The Study Of Magnetic Properties And Quantum Critical Phenomena In Several Strongly-Correlated Materials

Research on the quantum critical phase transitions has always been a very impor-tant branch in the field of condensed matter physics.Quantum phase transitions exist widely in strongly-correlated materials such as heavy fermion compounds,iron-based superconductors and copper oxide high-temperature superconductors.As a kind of typ-ical strongly-correlated materials,heavy fermion systems exhibit abundant properties such as non-Fermi liquid behavior,unconventional superconductivity,hidden order and multipolar order.The characteristic energy scales of various ground states in heavy fermion materials are very low,making them one of the systems for studying quantum critical phenomena.Many quantum critical phenomena have also been found in iron based superconductors of Ba122 systems.Studying Ba122 systems is helpful to un-derstand the relationship between quantum critical fluctuations and superconductivity.Currently,there are two main quantum critical images of heavy fermion systems.One is based on Landau Fermi liquid theory,which is called a conventional spin density wave（SDW）quantum critical point.Another one is the unconventional Kondo destruc-tive quantum critical quantum phase transition,which breaks out of the framework of Landau Fermi’s liquid theory.This critical phenomenon cannot be described by fluc-tuations of order parameters,and conforms to the description of local quantum phase transition theory.For experimental work,finding more materials with quantum crit-ical points and studying related quantum critical phenomena are of great significance for the development and improvement of quantum phase transition theory.Our research focuses on the quantum critical phenomena and magnetic properties of several strongly-correlated materials.In this paper,I will introduce how we use the neutron scattering and uniaxial pressure techniques to study the quantum critical phenomena and magnetic properties of heavy fermion materials of Nd Fe₂Ga₈、Ce Co₂Ga₈、URu₂Si₂and equiv-alently doped Ba122 system Ba(Fe_1-Cr)₂(As_1-P)₂.This paper are mainly divided into the following parts:1.We studied the magnetic structure of Nd Fe₂Ga₈by neutron single-crystal diffra-ction and neutron powder diffraction measurements and carefully analyzed the magnetic phase transition of Nd Fe₂Ga₈.The measurement results of specific heat,resistance,and magnetic susceptibility show that there are two magnetic phase transitions,and many behaviors are consistent with quantum critical behaviors of the conventional three-dimensional SDW type QCP.Both refinements give the same ground magnetic structure of Nd Fe₂Ga₈,which is the magnetic moments aligned ferromagnetically within the ab plane but antiferromagnetically along the c axis.We also find that with the increasing of temeprature,incommensurate antiferromagnetic peaks appear near the position of the conmmensurate magnetic peak.Considering the neutron experiment results with the measurement of linear Grüneisen parameter,we speculate that there may be a multipolar order in this material and it may be the origin of the incommensurate magnetic peaks.2.We studied the spin dynamics of the f-electron system Ce Co₂Ga₈by low-energy inelastic neutron scattering（INS）technique.Both the experimental results of transport andSR measurements indicate that Ce Co₂Ga₈may in the close vicinity of a quan-tum critical points at ambient pressure and it is on the non-magnetic side.We mea-sured the magnetic excitation spectrum of Ce Co₂Ga₈by INS.The result reveals that there are incommensurate antiferromagnetic spin excitations alongand the dynami-cal susceptibility obeys an/scaling relationship,suggesting that there is indeed an unconventional quantum critical point in this material.In addition,the intergrated in-tensity increases with the growth of scattering vectors||,which is different from the result calculated by magnetic form factors.Considering the similarities of Ce Co₂Ga₈and Nd Fe₂Ga₈in the crystal structure and the location of magnetic excitation,we guess that there may be multipolar order fluctuations just as in Nd Fe₂Ga₈.3.We studied the nematic quantum critical point in Ba(Fe_1-yCr_y)₂(As_1-xP_x)₂.There are both AFM and nematic quantum critical points in Ba Fe₂(As_1-xP_x)₂.The superconductivity in Ba Fe₂(As_1-xP_x)₂can be fully suppressed by just 3%of Cr substi-tution of Fe.In the meantime,the nematic quantum critical point disappears while the antiferromagnetic quantum critical point still exists and moves towards the direction of more P doping.In order to investigate whether the disappearance of the nematic quan-tum critical point is related to the disappearance of superconductivity,we meaured the elastic resistance of samples with 1.5%Cr doping.The experimental results show that the position with strongest nematic fluctuations does shift towards more P doping with the increase of Cr doping.The amplitude of nematic fluctuations gradually weakens with increasing Cr doping,and the nematic quantum critical point is completely can-celed with the disappearance of superconductivity until the amount of Cr doping reaches3%.4.We studied the uniaxial-pressure dependence of resistivity for URu_2-xFe_xSi₂samples with=0 and 0.2,which host a hidden order（HO）and a large-moment anti-ferromagnetic（LMAFM）phase,respectively.The order parameter of the hidden order in URu₂Si₂has puzzled people for a long time.Previous studies suggested that the hid-den order in this material is related to the nematic fluctuations,but subsequent studies showed that there is no symmetry breaking and the Curie-Weiss behavior,which is be-lieved to be asscocitated with nematic fluctuations comes from the bulk pressure effect rather than symmetry breaking.The role that the nemacity plays in the hidden order is still worth further research.We applied uniaxial pressure to measure the response of its resistance to pressure,and found that there would be an in-plane anisotropy of elastic resistance when entering the hidden order,and the anisotropy would disappear when entering the LMAFM phase.We also provide a simple explanation for the temperature dependence of elastoresistance.Therefore,the in-plane anisotropic response of the ela-storesistance when entering the hidden orderthat is a unique property of URu₂Si₂and it only exists when applying uniaxial pressure and without necessarily breaking the rota-tional lattice symmetry spontaneously.We also assumed that whether or not there are nematic fluctuations,they should play little role in the formation of the hidden order.