Study On The Transition-metal Chalcogenides Under High Pressure

Superconducting materials have great potential due to their physical characteristics such as zero resistivity and complete diamagnetism,thus it is significant to explore and adjust superconducting materials.Particularly,layered chalcogenide materials with weak Van der Waals interaction between interlayers can be easily adjusted to show superconductivity by chemical doping and high pressure which are beneficial for us to understand several basic physical problems such as competing orders and their attribution to superconductivity,the relationship between charge density wave(CDW)and superconductivity.In this thesis,the layered chalcogenide superconducting candidate materials and the topological semimetal Rh Sn were characterized in detail through high pressure,low temperature and magnetic field physical properties measurements to clarify the relationship between CDW and superconductivity.The innovative experimental results we obtained are as follows:1,The physical properties of the transition-metal chalcogenide 2H-TaSe_2-xS_x(x=0,0.2,1)single crystal,such as electrical resistivity,ac magnetic susceptibility,and X-ray diffraction under pressure were systematically studied.The complete temperature-pressure phase diagram shows that there is a clear competitive relationship between CDW and superconductivity(SC).Under pressure,the CDW is suppressed while the T_cof SC is enhanced significantly and saturated which is different from the previous reports.The highest superconducting phase transition temperature under pressure is twice than that of the optimal sulfur-doped TaSe₂at ambient pressure.In addition,we found that both physical pressure and chemical pressure can melt CDW and enhance SC through lattice contraction.However,the chemical doping introduces chemical disorders which destruct the CDW ground state faster and allow more electrons to participate in cooper pairing,but restrict the further enhancement of T_cafter the collapse of CDW.2,The effects of disorder and hydrostatic pressure on the CDW and SC in the2H-Ta S₂are studied.The superconducting phase transition temperature T_cexhibits“dome-like”pressure dependence for crystals in a clean limit and reaches the maximum of?9.15 K at critical pressure(P_c)where CDW collapses completely,which is accompanied by a sizable reduction in the temperature exponent and one order enhancement in magnitude of quadratic temperature coefficient of normal-state resistivity near P_cindicating an enhancement of density of state at Fermi level.Pressure-induced crossover from CDW to SC is evidenced by ac magnetic susceptibility indicating a strong competition between CDW and SC.For dirty crystals,there is no clear CDW at ambient pressure and the pressure dependences of the“dome-like”T_cbroaden up in comparison with the clean crystals possibly due to the competition between short range CDW and SC.3,We investigated the resistivity,magnetoresistance,Hall resistivity of1T-Cu_0.03Ti Se₂under various pressures and temperatures and obtained the phase diagrams of CDW and SC.1T-Cu_0.03Ti Se₂shows a CDW at 150 K and Hall coefficient is negative due to excessive electrons introduced by Cu doping.It is found that the dome-like SC emerging right after the collapse of CDW near the critical pressure indicating the complete competition relationship between CDW and SC.The energy bands are dominated by electron-type carriers before the collapse of the CDW phase transition and reverse into the hole-type one after the collapse of CDW.Meanwhile,the mobility increases up to nearly five times in magnitude and Kohler slope tends to saturate.Our results suggest the reconstruction of Fermi surface associates with pressure-induced collapse of CDW.In other words,both the CDW and SC phase are associated with the structure of Fermi surface.4,We reported the growth,physical properties at ambient pressure and high-pressure effects of high-quality single-crystal SnO.We found the single-crystal SnO shows a strong anisotropy.Its in-plane resistivity exhibits a metal-insulator transition while the out-of-plane resistivity shows insulating behaviour.The anisotropic resistivity ratio increases with decreasing temperature and can reach?400 at low temperature.Its multiband electronic character with dominant hole-type carriers is revealed via the Hall coefficient and the appearance of low-lying phonon modes are evidenced by specific heat.The appearance of a metal-insulator phase transition in single-crystal SnO is attributed to the slight difference in the lattice parameters ratio c/a,the atomic coordinate of Z(Sn),and the chemical pressure effect.Under hydrostatic pressures generated in a cubic anvil pressure cell,the insulating state melts at the critical pressure 3-4 GPa,and the temperature exponent n of resistivity in the metallic state increases gradually from 2 to 3 with increasing the pressure.A dome-like superconductivity of single-crystal SnO is induced by the diamond pressure cell under extremely low temperature and the highest superconducting transition temperature of single-crystal SnO is close to that of polycrystals.In addition,single-crystal SnO has a small upper critical field?500 Oe under pressure which might belong to weakly coupled phonon-mediated superconductor.5,We have studied the effect of pressure on the electrical transport properties of Rh Sn single crystal with chiral fermions.At ambient pressure,Rh Sn exhibits large positive magnetoresistance(MR)and field-induced resistivity upturn at low temperatures.We find that with increasing pressure the temperature-dependent resistivity of Rh Sn varies minutely,whereas the value of MR at low temperatures decreases significantly.Analyses of the nonlinear Hall conductivity with two-band model indicate that the carrier concentrations do not change significantly with pressure,but the mobilities for hole carriers are reduced quickly.The significant reduction of MR under high pressures is ascribed to the reduction of hole-type carrier mobility and residual resistivity ratio RRR.