Relevant Degree Programs
Affiliations to Research Centres, Institutes & Clusters
- controlling magnetic and electric properties with electric fields
- diffusion of Li in battery electrode materials
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2020)
It is well established that the properties of many materials change as their thicknessis shrunk to the nanoscale, often yielding novel features at the near-surface regionthat are absent in the bulk. Even though there are several techniques that can studyeither the bulk or the surface of these materials, there are very few that can scanthe near-surface region of crystals and thin films versus depth. Beta-detected NMR(b-NMR) is capable of this and therefore has been established as a powerful toolfor material science. This thesis aims to further develop the capabilities of b-NMR.The first part of this thesis demonstrates that by comparing the spin-lattice relaxationrates (SLR) of two radioactive Li isotopes (⁸,⁹Li) it is possible to distinguishwhether the source of SLR in a given situation is driven by magnetic or electricinteractions. This is an important development for b-NMR, since there are instanceswhere it is problematic to distinguish whether the measured relaxation is due tomagnetic or electric fluctuations. Using this method, it was found that the SLR inPt is (almost) purely magnetic in origin, whereas the spin relaxation in SrTiO₃ isdriven (almost) entirely by electric quadrupolar interactions.The second part of this thesis traces the development of a-radiotracer, that usesthe progeny a-particles from the decay of ⁸Li, in order to directly measure thenanoscale diffusivity of Li⁺ in Li-ion battery materials. To develop this technique,Monte Carlo simulations of the experimental configuration were carried out, a newapparatus and a new a-detector were designed and used for experiments on rutileTiO₂. In rutile, the measurements revealed that Li+ gets trapped at the (001) surface,a result that helps explain the suppressed intercalation of Li⁺ in bulk rutile. Moreover,the diffusion rate of Li⁺ in rutile was found to follow a bi-Arrhenius relationship,with a high-T activation energy in agreement with other reported measurementsand a low-T component of similar magnitude with the theoretically calculateddiffusion barrier as well as the activation energy of the Li-polaron complex foundwith b-NMR below 100 K.
This thesis reports measurements on the dynamics of isolated lithium in single crystal materials using ion-implanted ⁸Li β-detected nuclear magnetic resonance. From spin-lattice relaxation and resonance measurements, we identify the kinetic parameters describing the ion’s site-to-site hop rate – the elementary process in long-range solid-state diffusion – and compare the results with theoretical work in the literature, as well as experiments at higher concentration. In addition to these “ionic” details, the nuclear magnetic resonance probe provides information on the electronic properties of the host, whose most intriguing features are also discussed. In the one-dimensional ion conductor rutile TiO₂, we find two sets of thermally activated dynamics: one below 100 K and another at higher temperatures. We suggest the low temperature process is unrelated to lithium motion, but rather a consequence of electron polarons in the vicinity of the implanted ⁸Li⁺. Above 100 K, Li⁺ undergoes diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor that are in agreement with macroscopic diffusion measurements, but not with theory. In Bi₂Te₂Se, a topological insulator with layered tetradymite structure, implanted ⁸Li⁺ undergoes ionic diffusion above 150 K, likely in the van der Waals gap between adjacent Te planes. A comparison with structurally related materials reveals the mobility of isolated Li⁺ is exceptional. At lower temperature, we find linear Korringa-like relaxation, but with a field dependent slope and intercept, accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor. In the doped tetradymite topological insulators Bi₂Se₃:Ca and Bi₂Te₃:Mn, the onset of lithium dynamics is suppressed to above 200 K. At low temperatures, the nuclear magnetic resonance properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift. From this, we make a detailed comparison with isostructural Bi₂Te₂Se.
Modern physics research relies on particle accelerators and available beam time is a very limited resource. The ARIEL eLINAC will strengthen the rare isotope program at TRIUMF by providing an alternative way to create rare isotope beams (RIB). A possible way to add additional use to this machine is to create a return beam line and use the beam to excite a free electron laser (FEL). The remaining beam can be used to drive fields in the SRF cavities to reduce the required RF power.One limitation of these energy recovery LINACs (ERL) is beam-break up. Higher order modes (HOM), especially dipole modes, have a negative influence on the beam which can lead to beam loss. The design of the SRF cavity has to accommodate this to make sure a beam current of up to 10mA can be used for both RIB production and ERL operation.This thesis will go through the design process of the ARIEL 1.3 GHz nine-cell cavity. The design relies on simulations to calculate the fields inside the cavity and with it the shunt impedance of HOMs.The investigations showed that resistive beam line absorbers can be used to reduce the shunt impedance of HOMs sufficiently without interfering with the accelerating mode. The performance of the absorber material has been verified in dedicated low temperature measurements, while the HOM field distribution has been measured via beadpulling on a copper model of the cavity. These measurements showed good agreement with the simulations.The power dissipation in the SRF cavities is of vital importance. The cryogenic system is a signicant part of the capital investment for the accelerator and sets the power budget for each cavity to around 10 W. This corresponds to a Q₀ value of 1 x 10¹⁰ at an operational temperature of 2 K. The gradient goal is 10 MV/m to reach the design energy of 50 MeV with five cavities. Both Q₀ and Eacc specifications have been met in the first two cavities that are installed in cryomodules. Two more cavities have been built and are in their qualification phase.
One of the fundamental quantities of a superconductor is the magnetic penetration depth, λ, which is the characteristic length scale that a magnetic field penetrates into the surface of a superconductor while in the Meissner state. In the clean limit the absolute value of λ is directly relatedto the superfluid density ns via 1/λ² = μοe²ns/m∗ , where m∗ is the effective mass. Consequently, its variation as a function of temperature, doping and orientation are of central importance in testing microscopic theories of exotic superconductors. A low energy beam of spin polarized muons has recently been developed, at the Paul Scherrrer Institute, to directly measure λ in a superconductor. When a muon (μ+) decays, it emits a fast decay positron preferentially along the direction of its spin due to the parity violating decay.The time evolution of statistical average direction of the spin polarization of the muon ensemble depends sensitively on the local magnetic field which can be monitored as a function of the mean depth of implantation. In this way it is possible to measure the field profile near the surface of a superconductor and extract the magnetic penetration depth in a direct manner which is not otherwise possible with conventional bulk methods.In this thesis, accurate measurements of λ and its anisotropy (≡ λa /λb) have been made for three different oxygen (x = 6.52, 6.92 and 6.998) contents of YBa₂Cu₃O₆₊x as well as inBa(Co₀.₀₇₄Fe₀.₉₂₆₎₂As₂. The measured values of λ and the in-plane anisotropies are considerably different from that reported in the literature, using less direct methods. The a – b anisotropy is surprisingly insensitive to x in YBa₂Cu₃O₆₊x . We observe an exponential decay of the magnetic field and corresponding supercurrent density deep inside the crystals. Small deviations from the London model are observed which indicate there is a suppression of the supercurrent density close to the surface. The measured (λ) values are also found to depart substantially from the Uemura relation Tcα 1/λ² .
Beta-detected Nuclear Magnetic Resonance (β-NMR) uses highly spin polarized β-emitting nuclei as a probe. Besides its use in nuclear physics, it hasalso become a powerful and sensitive tool in condensed matter physics and materials science. At TRIUMF, β-NMR of ⁸Li+ has been developed to study materials in a depth-resolved manner, where the implantation depth of ⁸Li+ is controlled via electrostatic deceleration. In this thesis, β-NMR of ⁸Li+ has been used to study high-Tc cuprate superconductors (HTSC). The objectiveof this work is to search for spontaneous magnetic fields generated by a possible time-reversal symmetry breaking (TRSB) superconducting statenear the surface of hole-doped YBa₂Cu₃O₇−d (YBCO), and study the nature of the vortex lattice (VL) in YBCO and electron-doped Pr₂−xCexCuO₄−d(PCCO). For several advantages, our measurements were carried out by implanting ⁸Li+ in thin silver films evaporated on the superconductors.In our TRSB studies, the magnetic field distribution p(B) is measured 8 nm away from the Ag/YBCO interface in magnetic fields B₀ = 5 to 100 G, applied parallel to the interface. p(B) showed significant broadening below the Tc of ab- and c-axis oriented YBCO films. The broadening signals the existence of weak disordered magnetic fields near the surface of YBCO. From the broadening’s temperature and field dependence we draw an upper limit of 0.2 G on the magnitude of spontaneous magnetic fields associated with TRSB.To study the VL, p(B) is measured at average implantation depths ranging from 20 to 90 nm away from the Ag/YBCO or Ag/PCCO interface in B₀ = 0.1 to 33 kG, applied perpendicular to the surface. p(B) showed a dramatic broadening below Tc as expected from the emerging field lines ofthe VL in the superconductor. In YBCO, p(B) is symmetric and the dependence on B0 is much weaker than expected from an ideal VL, indicatingthat the vortex density varies across the face of the sample on a long length scale, likely due to vortex pinning at twin boundaries. In PCCO, a 2D VLis established due to the high anisotropy of the superconductor leading to a nearly symmetric p(B).
Master's Student Supervision (2010 - 2018)
μSR was used to investigate the effect of sample preparation on the magnetic field penetration of high purity niobium to improve the fabrication and preparation of superconducting niobium radio frequency (RF) cavities for use in particle acceleration. The sample preparations tested were (1) electro-polish etching, (2) buffered chemical polish etching, (3) nitrogen doping, (4) plating with Nb₃Sn, (5) baking at 120⁰C, (6) baking at 800⁰C, and (7) baking at 1400⁰C. Three different sample geometries and two different applied magnetic field orientations were used in order to observe the effect of sample shape on the μSR measurements and to minimize the effect of the demagnetization factor on the results. The results showed that etching caused flux to enter the center of the samples at a lower applied magnetic field; however, a 120⁰C bake caused the etched samples to reach higher field before experiencing flux penetration. These results correlate with RF cavity test results using the same treatment method. Higher heat treatments caused a reduction in the pinning strength of the niobium samples and caused flux to enter the center of the sample at lower applied magnetic fields. Impurities and vacancies in a sample were suspected of acting as pinning centers and increasing the pinning strength; certain impurities and vacancies are also thought to prevent hydride formation in samples and prevent high field RF losses in cavities. If the same impurities that prevent RF losses in cavities also create pinning centers in the μSR samples, it could explain why the DC field μSR measurements are showing similar results to AC field RF cavity tests. The perpendicular field results for the Nb₃Sn plated and nitrogen doped samples showed no difference compared with regular niobium samples that had undergone similar heat treatments; however, the parallel field measurements of the Nb₃Sn plated sample show an increase in the field of first flux entry. Parallel field measurements are less affected by pinning strength than the perpendicular field measurements and give a better indication of when the sample first experiences flux entry. Plating niobium with Nb₃Sn could increase the effective HC1 and thereby accelerating gradient of cavities.