Research On Terahertz Time Domain Spectroscopy Of Strong Correlation System Materials

Terahertz(THz)pulse is the electromagnetic wave between 0.1-10 THz(or the corresponding wavelength range is 3 mm-30?m).It is located in the band of photonics and electronic,and it is a bridge connecting macroscopic electronics and microscopic photonics.Terahertz optoelectronic technology has a great practical application prospect in electronic,information,life,national defense,public safety,space and other aspects.Optical experiment as a non-contact does not need to make electric contacts on samples and not affected by the shape and size and is an important tool to characterize and study the properties of the materials and device physics.THz TDS has great advantages and cannot be irreplaceable as a measurement technology.For example,THz TDS enables one to determine the real and imaginary of optical parameters directly without the need for the Kramers-Kronig transformation and accurately obtain more sample materials parameters.Therefore,we use terahertz time-domain spectroscopy to carry out terahertz waveband spin dynamics measurement research on La_0.33Pr_0.34Ca_0.33MnO₃ film and MnCr₂O₄ strong correlation system materials,and systematically study terahertz waves and strong correlation system materials under strong magnetic fields.The terahertz transmission spectrum of the interaction.By deriving the formula and fitting of the terahertz transmission spectrum,the optical parameters of the sample can be analyzed without the Kramers-Kronig relationship,so as to explore the physical mechanism related to the material of the strong correlation system.The main research contents of this paper are as follows:(1)The metal insulator transition process of La_0.33Pr_0.34Ca_0.33MnO₃ film was studied using terahertz time-domain spectroscopy,and the La_0.33Pr_0.34Ca_0.33MnO₃ film with nano-size thickness was observed using terahertz transmission spectroscopy.The insulator facing metal occurred with the decrease of temperature.The process of phase transition.This transformation process occurs in a wide temperature range(30-105K).The conductivity of the La_0.33Pr_0.34Ca_0.33MnO₃ film in the terahertz band was also obtained experimentally,and it was found that the conductivity of the La_0.33Pr_0.34Ca_0.33MnO₃ film at low temperature(15?60 K)does not show the non-ideal metallic properties.Drude behavior is better to obey the Drude-Smith model with backscatter.It can be indirectly concluded that there is not only a ferromagnetic metal phase in the low temperature region,but also an orderly insulating phase of antiferromagnetic charges.However,in the high temperature region,such as above 105K,the La_0.33Pr_0.34Ca_0.33MnO₃ film is dominated by the antiferromagnetic ordered insulating phase,and the conductivity no longer meets the Drude-Smith model.(2)Using terahertz time-domain spectroscopy to study the terahertz transmission spectra of MnCr₂O₄ polycrystalline samples at low temperatures under a magnetic field of up to 9T,we have observed resonance absorption in the sub-THz range.Under the magnetic field of 4T to 9T,the resonant absorption peak has an obvious linear blue shift with the increase of the magnetic field,that is,the resonant absorption peak moves to the high frequency band with the increase of the external magnetic field.The measurement of electron spin resonance(ESR)and the fitting of ferromagnetic resonance theory confirmed that the magnetic field-driven resonance absorption originated from ferromagnetic resonance.Our work provides new insights for the spin dynamics in THz region in the perovskite manganite materials and chromite spinel compounds.