Graeme Luke - US grants

9510454 Uemura Muon spin relaxation (muSR) measurements are proposed to study ground state properties and spin fluctuations in geometrically frustrated spin systems, such as the Kagome lattice and triangular lattice antiferromagnets strontium chromium (x) gallium (12-x) oxygen (19) and lithium nickel oxygen (2), as well as in low-dimensional spin systems, including the doped Haldane spin chain system (yttrium, calcium) (2) barium nickel oxygen (5), the spin-Peierls system (copper, zinc) germanium oxygen (3), the one-dimensional organic conductor aluminum carbon (60), and the spin-ladder cuprates, strontium (n-1) copper (n+1) oxygen (2n). This proposal aims to reveal details of suggested spin-liquid ground states as well as to elucidate the process of destruction of the many-body singlet ground state due to the introduction of imperfections or doped charge carriers. The muSR experiments will be performed at the TRIUMF facility in Vancouver, Canada. %%% Certain configurations of atomic arrangements in solids, such as the triangular or Kagome lattice, tend to prevent the alignment of the magnetic moments of the atoms placed on it, promoting dynamic fluctuations of the moments (spins). Similarly, magnetic ordering is suppressed in low-dimensional arrangements of spins, such as a chain of magnetic moments. In these systems, novel magnetic behavior is expected at low temperatures, including the possibility for the occurrence of a "quantum spin liquid", which is a disordered liquid- like dynamic state of the moments due to quantum-mechanical effects. This proposal aims to reveal the nature of such exotic magnetic behavior by utilizing a novel technique called muon spin relaxation (muSR), which probes the magnetism within solids with radioactive elementary particles, muons, produced at a high-intensity proton accelerator (the TRIUMF facility in Vancouver , Canada).

9510453 Luke Muon spin relaxation will be used to study the interplay between various interactions and phenomena in rare earth and actinide compounds. Many of these systems exhibit interesting magnetic behavior due to the combination of the Kondo effect, Rudman-Kittel-Kasuya Yosida interaction and the effect of crystalline fields. In materials where these interactions are present, such phenomena as heavy fermion superconductivity and quadrupole Kondo effect are observed. Superconductivity and antiferromagnatic ordering coexist in uraniumplatinum-three compound. This antiferromagnetism contributes to multiple superconducting phases. This research will help elucidate the microscopic nature of the different superconducting states. In another compound, cerium-copper2-silicon2, superconductivity and magnetism do not coexist but compete for the ground state. This work will investigate the competition between the two states. The competition between Kondo screening and exchange interactions will be studied in cerium-copper-two- tin-two. The relative importance of the crystalline electric fields, Kondo screening and exchange interactions will be determined in uranium doped yttrium-palladium- three. Theoretical models based on the anomalous low temperature linear specific heat in neodymium-cerium-copper- oxygen compound will be tested by way of the measurement of spin fluctuations in this compound. %%% Muon Spin Relaxation experiments will be conducted on a number of rare earth and actinide compounds which exhibit a rich interplay between various interactions and phenomena such as superconductivity and magnetic ordering. Five different studies will be conducted: (1) the coexistence between antiferromagnetic ordering and the microscopic nature of the resulting multiple superconducting phases, (2) the competition between magnetic ordering and superconductivity in a compound where they do not coexist, (3) the competition between Kondo screening and exchange interaction, (4) the relative importance between crystalline electric fields, Kondo screening and exchange interactions, and (5) fluctuation effects in materials which exhibit anomalous thermodynamic behavior. Each of the above study will be conducted on the appropriate compound. ***

w:\awards\awards96\*.doc 9802000 Uemura This experimental research project is devoted to unusual magnetic behavior in pure and doped spin-gap systems, and in spin-liquid systems, as studied by muon spin relaxation. Of principal interest are low-dimensional and/or geometrically frustrated spin systems. Of interest are conditions for the formation/absence of spin freezing, the role of frustration, and effects of charge doping and magnetic dilution. In more detail, four topics and systems of interest are: 1) questions of ordering temperature and/or ordered moment size in quasi 1-d zig-zag spin chain SrCuO2 and LixV2O5; 2) charge/vacancy doping effects in 2-leg ladders Sr2(Cu,Zn)4O6, LaCuO2.5, and (Ca,Mg)(V,Ti)2O5; 3) ground state of pure and perturbed geometrically frustrated systems LiV2O4, (Y,Sc)Mn2, CePt2Sn2; and 4) magnetic properties of linear chains and 2-leg ladders in the hybrid (La,Sr,Ca)14Cu24O41. The muon spin relaxation experiments are performed at TRIUMF, magnetic susceptibility measurements at Columbia and neutron scattering studies at JAERI. This research program is interdisciplinary in nature and involves one or more graduate or postgraduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. %%% This experimental basic research project is devoted to unusual magnetic behavior in crystalline specimens of a variety of inorganic chemical compounds. The questions of interest are the local behavior of the magnetic species in the samples, and an advanced experimental method called "muon spin relaxation" is used in this work. The measurement has to be made at a special accelerator facility where the muons (mu mesons), are produced and injected into the sample of interest. The measurement reveals the magnetic field strength inside the sample at those loca tions, often near internal magnetic moments, where the muons become trapped before they undergo decay. Of interest are conditions for the formation/absence of spin freezing, the role of frustration, and effects of charge doping and magnetic dilution. In more detail, four topics and systems of interest are: 1) questions of ordering temperature and/or ordered moment size in quasi 1-d zig- zag spin chain SrCuO2 and LixV2O5; 2) charge/vacancy doping effects in 2-leg ladders Sr2(Cu,Zn)4O6, LaCuO2.5, and (Ca,Mg)(V,Ti)2O5; 3) ground state of pure and perturbed geometrically frustrated systems LiV2O4, (Y,Sc)Mn2, CePt2Sn2; and 4) magnetic properties of linear chains and 2-leg ladders in the hybrid (La,Sr,Ca)14Cu24O41.The muon spin relaxation experiments are performed at TRIUMF, magnetic susceptibility measurements at Columbia and neutron scattering studies at JAERI. The results of this research will be of importance in magnetism, and in physical theory. They may lead to new methods that can be applied in chemistry and in nanoscale electronic devices. This research program is interdisciplinary in nature and involves one or more graduate or postgraduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. ***