Studies Of Pressure-Induced Quantum Phase Transitions

Quantum phase transition is one of the essential scientific issues and research focuses in the realm of condensed matter physics in the century.Quantum phase transition occurs only at zero temperature,but its influence on physical properties can be felt at finite temperatures,even above room temperature.Studies into various types of quantum phase transitions can provide an essential framework of reference for the universality of quantum phase transitions.By utilizing pressure-tuning,the thesis have investigated possible existence of quantum phase transitions and related properties of a ferromagnetic compound CeRh₆Ge₄,an antiferromagnetic compound CePdIn and a charge-density-wave?CDW?compound LaPt₂Si₂.1.Antiferromagnetic quantum critical point?AFM QCP?has been found in many strongly correlated electron systems.Moreover,strange metal behaviors can be observed in the vicinity of an AFM QCP of some compounds.However,the search for solid evidence for the presence of a ferromagnetic quantum critical point?FM QCP?is still going on.Furthermore,the idea that FM QCP doesn't exist in a clean system is backed in theoretical grounds.We found a pressure-induced FM QCP in CeRh₆Ge₄ single crystals of high sample quality,accompanied with strange metal behaviors in the vicinity of the FM QCP.Our discovery overturns previous understanding towards ferromagnetic quantum phase transition,and opens up a whole new category for the study of ferromagnetic quantum critical points and strange metal behaviors.2.Antiferromagnetic quantum critical points of most heavy fermion compounds can be understood in the framework of conventional HMM theory.However,there are antiferromagnetic quantum critical points of several compounds are found beyond HMM theory.Scientists attribute it to the important role of frustration.Antiferromagnetic compound CePdln crystallizes in the hexagonal ZrNiAl-type structure,which is assumed frustrated.In order to investigate the influence of frustration on the properties of CePdIn,we explored the magnetic field effect and pressure effect on CePdIn,and drew a comparison with a quasi-two-dimensional compound CePdAl of the same crystal structure.We found that CePdIn is prone to be three dimensional,thus its strength of frustration is reduced.Furthermore,novel phase diagrams similar to that of CePdAl are not found in CePdIn.We argue that it can be accounted for by the weakened frustration in CePdIn.3 LaPt₂Si₂ is a CDW compound,and it also exhibits a superconducting transition at low temperature.There has been growing studies into CDW quantum phase transitions.There are basically two categories concerning these phase diagrams.In the first category,there is a sudden suppression of the CDW order accompanied with a rapid jump of the superconducting transition temperature.In the second one,CDW order can be continuously suppressed,and a CDW QCP appears with a maximum of the superconducting transition temperature in the vicinity of the CDW QCP.Furthermore,the first category can be explained in the BCS framework,while the second one has not yet been understood soundly,which awaits investigations into new systems.Through pressure tuning,we found that LaPt₂Si₂ shows a first-order quantum phase transition of the CDW order with a maximum value of the superconducting transition temperature around the critical pressure.Moreover,the trend of superconductivity under pressure can be interpreted in the BCS framework.