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Instruments / Technical Area	Publication Details	Published
I19-Small Molecule Single Crystal Diffraction	High-pressure crystal structure and unusual magnetoresistance of a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) Hengbo Cui , Hamish H.-M. Yeung , Yoshitaka Kawasugi , Takaaki Minamidate , Lucy K. Saunders , Reizo Kato Crystals 11 DOI: 10.3390/cryst11050534 Diamond Proposal Number(s): [20934] Open Access Show Abstract ▼ Abstract: A single-component molecular crystal [Pd(dddt)2] has been shown to exhibit almost temperature-independent resistivity under high pressure, leading theoretical studies to propose it as a three-dimensional (3D) Dirac electron system. To obtain more experimental information about the high-pressure electronic states, detailed resistivity measurements were performed, which show temperature-independent behavior at 13 GPa and then an upturn in the low temperature region at higher pressures. High-pressure single-crystal structure analysis was also performed for the first time, revealing the presence of pressure-induced structural disorder, which is possibly related to the changes in resistivity in the higher-pressure region. Calculations based on the disordered structure reveal that the Dirac cone state and semiconducting state coexist, indicating that the electronic state at high pressure is not a simple Dirac electron system as previously believed. Finally, the first measurements of magnetoresistance on [Pd(dddt)2] under high pressure are reported, revealing unusual behavior that seems to originate from the Dirac electron state.	May 2021
I19-Small Molecule Single Crystal Diffraction	Electronic structure of a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) under high pressure Reizo Kato , Hengbo Cui , Takaaki Minamidate , Hamish H.-M. Yeung , Yoshikazu Suzumura Journal Of The Physical Society Of Japan 89 DOI: 10.7566/JPSJ.89.124706 Diamond Proposal Number(s): [20934] Show Abstract ▼ Abstract: We examined the high-pressure electronic structure of a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) at room temperature, on the basis of the crystal structure determined by single-crystal synchrotron X-ray diffraction measurements at 5.9 GPa. The monoclinic unit cell contains four molecules that form two crystallographically independent molecular layers. A tight-binding model of an 8 × 8 matrix Hamiltonian gives an electronic structure as a Dirac electron system. The Dirac point describes a loop within the first Brillouin zone, and a nodal line semimetal is obtained. The noticeable property of the Dirac cone with a linear dispersion is shown by calculating the density of states (DOS). The Dirac cone in this system is associated with the crossing of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands, which originates from the direct interaction between different molecular layers. This is a newly found mechanism in addition to the indirect interaction [J. Phys. Soc. Jpn. 86, 064705 (2017)]. The Dirac points emerge as a line when the HOMO and LUMO bands meet on the surface and the HOMO–LUMO couplings are absent. Such a mechanism is verified using a reduced model of a 4 × 4 matrix Hamiltonian. The deviation of the band energy (δE) at the Dirac point from the Fermi level is very small (δE < 0.4 meV). The nodal line is examined by calculating the parity of the occupied band eigenstates at time reversal invariant momentum (TRIM), which shows that the topological number is 1.	Dec 2020
I19-Small Molecule Single Crystal Diffraction	High pressure crystal structure and electrical properties of a single component molecular crystal [Ni(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) Hengbo Cui , Takao Tsumuraya , Hamish H.-M. Yeung , Chloe S. Coates , Mark R. Warren , Reizo Kato Molecules 24 DOI: 10.3390/molecules24101843 Diamond Proposal Number(s): [15000] Open Access Show Abstract ▼ Abstract: Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.	May 2019

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