High-pressure Regulation On Novel 3d-transition-metal-based Superconductors

Superconducting materials and physics have been studied for more than 100 years,and it has always been a frontier topic in condensed matter physics.Moreover,the high-temperature superconducting mechanism has been selected as one of the 125major problems by science magazine.Due to the potentially important applications of superconductivity,the exploration of high-temperature superconductors and new superconductors has attracted widespread attention in the past few decades.As one of the three important parameters（pressure,temperature and volume）that determine the thermodynamic state of materials,pressure has unique advantages in the exploration of new superconducting materials and the study on the physical properties.On the one hand,high-pressure regulation is a relatively clean means.In principle,lattice disorder and charge carriers will not be introduced by pressure.On the other hand,it can achieve precise small-step regulation,so that the physical properties of quantum materials can be continuously controlled.Therefore,the study on the evolution of normal state and superconducting behavior of superconductors with pressure can not only help to clarify the key parameters closely related to superconducting pairing and reveal the microscopic mechanism of superconductivity,but also can be as a guide to design and synthesize the new superconducting materials.In this dissertation,twoη-carbide Ti₄X₂O（X=Co and Ir）superconductors with high upper critical magnetic fields,antiperovskite Sr(Pt_0.9Pd_0.1)₃P and spinel Mg Ti₂O₄thin film superconductors were selected as the candidates.We performed the detailed high-pressure transport measurements,and then summarized the experimental results to explain the mechanism of the upper critical magnetic field by combining the first-principal calculations.In addition,two new superconducting materials,Cu S₂ and Cu Te₂,were synthesized by high temperature and high-pressure techniques,and the detailed evolution of its superconducting states was studied by high-pressure transport measurements.Key findings are listed as follows:1.At ambient pressure,Ti₄Ir₂O shows the superconducting transition at T_c～5 K,and its upper critical fieldμ₀H_c2（0）is about 18.2 T.Through the high-pressure electrical transport measurements,we found that pressure can suppress T_c and it shows a very small pressure coefficient,i.e.,d T_c/d P≈-0.047 K/GPa（P<15 GPa）and-0.017 K/GPa（15<P<50 GPa）.Moreover,Ti₄Ir₂O has a larger bulk modulus B₀≈252 GPa and a higher Vicker hardness H_v≈10.5 GPa by measuring the high-pressure synchrotron XRD and hardness,which indicates that Ti₄Ir₂O is a strong coupling superconductor with low compressibility.Thus,the T_c is not sensitive to pressure.In addition,the upper critical field from beyond the Pauli limit at ambient pressure to less than the Pauli limit at 35.6 GPa.The first-principle calculations show that the bands associated with spin-orbit coupling（SOC）near the K point splits,accompanied by the increase of the splitting energy and the movement of band with increasing pressure.Finally,the Fermi surface exhibits the reconstruction between 31 and 41GPa,and it corresponds to the pressure at which the upper critical field changes.Our findings can provide important information for exploring new superconducting materials with both good mechanical properties and large upper critical field.2.At ambient pressure,Ti₄Co₂O exhibits the superconducting transition at T_c～2.8K and its upper critical fieldμ₀H_c2（0）is about 7.8 T.Unlike Ti₄Ir₂O,the T_c（P）exhibits the nonmonotonic evolution that it first shows an increase and then decreases again,which exhibits the dome-shaped superconducting phase below 20 GPa.When P>20GPa,the T_c shows a linear increase without saturation in the studied pressure range.However,the pressure coefficient is very small,i.e.,d T_c/d P≈+0.022 K/GPa.The pressure dependence of upper critical field can be tracked by employing the transport measurements under different magnetic fields and pressures.It is found that the critical field displays a different evolution before and after 20 GPa,which may correspond to two different superconducting phases.In addition,the evolution of T_cbetween 1.5 and 20 GPa may correlated with the change of carrier effective mass.The hardness results show that Ti₄Co₂O has a similar value H_v≈9.7 GPa with Ti₄Ir₂O.Therefore,the extremely small pressure coefficient may have direct correlation with the small changes of lattice parameters under high pressure.3.The pressure effect on the anti-perovskite superconductor Sr(Pt_0.9Pd_0.1)₃P has been reported,and the temperature-pressure phase diagram was constructed.It is found that pressure can suppress T_c and it shows the largest pressure coefficient among the reported perovskite-like superconductors,i.e.,d T_c/d P≈-0.33 K/GPa for P<10 GPa.By analyzing its normal and superconducting states,we found that Sr(Pt_0.9Pd_0.1)₃P belongs to the phonon dominated BCS-type superconductor.The evolution of T_c should have direct correlation with the decrease of density of states near the Fermi surface and the harden of phonon.4.The spinel Mg Ti₂O₄ is a newly discovered thin film superconductor with T_c～5.3 K.By employing the high-pressure transport measurements,the detailed evolution of its normal and superconducting states has been explored.Below 2.3 GPa,the T_cshows a gradual decrease,and it is consistent with the reported relation that the T_cdisplays a decline with decreasing the c-axis at ambient pressure.However,the normal state resistance exhibits an increase and shows the opposite trend to T_c with increasing pressure.This may correlate with the change of electron-electron interactions and electron-phonon interactions under high pressure.When we increase the pressure to above 2.3 GPa,the T_c exhibits an anomalous increase.Finally,the T_cwas enhanced to～6.3 K at～12 GPa,and it may have direct correlation with the rotation of Ti-O octahedron.5.The pyrite-type Cu S₂ polycrystals were synthesized by using the high temperature and high-pressure techniques,and the charge density wave（CDW）was observed in its resistivity and magnetic susceptibility at～145 K.Be detailed employing the transmission electron microscope,the existence of CDW was confirmed with showing the 2×2×2 superlattice at low temperatures,and this is the first time that the direct evidence was provided from experiments.Then we performed the detailed high-pressure study to further reveal the relationship between CDW and SC.Our results show that pressure can effectively suppress the CDW order and enhance the T_c,demonstrating the competing character between CDW and SC.In addition,we also grown the pyrite-type Cu Te₂ single crystal for the first time and characterized its structure,resistivity,specific heat and magnetic properties.The results show that Cu Te₂ belongs to the weak coupling type-II superconductor.