| A universally emergent electronic states,which break rotational symmetry from C4to C2,in strongly correlated electronic systems indicate a possible elec-tronic nematic phase.The discovery of electronic nematic phase and determina-tion of nematic quantum critical point play an important role in understanding the phase diagrams of high-Tcsuperconductors.This thesis work mainly focuses on s-tudying not only the temperature and doping revolution of nematic fluctuations in normal-state tetragonal phase of electron-doped iron-based superconductors,but also the interplay between nematic fluctuations and superconductivity.To mea-sure the response of elastoresistance which represents the nematic fluctuations,I have designed a compact in-situ uniaxial pressure device based on a multilay-ered piezoelectric plate bender.This device has improved measurement precision and been widely used in elastoresistance measurement under uniaxial pressure on mm-size samples.In Chap.1,we introduce the structure of single crystal and phase diagram-s of copper-oxide and iron-based high-Tcsuperconductors.Moreover,we have discussed the origins of nematic phase and the pairing mechanism of supercon-ductivity and sketched some basic concepts of quantum phase transition.The main context this thesis contains is as following:?1?We introduce single crystal growth of BaFe2-xNixAs2and give a detailed description of my in-situ uniaxial pressure device based on piezoelectric bender in Chap.2.Compared with other piezoelectric devices,we display the advan-tages/disadvantages in sample mounting,measurement precision and also suggest some improvements.?2?With utilization of this device,we have systematically studied the tem-perature dependence of nematic fluctuations in BaFe2-xNixAs2.The nematic susceptibility??110?=d??R/R0?/dp,follows a Curie-Weiss-like behavior:??110?=A/?T-T??+y0,in underdoped regime with pressure along tetragonal[110]di-rection.The mean field nematic phase transition temperature T?decreases with doping increasing and vanished near optimal doping which implicates a possible nematic quantum critical point?NQCP?.This result supports the previous work.?3?Due to enhancing measurement precision of my device,we first observe the disordered behavior of nematic susceptibility in overdoped regime.It presents a hump feature whose peak at Th.The extrapolated line from doping dependence of Thalso ends at NQCP.Both T?and Thgive the funnel shape of quantum critical region.So far we map out the most extensive phase diagram of nematic fluctuations.It demonstrates that superconductivity should related to nematic fluctuations which covers the whole superconducting dome.?4?We measured the signal with pressure along[100]and amazingly discov-ered a similar response as[110]signal with much smaller intensity.By studying the doping and temperature evolution of??100?,we find it shows strongest at NQCP.The hump feature is similar as??110?'s and Thalmost same in overdoped samples.The signals in underdoped regime disagree with 1/T divergence.In comparison with FeSe1-xSxsystem which has no antiferromagnetic?AF?order,we suggest the strong signals of??110?in underdoped range come from thermal fluctuations of AF order and??100?relates to nematic quantum critical fluctuations.The??100?which disobeys an Ising-type nematic model may result from isotropic scattering near NQCP.?5?The research on interplay between nematic fluctuation and supercon-ductivity demonstrates that,for overdoped samples,Tcvariations under uniaxial pressure can be fitted by Tc?p?=Tc0+Bp+Cp2?1-Dp2?.The constant B refers to response of superconductivity to isotropic strain mode and C contains information about nematicity-superconductivity coupling.?6?Further study of samples near the NQCP,we find that the sample-dependent response of Tc.Detailed analysis indicates the Tc?p?behavior relates to nematic phase transition temperature T?.Constant B shows a linear relation-ship with T?and continuously crosses zero at T?=0 where referred to a putative NQCP.Constant C presents almost unchanged with T?and keeps negative value which reveals competition between nematic and superconducting order parame-ters in BaFe2-xNixAs2.These results confirm the analysis in overdoped samples.In addition,based on a phenomenological Ginsburg-Landau free energy analysis,C may refer to superconducting pairing symmetry.Additionally,in Chap.5,we have successfully grown high quality single crys-tals of overdoped Pr1-xLaCexCuO4-??PLCCO?by travelling-solvent floating zone method via replacing the original lamp.Some neutron scattering experimental results about magnetic field induced AF order and low energy spin excitations are listed in Chap.6 at last. |
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