Robert Kiefl

Professor

Research Classification

Quantum Phenomena

Research Interests

condensed matter physics
polarons
multiferroics
superconductivity
magnetism

Relevant Degree Programs

 

Research Methodology

muon spin rotation
beta dtetected nuclear magnetic resonance

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Master's students
Any time / year round

controlling magnetic and electric  properties with electric fields

diffusion of Li in battery  electrode materials

I support experiential learning experiences, such as internships and work placements, for my graduate students and Postdocs.

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
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)

No abstract available.

Magnetic properties near the surface of cuprate superconductors studied 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 (2010 - 2018)
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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