210
10 Importance of Molecular Crystals
a)
b)
Fig. 10.6 Localization of the soft mode in doped BP crystals (schematic). a weak localization with
a large radius, b strong localization with a small radius. Reproduced from J. Phys. Soc. Jpn., 67,
1649 (1998) [63]
single-particle potential, the resemblance seems reasonable if we remember the vital
role of the lattice vibration in displacive phase transitions, which takes place as a result
of the softening of a relevant vibrational mode. That is, the relevant vibrational mode
(twisting mode) scarcely changes in the NA-doped BP crystals at the doping level of
1%. On the other hand, the mode weakly localizes around DFBP molecules. It seems
important to remind that the frequency of the twisting vibration of a DFBP molecule
in the crystal lattice is higher than BP molecules in the high-temperature phase, in
which molecules are planar. The localization, though weak, plausibly prevents the
softening because of the higher stability of the planar forms. Then, the overlapping of
the affected regions will probably cause the suppression of the softening. That is, the
marked difference in the impurity effect originates in a significant difference in the
localization radius, as illustrated in Fig. 10.6. Molecular dynamics (MD) simulations
mimicking the doped BP crystals reproduced the observations [66].
10.3 Molecular Crystals as Stage for Novelties
10.3.1 Coupling Between Molecular Dynamics and
Electronic System
Physical properties that originate in the electronic system inside the crystal suffer
from the effects of the lattice motional degrees of freedom. For example, electrical
resistivity due to the scattering of electric carriers (electrons and holes) by lattice
vibrations (phonon) is a ubiquitous example. Also, the superconductivity described
well by the BCS theory occurs via the formation of the Cooper pairs of electrons
mediated by phonons. When we compare molecular crystals with other kinds of
crystals, noteworthy is the presence of the orientation degrees of freedom. We can
10 Importance of Molecular Crystals
a)
b)
Fig. 10.6 Localization of the soft mode in doped BP crystals (schematic). a weak localization with
a large radius, b strong localization with a small radius. Reproduced from J. Phys. Soc. Jpn., 67,
1649 (1998) [63]
single-particle potential, the resemblance seems reasonable if we remember the vital
role of the lattice vibration in displacive phase transitions, which takes place as a result
of the softening of a relevant vibrational mode. That is, the relevant vibrational mode
(twisting mode) scarcely changes in the NA-doped BP crystals at the doping level of
1%. On the other hand, the mode weakly localizes around DFBP molecules. It seems
important to remind that the frequency of the twisting vibration of a DFBP molecule
in the crystal lattice is higher than BP molecules in the high-temperature phase, in
which molecules are planar. The localization, though weak, plausibly prevents the
softening because of the higher stability of the planar forms. Then, the overlapping of
the affected regions will probably cause the suppression of the softening. That is, the
marked difference in the impurity effect originates in a significant difference in the
localization radius, as illustrated in Fig. 10.6. Molecular dynamics (MD) simulations
mimicking the doped BP crystals reproduced the observations [66].
10.3 Molecular Crystals as Stage for Novelties
10.3.1 Coupling Between Molecular Dynamics and
Electronic System
Physical properties that originate in the electronic system inside the crystal suffer
from the effects of the lattice motional degrees of freedom. For example, electrical
resistivity due to the scattering of electric carriers (electrons and holes) by lattice
vibrations (phonon) is a ubiquitous example. Also, the superconductivity described
well by the BCS theory occurs via the formation of the Cooper pairs of electrons
mediated by phonons. When we compare molecular crystals with other kinds of
crystals, noteworthy is the presence of the orientation degrees of freedom. We can