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We present time-of-flight neutron-scattering measurements on single crystals of La2-xBaxCuO4 (LBCO) with 0 ≤ x ≤ 0.095 and La2-xSrxCuO4 (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional commensurate long range antiferromagnetic order for x ≤ 0.02 to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x ≥ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Lastly spin gaps, suppression of low energy magnetic spectral weight, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.
We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-Tc superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime. We observed resonance mode in the electron doped cuprate Nd[subscript 1.85]Ce[subscript 0.15]CuO[subscript 4], demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr[subscript 0.88]LaCe[subscript 0.12]CuO[subscript 4], the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates. Shortly after the discovery of high-Tc superconductivity in the Fe pnictides, we discovered that the magnetic phase diagram of CeFeAsO[subscript 1-x]F[subscript x] is remarkably similar to that of the cuprates. Besides CeFeAsO, similar magnetic and lattice structures are also observed in PrFeAsO and SrFe[subscript 2]As[subscript 2] systems. Neutron scattering measurements show that in SrFe[subscript 2]As[subscript 2], the spectrum of magnetic excitations consists of a Bragg peak at the elastic position, a spin gap, and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimated the effective magnetic exchange coupling using a Heisenberg model. In order to study the nature of the exchange interactions in the parent compound of Fe pnictides, we studied the high energy spin-wave excitations in CaFe2As2. Although the spin waves in the entire Brillouin zone can be described by an effective three-dimensional anisotropic Heisenberg Hamiltonian, the magnetism in this system is neither purely local nor purely itinerant; rather it is a complicated mix of the two. When the Fe pnictide is tuned into superconducting regime with doping, the low energy spin fluctuation is dominated by a resonance mode. In the optimally electron doped BaFe[subscript 1.9]Ni[subscript 0.1]As[subscript 2], application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of high Tc superconductivity.
We present comprehensive neutron scattering studies on the electron-doped superconductors Nd2[subscript x]Ce[subscript x]CuO4 and Pr1[subscript x]LaCe[subscript x]CuO4 to understand how magnetic properties and phonon are related to superconductivity. For superconducting Nd1[subscript point]5Ce0[subscript point]15CuO4, a[italicized c]-axis magnetic field enhances the commensurate antiferromagnetic order, while an [italicized ab]-plane magnetic field induces spin-flop transition from a noncollinear to a collinear magnetic structure. The [italicized c]-axis field-induced effect is absent in both parent Nd2CuO4 and the as-grown nonsuperconducting Nd1[subscript point]5Ce0[subscript point]15CuO4 samples. These results, combined with those on the hole-doped La2−[subscript x]Sr[subscript x]CuO4 suggest that the antiferromagnetism competes with supercondictivity. In the Pr1−[subscript x]LaCe[subscript x]CuO4 system, we find that quasi-2 dimensional commensurate antiferromagnetism coexists with superconductivity and 3 dimensional antiferromagnetism. To determine whether the antiferromagnetic order is the competing order in the high temperature superconductors, regardless of hole-doping or electron-doping, we performed field-induced experiments on the electron-doped superconductor Pr0[subscript point]88LaCe0[subscript point]12CuO4[subscript plus or minus delta] by applying magnetic fields along the [italicized c]-direction and in the [italicized ab]-plane direction. The anisotropy of the field-induced moment demonstrates the competing nature of these two order parameters. We find that the spin-flop transition induced by an [italicized ab]-plane magnetic field in the lightly electron-doped Pr1[subscript point]29La0[subscript point]--Ce0[subscript point]01CuO4 affects significantly both in-plane and out-of-plane resistivity. No magnetoresistance anomaly is observed when a [italicized c]-axis magnetic field is applied, consistent with the absence of a spin-flop transition for such field direction. This anisotropic magnetoresistance effect suggests that there is spin-charge coupling in the electron-doped superconductors as well as in the hole-doped ones. Inelastic neutron scattering has been performed to study the generalized phonon density of states (GDOS) of electron-doped Nd2−[subscript x]Ce[subscript x]CuO4 with various levels of doping. Upon doping, the softening of phonon around 70 me V has been observed. However, most of the softening occurs within a few percent of Ce doping, and it is not related to the doping induced nonsuperconducting-superconducting transition. Therefore, the electron-lattice coupling in the electron-doped Nd2−[subscript x]Ce[subscript x]CuO4 is different from that in the hole-doped materials.
This thesis describes neutron scattering experiments on strongly correlated systems exhibiting a range of emergent phenomena: antiferromagnetism, charge order, superconductivity and multiferroicity. I have examined the La_{2}CoO_{4} compound which is a Mott insulator and orders antiferromagnetically near room temperature. The La_{2}CoO_{4} sample was studied using spherical neutron polarimetry and I present magnetic structure models to describe the two antiferromagnetic phases of the compound. Furthermore, the magnetic fluctuations have been investigated using neutron time-of-flight technique. This has allowed us to extract the dominant exchange interactions in the system. More interestingly, the work on La_{2}CoO_{4} presented in this thesis provides a basis for the experimental evidence of an hourglass dispersion in La_{5/3}Sr_{1/3}CoO_{4}, previously only observed in the copper oxide based superconductors. This dispersion has been understood in terms of a stripe ordered magnetic phase and was found to be well described by a linear spin-wave model. Neutron scattering experiments were also carried out on the new iron-based high-temperature superconductors, FeSe_{x}Te_{1-x}. A range of compositions were studied, including both antiferromagnetically ordered and superconducting. Below the superconducting phase transition temperature, a spin resonance mode was found centred on the antiferromagnetic wavevector. This is an important feature shared by many unconventional superconductors. The spin resonance intensity was found to reflect the order parameter of the superconducting state. Polarised inelastic neutron scattering experiments have revealed a small anisotropy between the in-plane and out-of-plane magnetic fluctuations at the resonance. This anisotropy cannot be readily explained by the usual anisotropic terms in the Hamiltonian. This could be evidence of new physics in the FeSe_{x}Te_{1-x} superconductors. Finally, I have studied CuO - a high-temperature multiferroic. Analysis of polarised neutron diffraction experiments shows that the magnetic domain population can be varied using an externally applied electric field. This unambiguously demonstrates coupling between the magnetic and ferroelectric degrees of freedom. Using representation analysis I derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction.