The Study Of Structure And Superconductivity On Low Dimensional Materials Under Pressure

Superconductivity is an important research in condensed-matter physics.Since the discovery of high temperature cuprate superconductors,the research of high temperature superconductivity has attracts wide concern from scientists.In this dissertation,we mainly focus on the study of structure and superconductivity on low dimensional materials under pressure.The first kind is quasi two dimensional polycyclic aromatic hydrocarbons organic superconductors.Superconductivity can be achieved from this system by doping alkali metals.The structure of the parent is simple,however,the physical properties are complex.The famous scientist Little has predicted that room temperature superconductors are likely to be achieved from organic compounds,and previous extensive theoretical and experimental studies found that these materials have several structural and electrical phase transitions under pressure.In this work,we choose quasi two dimensional polycyclic aromatic hydrocarbons as candidates,which possess strong electron-phonon and electron-electron interactions.We have synthetized Kxphenanthrene with different ratio of K and phenenthrene,aiming at exploring the condition of the emergence of superconductivity.Meanwhile,we have studied the vibrational and structural properties of parents under pressure by using Raman scattering and synchrotron Xray diffraction(XRD)measurements,which are helpful to understand the mechanism of superconductivity.Additionally,we have studied the lattice and electronic properties on similar organic materials and estimated the pressure dependence of band gap,exploring the possible metallization at high pressures.The second kind is two dimensional transition metal chalcogenide,this system are typical layered materials.The transition metals are connected with chalcogenide with covalent bond,weak van der Waals interaction plays an important role between interlayers.Lattice and electronic structure are easy to modify with the application of pressure,which can induce novel physical properties and phenomenon.We have studied the vibrational,crystal and electronic properties of MoX2(S,Te),and found that these materials undergo the transition from semiconductor to metal under pressure.For MoS2,during this transition,it undergoes first order structural transition.The transition metal chalcogenide with low dimensional structure are the typical charge density wave materials.In this dissertation,we explored the high pressure behavior of charge density wave and superconductivity of TaS2,and investigated the structure evolution of TaS2 under pressure.The detailed contents are as follows:1.We have synthetized Kxphenanthrene through solid phase method by doping different content of potassium,then we performed Raman scattering and magnetization measurements on Kxphenanthrene.The results show that only the K3 phenanthrene phase is found to exhibit the superconducting transition.Theoretical research demonstrates that the metallic feather is absent in the system where deviating from x = 3.Meanwhile,we carried out high pressure Raman and XRD measurements on coronene and phenanthrene.For coronene,the spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa.The structures of corresponding phases are indexed with space groups of P21/a for phase I(P < 1.5 GPa),P2/m for phase II(1.5 ? P ? 12.2 GPa),Pmmm for phase III(P > 12.2 GPa).While,the molecule Phenanthrene undergoes three structural transitions under pressure.As pressure is increased to 30.8 GPa,X-ray diffraction peaks of phenanthrene are clearly observed,suggesting that the molecules may still remain on lattice sites,it may amorphizes above 30.8 GPa.2.Vibrational properties of isoviolanthrone are investigated by Raman scattering.The results show the onset of a phase transition at 11.0 GPa and the formation of a new phase above 13.8 GPa.The transition is proposed to result from the changes of intra-and intermolecular bonding.The absence of the changes in the lattice modes indicates that the observed phase transition is probably a result of the structural distortions or reorganizations.The critical pressure is in good agreement with that of resistance measurements,indicating that it may associate to electron phase transition.Accordingly,we have studied optical and structural properties of tripheneylene under pressure theoretically and experimentally.The spectroscopic results demonstrate substantial change of the molecular configuration at 1.4 GPa.The structure of triphenylene is found be to stable in the pressure region studied without any evidence of structural transition from the X-ray diffraction patterns.The lattice parameters from calculation show a good agreement with experimental results.With the application of pressure,the optical absorption spectra indicate that the obtained band gap systematically decreases.The theoretical calculations show pressure-induced increase of the intermolecular overlap results in a vanishing of the band gap at 180 GPa,yielding a pressure-induced metallization.3.X-ray diffraction,Raman spectroscopy,and electrical conductivity measurements of MoS2 are performed.Above 20.0 GPa,we find discontinuous changes in Raman spectra and XRD patterns which provide evidence for isostructural phase transition from 2Hc to 2Ha modification through layer sliding.This first-order transition,which is completed around 40.0 GPa,is characterized by a collapse in the c-lattice parameter and volume.After the phase transition completion,MoS2 becomes metallic in new 2Ha phase.4.We have studied the vibrational,structural and electronic properties of 2H-MoTe2 by using multiple experimental techniques and ab initio calculations.Both the experiments and calculations consistently demonstrate that MoTe2 undergoes a semiconductor-to-metallic transition above 10.0 GPa.Unlike MoS2,the semiconductor to metal transition is driven by the gradual tunability of electric structure and band gap without structural transition.The discontinuities of Raman shifts at 10.0 GPa is interpreted in terms of the change of compressibility.During the transition from semiconductor to metal,MoTe2 possesses highly tunable transport properties under pressure including the almost 10 orders of magnitude decrease in resistivity,5 orders increase in carrier concentration,and 3 orders decrease in mobility.5.We have performed the pressure dependence of transport properties,Raman spectra an X-ray diffraction measurements on 2H-TaS2.The outcomes demonstrate superconducting transition temperature(TC)is remarkable enhanced nearly six times than the initial value with the application of pressure,while the charge density wave state was suppressed.Above 7.0 GPa,charge density wave vanished,accompanied with the change of carrier concentration from negative to positive,probably due to the reconstruction of the Fermi surfaces.Through the standard resistivity fit in normal state,the decreasing coefficient A and the increasing residual resistivity will account for the decrease in TC.The study of high pressure XRD indicates that no structural transition take place in the pressure region studied.By fitting the crystal structure,we found that the obtained ratio of a/c shows a close relationship with TC and TCDW.With the increase of the ratio of a/c,TC increases,however,charge density wave transition temperature(TCDW)decreases.