Theory of Discommensurations and Re-Entrant Lock-in Transition in the Charge-Density-Wave State of 2H-TaSe₂

Abstract

Charge density wave (CDW) states of 2H-TaSe₂ are studied on the basis of Jacobs and Walker’s free energy. Discommensuration (DC) structures are inspected in detail under the situation where the stable commensurate (C) state change from one to another as the interlayer coupling constant is increased. Numerical calculations, in which a sufficiently large number of harmonics are taken into account, exhibit double honeycomb, triangular and hexagonal DC structures. The formation mechanism of various DC structures is discussed and compared with McMillan’s DC diagram. Crossing over between different DC structures is also studied. Phase diagrams in the temperature and interlayer coupling plane are constructed to explain the re-entrant lock-in transition of 2H-TaSe₂. In this connection a suggestion is made on the nature of the high-pressure C state.