Study Of Structure And Electrical Transport Properties On Low-dimensional Materials Under High Pressure

As a fundamental thermodynamic parameter,pressure is an important tool to modify physical properties.By compressing the atomic spacing effectively,pressure can modulate the band structure,band width and phonon frequency,thus achieving the purpose of modifying the physical properties of functional materials.Compared with chemical doping,high pressure does not change the stoichiometric ratio of the material,either not increase lattice disorder or additional charge carrier.Thus,pressure achieves more efficient modification on electronic material.Low-dimensional materials have attracted widely attention from many novelty physical properties,such as charge density wave(CDW),unconventional superconductivity,and topological states.In this paper,two low-dimensional materials,1T-VSe₂ and Fe Se,were synthesized and studied systematically under high pressure.The details are as follows:(1)The structure of 1T-VSe₂ under high-pressure was studied by synchrotron X-ray diffraction.The result shows that the structure is stable below 14.6 GPa and new diffraction peaks appear under further compression,which means a structural phase transition,and further refinement shows that the space group changed to C2/m(1T'phase)from P-3m1(1T phase).The ratio c/a of the lattice constant associated with CDW transition temperature(T_CDW)is found to decrease under compression,and no clear volume change is observed across the 1T-1T'transition,which suggests a pressure induced second-order phase transition.(2)By combing the electrical transport properties measurement and Hall effect measurement,the effect of pressure on the resistivity and carriers concentration of1T-VSe₂ were studied.The results show that Kondo effect was suppressed while T_CDWis enhanced under high pressure,which reaches about 358 K at 14.6 GPa..CDW phase disappears under higher pressure,and the sample does not show superconductivity up to 29.6 GPa.By measuring Hall effect,we obtain the carrier concentration.It decreases gradually from?1.0×10²² cm^-3 at 1.5 GPa to?8.5×10²¹cm^-3 at 14.6 GPa.Above 14.6 GPa,the carrier concentration shows a sharp decrease and reduces to 5.3×10²¹ cm^-3 at 18.1 GPa.(3)Fe Se single crystal with T_c=9.5 K have been synthesized via CVT method,and the electrical transport and magnetoresistance were studied under high pressure.The temperature dependence of resistivity shows that the T_c of Fe Se single crystal increases from 16.8 K to 25 K when pressure increases from 0.5 GPa to 1.6 GPa,and the temperature of the resistivity upward,which related to the antiferromagnetic,increases from 26.8 K to 38.9 K.Magnetoresistance results show pressure induced enhancement of anti-ferromagnetic fluctuations,and the violation of Kohler's rule indicates the multiband characteristics of Fe Se.