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W. A. MacFarlane

Publications and source records attributed to W. A. MacFarlane.

At least 37 records · Page 2Linked to original sources
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

NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBa$_2$Cu$_3$O$_{6.5}$

Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (YBCO6.5) single crystals reveal unusual and remarkable properties. The chain Cu resonance broadens significantly, but gradually, on cooling from room temperature. The lineshape and its temperature dependence are substantially different from that of a conventional spin/charge density wave (S/CDW) phase transition. Instead, the line broadening is attributed to small amplitude static spin and charge density oscillations with spatially varying amplitudes connected with the ends of the finite length chains. The influence of this CuO chain phenomenon is also clearly manifested in the plane Cu NMR.

cond-mat.supr-con↗
arXiv2006

Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets

We present longitudinal field muon spin relaxation ($μ$SR) measurements in the unilluminated state of the photo-sensitive molecular magnetic Co-Fe Prussian blue analogues M$_{1-2x}$Co$_{1+x}$[Fe(CN)$_6$]$\cdot z$ H$_2$O, where M=K and Rb with $x=0.4$ and $\simeq 0.17$, respectively. These results are compared to those obtained in the $x=0.5$ stoichiometric limit, Co$_{1.5}$[Fe(CN)$_6$]$\cdot 6$ H$_2$O, which is not photo-sensitive. We find evidence for correlation between the range of magnetic ordering and the value of $x$ in the unilluminated state which can be explained using a site percolation model.

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

$β$-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↗
arXiv2004

Correlation length in cuprates deduced from the impurity-induced magnetization

We report a new multi-nuclei based NMR method which allows us to image the staggered polarization induced by nonmagnetic Li impurities in underdoped O6.6 and slightly overdoped O7 YBa2Cu3O6+y above T_C. The spatial extension of the polarization xi_imp approximately follows a Curie law, increasing up to six lattice constants at T=80K at O6.6 in the pseudogap regime. Near optimal doping, the staggered magnetization has the same shape, with xi_imp reduced by a factor 2. xi_imp is argued to reveal the intrinsic magnetic correlation length of the pure system. It is found to display a smooth evolution through the pseudogap regime.

cond-mat.supr-con↗
arXiv2004

Antiferromagnetic properties of a water vapor-inserted $YBa_2Cu_3O_{6.5}$ compound studied by NMR, NQR and $μ$ SR

We present a detailed NQR, NMR and $μ$SR study of a magnetic phase obtained during a topotactic chemical reaction of YBa$_{2}$Cu$_{3}$O$_{6.5}$ high- temperature superconductor with low-pressure water vapor. Our studies give straightforward evidence that the ''empty'' Cu(1) chains play the role of an easy water insertion channel. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and completely disappears when one H$_{2}$O molecule is inserted per unit cell. Similarly, a Cu ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are non-superconducting antiferromagnetic phases in which the bilayers are ordered. These antiferromagnetic phases are felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss respectively. Two muon sites are also evidenced with similar local fields which vanish at $T\approx 400$ K. This indicates that the magnetic phases have similar Néel temperatures as the other bilayer undoped compounds. An analysis of the internal fields on different sites of the structure suggests that they can be all assigned to a single magnetic phase at large water content in which the Cu(1) electron spins order with those of the Cu(2). It appears that even samples packed in Stycast epoxy resin heated moderately at a temperature (200$^{0}$C) undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is then quite clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in such materials, as those performed in some recent neutron scattering experiments.

cond-mat.supr-con↗
arXiv2003

Cu NMR Study of Detwinned Single Crystals of Ortho--II YBCO6.5

Copper NMR has been used as a local probe of the oxygen ordering in Ortho--II YBa2Cu3O6.5 crystals grown in BaZrO3 crucibles. Line assignments have been made to each of the expected crystallographically inequivalent sites. The presence of distinct and narrow lines for these sites as well as the lack of a line known to be associated with oxygen defects indicates that these crystals are highly stoichiometric. Our estimate of the lower limit on the chain length is consistent with that derived from X-ray diffraction measurements. In addition, we have found no evidence for static magnetic moments, in contrast to some previous results.

cond-mat.supr-con↗
arXiv2003

Local Magnetic Susceptibility of the Positive Muon in the Quasi 1D S=1/2 Antiferromagnet dichlorobis (pyridine) copper (II)

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with Transfer Matrix DMRG numerical simulations.

cond-mat.str-el↗
arXiv2002

Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}

We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x}, which at x=1 exhibits a characteristic pseudogap temperature dependence, confirming that Pr reduces the concentration of mobile holes in the CuO_{2} planes. Our estimate of the rate of this counterdoping effect, obtained by comparison with the shift in pure samples with reduced oxygen content, is found insufficient to explain the observed reduction of T_c. From the temperature dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment and the local magnetic defect induced in the CuO_2 planes produce a long range spin polarization in the planes, which is likely associated with the extra reduction of T_c. We find a qualitatively different behaviour in the oxygen depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T$_c$ is nearly the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This difference may signal a weaker coupling of the Pr to the planes in the underdoped compound, which might be linked with the larger Pr to CuO_2 plane distance, and correspondingly weaker hybridization.

cond-mat.supr-con↗
arXiv2002

Elastic Tensor of Sr$_2$RuO$_4$

The six independent elastic constants of Sr$_2$RuO$_4$ were determined using resonant ultrasound spectroscopy on a high-quality single-crystal specimen. The constants are in excellent agreement with those obtained from pulse-echo experiments performed on a sample cut from the same ingot. A calculation of the Debye temperature using the measured constants agrees well with values obtained from both specific heat and Mössbauer measurements.

cond-mat.supr-con↗
arXiv2001

Ultrasound Attenuation in Sr$_2$RuO$_4$: an Angle-Resolved Study of the Superconducting Gap Function

We present a study of the electronic ultrasound attenuation $α$ in the unconventional superconductor Sr$_2$RuO$_4$ . The power law behavior of $α$ at temperatures down to $T_c/30$ clearly indicates the presence of nodes in the gap. In the normal state, we find an enormous anisotropy of $α$ in the basal plane of the tetragonal structure. In the superconducting state, the temperature dependence of $α$ also exhibits significant anisotropy. We discuss these results in relation to possible gap functions.

cond-mat.supr-con↗
arXiv2001

Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates

We measure the magnetic susceptibility nearby Li spinless impurities in the superconducting phase of the high Tc cuprate YBaCuO. The induced moment which was found to exist above Tc persists below Tc. In the underdoped regime, it retains its Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (T_K=135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. This moment still extends essentially on its 4 near neighbour Cu, showing the persistence of AF correlations in the superconducting state. A direct comparison with recent STM results of Pan et al. is proposed.

cond-mat.supr-con↗
arXiv2000

Dynamics of the Local Moment Induced by Nonmagnetic Defects in Cuprates

We present a study of the spin dynamics of magnetic defects induced by Li substitution of the plane Cu in the normal state of YBa$_2$Cu$_3$O$_{6+x}$. The fluctuations of the coupled Cu magnetic moments in the vicinity of Li are probed by near-neighbour $^{89}$Y {\it and} $^7$Li NMR spin lattice relaxation. The data indicates that the magnetic perturbation fluctuates as a single entity with a correlation time $τ$ which scales with the local static susceptibility. This behaviour is reminiscent of the low $T$ Kondo state of magnetic impurities in conventional metals. Surprisingly it extends well above the ``Kondo'' temperature for the underdoped pseudogapped case.

cond-mat.str-el↗
arXiv1999

Spinless impurities in high Tc cuprates: Kondo-like behavior

We compare the effects of in-plane non magnetic Li$^{+}$ and Zn$^{2+}$ impurities on the normal state of high-T$_{c}$ cuprates. Y NMR shows that the extra hole introduced by Li is not localized in its vicinity. The Tc depression and induced moments on near neighbour Cu sites of Zn or Li are found identical. These universal effects of spinless impurities establish the major influence of the spin perturbation with respect to the charge defect. The susceptibility of the induced moment measured by Li NMR displays a 1/(T+Theta) behavior. Theta increases with doping up to about 200 K in the overdoped regime. We attribute this to a "Kondo like" effect.

cond-mat.supr-con↗
arXiv1999

Melting and Dimensionality of the Vortex Lattice in Underdoped YBa2Cu3O6.60

Muon spin rotation measurements of the magnetic field distribution in the vortex state of the oxygen deficient high-Tc superconductor YBa{2}Cu{3}O{6.60} reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated material. The transition is best described by a model in which adjacent layers of ``pancake'' vortices decouple in the liquid phase. Evidence is also found for a pinning-induced crossover from a solid 3D to quasi-2D vortex lattice, similar to that observed in the highly anisotropic superconductor Bi{2+x}Sr{2-x}CaCu{2}O{8+y}.

cond-mat.supr-con↗