Structural Phase Transitions in Layered Transition Metal Compounds

The structural phase transition is one of the most fundamental problems in solid state physics. Layered transition-metal dichalcogenides provide us with a most exciting area for the study of structural phase transitions that are associated with the charge density wave (CDW). A large variety of structural phase transitions, such as commensurate and incommensurate transitions, and the physical proper ties related to the formation of a CDW, have been an object of intense study made for many years by methods employing modem microscopic techniques. Rather recently, efforts have been devoted to the theoretical understanding of these experimental results. Thus, McMillan, for example, has developed an elegant phenomenological theory on the basis of the Landau free energy expansion. An extension of McMillan's theory has provided a successful understanding of the successive phase transitions observed in the IT- and 2H-compounds. In addition, a microscopic theory of lattice instability, lattice dynamics, and lattice distortion in the CDW state of the transition-metal dichalcogenides has been developed based on their electronic structures. As a result, the driving force of the CDW formation in the IT- and 2H-compounds has become clear. Furthermore, the effect of lattice fluctuations on the CDW transition and on the anomalous behavior of various physical properties has been made clear microscopically.

Inhalt
Microscopic Theory of Structural Phase Transitions in Layered Transition-metal Compounds.- 1. Introduction.- 2. General theory of electronlattice interaction and lattice dynamics based on the nonorthogonal tight-binding approximation.- 3. 1T-type transition-metal dichalcogenides.- 4. 2H-type transition-metal dichalcogenides.- 5. Transition-metal trichlorides MCl3 (M = Ti, V, Cr).- Appendix A. Perturbation theory in nonorthogonal representation.- Appendix B. Electronic free energy expansion in the adiabatic approximation and derivation of generalized electronic susceptibility.- Appendix C. Fröhlich model and Bloch model.- Appendix D. Expressions for the overlap and transfer integrals and their derivatives in terms of SlaterKoster integrals.- Microscopic Theory of Effects of Lattice Fluctuation on Structural Phase Transitions.- 1. Introduction.- 2. Formulation.- 3. Effects of lattice fluctuation on CDW transition.- 4. Effects of lattice fluctuations on the electronic density of states, spin susceptibility, and electrical resistivity.- 5. Supplementary remarks.- Appendix A. Feynman rules for ? and ?.- Appendix B. Evaluation of frequency sum with the use of contour integral in the complex plane.- Phenomenological Landau Theory of Charge Density Wave Phase Transitions in Layered Compounds.- 1. Introduction.- 2. Construction of Landau free energy.- 3. A simple example: single-q CDW.- 4. 2H-TaSe2.- 5. 1T-TaS2 and TaSe2.- 6. Concluding remarks.- Charge Density Waves in the Transition-metal Dichalcogenides: Recent Experimental Advances.- 1. Introduction.- 2. Charge density wave transformations observed in the Group Vb compounds.- 3. The 2a0 superlattice in the Group IVb compound 1T-TiSe2.- 4. Recent developments.- Index of Names.- Index of Subjects.