Static and Dynamic Magnetism in the Electron-Doped High-Temperature Superconductor Pr_0.88LaCe_0.12CuO_4-δ and in the f-electron, Non-Fermi Liquid Alloy Sc_1-xU_xPd₃

Author

Stephen Dennis Wilson, University of Tennessee - Knoxville

Date of Award

5-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Jim Thompson, Takeshi Egami, Marianne Breinig

Abstract

Here we present detailed studies of the spin dynamics within the electron-doped high temperature superconductor Pr_.88LaCe_.12CuO_4-δ (PLCCO) and within the non-Fermi liquid metal Sc_1-xU_xPd₃ (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-T_c cuprate Pr_.88LaCe_.12CuO_4-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 (T_c=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-T_c 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 (T_c=24K). We find that the resonance mode in this system follows the universal relation E_R=5.8k_BT_c 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 (T_c=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-T_c superconductivity.

For our studies of Sc_1-xU_xPd₃, 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.

Recommended Citation

Wilson, Stephen Dennis, "Static and Dynamic Magnetism in the Electron-Doped High-Temperature Superconductor Pr_0.88LaCe_0.12CuO_4-δ and in the f-electron, Non-Fermi Liquid Alloy Sc_1-xU_xPd₃. " PhD diss., University of Tennessee, 2007.
https://trace.tennessee.edu/utk_graddiss/288