  Patterns and periods of charge density waves (CDW) in transition metal
dichalcogenides exhibit complex phase diagrams that depend on pressure,
temperature, metal intercalation, or chalcogen alloying. The phase diagrams
have been understood in the context of phenomenological Landau free energy
model, but the microscopic mechanisms underlying them are still not clear.
Here, we present a new microscopic theory based on the interatomic potential,
and have explicitly calculated temperature-dependent phase diagrams using the
interatomic potential extracted from first-principles calculations. With
detailed atomic structures, the calculated phase diagram of monolayer H-TaSe2
successfully reproduces the experimental features such as commensurate lock-in
and stripe phase. Our work shows the complex behaviors of charge density wave
are originated from the relatively simple structure of the interatomic
potential and elucidates the role of lattice anharmonicity on the CDW phase
transition.
