10.3 Molecular Crystals as Stage for Novelties
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imagine not only librational phonons but also other large-amplitude motions and
resulting disorders. In this respect, the coupling between such degrees and the electronic system inside the crystal is of interest. The direct coupling may be scarce due
to the significant difference in characteristic time scales in electronic and molecular
dynamics. However, we can list the following. The degree of conformational disorder in the organic superconductor significantly alters the transition temperature
[67, 68]. The electronic/magnetic properties and the instability intrinsic to the onedimensionality of the electronic system in the inorganic MMX complexes offer an
example of a systematic study [7, 8, 12]. The description of thermodynamic and
magnetic properties by a single statistical model can also be regarded as another
example [24].
10.3.2 Phenomena Involving Molecular Flexibility
The internal degrees of motional freedom is also intrinsic to molecules within the
hierarchy of the material world. In this respect, the phenomena involving molecular
flexibility are, in principle, new phenomena characteristic to molecular crystals. The
glass transition and phase transitions related to the molecular flexibility, described
in Chap. 9, belong to the category. The entropic stabilization of some mesophases
seems ready for practical utilization in the phenomena related to the large entropy
capacity of alkyl groups.
10.3.3 Dipolar Ising System
Spin systems have played an essential role in the study of phase transitions and critical
phenomena [69–71]. It is somewhat interesting to see the fact that the exact solutions
exist for the cases where the interaction between spins localizes in the neighboring
spins (two-dimensional Ising model [72]) and extends to infinitely (mean-field theory). Generally, the analysis becomes difficult with the elongation of the interaction
range. The dipolar interaction is one with a physical counterpart decaying as ∝ r
−3
and possesses unique anisotropy, ferroic along the dipolar axis, and antiferroic laterally (see Eq. 1.18). The study of spin models with dipolar interaction has a long
history [73, 74]. Its Ising version is the dipolar Ising model (DIM), most studies of
which mainly have paid attention to critical behaviors [75–78]. Although the DIM
intuitively applies to a broad class of crystals consisting of simple polar molecules,
they do not fit for studying the physical properties of the DIM because molecules
cannot change their orientation free from the crystal lattice. The full clarification of
its properties, therefore, requires a physical realization.
Since the DIM usually refers to a classical Ising model, real spin systems where
spins interact via magnetic dipolar interaction is not ones under discussion. An isolated bistable hydrogen (H) bond (O·H· · · O or O· · · H·O) is also unsuitable because
211
imagine not only librational phonons but also other large-amplitude motions and
resulting disorders. In this respect, the coupling between such degrees and the electronic system inside the crystal is of interest. The direct coupling may be scarce due
to the significant difference in characteristic time scales in electronic and molecular
dynamics. However, we can list the following. The degree of conformational disorder in the organic superconductor significantly alters the transition temperature
[67, 68]. The electronic/magnetic properties and the instability intrinsic to the onedimensionality of the electronic system in the inorganic MMX complexes offer an
example of a systematic study [7, 8, 12]. The description of thermodynamic and
magnetic properties by a single statistical model can also be regarded as another
example [24].
10.3.2 Phenomena Involving Molecular Flexibility
The internal degrees of motional freedom is also intrinsic to molecules within the
hierarchy of the material world. In this respect, the phenomena involving molecular
flexibility are, in principle, new phenomena characteristic to molecular crystals. The
glass transition and phase transitions related to the molecular flexibility, described
in Chap. 9, belong to the category. The entropic stabilization of some mesophases
seems ready for practical utilization in the phenomena related to the large entropy
capacity of alkyl groups.
10.3.3 Dipolar Ising System
Spin systems have played an essential role in the study of phase transitions and critical
phenomena [69–71]. It is somewhat interesting to see the fact that the exact solutions
exist for the cases where the interaction between spins localizes in the neighboring
spins (two-dimensional Ising model [72]) and extends to infinitely (mean-field theory). Generally, the analysis becomes difficult with the elongation of the interaction
range. The dipolar interaction is one with a physical counterpart decaying as ∝ r
−3
and possesses unique anisotropy, ferroic along the dipolar axis, and antiferroic laterally (see Eq. 1.18). The study of spin models with dipolar interaction has a long
history [73, 74]. Its Ising version is the dipolar Ising model (DIM), most studies of
which mainly have paid attention to critical behaviors [75–78]. Although the DIM
intuitively applies to a broad class of crystals consisting of simple polar molecules,
they do not fit for studying the physical properties of the DIM because molecules
cannot change their orientation free from the crystal lattice. The full clarification of
its properties, therefore, requires a physical realization.
Since the DIM usually refers to a classical Ising model, real spin systems where
spins interact via magnetic dipolar interaction is not ones under discussion. An isolated bistable hydrogen (H) bond (O·H· · · O or O· · · H·O) is also unsuitable because