There are two possible modes of vibrations of atoms in the crystal, longitudinal and transverse. When we examine phonon dispersion curve or its band structure, usually both the modes can be observed. However, is it possible for only one type of mode to exist?
No atoms have both transverse and longitudinal vibration. In phase and out phase
Ultrasonic transducers have crystals oriented to produce longitudinal or transverse wave propagations. Piezoelectric crystals are generally used for transducer elements.
Unless the temperature gets extremely low, the vibration may go on unidirectionally.
Thankyou Prof. Zhongwen Yao, Dr. Dibya Prakash Rai and Dr. Nelaturi V Raghavaiah
It depends upon the polarization selection rules for scattering configuration in which you measure the vibrational mode of either bulk or superlattice crystalline structure. Please see Cardona et. al. Phys. Rev. B 1993 and T. Mozume J. of Crystal Growth (2005).
It depends on the type of crystals. However, in solids, only Transverse mode is available.(Ref.Soild State Theory, Ulrich Rossler, Springer(2009)pg.55,$3.5).One need to consider the frequencies to have a detail effect. Please read this in detail.
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