arXiv subjects

Derek Fujimoto

Publications and source records attributed to Derek Fujimoto.

16 recordsLinked to original sources

arXiv2026

Contrasting impurity-induced magnetism and dynamics in 2H-MoTe2

We investigate the behavior of interstitial $^8$Li$^+$ implanted near the surface of 2H-MoTe$_2$ using $\beta$-detected NMR. We find that, unlike the muon, $^8$Li$^+$ does not show any signature of induced magnetism. This result is consistent with density functional theory, which identifies the Li stopping site at the 2a Wyckoff position in the van der Waals gap and confirms the absence of detectable Li-induced electronic spin polarization. Both the spin-lattice relaxation and the resonance lines show evidence of strong spin dynamics above $\sim 200$ K, reminiscent of local stochastic $^8$Li$^+$ motion within a cage. The resonance line shape consists of quadrupolar satellites on top of a broad central peak. To better understand the interaction of $^8$Li$^+$ with the host material, we employ a frequency-comb measurement, by simultaneously exciting four frequencies corresponding to the first-order quadrupolar satellite transitions, $\nu_0 \pm 3\nu_{\mathrm{comb}}$ and $\nu_0 \pm\nu_{\mathrm{comb}}$ around the Larmor frequency $\nu_0$ as a function of $\nu_{\mathrm{comb}}$. This offers an enhanced sensitivity to the quadrupolar split portion of the line. Using this method, we find a small decrease of the quadrupolar frequency with increasing temperature, showing the typical behavior associated with thermally excited phonons and the absence of any magnetic response which was observed with other defects in 2H-MoTe$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Lithium-ion dynamics in synthetic quartz studied via the NMR of implanted $^{8}$Li$^{+}$

We report $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) measurements of implanted $^{8}$Li$^{+}$ in a synthetic single crystal of $\alpha$-SiO$_2$ (quartz). At 6.55 Tesla, the spectrum is comprised of a large amplitude broad resonance and a quadrupolar multiplet that is only revealed by an RF comb excitation. The quadrupole splitting is surprisingly small, increases with temperature, and provides information on the implantation site. Supercell density functional theory calculations show that the small EFG is consistent with an in-channel interstitial site (Wyckoff 3$a$). The spin-lattice relaxation is unexpectedly fast and strongly temperature dependent with a diffusive peak above 200 K and a second more prominent relaxation peak at lower temperature. Analysis of the diffusive relaxation yields an activation barrier 178(43) meV for the isolated Li$^{+}$, in the range of other measurements and calculations. To account for many of the other features of the data, it is suggested that some of the implanted ions trap an electron forming the neutral Li$^{0}$, which is stable over a narrow range of temperatures.

cond-mat.mtrl-sci↗

arXiv2024

QuSpin Zero-Field Magnetometer Characterization for the TUCAN Experiment

The TUCAN nEDM experiment characterizes the QuSpin Zero-Field Magnetometer (QZFM) to accurately map residual fields within a large magnetically shielded room. The magnetometer's intrinsic offset was measured to be within 3 nT and stable over a period of one year. The response was shown to be within 2 percent of linearity in the zero-field regime, up to 2 nTpp, and then follows a smooth dispersion curve. Crosstalk effects induced by multisensor operation were determined to have a small effect, and inconsequential with a separation above 6 cm. These results enable the QZFM for accurate measurement of DC fields, increase the operational range of QZFM by a factor of more than an order of magnitude, and allow for higher efficiency and flexibility by green-lighting simultaneous operation of multiple QZFMs.

physics.ins-det↗

arXiv2024

Polarized cold-neutron reflectometry at JRR-3/MINE2 for the development of ultracold-neutron spin analyzers for a neutron EDM experiment at TRIUMF

The neutron electric dipole moment (EDM) is a sensitive probe for currently undiscovered sources of charge-parity symmetry violation. As part of the TRIUMF Ultracold Advanced Neutron (TUCAN) collaboration, we are developing spin analyzers for ultracold neutrons (UCNs) to be used for a next-generation experiment to measure the neutron EDM with unprecedented precision. Spin-state analysis of UCNs constitutes an essential part of the neutron EDM measurement sequence. Magnetized iron films used as spin filters of UCNs are crucial experimental components, whose performance directly influences the statistical sensitivity of the measurement. To test such iron film spin filters, we propose the use of polarized cold-neutron reflectometry, in addition to conventional UCN transmission experiments. The new method provides information on iron film samples complementary to the UCN tests and accelerates the development cycles. We developed a collaborative effort to produce iron film spin filters and test them with cold and ultracold neutrons available at JRR-3/MINE2 and J-PARC/MLF BL05. In this article, we review the methods of neutron EDM measurements, discuss the complementarity of this new approach to test UCN spin filters, provide an overview of our related activities, and present the first results of polarized cold-neutron reflectometry recently conducted at the MINE2 beamline.

physics.ins-det↗

arXiv2024

Depth-resolved Characterization of Meissner Screening Breakdown in Surface Treated Niobium

We report direct measurements of the magnetic field screening at the limits of the Meissner phase for two superconducting Nb samples. The samples are processed with two different surface treatments that have been developed for superconducting radio-frequency cavity applications -- a "baseline" treatment and an oxygen-doping ("O-doping") treatment. The measurements show: 1) that the screening length is significantly longer in the "O-doping" sample compared to the "baseline" sample; 2) that the screening length near the limits of the Meissner phase increases with applied field; 3) the evolution of the screening profile as the material transitions from the Meissner phase to the mixed phase; and 4) a demonstration of the absence of any screening profile for the highest applied field, indicative of the full flux entering the sample. Measurements are performed utilizing the $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) technique that allows depth resolved studies of the local magnetic field within the first 100 nm of the surface. The study takes advantage of the $\beta$-SRF beamline, a new facility at TRIUMF, Canada, where field levels up to 200 mT are available parallel to the sample surface to replicate radio frequency (RF) fields near the Meissner breakdown limits of Nb.

cond-mat.supr-con↗

arXiv2023

Ion-Implanted $^8$Li Nuclear Magnetic Resonance in Highly Oriented Pyrolytic Graphite

We report $\beta$-detected nuclear magnetic resonance of ultra-dilute $^{8}$Li$^{+}$ implanted in highly oriented pyrolytic graphite (HOPG). The absence of motional narrowing and diffusional spin-lattice relaxation implies Li$^+$ is not appreciably mobile up to 400 K, in sharp contrast to the highly lithiated stage compounds. However, the relaxation is remarkably fast and persists down to cryogenic temperatures. Ruling out extrinsic paramagnetic impurities and intrinsic ferromagnetism, we conclude the relaxation is due to paramagnetic centers correlated with implantation. While the resulting effects are not consistent with a Kondo impurity, they also differ from free paramagnetic centers, and we suggest that a resonant scattering approach may account for much of the observed phenomenology.

cond-mat.mtrl-sci↗

arXiv2022

A New High Parallel-Field Spectrometer at TRIUMF's $\beta$-NMR Facility

A new high field spectrometer has been built to extend the capabilities of the $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) facility at TRIUMF. This new beamline extension allows $\beta$-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials, close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described.

cond-mat.mtrl-sci↗

arXiv2022

The Precision nEDM Measurement with UltraCold Neutrons at TRIUMF

The TRIUMF Ultra-Cold Advanced Neutron (TUCAN) collaboration aims at a precision neutron electric dipole moment (nEDM) measurement with an uncertainty of $10^{-27}\,e\cdot\mathrm{cm}$, which is an order-of-magnitude better than the current nEDM upper limit and enables us to test Supersymmetry. To achieve this precision, we are developing a new high-intensity ultracold neutron (UCN) source using super-thermal UCN production in superfluid helium (He-II) and a nEDM spectrometer. The current development status of them is reported in this article.

physics.ins-det↗

arXiv2021

Nuclear magnetic resonance of ion implanted $^8$Li in ZnO

We report on the stability and magnetic state of ion implanted $^8$Li in single crystals of the semiconductor ZnO using $\beta$-detected nuclear magnetic resonance. At ultradilute concentrations, the spectra reveal distinct Li sites from 7.6 to 400 K. Ionized shallow donor interstitial Li is stable across the entire temperature range, confirming its ability to self-compensate the acceptor character of its (Zn) substitutional counterpart. Above 300 K, spin-lattice relaxation indicates the onset of correlated local motion of interacting defects, and the spectra show a site change transition from disordered configurations to substitutional. Like the interstitial, the substitutional shows no resolved hyperfine splitting, indicating it is also fully ionized above 210 K. The electric field gradient at the interstitial $^8$Li exhibits substantial temperature dependence with a power law typical of non-cubic metals.

cond-mat.mtrl-sci↗

arXiv2020

Energy barriers and cooperative motion at the surface of freestanding glassy polystyrene films

We investigate the near-surface relaxation of freestanding atactic \glsdesc{ps} films with molecular dynamics simulations. As in previous coarse-grained simulations, relaxation times for backbone segments and phenyl rings are linked to their bulk relaxation times via a power law coupling relation. 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 at the interface which can be interpreted in the framework of cooperative models for glassy dynamics.

cond-mat.soft↗

arXiv2020

Digging Into MUD With Python: mudpy, bdata, and bfit

Used to store the results of $μ$SR measurements at TRIUMF, the Muon Data (MUD) file format serves as a useful and flexible scheme that is both lightweight and self-describing. The application programming interface (API) for these files is written in C and FORTRAN, languages not known for their ease of use. In contrast, Python is a language which emphasizes rapid prototyping and readability. This work describes three Python 3 packages to interface with MUD files and analyze their contents: mudpy, bdata, and bfit. The first enables easy access to the contents of any MUD file. The latter two are implemented specifically for the implanted-ion $β$-detected NMR ($β$-NMR) experiment at TRIUMF. These tools provide both an API and graphical user interface (GUI) to help users extract and fit $β$-NMR data.

physics.data-an↗

arXiv2020

Investigation of Ionic and Anomalous Magnetic Behavior in CrSe$_2$ Using $^8$Li $β$-NMR

We have studied a mosaic of 1T-CrSe$_2$ single crystals using $β$-detected nuclear magnetic resonance of $^{8}$Li from 4 to 300 K. We identify two broad resonances that show no evidence of quadrupolar splitting, indicating two magnetically distinct environments for the implanted ion. We observe stretched exponential spin lattice relaxation and a corresponding rate ($1/T_1$) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in $1/T_1$ is observed at the low temperature magnetic transition near 20 K. Between these limits, $1/T_1$ instead exhibits a broad minimum with a remarkable absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest $^{8}$Li$^{+}$ site occupation within the van der Waals gap between CrSe$_2$ trilayers. Possible origins of the two environments are discussed.

cond-mat.mtrl-sci↗

arXiv2019

Local electronic and magnetic properties of the doped topological insulators Bi$_{2}$Se$_{3}$:Ca and Bi$_{2}$Te$_{3}$:Mn investigated using ion-implanted $^{8}$Li $\beta$-NMR

We report $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) measurements in Bi$_{2}$Se$_{3}$:Ca (BSC) and Bi$_{2}$Te$_{3}$:Mn (BTM) single crystals using $^{8}$Li$^{+}$ implanted to depths on the order of 100 nm. Above $\sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of $^{8}$Li$^{+}$, with activation energies of $\sim 0.4$ eV ($\sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (NMR) properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi$_{2}$Te$_{2}$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the $^{8}$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the $^{8}$Li NMR response is an important step towards using depth-resolved $\beta$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi$_{2}$Ch$_{3}$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $\beta$-NMR remains strong.

cond-mat.mtrl-sci↗

arXiv2019

The dynamics of liquid 1-ethyl-3-methylimidazolium acetate measured with implanted-ion $^8$Li $β$-NMR

We demonstrate the application of implanted-ion $β$-detected NMR as a probe of ionic liquid molecular dynamics through the measurement of $^8$Li 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, $1/T_1$, imply a sensitivity to sub-nanosecond Li$^+$ solvation dynamics. From an analysis of $1/T_1$, we extract an activation energy ${E_A = 74.8 \pm 1.5}$ meV and Vogel-Fulcher-Tammann constant ${T_{\mathrm{VFT}} = 165.8 \pm 0.9}$ K, 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. The depth resolution of this technique may later provide a unique means of studying nanoscale phenomena in ionic liquids.

cond-mat.soft↗

arXiv2019

Local Metallic and Structural Properties of the Strongly Correlated Metal LaNiO$_{3}$ using $^{8}$Li $β$-NMR

We report $β$-detected NMR of ion-implanted $^{8}$Li in a single crystal and thin film of the strongly correlated metal LaNiO$_{3}$. In both samples, spin-lattice relaxation measurements reveal two distinct local metallic environments, as is evident from $T$-linear Korringa $1/T_{1}$ below 200 K with slopes comparable to other metals. A small, approximately temperature independent Knight shift of $\sim 74$ ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations, but, we find no evidence for a magnetic transition from 4 to 310 K. Two distinct, equally abundant $^{8}$Li sites is inconsistent with the widely accepted rhombohedral structure of LaNiO$_{3}$, but cannot be simply explained by either of the common alternative orthorhombic or monoclinic distortions.

cond-mat.str-el↗

arXiv2019

Ionic and electronic properties of the topological insulator Bi$_2$Te$_2$Se investigated using $β$-detected nuclear magnetic relaxation and resonance of $^8$Li

We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $β$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $τ_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is 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, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.

cond-mat.mtrl-sci↗