Nematic Fluctuations Under Uniaxial Pressure In Iron-based Superconductors

It has been hundreds of years since superconductor was discovered in 1911.Superconductors show various physical properties from both experiments and the-ories.The discovery of iron-based superconductors in 2008 opens a new route for superconductivity.Comparing to cuprates,iron-based superconductors have rel-atively high temperature for superconducting transition,high upper critical field and low anisotropy.Also,the antiferromagnetism coexist with superconductivi-ty and multiply band structure have been found in iron-based superconductors.All the peculiar physical properties make it more complicated for the study of superconductivity mechanism.The concept of parent compound for iron-based superconductors is from cuprates.Four types of parent compounds for iron-based superconductors show different properties,such as the existence of superconduc-tivity and antiferromagnetism.When dop carriers into parent compounds,the structure and antiferromagnetism are suppressed with the potential emergence of the nematic order and quantum critical points.Carriers doping as a usually way for tuning superconductivity,the contents and types of the carriers show different effects in iron based superconductors.The role of doping is not clearly now.Ne-maticity as an electronic state,proximate to superconductivity and antiferromag-netism,may have a similar origin of superconductivity and antiferromagnetism.Nematicity is a universal property of iron-based superconductors.The interplay between antiferromagnetism,nematicity and superconductivity are complex and closely associated with each other.In spite of the origin of antiferromagnetism,nematicity and superconductivity are still unclear,the electron-electron interac-tions are important to the normal state,superconductivity state and other order states.The contents of this thesis are as below.?1?A unified phase diagram for iron-based superconductors has been estab-lished by measuring nematic fluctuations in iron-based superconductors.We mea-sured different kinds of iron-based superconductors,include the parent compounds of”1111”,”111”,”122”,”11”and the doped compounds of”111”,”122”,”11”,”112”.We perform a detailed investigation on the nematic susceptibility of a variety of iron-based superconductors by studying the uniaxial pressure effect on the in-plane resistivity.The temperature dependence of the nematic susceptibility above the structural or nematic transition temperature T_scan be well fitted by a Curie-Weiss-like function,which gives a nematic Curie constant A_n.A linear rela-tionship exists between|A_n|^-1and the ordered moments suggesting the magnetic ground states of a variety of iron-based superconductors can be achieved by tuning the strength of nematic fluctuations.Accordingly,a unified phase diagram can be established where superconductivity comes from the suppression of the AFM order in a hypothetical parent compound by enhancing nematic fluctuations.?2?Nematic fluctuations in nonsuperconducting”Ba122”systems.We mea-sured nematic fluctuations of Ba(Fe_1-xTm_x)₂As₂?Tm=Mn,Cr,V,Cu?,Ba(Fe_0.97Cr_0.03)₂(As_1-xP_x)₂and Ba(Fe_1-xTm_x)₂(As_0.69P_0.31)₂?Tm=Mn,Cu?.Expect the Ba(Fe_1-xCu_x)₂As₂system,all the measured samples are not obey the linear relationship between|A_n|^-1and the ordered moments which is consistent with the expectation that the nematic fluctuations are too weak to induce su-perconductivity in these materials.For Ba(Fe_1-xCu_x)₂As₂,the long range AFM order change into short range AFM order and only low-temperature nematic fluc-tuations are suppressed which may explain why Ba(Fe_1-xCu_x)₂As₂don't show su-perconductivity.We also study the nematic fluctuations in NaFe_1-xNi_xAs system.There are two different nematic fluctuations exist in this system.The deviation from power-law temperature dependence in resistivity accompanied with the en-hancement of nematic fluctuations which indicates that the nematic fluctuations are important for studying the correlation of electrons.?3?We have successfully grown the superconducting La_0.34Na_0.66Fe₂As₂single crystals by the flux method for the first time with superconducting transition temperature at 27 K and antiferromagnetic transitions at 106 K.We also do some neutron scattering works include studying the phonon dispersion in Na₂Ti₂Sb₂O material with inelastic neutron scattering method for the origin of the charge density wave and search for spin liquid state in Ba₈TmNb₆O₂₄?Tm=Co,Mn?with neutron diffraction method.