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Register a new user$beta$-Detected NMR of $^8$Li in the Normal State of 2H-NbSe$_2$
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$beta$-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.
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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 $beta$-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.
Using ion-implanted $^8$Li $beta$-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.
A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature T$_{cdw}$. Here we investigate, using photoemission, X-ray scattering and scanning tunneling microscopy, the canonical CDW compound 2H-NbSe$_2$ intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude that impacts the electronic dispersion, giving rise to an energy gap. The phase transition at T$_{cdw}$ marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in `pseudogap states.
We report {beta} detected nuclear magnetic resonance ({beta}NMR) measurements of 8Li+ implanted into high purity Pt. The frequency of the 8Li {beta}NMR 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+.
We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $beta$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) times 10^{10}$ s$^{-1}$. We suggest this is unrelated to Li$^{+}$ motion, and rather is a consequence of electron polarons in the vicinity of the implanted $^{8}$Li$^{+}$ that are known to become mobile in this temperature range. Above 100 K, Li$^{+}$ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of $0.32(2)$ eV and $1.0(5) times 10^{16}$ s$^{-1}$, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li$^{+}$ concentration does not limit the diffusivity even up to high concentrations, but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li$^{+}$ and polaron motion.
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