Rapid Suppression of the Spin Gap in Zn-doped CuGeO_3 and SrCu_2O_3

The influence of non-magnetic impurities on the spectrum and dynamical spin structure factor of a model for CuGeO$_3$ is studied. A simple extension to Zn-doped ${\rm Sr Cu_2 O_3}$ is also discussed. Using Exact Diagonalization techniques and intuitive arguments we show that Zn-doping introduces states in the Spin-Peierls gap of CuGeO$_3$. This effect can beunderstood easily in the large dimerization limit where doping by Zn creates ``loose'' S=1/2 spins, which interact with each other through very weak effective antiferromagnetic couplings. When the dimerization is small, a similar effect is observed but now with the free S=1/2 spins being the resulting S=1/2 ground state of severed chains with an odd number of sites. Experimental consequences of these results are discussed. It is interesting to observe that the spin correlations along the chains are enhanced by Zn-doping according to the numerical data presented here. As recent numerical calculations have shown, similar arguments apply to ladders with non-magnetic impurities simply replacing the tendency to dimerization in CuGeO$_3$ by the tendency to form spin-singlets along the rungs in SrCu$_2$O$_3$.Comment: 7 pages, 8 postscript figures, revtex, addition of figure 8 and a section with experimental predictions, submmited to Phys. Rev. B in May 199

Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments

Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by $Zn$ doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational techniques which includes Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a ``pruning'' of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems $Cu(NO_3)_2\cdot2.5H_2O$, $CuWO_4$, $(VO)_2P_2O_7$, and $Sr_{14}Cu_{24}O_{41}$, in ladder materials such as $Sr Cu_2 O_3$, in the spin-Peierls systems $CuGeO_3$ and $NaV_2 O_5$, and in several others since it is a universal effect common to a wide variety of models and compounds.Comment: 18 pages revtex with 26 figures include

Concurrent Performances: Reinforcement By Different Doses Of Intravenous Cocaine In Rhesus Monkeys 1

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96683/1/jeab.1974.22-179.pd

Competitive and uncompetitive N -methyl- d -aspartate antagonist discriminations in pigeons: CGS 19755 and phencyclidine

The purpose of the present studies was to examine representative uncompetitive and competitive NMDA antagonists, as well as the glycine/NMDA antagonist, HA 966, in pigeons trained to discriminate either PCP or CGS 19755 from saline. Separate groups of pigeons were trained to discriminate either the uncompetitive, phencyclidine (PCP; 0.32 and 1.0 mg/kg, IM), or the competitive, CGS 19755 ( cis -4-phosphonomethyl-2-piperidine-carboxylic acid; 1.8 mg/kg, IM), NMDA antagonists from saline. Uncompetitive and competitive NMDA antagonists were examined in generalization studies, as were the racemate and the (+) and (−) stereoisomers of HA 966 (3-amino-1-hydroxypyrrolid-2-one). Dizocilpine (MK 801) was fully generalized to PCP but not to CGS 19755. All competitive NMDA antagonists tested were fully generalized to CGS 19755, but not to PCP. The competitive antagonists, however, produced >50% PCP-appropriate responding. The (+) isomer of HA 966 was fully generalized by three of four pigeons discriminating PCP (1.0 mg/kg) or CGS 19755, whereas the racemate and the (−) isomer produced 10% drug-appropriate responding in either discrimination group. The competitive antagonists tended to produce peak drug-appropriate responding at times greater than 60 min after administration, whereas uncompetitive antagonists produced peak drug-appropriate responding at earlier times. HA 966 also had a relatively slow onset of action as compared to PCP. These results suggest that antagonists acting at different modulatory sites of the NMDA receptor complex produce similar, but not identical, discriminative stimuli.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46346/1/213_2005_Article_BF02245248.pd

Phono-spectrographic analysis of heart murmur in children

This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens

Methylphenidate Attenuates Limbic Brain Inhibition after Cocaine-Cues Exposure in Cocaine Abusers

Dopamine (phasic release) is implicated in conditioned responses. Imaging studies in cocaine abusers show decreases in striatal dopamine levels, which we hypothesize may enhance conditioned responses since tonic dopamine levels modulate phasic dopamine release. To test this we assessed the effects of increasing tonic dopamine levels (using oral methylphenidate) on brain activation induced by cocaine-cues in cocaine abusers. Brain metabolism (marker of brain function) was measured with PET and 18FDG in 24 active cocaine abusers tested four times; twice watching a Neutral video (nature scenes) and twice watching a Cocaine-cues video; each video was preceded once by placebo and once by methylphenidate (20 mg). The Cocaine-cues video increased craving to the same extent with placebo (68%) and with methylphenidate (64%). In contrast, SPM analysis of metabolic images revealed that differences between Neutral versus Cocaine-cues conditions were greater with placebo than methylphenidate; whereas with placebo the Cocaine-cues decreased metabolism (p<0.005) in left limbic regions (insula, orbitofrontal, accumbens) and right parahippocampus, with methylphenidate it only decreased in auditory and visual regions, which also occurred with placebo. Decreases in metabolism in these regions were not associated with craving; in contrast the voxel-wise SPM analysis identified significant correlations with craving in anterior orbitofrontal cortex (p<0.005), amygdala, striatum and middle insula (p<0.05). This suggests that methylphenidate's attenuation of brain reactivity to Cocaine-cues is distinct from that involved in craving. Cocaine-cues decreased metabolism in limbic regions (reflects activity over 30 minutes), which contrasts with activations reported by fMRI studies (reflects activity over 2–5 minutes) that may reflect long-lasting limbic inhibition following activation. Studies to evaluate the clinical significance of methylphenidate's blunting of cue-induced limbic inhibition may help identify potential benefits of this medication in cocaine addiction

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity— have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of ¹⁵N to ¹⁴N, termed δ¹⁵N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ¹⁵N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ¹⁵N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ¹⁵N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ¹⁵N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions

Behavior maintained by intravenous injection of codeine, cocaine, and etorphine in the rhesus macaque and the pigtail macaque

Lever-pressing behavior of two species of macaque, the rhesus macaque ( M. mulatta ) and the pigtail macaque ( M. nemestrina ) was maintained by intravenous injection of codeine, etorphine, or cocaine. Monkeys responded under a fixed-ratio 30 timeout 600 s schedule of drug injection during two daily experimental sessions. Drug-maintained behavior was studied under two access conditions. Under the first condition, selected doses of codeine or cocaine were available for ten consecutive sessions. Under the second condition, responding was maintained by 0.32 mg/kg codeine or 0.32 mg/kg cocaine, and saline and selected doses of codeine, etorphine, and cocaine were substituted during single experimental sessions. Performance varied with drug and injection dose, access condition, and macaque species. For all three drugs, response rate increased and then decreased as injection dose increased. Maximal rates were maintained by 0.10–0.32 mg/kg codeine, 0.0003–0.001 mg/kg etorphine, and 0.10–0.32 mg/kg cocaine. A cocaine dose of 0.32 mg/kg maintained higher rates than any dose of codeine or etorphine, and maintained higher rates when available during consecutive sessions than when substituted for codeine for a single session. Codeine maintained similar rates under all access conditions. The pigtail macaques had short catheter lives, did not readily acquire codeine-maintained responding, and displayed lower rates of drug-maintained lever pressing than the rhesus macaques.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46415/1/213_2004_Article_BF00427883.pd

Fenfluramine and N-ethyl amphetamine: Comparison of the reinforcing and rate-decreasing actions in the rhesus monkey

N-ethyl amphetamine HCl (NEA) and fenfluramine HCl ( meta -trifluoromethyl N-ethyl amphetamine) were evaluated as reinforcers in rhesus monkeys that had been previously trained to press a lever using food presentations and cocaine HCl injections as reinforcers. Each daily session consisted of episodic opportunities to obtain reinforcers under a fixed-ratio schedule of 30. A drug period was interpolated between two periods in which lever-press responding was maintained by food presentations. Compared to saline, none of the drugs altered the rate of responding in the food periods which preceded the drug sessions, indicating the absence of residual response-disrupting drug actions from previous sessions. However, NEA and fenfluramine self-injection resulted in dose-related decreases in response rates during the food periods which immediately followed the drug sessions. Cocaine HCl (30 Μg/kg/injection) maintained high response rates at over one response/second during the drug periods, as did the same dose of NEA. Doses of 10 and 100 Μg/kg/injection of NEA as well as all doses of fenfluramine HCl (10 through 300 Μg/kg/injection) maintained rates that were not different from those associated with saline injections. These results substantiate and extend earlier findings with fenfluramine and indicate that its failure to act as a reinforcer is attributable to its meta -trifluoromethyl group.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46392/1/213_2004_Article_BF00429257.pd

Supraleitung, elektrischer Widerstand und strukturelle Eigenschaften des Lanthans unter Druck

High precision resistivity measurements of lanthanum between 300 K and liquid helium temperature under pressures up to 200 kbar reveal anomalies both in theresidual and the phonon resistivity. The transition temperature to the superconducting state, T$_{c}$ increases with pressure, showing anomalies at about 25 and about 53 kbar. T$_{c}$ reaches a limiting value of 12.9 K just under 200 kbar. Our measurements indicate two new first-order crystallographic phase transitions at low temperatures. A tentative T-P phase diagram is constructed suggesting that it is possibly isomorphic to that of cerium. In addition, the phase transition of AgCl occuring at room temperature and 76 kbar was followed down to 4 K and 52 kbar