The interplay between the strong intralayer covalent-ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in α and β polytypes of In 2 Se 3 are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in αand β-In 2 Se 3 , suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in αand β-In 2 Se 3 . The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra-and interlayer forces in complex crystal structures, such as the polytype phases of In 2 Se 3 .
N RECENT YxAS courts have developed doctrines and attitudes which have greatly enlarged the liability of manufacturers, sellers and distributors.' Caveat emptor is dead. The new watchword is that the cost of the product should bear the blood of the consumer. 2 However, this trend has not extended to automobile manufacturers' liability for unsafe design of passenger cars. While a number of suits involving unsafe design have been settled, 3 plaintiffs have had little luck on the trial court level. 4 On appeals, plaintiffs are batting zero. The appellate courts have * Member of the Connecticut and Massachusetts Bars.
With this point let us close. The book is a magnificent achievement, drawing on a plethora of evidence and abrim with fresh insights and innovative approaches. These qualities should not, however, cause the book to be treated as holy writ. Its greatest success will be if it triggers a wave of scholarship that probes its findings and develops its theoretical insights.
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