneered by works on the semiconducting properties of phthalocyanines and condensed aromatic hydrocarbons in the middle of the last century. Around three decades ago, the first organic superconductor was reported. Since then, various new molecular superconductors based on multichalcogen π molecules such as bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), dimethyl(ethylenedithio)diselenadithiafulvalene (DMET), BETS (or BEDT-TSF = bis(ethylenedithio)tetraselenafulvalene), and [M(dmit)(2) ] (M = Ni, Pd; dmit = 4,5-dimercapto-1,3-dithiole-2-thione) including unprecedented antiferromagnetic and field-induced organic superconductors and the first single-component molecular metals were developed by the members in the Department of Chemistry of the University of Tokyo along with outside collaborators. Studies on the physical properties--especially optical properties--of various types of molecular conductors are also extensively examined.

Single-Component Molecular Metals as Multiband π-d Systems

Electronic states of single-component molecular metals M (tmdt) 2 ( M = Ni, Au) are studied theoretically. We construct an effective three-band Hubbard model for each material by numerical fitting to first-principles band calculations, while referring to molecular orbital calculations for the isolated molecules. The model consists of two kinds of base orbital for each molecule with hybridization between them, i.e., a π-character orbital for each of the two tmdt ligands, and, a p d π-orbital for M = Ni or a p d σ-orbital for M = Au centered on the metal site; this indicates that these materials can be considered as novel multiband π– d systems. We find that both orbitals contribute to realize the metallic character in Ni(tmdt) 2 . The origin of the antiferromagnetic transition observed in Au(tmdt) 2 is also discussed based on this model.

New π-extended organic donor containing a stable TEMPO radical as a candidate for conducting magnetic multifunctional materials

A novel organic electron donor containing a stable TEMPO radical (TEMPOET) was synthesized and its magnetic and electrochemical properties were investigated; furthermore, the crystal structure and physical properties of the Au(CN)2– salt of TEMPOET were clarified.

Role of anion size, magnetic moment, and disorder on the properties of the organic conductor κ-(BETS)2Ga1-xFexCl4-yBry

Abstract Shubnikov–de Haas and angular dependent magnetoresistance oscillations have been used to explore the role of anion size, magnetic moment, and disorder in the organic conductors κ ‐ ( BETS ) 2 GaBr 4 and κ ‐ ( BETS ) 2 FeCl 2 Br 2 in the isomorphic class κ ‐ ( BETS ) 2 Ga 1 ‐ x Fe x Cl 4 ‐ y Br y . The results, combined with previous work, show correlations between the anion composition ( Ga 1 ‐ x Fe x Cl 4 ‐ y Br y ) and the superconducting transition temperature, effective mass, Fermi surface topology, and the mean free path.

Partial Breakdown of the Field-induced Superconductivity by Dynamical Spin Reversal

has attracted considerable interests since the application of a sufficiently strong magnetic field usually destroys the superconducting state [1]. The origin of FISC can be explained by the Jaccarino-Peter compensation effect where the internal field, created by the Fe (III) moments, is compensated by the external field. Hence, Zeeman effect, that normally destroys the superconductivity, is suppressed under this condition [2]. To have more detailed microscopic information of the FISC state, we have employed ESR on λ-(BETS)