CDW in NbSe3

CDW in \(NbSe_3\)¶

\(NbSe_3\) is the first inorganic linear chain material in which CDW transitions were found. The structure is shown below. The units form infinite and relatively well-separated chains. They are linked together with Nb-Se bonds in perpendicular direction.

The compound shows sharp increase in resistivity \(\rho(T)\), measured along the chain direction, at T = 144K and T = 59K as shown. This indicates a partial destruction of the Fermi surface at these temperatures.

Resistivity	Normalized Resisitivity
Temperature dependence of the electrical resistivity and normalized resistivity of \(NbSe_3\).

The two phase transitions, and the metal-like character of \(\rho(T)\) is believed to be the consequences of three structurally different chains in the material. One chain remains metallic down to low temperatures, the whole the two phase transitions reflect the development of lattice distortions in the remaining two chains.

Physical Properties : Explanation¶

The physical properties of \(NbSe_3\) can be explained by assuming the formation of CDW. We know that when CDW forms, gaps open at the Fermi surface at those portions that satisfy the nesting condition.

The formation of CDW is determined by the competition between two terms in the free energy of the system: the strain energy, which increases (due to stiffening of the lattice) with the formation of superlattice distortions, and the gain in the electronic energy resulting from the opening of the gaps. The gain in the electronic energy increases with decreasing temperature because the Fermi surface is sharper at the lower temperatures.

To offset the increase in energy, the electronic gain must be larger for the CDW state to be stable. Consequently, the critical temperature is lowered. This explains the decreasing \(T_{CO}\) (see this)in the Phase diagram of AV3Sb5.