1,131 research outputs found
Neutron scattering study of a quasi-2D spin-1/2 dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure
We report inelastic neutron scattering study of a quasi-two-dimensional S=1/2
dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure. The
spin gap {\Delta} becomes softened with the increase of the hydrostatic
pressure up to P= 9.0 kbar. The observed threefold degenerate triplet
excitation at P= 6.0 kbar is consistent with the theoretical prediction and the
bandwidth of the dispersion relation is unaffected within the experimental
uncertainty. At P= 9.0 kbar the spin gap is reduced to 0.55 meV from 1.0 meV at
ambient pressure.Comment: 4 pages, 5 figure
Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite
Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown
impressive power conversion efficiencies of above 20%. However, the microscopic
mechanism of the high photovoltaic performance is yet to be fully understood.
Particularly, the dynamics of CH3NH3+ cations and their impact on relevant
processes such as charge recombination and exciton dissociation are still
poorly understood. Here, using elastic and quasi-elastic neutron scattering
techniques and group theoretical analysis, we studied rotational modes of the
CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K)
and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold
rotational symmetry of the C-N axis (C4) along with three-fold rotation around
the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is
present. Around room temperature, the characteristic relaxation times for the
C4 rotation is found to be ps while for the C3 rotation ps. The -dependent
rotational relaxation times were fitted with Arrhenius equations to obtain
activation energies. Our data show a close correlation between the C4
rotational mode and the temperature dependent dielectric permittivity. Our
findings on the rotational dynamics of CH3NH3+ and the associated dipole have
important implications on understanding the low exciton binding energy and slow
charge recombination rate in CH3NH3PbI3 which are directly relevant for the
high solar cell performance
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