Robert Kiefl

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Research Classification

Research Interests

condensed matter physics
quantum phenomena

Relevant Thesis-Based Degree Programs

Affiliations to Research Centres, Institutes & Clusters

Research Options

I am available and interested in collaborations (e.g. clusters, grants).
I am interested in and conduct interdisciplinary research.

Research Methodology

muon spin rotation
beta dtetected nuclear magnetic resonance

Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Finite size effects in strongly correlated transition metal oxides probed by beta-detected NMR (2022)

In this thesis, the strongly correlated metal LaNiO3 was studied in bulk and heterostructures using8Li beta-NMR. The main objective of the thesis was to probe the metallic state and how it changes on approach to a dimensionality-induced metal-insulator transition (MIT) in heterostructures. Withits sensitivity to the metallic state, akin to conventional NMR, and ability to study thin films andheterostructures, 8Li beta-NMR is uniquely suited to tackle this challenging subject. In bulk LaNiO3, spin-lattice relaxation (SLR) measurements reveal two equally abundantcomponents with linear temperature dependence below 200K. The linearity is consistent with aKorringa mechanism and is evidence of a conventional metallic state. The resonance is characterized by a single broad line with a small temperature independent Knight shift. The normalized Korringa product indicates substantial antiferromagnetic correlations. In LaNiO3 heterostructures with insulating LaAlO3, an MIT and Néel order has been observedwhen the thickness of LaNiO3 is reduced to 2 u.c.. We used 8Li beta-NMR to study heterostructures with varying thicknesses of LaNiO3. SLR measurements show two equally abundant components with distinct temperature dependences. One component is linear with temperature, and only weakly affected by LaNiO3 thickness. In contrast, the second component is non-linear, and strongly depends on thickness. We attribute the two component relaxation in the heterostructures and bulk LaNiO3 to microscopic phase separation of the electronic state. Finally, we studied some novel LaNiO3 heterostructures where LaAlO3 was replaced with themagnetic insulator La2CuO4. Bulk La2CuO4 is characterized by long-range antiferromagnetic order below TN = 300K which is extremely sensitive to doping. In a La2CuO4 film, SLR measurements reveal fast but measurable 1/T1. We see evidence of magnetic order not from a peak in 1/T1 but instead by a gradual loss of asymmetry beginning at 140K. We attribute the suppression of TN and the broad transition to inhomogeneous hole doping in the film. Below 40K, an upturn in 1/T1 suggests freezing of mobile holes from the more doped regions of the sample. The behavior in the heterostructures is similar to the La2CuO4 film.

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Charge-neutral muon centers in magnetic and non-magnetic materials: implications and applications (2021)

Spin polarized muons are widely known as extremely sensitive local probes of magnetism. Muon spin rotation (μSR) spectroscopy has made key contributions in the study of complex condensed matter systems such as frustrated and dilute magnetic systems and superconductors. Additionally, positively charged muons implanted into semiconductors and insulators often bind an electron to form muonium (Mu=[μ+e-]), a charge-neutral muon-electron bound state. Muonium has been studied extensively in a wide range of semiconducting and insulating materials, motivated by the fact that its electronic structure inside a material is virtually identical to that of isolated hydrogen defects, one of the most ubiquitous impurities in semiconductors. However, such measurements are thought to be limited to non-magnetic compounds; in magnetic materials, muonium is widely assumed to be unobservable, and charge-neutral muon states are generally not considered relevant.Here, we present strong evidence that charge-neutral muon centers do exist in magnetic compounds. Detailed μSR investigations of the prototypical antiferromagnets Cr2O3, Fe2O3 and MnF2 reveal that charge-neutral muon states can form and take on different shapes, including muon-polaron complexes and interstitial centers with large muon-electron hyperfine coupling. Crucially, we find that in magnetic materials, charge-neutral muon states do not display any signatures conventionally associated with muonium, effectively “hiding” their presence. Despite their inconspicuous signals, charge-neutral centers can significantly change how the muons interact with their host material and thus significantly alter the μSR signals. In addition, we clearly demonstrate for MnF2 that the charge-state of the muon and the magnetic properties measured by μSR are closely related, and both aspects have to be considered when using μSR to determine the intrinsic magnetic properties. These results indicate that μSR may be useful to study not only the electronic impact of hydrogen defects, but also their role as magnetic impurities in non-conductive magnetic compounds.For comparison, we also investigate charge-neutral muon-polaron complexes in non-magnetic TiO2 as well as vacuum-like muonium diffusing through the voids of an amorphous silica aerogel. These examples are used to highlight the differences and similarities between charge-neutral muon states in magnetic and non-magnetic materials.

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The interfacial dynamics of amorphous materials as revealed by beta-NMR measurements and molecular simulations (2021)

The free surface is important for developing a fundamental understanding of dynamical length scales in glasses. We first investigate the relaxation of freestanding atactic polystyrene (aPS) thin films with molecular dynamics simulations. As in previous coarse-grained simulations, the surface modification on the relaxation times for backbone segments and phenyl rings may be expressed as a power law relation, wherein the bulk dynamics fully encapsulate the temperature-dependence. Variation of the coupling exponent with distance from the surface is consistent with depth-dependent activation barriers. We also quantify a reduction of dynamical heterogeneity, transient spatial fluctuations of the dynamics, at the interface which can be interpreted in the framework of cooperative models for glassy dynamics.Capable of depth-resolved measurements near the surface, implanted-ion beta-detected nuclear magnetic resonance (?-NMR) has been a powerful technique for the study of dynamics in aPS thin films. We have completed and commissioned an upgrade to the ?-NMR spectrometer, extending the accessible upper temperature, and enabling a direct comparison between this experimental technique and the molecular dynamics simulations. We demonstrate that the modified spectrometer is now capable of operation to at least 400 K, an improvement of more than 80 K. We also demonstrate the application of ?-NMR as a probe of ionic liquid molecular dynamics through the measurement of ⁸Li⁺ spin-lattice relaxation (SLR) and resonance in 1-ethyl-3-methylimidazolium acetate. The motional narrowing of the resonance, and the local maxima in the SLR rate, 1/T₁, imply a sensitivity to sub-nanosecond Li⁺ solvation dynamics. From an analysis of 1/T₁, we extract an activation energy and Vogel-Fulcher-Tammann constant in agreement with the dynamic viscosity of the bulk solvent. Near the melting point, the lineshape is broad and intense, and the form of the relaxation is non-exponential, reflective of our sensitivity to heterogeneous dynamics near the glass transition. We also employ the depth resolution capabilities of this technique to probe the subsurface dynamics with nanometer resolution. We show modified dynamics near the surface in, and above, the glassy state.

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Diffusion and surface trapping of 8Li in rutile TiO2 and the comparison on 8Li and 9Li spin relaxation using ?-NMR (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.

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Microscopic dynamics of isolated lithium in crystalline solids revealed by nuclear magnetic relaxation and resonance of xLi (2020)

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.

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The TRIUMF nine-cell SRF cavity for ARIEL (2016)

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.

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Absolute Value of the magnetic Penetration Depth and Field Profiel in the Meissner State of Exotic Superconductors YBA2Cu3O6+x and Ba(Co0.074Fe0.926)2As2 (2012)

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 .

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Magnetic Properties Near the Surface of Cuprate Superconductors Probed Using Beta Detected NMR (2010)

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).

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Muon Spin Rotation Characterization of Superconducting Niobium for Applications in High Field Superconducting Radio Frequency Cavities (2016)

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.

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