We report β-detected NMR of ion-implanted Li8 in a single crystal and thin film of the strongly correlated metal LaNiO3. Spin-lattice relaxation measurements reveal two distinct local environments, ...both metallic as evident from T-linear Korringa 1/T1 below 200K with slopes comparable to other metals. A small approximately temperature-independent Knight shift of ∼74ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations. We find no evidence for a magnetic transition from 4 to 310K. The similarity of these features in the two very different samples indicates that they are intrinsic and unrelated to dilute oxygen vacancies. We attribute the two environments to two distinct but similar crystallographic Li8 sites and not to any form of phase inhomogeneity, but this is inconsistent with the conventional rhombohedral structure of LaNiO3, and also cannot be simply explained by the common alternative orthorhombic or monoclinic distortions.
Understanding patterns of biodiversity in microbial communities is severely constrained by the difficulty of adequately sampling these complex systems. We illustrate the problem with empirical data ...from small surveys (200-member 16S rRNA gene clone libraries) of four bacterial soil communities from two locations in Arizona. Among the four surveys, nearly 500 species-level groups (Dunbar et al., Appl. Environ. Microbiol. 65:662-1669, 1999) and 21 bacterial divisions were documented, including four new candidate divisions provisionally designated SC1, SC2, SC3, and SC4. We devised a simple approach to constructing theoretical null models of bacterial species abundance. These null models provide, for the first time, detailed descriptions of soil bacterial community structure that can be used to guide experimental design. Models based on a lognormal distribution were consistent with the observed sizes of the four communities and the richness of the clone surveys. Predictions from the models showed that the species richness of small surveys from complex communities is reproducible, whereas the species composition is not. By using the models, we can now estimate the required survey scale to document specified fractions of community diversity. For example, documentation of half the species in each model community would require surveys of 16,284 to 44,000 individuals. However, quantitative comparisons of half the species in two communities would require surveys at least 10-fold larger for each community.
Abstract
The molecular dynamics of inelastic strain in glasses remains poorly understood, in contrast to the plasticity of crystalline materials that is well-characterized by measurements of ...dislocation activity. We report initial results on a 300 nm thick atactic polystyrene film undergoing plastic strain in its glassy state. This physical modification was applied by nanoimprint stamping with a 1 mm ultra-smooth spherical die to induce a stress exceeding mechanical yield (0.8% residual strain). Using
8
Li implanted-ion
β
NMR, we monitor the spin-lattice relaxation to infer depth-resolved rates of molecular dynamics. We find a significant change in the bulk molecular dynamics of the imprinted film (away from the surface) compared to an identically prepared control film. The relaxation is ∼ 20% slower in the film left densified by imprinting. We expect this relaxation to be coupled to the motion of the phenyl side rings; wherein slower dynamics due to densification is reasonable, as tighter packing should increase the energy barrier to molecular motion. In addition, we see an increase in the apparent thickness of a nanometric mobile surface layer, but this may be an artefact of surface roughening caused by imprinting.
The molecular dynamics of inelastic strain in glasses remains poorly understood, in contrast to the plasticity of crystalline materials that is well-characterized by measurements of dislocation ...activity. We report initial results on a 300 nm thick atactic polystyrene film undergoing plastic strain in its glassy state. This physical modification was applied by nanoimprint stamping with a 1 mm ultra-smooth spherical die to induce a stress exceeding mechanical yield (0.8% residual strain). Using 8Li implanted-ion βNMR, we monitor the spin-lattice relaxation to infer depth-resolved rates of molecular dynamics. We find a significant change in the bulk molecular dynamics of the imprinted film (away from the surface) compared to an identically prepared control film. The relaxation is ∼ 20% slower in the film left densified by imprinting. We expect this relaxation to be coupled to the motion of the phenyl side rings; wherein slower dynamics due to densification is reasonable, as tighter packing should increase the energy barrier to molecular motion. In addition, we see an increase in the apparent thickness of a nanometric mobile surface layer, but this may be an artefact of surface roughening caused by imprinting.
The inverse perovskite Sr3SnO is a 3D cubic Dirac semimetal with a very small energy gap1. Its unusual electronic structure confers a variety of novel properties, such as chiral topological surface ...states, and very strong itinerant electron orbital magnetism. Remarkably, when doped it also becomes superconducting2. In the lowest carrier density samples, the Fermi level lies close to the Dirac points, and orbital magnetism is maximal. Here we report the results of ion-implanted 8Li+ βNMR in Au-capped epitaxial thin films of Sr3SnO as a function of carrier content. In addition, we stop the 8Li in the Au overlayer to seek proximal evidence of the chiral surface state.In high magnetic field (6.55 T), we find remarkably little contrast in spin-lattice relaxation (SLR) between low carrier density Sr3SnO and the Au overlayer. In the inverse perovskite layer, 1/T1 ∼ 0.14 s-1, slightly faster than Au at 300 K, while in the overlayer, there is a small but systematic enhancement in 1/T1 compared to a control film of Au. The resonance in the Sr3SnO layer is broad with a long tail towards negative shift without resolved quadrupolar splitting.
Abstract
The inverse perovskite Sr
3
SnO is a 3D cubic Dirac semimetal with a very small energy gap1. Its unusual electronic structure confers a variety of novel properties, such as chiral ...topological surface states, and very strong itinerant electron orbital magnetism. Remarkably, when doped it also becomes superconducting2. In the lowest carrier density samples, the Fermi level lies close to the Dirac points, and orbital magnetism is maximal. Here we report the results of ion-implanted
8
Li
+
β
NMR in Au-capped epitaxial thin films of Sr
3
SnO as a function of carrier content. In addition, we stop the
8
Li in the Au overlayer to seek proximal evidence of the chiral surface state.
In high magnetic field (6.55 T), we find remarkably little contrast in spin-lattice relaxation (SLR) between low carrier density Sr
3
SnO and the Au overlayer. In the inverse perovskite layer, 1/
T
1
∼ 0.14 s
-1
, slightly faster than Au at 300 K, while in the overlayer, there is a small but systematic enhancement in 1/
T
1
compared to a control film of Au. The resonance in the Sr
3
SnO layer is broad with a long tail towards negative shift without resolved quadrupolar splitting.
We have studied a mosaic of 1T-CrSe2 single crystals using β-detected nuclear magnetic resonance of 8Li 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/T1) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in 1/T1 is observed at the low temperature magnetic transition near 20 K. Between these limits, 1/T1 exhibits a broad minimum with an anomalous absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest 8Li+ site occupation within the van der Waals gap between CrSe2 trilayers. Possible origins of the two environments are discussed.
PDF
