Status and progress of ion-implanted β NMR at TRIUMF

We report the β-detected NMR of implanted 8Li+ in the rhombohedrally distorted perovskite LaAlO3. As observed in other insulating perovskites, the resonance has large quadrupolar splitting. However, it exhibits additional splitting due to the rhombohedral distortion. In addition, the magnitude of the electric field gradient at the 8Li site is larger than in cubic perovskites, such as SrTiO3, with vQ ≈ 191.3 kHz.

Section: Methodsmentioning

confidence: 99%

Effects of the rhombohedral distortion in LaAlO₃ on the quadrupolar splitting of the implanted ⁸Li⁺ NMR

Karner

Chatzichristos²,

Fujimoto³

et al. 2023

“…where Σ is a diagonal matrix of the reciprocal uncertainties 1/δA 2 (t) (i.e., the ILT ρ minimizes the χ 2 statistic ||Σ (Kρ − A) || 2 ). This is thus a weighted linear least-squares problem, but the large number of parameters (1000) present in ρ makes the solution ill-defined: many choices of ρ provide equally good candidates for the minimization of Equation (5). Regularization is introduced to increase the stability of this inversion.…”

Section: Methodsmentioning

confidence: 99%

“…In βNMR [5], like µSR [6], the spin probe is implanted into the sample. There are several possible origins for non-single exponential relaxation.…”

Section: Introductionmentioning

confidence: 99%

Inverse Laplace Transform Approaches to βNMR Relaxation

MacFarlane

Fujimoto²,

McFadden³

2023

Spin lattice relaxation is the simplest type of βNMR measurement. The usual approach is to implant a pulse of hyperpolarized nuclei and monitor the time-resolved β-decay asymmetry, yielding the ensemble average spin-lattice relaxation. In the simplest case, the asymmetry decays exponentially with a characteristic time constant T 1, but this ideal is rarely obtained in practice. In most data, the relaxation is more complicated. This can be the result of multiple crystallographic sites for the implanted probe each having a distinct T 1. The sample may also be inhomogeneous due to: impurities or defects (including interfaces that are particularly important for thin films), intrinsic phase separation, or, if it is a glass. There may also be a background signal from probe ions that stop outside the sample. The general approach to this problem has been the ad hoc development of an appropriate relaxation model that avoids overparametrization. Given the prevalence of more complicated relaxation, it is crucial to develop a systematic approach to relaxation modelling. The decomposition of a relaxing signal into exponentials is, however, a mathematically ill-posed problem[1]. This feature is intrinsic and unavoidable, but there are a number of methods to accommodate it for noisy real-world data, including nuclear spin relaxation[2, 3, 4]. Here we demonstrate one of the best and most commonly used methods, Tikhonov regularization for the inverse Laplace transform, implemented for the particular features of βNMR relaxation data, most importantly the strong time dependence of the statistical uncertainty stemming from the radioactive lifetime of the probe.

“…β-NMR experiments were conducted at TRIUMF's ISAC facility in Vancouver, Canada. 6,17 A beam of 8 Li + was spin-polarized to ∼70 % via in-flight colinear optical pumping with a circularly polarized laser. 18 Mean implantation depths were determined by Monte-Carlo simulation.…”

Section: Methodsmentioning

confidence: 99%

“…We employ a technique called implanted-ion β-detected nuclear magnetic resonance (β-NMR). 5,6 In many ways similar to the well-established technique of low energy muon spin rotation (LE-µSR), β-NMR differs in that it uses a radioactive atomic ion as its probe. The use of the much longer-lived atomic probe allows β-NMR access to much slower dynamics, as well as a variety of chemically-relevant interactions.…”

Section: Introductionmentioning

confidence: 99%

Near-surface dynamics of the ionic liquid EMIM-Ac above and below the glass transition

Fujimoto

Karner

Dehn

et al. 2023

In our prior work, we showed that β-detected nuclear magnetic resonance (β-NMR) was a good probe of bulk room temperature ionic liquid (RTIL) dynamics and dynamic heterogeneity. We now investigate how the surface modifies these properties, presenting the first depth-resolved β-NMR measurements in 1-ethyl-3-methylimidazolium acetate as a liquid, supercooled liquid, and glass. This interfacial region is important for understanding how constrained dimensionality affects dynamics. We show that both the surface and the glass transition have a large impact on molecular dynamics, which in many aspects differs greatly from our expectations based on polymer glasses. For example, in the glassy phase the surface dynamics appear to be faster than in the bulk (i.e., liquid-like), yet just as heterogeneous (i.e., glass-like).