Hengbo Cui

A Single-Component Molecular Superconductor

The pressure dependence of the resistivities of a single-component molecular conductor, [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate) with semiconducting properties at ambient pressure was examined. The four-probe resistivity measurements were performed up to ∼10 GPa using a diamond anvil cell. The low-temperature insulating phase was suppressed above 7.5 GPa and the resistivity dropped, indicating the superconducting transition occurred around 7.5-8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. The high-pressure crystal and electronic band structures were derived by the first-principle calculations at 6-11 GPa. The crystal was found to retain the semiconducting band structure up to 6 GPa. But the electron and hole Fermi surfaces appear at 8 GPa. These results of the calculations agree well with the observation that the pressure-induced superconducting phase of [Ni(hfdt)2] appeared just above the critical pressure where the low-temperature insulating phase was suppressed.

Direct observation of collective modes coupled to molecular orbital-driven charge transfer

The making of a molecular movie Phase transitions familiar from everyday life, such as boiling or melting, are caused by changing the temperature. In the laboratory, however, researchers can also change the phase of a material by shining intense light on it. During such transitions, changes occur in both the electronic and lattice structure of the material. Ishikawa et al. used ultrafast optical and electron diffraction probes to monitor both types of change simultaneously during a photo-induced phase transition in a molecular crystal. The resulting molecular movies showed expansion of the intermolecular distance, flattening of the molecules, and tilting of molecular dimers. Science, this issue p. 1501 Ultrafast spectroscopy and electron diffraction are used to create molecular movies of a phase transition in Me4P[Pt(dmit)2]2. Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Metallization of the single component molecular semiconductor [Ni(ptdt)2] under very high pressure.

The four-probe electrical resistivity measurements on a single-component molecular semiconductor [Ni(ptdt)(2)] (Ni(S(8)C(9)H(6))(2)) was performed up to 20.7 GPa by using a diamond anvil cell. A newly improved method was employed to reduce the effect of uniaxial pressure. The semiconducting behavior persisted up to 17.9 GPa. The pressure-induced metallization began to appear at 18.9 GPa, and the complete metallic behavior down to 1 K was observed at 19.9 GPa.

Electrical Resistivity of Tetramethyltetratelluronaphtalene Crystal at Very High Pressures : Examination of the Condition of Metallization of π Molecular Crystal

Four-probe resistivity measurements were performed on the TMTTeN crystal by using a diamond anvil high-pressure cell up to 30 GPa. The crystal could not be metallized though the room-temperature resistivity decreased to a very small value (1.5 x 10-3 Omega cm). Although single-component molecular metals are composed of molecules, the pressure-induced metallization of a single-component pi molecular crystal while maintaining the initial molecular structure appears to be very difficult.

Supramolecular Ni(dmit)2 salts with halopyridinium cations -development of multifunctional molecular conductors with the use of competing supramolecular interactions

Halopyridinium cations with multiple halogen and hydrogen bonding donor sites have been used as the counter ions for the Ni(dmit)2 anion (dmit = 1,3-dithiole-2-thione-4,5-dithiolate). Because of the competing supramolecular interactions, such as halogen and hydrogen bondings, some of the Ni(dmit)2 salts showed complicated structures with multiple functionality that originates from the intriguing molecular arrangements. In these multifunctional salts, Ni(dmit)2 molecules play two roles, conducting and magnetic, depending on the molecular arrangement in the layers that they belong to. The physical properties of these salts have been examined by conductivity measurements, with or without hydrostatic/uniaxial pressure, as well as magnetic susceptibility and band calculations. By analysing the low temperature conductivity, it can be concluded that the itinerant electrons in the conducting layer have magnetic coupling with the localized spins in the magnetic layers to result in a Kondo singlet formation.

Structural anomalies associated with antiferromagnetic transition of single-component molecular metal [Au(tmdt)2].

The crystal structure of the single-component molecular metal [Au(tmdt)(2)] was examined by performing powder X-ray diffraction experiments in the temperature range of 9-300 K using a synchrotron radiation source installed at SPring-8. The structural anomalies associated with antiferromagnetic transition were observed around the transition temperature (T(N) = 110 K). The continuous temperature dependence of the unit cell volume and the discontinuous change in the thermal expansion coefficient at T(N) suggested that the antiferromagnetic transition of [Au(tmdt)(2)] is a second-order transition. Au(tmdt)(2) molecules are closely packed in the (021) plane with two-dimensional lattice vectors of a and l (= 2a + b + 2c). The shortest intermolecular S...S distance along the a axis shows a sharp decrease at around T(N), while the temperature dependence of l exhibits a characteristic peak in the same temperature region. A distinct structure anomaly was not observed along the direction perpendicular to the (021) plane. These results suggest that the molecular arrangement in only the (021) plane changes significantly at T(N). Thus, the intermolecular spacing shows anomalous temperature dependence at around T(N) only along that direction where the neighboring tmdt ligands have opposite spins in the antiferromagnetic spin structure model recently derived from ab initio band structure calculations. The results of single-crystal four-probe resistance measurements on extremely small crystals (approximately 25 microm) did not show a distinct resistance anomaly at T(N). The resistance anomaly associated with antiferromagnetic transition, if at all present, is very small. The Au-S bond length decreases sharply at around 110 K; this is consistent with the proposed antiferromagnetic spin distribution model, where the left and right ligands of the same molecule possess opposite spin polarizations. The tendency of the Au-S bond to elongate with decreasing temperature is ascribed to the small energy gap between the pd sigma(-) (or SOMO + 1) and the asym-Lpi(d) (or SOMO) states of the Au(tmdt)(2) molecule.

The pressure effect on the antiferromagnetic and superconducting transitions of κ-(BETS)2FeBr4

The temperature–pressure phase diagram of the first antiferromagnetic organic superconductor κ-(BETS)2FeBr4 shows that the Neel temperature increases with pressure, while the superconducting transition temperature decreases rapidly around 3 kbar.

Phase diagram and anomalous constant resistivity state of a magnetic organic superconducting alloy, λ-(BETS)2FexGa1−xCl4

The combination of an organic superconductor, λ-(BETS)2GaCl4, and a field-induced organic superconductor, λ-(BETS)2FeCl4, provided unprecedented organic alloys, λ-(BETS)2FexGa1−xCl4, with the superconducting phase expanded by intruding into the adjacent antiferromagnetic insulating phase and an anomalous constant resistivity state located between the normal metallic and zero-resistivity states.

Emergence of the Dirac Electron System in a Single-Component Molecular Conductor under High Pressure

Single-component molecular conductors can provide a variety of electronic states. We demonstrate here that the Dirac electron system emerges in a single-component molecular conductor under high pressure. First-principles density functional theory calculations revealed that Dirac cones are formed in the single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate), which shows temperature-independent resistivity (zero-gap behavior) at 12.6 GPa. The Dirac cone formation in [Pd(dddt)2] can be understood by a tight-binding model. The Dirac points originate from the HOMO and LUMO bands, each of which is associated with different molecular layers. Overlap of these two bands provides a closed intersection at the Fermi level (Fermi line) if there is no HOMO-LUMO coupling. Two-step HOMO-LUMO couplings remove the degeneracy on the Fermi line, resulting in gap formation. The Dirac cones emerge at the points where the Fermi line intersects with a line on which the HOMO-LUMO coupling is zero.

Compensation of Effective Field in the Field-Induced Superconductor kappa-(BETS)~2FeBr~4 Observed by ^7^7Se NMR

We report results of 77Se NMR frequency shift in the normal state of the organic charge-transfer salt kappa-(BETS)2FeBr4 which shows magnetic field-induced superconductivity (FISC). From a simple mean-field analysis, we determined the field and the temperature dependences of the magnetization m(pi) of the pi conduction electrons on BETS molecules. We found that the Fe spins are antiferromagnetically coupled to the pi electrons and determined the exchange field to be J = -2.3T/microB. The exchange field from the fully saturated Fe moments (5 microB) is compensated by an external field of 12 T. This is close to the central field of the FISC phase, consistent with the Jaccarino-Peter local field-compensation mechanism for FISC [Phys. Rev. Lett. 9, 290 (1962)].