Photodarkening in glassy As2S3 Hari, P; Su, T; Taylor, P.C ...
Journal of non-crystalline solids,
05/2000, Letnik:
266-269
Journal Article
Recenzirano
NMR of 75As at 17 T has been employed to study photodarkening (the shift of the optical absorption edge to lower energies after excitation with light of energy near the optical band edge) in glassy ...As2S3. After irradiation at 514.5 nm for 230 h with 170 mW/cm2 the average asymmetry parameter of the electric field gradient (EFG) tensor increases from about 0.09plus/minus 0.01 to about 0.12plus/minus 0.01. This change is reversible on annealing at 200 C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry in the bonding at the As pyramidal sites. 4 refs.
NMR Phase Noise in Bitter Magnets Sigmund, E.E.; Calder, E.S.; Thomas, G.W. ...
Journal of magnetic resonance (1997),
02/2001, Letnik:
148, Številka:
2
Journal Article
Recenzirano
We have studied the temporal instability of a high field resistive Bitter magnet through nuclear magnetic resonance (NMR). This instability leads to transverse spin decoherence in repeated and ...accumulated NMR experiments as is normally performed during signal averaging. We demonstrate this effect via Hahn echo and Carr–Purcell–Meiboom–Gill (CPMG) transverse relaxation experiments in a 23-T resistive magnet. Quantitative analysis was found to be consistent with separate measurements of the magnetic field frequency fluctuation spectrum, as well as with independent NMR experiments performed in a magnetic field with a controlled instability. Finally, the CPMG sequence with short pulse delays is shown to be successful in recovering the intrinsic spin–spin relaxation even in the presence of magnetic field temporal instability.
Although the frustrated spin chain (zigzag chain) is a Drosophila of frustrated magnetism, the understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is ...incomplete. We address this problem through nuclear magnetic resonance (NMR) experiments on \(\text{BiCu}_2\text{PO}_6\) in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in \(\text{BiCu}_2\text{PO}_6\). An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice gives rise to a new kind of spin-Peierls instability.
Inductive shielding of NMR phase noise Sigmund, E.E; Mitrović, V.F; Calder, E.S ...
Journal of magnetic resonance (1997),
12/2002, Letnik:
159, Številka:
2
Journal Article
Recenzirano
We report on a solution to the problem of phase noise in nuclear magnetic resonance (NMR) experiments. Phase noise refers to the variation in the phases of NMR signals from successive acquisitions ...due to an unstable applied field. Such a situation exists in high-field resistive Bitter magnets and, for sufficiently long timescales, can cause serious signal degradation upon signal averaging. An inductive shield, formed by a highly conducting metal tube placed around the sample and along the applied field, provides screening of the AC components of the applied field and thereby retains phase coherence over long periods. Although simple in principle there are technical difficulties for practical implementation of this method. We present demonstrations of the utility of this approach. In particular, we show a significant extension of the effective transverse coherence time of the
13
C
resonance in doubly
13
C
-labeled glycerol in a resistive Bitter magnet. This was accomplished through the use of a highly conducting aluminum shield, cooled to 4
K with liquid helium.
This volume brings together prominent archaeologists working in areas outside Western Europe to discuss the most recent evidence for the origins of the early Upper Paleolithic and its relationship to ...the origin of modern humans. With a wealth of primary data from archaeological sites and regions that have never before been published and discussions of materials from difficult-to-find sources, the collection urges readers to reconsider the process of modern human behavioral origins. Archaeological evidence continues to play a critical role in debates over the origins of anatomically modern humans. The appearance of novel Upper Paleolithic technologies, new patterns of land use, expanded social networks, and the emergence of complex forms of symbolic communication point to a behavioral revolution beginning sometime around 45,000 years ago. Until recently, most of the available evidence for this revolution derived from Western European archaeological contexts that suggested an abrupt replacement of Mousterian Middle Paleolithic with Aurignacian Upper Paleolithic adaptations. In the absence of fossil association, the behavioral transition was thought to reflect the biological replacement of archaic hominid populations by intrusive modern humans. The contributors present new archaeological evidence that tells a very different story: The Middle-Upper Paleolithic transitions in areas as diverse as the Levant, Eastern-Central Europe, and Central and Eastern Asia are characterized both by substantial behavioral continuity over the period 45,000-25,000 years ago and by a mosaic-like pattern of shifting adaptations. Together these essays will enliven and enrich the discussion of the shift from archaic to modern behavioral adaptations. Contributors: O. Bar-Yosef, A. Belfer-Cohen, R. L. Bettinger, P. J. Brantingham, N. R. Coinman, A. P. Derevianko, R. G. Elston, J. R. Fox, X. Gao, J. M. Geneste, T. Goebel, E. Güleç, K. W. Kerry, L. Koulakovskaia, J. K. Kozlowski, S. L. Kuhn, Y. V. Kuzmin, D. B. Madsen, A. E. Marks, L. Meignen, T. Meshveliani, K. Monigal, P. E. Nehoroshev, J. W. Olsen, M. Otte, M. C. Stiner,J. Svoboda, A. Sytnik, D. Tseveendorj, L. B. Vishnyatsky
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