Research On The Metal-insulator Transition Of 4d/5d Transition Metal Oxides

Metal-insulator transition?MIT?is one of the most important manifiestation of quantum phenomena in strongly correlated electron systems.The Mott insulating state frequently observed in the 3d transition-metal oxides can be convereted to metallic via doping charge carriers or applying chemical/physical pressure.In the transition region,the 3d electrons acquire both localized and itinerant characters,and the strong interplay of charge,spin,orbital,and lattice degrees of freedom can induce many intriguing quantum states of matter,which are prone to external stimuli and can give rise to some exotic phenomena such as high-T_c superconductivity or colossal magnetoresistance.Therefore,MIT and its manipulation ha ve attracted considerable interest in the strongly correlated electron systems.In comparison with the well-studied 3d transition-metal oxides,the electronic states of 4d/5d conterparts are closer to the localized-to-itinerant transition due to the spatially more extended 4d and 5d orbitals.In addition,the enhanced spin-orbit interaction,comparable with the onsite Coulomb interaction U and electronic bandwidth W,gives rise to many novel phenomena that are the subject of extensive investigations in recent years.In this diseration,we investigated the MIT in several4d/5d transition-metal oxides with an aim to uncover the mechanism and associated unusual phenomena.The main results include:1.The cubic pyrochlore Cd₂Ru₂O₇ develops a peculiar metallic state below the antiferromagnetic?AF?order around T_N=100 K.We have studied the effects of hydrostatic pressure and the isovalent chemical substitutions on the electrical transport properties of Cd₂Ru₂O₇ synthesized under high pressure.We found that the metallic state below T_N is very fragile and can be immediately convereted to an insulating state by either application of hydrostatic pressure¹ GPa or replacing¹0%Ca²⁺or Pb²⁺for Cd²⁺.In comparison with other Ru pyrochlores,we proposed that Ru⁵⁺-4d³ electrons are located near the crossover from localized to itinerant state and thoese external stimuli can enhance the localized character by strengthening the Cd-O valvency under pressure or introducing chemical disorders via doping.2.Another cubic pyrochlore Tl₂ Ru₂O₇ exhibits a sharp MIT around 120 K accompanied with a structural transition.Previous studies have shown that the development of orbital ordering below the structural transition might facilitate the formation of quasi-1D S=1 Haldane chain with a spin gap.We have synthesized the Tl₂Ru₂O₇ pyrochlore under high pressure and invistiaged the effect of hydrostatic pressure and the isovalent chemical substitution.We found that the application of pressure first reduces slightly the MIT and then shifts it to higher temperatures gradaully,with the MIT less pronounaced.It is likely that the pressure can stabilize the orbital order and enlarge the spin gap of Q1D Haldane chains.On the other hand,substitutions of Bi³⁺for Tl³⁺can suppress the MIT and stbalize a metallic ground state gradually,presemubaly due to the obital hybridization between the Bi³⁺-6p orbital with the Ru⁴⁺-t_2g orbitals as well as the introduction of lone pair Bi³⁺-6s² electrons.3.We also studided the doping-induced MIT in the series of cubic pyrochlores Y_2-x-x R_x Ru₂O₇?R=Bi or Pb?synthesized at ambient pressure.In both cases,the AF insulating state of Y₂Ru₂O₇ can be continuous transformed to the paramagnetic state upon doping.For the isovalent Bi³⁺doping,the introduction of lone-pair electrons and the orbital hybridizations are important for the MIT,while the hole doping in the Pb²⁺-doped samples should also contribute to the MIT.4.We also synthesized cation-ordered inverse trirutile Cr₂ReO₆ and monoclinic birutile CrReO₄ under moderate high-pressure and high-temperaure conditions,and performed detailed characterizations on their structural,transport,and magnetic properties.A second-order AF transition is found to take place at T_N=67 K for Cr₂ReO₆ and T_N=98 K for CrReO₄,respectively.Their magnetic structures were determined from the analysis of the NPD patterns.Both compounds are confirmed to display a semiconducting behavior following the Mott's VRH conduction mechanism.A weak anomaly can be discerned around T_N from the temperature derivative of resistivity for both compounds,signaling a coupling between spin and charge degrees of freedom for these 3d-5d coupled electron systems.5.CrSb₂ is a narrow-band semiconductor.We obtained its high-pressure form and investigated its physical properties in detail.The high-temperature phase of CrSb₂ is metallic,and undergoes two successive magnetic transition upon cooling,i.e.a ferromagnetic?FM?transition at¹60 K and an AF-like transition at 86 K.The AF transiton can be suppressed gradually by applying external magneic fields at ambient pressure,resulting in a ferromagnetic metal ground state above 3 T.O n the other hand,the AF transition can be enhanced quickly by pressure and meet with FM transition,forming a new AF state,which can be eventually suppressed by pressure of⁹ GPa,forming an AF quantum critical point.