Optical Spectroscopy Study Of R₃T₄X₁₃

A₃T₄X₁₃?where A stands for Sr,Ca,La or rare earth element,T for a transition metal and X for Ge or Sn?with cubic Yb₃Rh₄Sn₁₃-type structure shows rich broken symmetry states.Superconductivity?SC?,charge density wave?CDW?,complex magnetism,heavy fermions as well as mixed-valence behaviors have been found in this class of materials.LaSb has also attracted high attention due to the existence of extremely large magnetoresistance.Infrared optical spectroscopy is a powerful experimental technique in the exploration of charge dynamics and band structures of the materials,which has particular advance in identifying energy gaps in the broken symmetry states and revealing the correlation effect.So in this thesis,we perform optical spectroscopy study on single-crystal samples Sr₃Rh₄Sn₁₃,(Sr_0.5Ca_0.5)₃Rh₄Sn₁₃,La₃Co₄Sn₁₃,Ce₃Co₄Sn₁₃,Lu₃Ir₄Ge₁₃ and Yb₃Ir₄Ge₁₃which belong to A₃T₄X₁₃ and rock salt rare earth monopnictides LaSb.The main contents as follows:1,Sr₃Rh₄Sn₁₃and(Sr_0.5Ca_0.5)₃Rh₄Sn₁₃,which show interesting coexistence of superconductivity?SC?and structural phase transition,the structural phase transition temperatures TCDW? 138 K and 55 K,and superconducting transition temperatures TC? 4.2 K and 7K,respectively.Moreover it has been argued that this structural phase transition is associated with a charge density wave?CDW?transition of the conduction electron system.Our optical spectroscopy study observes a depletion of low frequency R???accompanied by an upturn at a little higher frequency for both samples when they undergo the CDW phase transitions.In the mean time,the real part of conductivity ?1???also show a significant spectral weight suppression in the Drude components and a broad peak-like feature forms near 2? which becomes more and more obvious as the temperature decreases.These features clearly indicate the formation of CDW energy gap for the two samples.The existence of residual Drude components in?1???below the CDW phase transition temperature TCDWindicates that the Fermi surface is only partially gapped in the CDW state.For Drude-Lorentz fitting,an additional Lorentz component appears at T < TCDW,the peak position of the additional Lorentz component approximately correspond to the value of the CDW energy gap 2?.Therefore we obtain the value of 2?/KBTCDWis roughly13 for both Sr₃Rh₄Sn₁₃and(Sr_0.5Ca_0.5)₃Rh₄Sn₁₃compounds,which is resembles to many other CDW systems but considerably larger than the mean-field value of3.52 based on the weak-coupling BCS theory.McMillan proposed an assumption that the coherence length is small,this means that phonons over a substantial part of the Brillouin zone are affected near the transition and that their contribution to the entropy can be much larger than the electronic entropy,If the phonon entropy is large enough,it reduces the transition temperature sufficiently,then results in the large 2?/KBTCDWvalue.The strong coupling theory was proposed by Varma et al.,which explains why the transition temperature is depressed an order of magnitude from the predictions of the weak-coupling theory?thereby giving the small correlation length?,The essential ingredients of the theory are the strong wave-vector dependence of electronically induced anharmonicity and mode-mode coupling.Our measurements provide optical evidence for the strong coupling characteristics of the CDW phase transition.2,La₃Co₄Sn₁₃ and Ce₃Co₄Sn₁₃were grown by Sn self-flux method,both show clearly phase transition at about 150 K,Lue et al.have proposed the phase transition belongs to a charge density wave?CDW?transition based on the specific heat and nuclear magnetic resonance measurements as the case of?Sr,Ca?₃?Rh,Ir?4Sn₁₃.Besides,La₃Co₄Sn₁₃ shows a superconducting transition at about 3K,whereas Ce₃Co₄Sn₁₃ is known as a non-superconducting heavy fermion dense Kondo system with a Kondo temperature of about 1.5 K.From the optical spectroscopy study,we observe the R???develops a dip near 1500-2000 cm-1for the two samples,which becomes more and more obvious as the temperature decreasing.The spectral changes may signal the opening of a pseudogap,as we see more clearly in the optical conductivity spectra.Upon cooling the ?1???between 600 and 4000 cm-1is gradually suppressed,the Drude component becomes narrower and the gradual removal of the spectral weight near 2000cm-1in ?1???represents the progressive formation of a pseudogap feature.But the pseudogap feature already present at 300 K.No any other specific structure could be identified across T*.Furthermore,for the density wave type energy gap the spectral weight transfer does not extend to very high energy scale.So our finding can not be related to the charge density wave transition.What's more,the suppressed spectral weight transfers mostly to the higher energy region reflects the presence of correlation effect in the compounds.We elaborate that the correlation effect should be attributed to the Co 3d electrons.3,The resistivity of Lu₃Ir₄Ge₁₃ and Yb₃Ir₄Ge₁₃show interesting “semiconductorlike”behavior unlike the metalic behavior of their isostructure compounds R₃T₄Sn₁₃.E.Morosan et al.proposed that the “semiconductor-like”behavior should ascribed to the large atomic displacement parameters?ADPs?ratio comes from the crystallographic and site disorder.What' more,Lu₃Ir₄Ge₁₃ belongs to a superconductor with low carrier density.Our optical measurement indicates that they are low carrier density metal in nature.Moreover,the study reveals that the plasma frequency decreases with decreasing temperature.This phenomenon suggests either an decrease of the conducting carrier density or/and a increase of the effective mass of carriers with decreasing temperature.The decrease of the effective mass of carriers can be explained by the change of dispersion near EF and a newly proposed “three-band” mode,this two explanations may be commonly used in explain the shift of the plasma frequency in optical spectroscopic studies.4,LaSb being identified to exhibit extremely large magnetoresistance is found to have low carrier density of the order of 1020cm-3,but ultrahigh mobility of the order of 104-105cm2V-1s-1.The electron and hole carriers densities are perfectly compensated and unaffected by temperature as the FSs show no changes with temperature based on ARPES results.More interestingly,the ARPES measurement also reveale that LaSb has a linearly dispersive bulk band and shares many similarities with the Weyl semimetal TaAs family which consists of hole FSs from the normal parabolic bands and electron FSs from the linear Weyl bands in the bulk electronic structure.A recent ARPES measurement even revealed nontrivial surface state.Our optical measurement indicates that the plasma edge locates at about 3000 cm-1,which is considerably low,the low energy scale of the plasma edge reveals a relatively small value of carrier density.More interestingly,the study reveals that the screened plasma edge increases with decreasing temperature.This phenomenon suggests either an increase of the conducting carrier density or/and a decrease of the effective mass of carriers with decreasing temperature.We attribute it primarily to the latter effect.We elaborate that the decrease of the effective mass of carriers in such low carrier density systems can be explained by the presence of either peculiar nonparabolic bands crossing EFor a heavy electron band above but close to the EF.The study offers new insight into the electronic structure of the compound.