arXiv subjects

C. D. P. Levy

Publications and source records attributed to C. D. P. Levy.

18 recordsLinked to original sources

arXiv2021

Evolution of the metallic state of LaNiO$_3$/LaAlO$_3$ superlattices measured by $^8$Li $β$-detected NMR

Using ion-implanted $^8$Li $β$-detected NMR, we study the evolution of the correlated metallic state of LaNiO$_3$ in a series of LaNiO$_3$/LaAlO$_3$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of 6.55 T reveal two equal amplitude components: one with metallic ($T$-linear) $1/T_{1}$, and a second with a more complex $T$-dependence. The metallic character of the slow relaxing component is only weakly affected by the LaNiO$_3$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing towards low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed.

cond-mat.str-el↗

arXiv2017

Determination of the nature of fluctuations using $^8Li$ and $^9Li$ $β$-NMR and spin-lattice relaxation

We report a comparison of the $1/T_1$ spin lattice relaxation rates (SLR) for $^9Li$ and $^8Li$ in Pt and SrTiO$_{3}$, in order to differentiate between magnetic and electric quadrupolar relaxation mechanisms. In Pt, the ratio of the $1/T_{1}$ spin relaxation rates $R_{Pt}$ was found to be 6.82(29), which is close to but less than the theoretical limit of $\sim7.68$ for pure magnetic relaxation. In SrTiO$_{3}$ this ratio was found to be 2.7(3), which is close but larger than the theoretical limit of $\sim2.14$ expected for pure electric quadrupolar relaxation. These results bring new insight into the nature of the fluctuations in the local environment of implanted $^8Li$ observed by $β$-NMR.

cond-mat.mtrl-sci↗

arXiv2012

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers

We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3 using beta-detected nuclear magnetic resonance. The spin lattice relaxation rate of 8 Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO3 exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO3/SrTiO3 interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ~4-6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (~0.002 muB) in the two samples with a larger LaAlO3 spacer thickness.

cond-mat.mtrl-sci↗

arXiv2012

$β$ NMR study of Isolated $^8$Li in the enhanced paramagnet Platinum

We report β detected nuclear magnetic resonance (βNMR) measurements of 8Li+ implanted into high purity Pt. The frequency of the 8Li βNMR resonance and the spin-lattice relaxation rates 1/T1 were measured at temperatures ranging from 3 to 300 K. Remarkably, both the spin-lattice relaxation rate and the Knight shift K depend linearly on temperature T although the bulk susceptibility does not. K is found to scale with the Curie-Weiss dependence of the Pt susceptibility extrapolated to low temperatures. This is attributed to a defect response of the enhanced paramagnetism of Pt, i.e. the presence of the interstitial Li+ locally relieves the tendency for the Curie-Weiss susceptibility to saturate at low T . We propose that the low temperature saturation in \c{hi} of Pt may be related to an interband coupling between the s and d bands that is disrupted locally by the presence of the Li+.

cond-mat.str-el↗

arXiv2011

First experimental determination of the charge radius of $^{74}$Rb and its application in tests of the unitarity of the CKM matrix

Collinear-laser spectroscopy with bunched-beams technique was used for the study of neutron deficient Rb isotopes, out to $^{74}$Rb ($N=Z=37$) at TRIUMF. The measured hyperfine coupling constants of $^{76,78m}$Rb were in agreement with literature values. The nuclear spin of $^{75}$Rb was confirmed to be $I=3/2$, and its hyperfine coupling constants were measured for the first time. The mean-square charge radius of $^{74}$Rb was determined for the first time. This result has improved the isospin symmetry breaking correction term used to calculate the $\mathcal{F}t$ value, with implications for tests of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

nucl-ex↗

arXiv2011

Search for broken time-reversal symmetry near the surface of YBCO films using Beta-NMR

Weak spontaneous magnetic fields are observed near the surface of YBCO films using Beta-detected Nuclear Magnetic Resonance. Below Tc, the magnetic field distribution in a silver film evaporated onto the superconductor shows additional line broadening, indicating the appearance of small disordered magnetic fields. The line broadening increases linearly with a weak external magnetic field applied parallel to the surface, and is depth-independent up to 45 nm from the Ag/YBCO interface. The magnitude of the line broadening at 10 K extrapolated to zero applied field is less than 0.2 G, and is close to nuclear dipolar broadening in the Ag. This indicates that any fields due to broken time-reversal symmetry are less than 0.2 G.

cond-mat.supr-con↗

arXiv2010

Isotope Shift Measurements of Stable and Short-Lived Lithium Isotopes for Nuclear Charge Radii Determination

Changes in the mean-square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a new laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8 ms lifetime isotope with production rates on the order of only 10,000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope Li-11 at the on-line isotope separators at GSI, Darmstadt, Germany and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.

physics.atom-ph↗

arXiv2010

Vortex lattice disorder in YBCO probed using Beta-NMR

Beta-NMR has been used to study vortex lattice disorder near the surface of the high-Tc superconductor YBCO. The magnetic field distribution from the vortex lattice was detected by implanting a low energy beam of highly polarized 8Li into a thin overlayer of silver on optimally doped, twinned and detwinned YBCO samples. The resonance in Ag broadens significantly below the transition temperature Tc as expected from the emerging field lines of the vortex lattice in YBCO. However, the lineshape is more symmetric and the dependence on the applied magnetic field is much weaker than expected from an ideal vortex lattice, indicating that the vortex density varies across the face of the sample, likely due to pinning at twin boundaries. At low temperatures the broadening from such disorder does not scale with the superfluid density.

cond-mat.supr-con↗

arXiv2009

Quadrupole Moments of Neutron-Deficient $^{20, 21}$Na

The electric-quadrupole coupling constant of the ground states of the proton drip line nucleus $^{20}$Na($I^π$ = 2$^{+}$, $T_{1/2}$ = 447.9 ms) and the neutron-deficient nucleus $^{21}$Na($I^π$ = 3/2$^{+}$, $T_{1/2}$ = 22.49 s) in a hexagonal ZnO single crystal were precisely measured to be $|eqQ/h| = 690 \pm 12$ kHz and 939 $\pm$ 14 kHz, respectively, using the multi-frequency $β$-ray detecting nuclear magnetic resonance technique under presence of an electric-quadrupole interaction. A electric-quadrupole coupling constant of $^{27}$Na in the ZnO crystal was also measured to be $|eqQ/h| = 48.4 \pm 3.8$ kHz. The electric-quadrupole moments were extracted as $|Q(^{20}$Na)$|$ = 10.3 $\pm$ 0.8 $e$ fm$^2$ and $|Q(^{21}$Na)$|$ = 14.0 $\pm$ 1.1 $e$ fm$^2$, using the electric-coupling constant of $^{27}$Na and the known quadrupole moment of this nucleus as references. The present results are well explained by shell-model calculations in the full $sd$-shell model space.

nucl-ex↗

arXiv2008

Local Magnetic Properties of a Monolayer of Mn12 Single Molecule Magnets

The magnetic properties of a monolayer of Mn12 single molecule magnets grafted onto a Si substrate have been investigated using depth-controlled $β$-detected nuclear magnetic resonance. A low energy beam of spin polarized radioactive 8Li was used to probe the local static magnetic field distribution near the Mn12 monolayer in the Si substrate. The resonance linewidth varies strongly as a function of implantation depth as a result of the magnetic dipolar fields generated by the Mn12 electronic magnetic moments. The temperature dependence of the linewidth indicates that the magnetic properties of the Mn12 moments in this low dimensional configuration differ from bulk Mn12.

cond-mat.mes-hall↗

arXiv2008

Hyperfine Fields in an Ag/Fe Multilayer Film Investigated with 8Li beta-Detected Nuclear Magnetic Resonance

Low energy $β$-detected nuclear magnetic resonance ($β$-NMR) was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 Å)/Ag(200 Å)/Fe(140 Å) (001) magnetic multilayer (MML) grown on GaAs. The resonance lineshape in the Ag layer shows dramatic broadening compared to intrinsic Ag. This broadening is attributed to large induced magnetic fields in this layer by the magnetic Fe layer. We find that the induced hyperfine field in the Ag follows a power law decay away from the Ag/Fe interface with power $-1.93(8)$, and a field extrapolated to $0.23(5)$ T at the interface.

cond-mat.str-el↗

arXiv2007

Giant Vortices Below the Surface of NbSe$_2$ Detected Using Low Energy $β$-NMR

A low energy radioactive beam of polarized $^8$Li has been used to observe the vortex lattice near the surface of superconducting NbSe$_2$. The inhomogeneous magnetic field distribution associated with the vortex lattice was measured using depth-resolved $β$-detected NMR. Below $T_c$ one observes the characteristic lineshape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with magnetic field. In particular in a low field of 10.8 mT the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed.

cond-mat.str-el↗

arXiv2007

Beta-NMR of Isolated Lithium in Nearly Ferromagnetic Palladium

The temperature dependence of the frequency shift and spin-lattice relaxation rate of isolated, nonmagnetic Li-8 impurities implanted in a nearly ferromagnetic host (Pd) are measured by means of beta-detected nuclear magnetic resonance (b-NMR). The shift is negative, very large and increases monotonically with decreasing T in proportion to the bulk susceptibility of Pd for T > T*~ 100 K. Below T*, an additional shift occurs which we attribute to the response of Pd to the defect. The relaxation rate is much slower than expected for the large shift and is linear with T below T*, showing no sign of additional relaxation mechanisms associated with the defect.

cond-mat.mtrl-sci↗

arXiv2006

$β$-NMR of Isolated $^{8}$Li$^{+}$ Implanted into a Thin Copper Film

Depth-controlled $β$-NMR was used to study highly spin-polarized $^8$Li in a Cu film of thickness 100 nm deposited onto a MgO substrate. The positive Knight Shifts and spin relaxation data show that $^8$Li occupies two sites at low temperatures, assigned to be the substitutional ($S$) and octahedral ($O$) interstitial sites. Between 50 to 100 K, there is a site change from $O$ to $S$. The temperature dependence of the Knight shifts and spin-lattice relaxation rates at high temperatures, i.e. when all the Li are in the $S$ site, is consistent with the Korringa Law for a simple metal.

cond-mat.str-el↗

arXiv2006

Near-Surface Structural Phase Transition of SrTiO$_3$ Studied with Zero-Field $β$-Detected Nuclear Spin Relaxation and Resonance

We demonstrate that zero-field $β$-detected nuclear quadrupole resonance and spin relaxation of low energy $^8$Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO$_3$ single crystal occurs at $T_c \sim 150$ K, i.e., 45 K higher than $T^{bulk}_c$, and that the tetragonal domains formed below $T_c$ are randomly oriented.

cond-mat.stat-mech↗

arXiv2005

Nuclear Charge Radius of Li-9, Li-11: Halo Neutron: the influence of Halo Neutrons

The nuclear charge radius of Li-11 has been determined for the first time by high precision laser spectroscopy. On-line measurements at TRIUMF-ISAC yielded a Li-7 - Li-11 isotope shift (IS) of 25101.23(13) MHz for the Doppler-free 2s - 3s transition. IS precision for all other bound Li isotopes was also improved. Differences from calculated mass-based IS yield values for change in charge radius along the isotope chain. The charge radius decreases monotonically from Li-6 to Li-9, and then increases from 2.217(35) fm to 2.467(37) fm for Li-11. This is compared to various models, and it is found that a combination of halo neutron correlation and intrinsic core excitation best reproduces the experimental results.

physics.atom-ph↗

arXiv2005

$β$-Detected NMR of $^8$Li in the Normal State of 2H-NbSe$_2$

$β$-NMR of isolated $^8$Li has been investigated in the normal state of 2H-NbSe$_2$. In a high magnetic field of 3T a single resonance is observed with a Gaussian line width of 3.5 kHz. The line shape varies weakly as function of magnetic field and temperature but has a strong orientation dependence. The nuclear electric quadrupole splitting is unresolved implying that the electric field gradients are 10-100 times smaller than in other non-cubic crystals. The nuclear spin relaxation rate is also anomalously small but varies linearly with temperature as expected for Korringa relaxation in a metal. These results suggest that Li adopts an interstitial position between the weakly coupled NbSe$_2$ layers and away from the conduction band.

cond-mat.str-el↗

arXiv2005

Beta-Detected NQR in Zero Field with a Low Energy Beam of \nuc{8}{Li}$^+$

Beta-detected nuclear quadrupole resonances (\bnqr) at zero field are observed using a beam of low energy highly polarized radioactive \nuc{8}{Li}$^+$. The resonances were detected in SrTiO$_3$, Al$_2$O$_3$ and Sr$_2$RuO$_4$ single crystals by monitoring the beta-decay anisotropy as a function of a small audio frequency magnetic field. The resonances show clearly that \nuc{8}{Li} occupies one site with non-cubic symmetry in SrTiO$_3, two in Al$_2$O$_3$ and three sites in Sr$_2$RuO$_4$. The resonance amplitude and width are surprisingly large compared to the values expected from transitions between the $|\pm 2> \leftrightarrow |\pm 1>$ spin states, indicating a significant mixing between the $|\pm m>$ quadrupolar split levels.

cond-mat.str-el↗