Date of Award
Doctor of Philosophy
Jim Thompson, Takeshi Egami, Marianne Breinig
Here we present detailed studies of the spin dynamics within the electron-doped high temperature superconductor Pr.88LaCe.12CuO4-δ (PLCCO) and within the non-Fermi liquid metal Sc1-xUxPd3 (ScUPd). Comprehensive neutron scattering experiments were carried out mapping the evolution of magnetism within these systems as they are tuned across their respective phase diagrams. The novel features of the magnetic spectra within these systems are correlated with known anomalies in their quasiparticle behaviors driven through either the appearance of a superconducting phase in PLCCO or the emergence of a non-Fermi liquid phase in ScUPd.
For the high-Tc cuprate Pr.88LaCe.12CuO4-δ, a detailed study of the evolution of the low energy spin excitations in this system is presented. In the unannealed, nonsuperconducting parent compound of PLCCO, the magnon excitations are well modeled as spin wave excitations arising from the long-range antiferromagnetic (AF) order in the system. As the system is doped into the superconducting phase towards optimal superconductivity, long-range AF order in the system is suppressed and low energy spin excitations behave drastically different than those observed in the NSC parent system. Instead of following the simple Bose statistics expected for spin wave magnon modes, the low energy excitations in superconducting concentrations show a form of hyperscaling in which the dynamic susceptibility is observed to scale as a function of ω/T. This likely reflects the influence of quantum critical excitations coupling to the spin degrees of freedom that arise from the quantum critical point in the phase diagram of PLCCO (where AF order is suppressed to 0K near optimal doping).
High energy spin excitations in an under-doped concentration of PLCCO (Tc=21K) are also reported. Our experiments show that in contrast to the seemingly universal pattern of dispersion reported the spin excitations of hole-doped cuprates, the high energy excitations in this n-type system instead resemble those observed in the parent compounds of the high-Tc cuprates. Rather than the “hourglass”-type dispersion observed in hole-doped cuprates, the dispersion in this n-type system remains a broadened commensurate spot at low energies that disperses outward into a ring-like excitation at higher energy transfers. The actual dispersion for this underdoped concentration at higher energies is anomalously sharper than that reported for the parent systems, Pr2CuO4 and La2CuO4.
Another facet of the spin excitations in PLCCO given particular focus is the newly discovered resonance mode in nearly optimally doped concentrations of PLCCO (Tc=24K). We find that the resonance mode in this system follows the universal relation ER=5.8kBTc for the resonance energies observed in all classes of cuprates. The resonance, when taken with the known commensurate response and high energy dispersion in the electron-doped cuprates, is therefore shown to stand as the long unifying feature in magnetic spectra of the cuprates, regardless of doped carrier-type.
Field-induced changes in the magnetic spectrum of this optimally doped concentration of PLCCO (Tc=24K) are also discussed. Particular focus is given to the suppression of the resonance mode and field-induced spin density wave order in this system. The suppression of the resonance mode is seen to track the condensation energy in relative magnitude as a function of applied magnetic field, thereby suggesting that the mode itself is fundamentally connected to the mechanism of high-Tc superconductivity.
For our studies of Sc1-xUxPd3, systematic neutron measurements mapped out magnetic excitations over a broad doping range. Concentrations near x=0.30, where the non-Fermi liquid (NFL) phase first appears, exhibit a seemingly local and nearly temperature-independent response in the spin excitation spectra. This parallels earlier results reported for another NFL system, UCu4Pd, whose spin dynamics acquire a local temperature insensitive character near the NFL phase boundary. We find that this spin behavior is characteristic of the proximity of a spin-glass quantum critical point in the phase diagram of these systems and possibly intrinsic to the appearance of NFL dynamics in these classes of materials.
Wilson, Stephen Dennis, "Static and Dynamic Magnetism in the Electron-Doped High-Temperature Superconductor Pr0.88LaCe0.12CuO4-δ and in the f-electron, Non-Fermi Liquid Alloy Sc1-xUxPd3. " PhD diss., University of Tennessee, 2007.