Infrared Study Of Structural Phase Transition In 1T'-MoTe₂

Weyl semimetal is a new quantum state of matter with topological non trivial band structure,which low-energy excitations are called Weyl feimions that it can be described by the Weyl equation.Duing to time reversal breaking or space inversion breaking,Dirac fermions split into two opposite chiral Weyl fermions.Theoretical predictions and experimental results verify the existence of Weyl feimions in condensed matter materials such as TaAs,TaP,NbAs,NbP,which is dubbed type-I Weyl semimetal with Vertical Dirac cone and respect Lorenz invariance.In recent years,WTe₂,Td-MoTe₂ has been proposed as type-II Weyl semimetal for it has a strongly tilted Dirac cone and break Lorenz invariance.Due to the strongly tilted energy band near the Weyl node in type-II Weyl semimetal,it has been paid more and more attention for its fantastic physical properties.The energy range of the infrared spectrum is between 1.2mev--1.8ev,which is approximately equal to the energy of the free electron intraband band transition,so it can be used to detect the absorption of electrons transition in the solid.By means of reflection or transmission infrared spectrum,we can obtaine the optical response to free electrons.Then according to the optical response to free electrons.On the one hand,we can get the interaction between electrons and other quasi particles in solids to study the charge dynamics problem.On the other hand,it can be used to detect the electron transition behavior in solid to obtain the electronic structure information in solids,especially near the Fermi surface.This paper explores the causes of structural phase transition from 1T'-MoTe₂ to Td-MoTe₂ with infrared spectra.The structural phase transition from 1T'-MoTe₂ to Td-MoTe₂,which is a central symmetric non topological state of high temperature phase changes to a non symmetric topological state of low temperature phase.We measured the resistivity of 1T'-MoTe₂ varies with temperature,it occurres structural phase transition about 250 K.Then we measured the infrared spectrum of 1T'-MoTe₂ at 300K and 8K that before and after structural phase transition.We can see that afterthe structural phase transition,a large part of free carrier in Td-MoTe₂ whose Drude component of spectral weight is lost at low energy.The loss of spectrum weight response loss of the Fermi surface,it open a new energy gap duing to band lost and recombined.We can see a new energy gap on the photoconductivity spectrum at 800 wavenumbers of 8K.The loss of Fermi surface means the decrease of the total energy of the system,the decrease of the total energy of the electron system is likely to cause the structural phase transition from 1T'-MoTe₂ to Td-MoTe₂.