132 research outputs found
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
Magnetic order and dynamics of the charge-ordered antiferromagnet La1.5Sr0.5CoO4
We describe neutron scattering experiments performed to investigate the
magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered
perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order
at temperatures below ~ 800 K. Magnetic correlations are observed at
temperatures below ~ 60 K but static magnetic order only becomes established at
31 K, a temperature at which a kink is observed in the susceptibility. On
warming above 31 K we observed a change in the magnetic correlations which we
attribute either to a spin canting or to a change in the proportion of
inequivalent magnetic domains. The magnetic excitation spectrum is dominated by
an intense band extending above a gap of approximately 3 meV up to a maximum
energy of 16 meV. A weaker band exists in the energy range 20-30 meV. We show
that the excitation spectrum is in excellent quantitative agreement with the
predictions of a spin-wave theory generalized to include the full magnetic
degrees of freedom of high-spin Co2+ ions in an axially distorted crystal
field, coupled by Heisenberg exchange interactions. The magnetic order is found
to be stabilized by dominant antiferromagnetic Co2+ -- Co2+ interactions acting
in a straight line through Co3+. No evidence is found for magnetic scattering
from the Co3+ ions, supporting the view that Co3+ is in the S = 0 state in this
material.Comment: 17 pages, 10 figures. Accepted for publication in Phys. Rev.
Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet
Superconductivity in layered copper-oxide compounds emerges when charge
carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers
destroy the antiferromagnetic order, but strong spin fluctuations persist
throughout the superconducting phase and are intimately linked to
super-conductivity. Neutron scattering measurements of spin fluctuations in
hole-doped copper oxides have revealed an unusual `hour-glass' feature in the
momentum-resolved magnetic spectrum, present in a wide range of superconducting
and non-superconducting materials. There is no widely-accepted explanation for
this feature. One possibility is that it derives from a pattern of alternating
spin and charge stripes, an idea supported by measurements on stripe-ordered
La1.875Ba0.125CuO4. However, many copper oxides without stripe order also
exhibit an hour-glass spectrum$. Here we report the observation of an
hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the
family of superconducting copper oxides. Our system has stripe correlations and
is an insulator, which means its magnetic dynamics can conclusively be ascribed
to stripes. The results provide compelling evidence that the hour-glass
spectrum in the copper-oxide superconductors arises from fluctuating stripes.Comment: 13 pages, 4 figures, to appear in Natur
Direct evidence for charge stripes in a layered cobalt oxide
Recent experiments indicate that static stripe-like charge order is generic to the hole-doped copper oxide superconductors and competes with superconductivity. Here we show that a similar type of charge order is present in La5/3 Sr1/3 CoO4 , an insulating analogue of the copper oxide superconductors containing cobalt in place of copper. The stripe phase we have detected is accompanied by short-range, quasi-one-dimensional, antiferromagnetic order, and provides a natural explanation for the distinctive hour- glass shape of the magnetic spectrum previously observed in neutron scattering mea- surements of La2âxSrx CoO4 and many hole-doped copper oxide superconductors. The results establish a solid empirical basis for theories of the hourglass spectrum built on short-range, quasi-static, stripe correlations
Investigation of the Spin Density Wave in NaxCoO2
Magnetic susceptibility, transport and heat capacity measurements of single
crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave
(SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac)
data in which a cusp is observed at 4 K and attributed to a low temperature
glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and
low temperature hysteresis are only visible along the c-axis. The system also
exhibits a substantial (~40%) positive magnetoresistance below this
temperature. Calculations of the electronic heat capacity gamma above and below
Tmag and the size of the jump in C indicate that the onset of the SDW brings
about the opening of gap and the removal of part of the Fermi surface. Reduced
in-plane electron-electron scattering counteracts the loss of carriers below
the transition and as a result we see a net reduction in resistivity below
Tmag. Sodium ordering transitions at higher temperatures are observable as
peaks in the heat capacity with a corresponding increase in resistivity.Comment: 14 pages, 6 figure
Agro-materials : a bibliographic review
Facing the problems of plastic recycling and fossil resources exhaustion, the use of biomass to conceive new materials appears like a reasonable solution. Two axes of research are nowadays developed : on the one hand the synthesis of biodegradable plastics, whichever the methods may be, on the other hand the utilization of raw biopolymers, which is the object of this paper. From this perspective, the âplasticâ properties of natural polymers, the caracteristics of the different classes of polymers, the use of charge in vegetable matrix and the possible means of improving the durability of these agro-materials are reviewed
Pain in the lumbar, thoracic or cervical regions: do age and gender matter? A population-based study of 34,902 Danish twins 20â71 years of age
Background. It is unclear to what extent spinal pain varies between genders and in relation to age. It was the purpose of this study to describe the self-reported prevalence of 1) pain ever and pain in the past year in each of the three spinal regions, 2) the duration of such pain over the past year, 3) pain radiating from these areas, and 4) pain in one, two or three areas. In addition, 5) to investigate if spinal pain reporting is affected by gender and 6) to see if it increases gradually with increasing age. Method. A cross-sectional survey was conducted in 2002 on 34,902 twin individuals, aged 20 to 71 years, representative of the general Danish population. Identical questions on pain were asked for the lumbar, thoracic and cervical regions. Results. Low back pain was most common, followed by neck pain with thoracic pain being least common. Pain for at least 30 days in the past year was reported by 12%, 10%, and 4%, respectively. The one-yr prevalence estimates of radiating pain were 22% (leg), 16% (arm), and 5% (chest). Pain in one area only last year was reported by 20%, followed by two (13%) and three areas (8%). Women were always more likely to report pain and they were also more likely to have had pain for longer periods. Lumbar and cervical pain peaked somewhat around the middle years but the curves were flatter for thoracic pain. Similar patterns were noted for radiating pain. Older people did not have pain in a larger number of areas but their pain lasted longer. Conclusion. Pain reported for and from the lumbar and cervical spines was found to be relatively common whereas pain in the thoracic spine and pain radiating into the chest was much less common. Women were, generally, more likely to report pain than men. The prevalence estimates changed surprisingly little over age and were certainly not more common in the oldest groups, although the pain was reported as more long-lasting in the older group
59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2-yH2O
We present our NMR studies of double-layer hydrated cobalt oxides
NaxCoO2-yH2O (x ~ 0.35, y ~ 1.3) with various Tc = 0 - 4.8 K and magnetic
transition temperatures. High-resolution H NMR spectrum served as an
evidence for the existence of HO oxonium ions. Na nuclear
spin-lattice relaxation rates served to detect local field fluctuations
sensitive to Tc. Co nuclear quadrupole resonance (NQR) spectra served to
classify the various Tc samples. From the classification by Co NQR
frequency, the double-layer hydrated compounds were found to have two
superconducting phases closely located to a magnetic phase. In the normal state
and at a magnetic field in the -plane, two Co NMR signals with
different Knight shifts and different Co nuclear spin-lattice relaxation
times were observed. The two Co NMR signals suggest
magnetic disproportionation of two Co sites or in-plane (XY) anisotropy of a
single Co site. Non Korringa behavior and power law behavior in zero-field NQR
1/ above and below Tc suggest non-Fermi liquid and unconventional
superconductivity.Comment: 12 pages, 7 figures, Submitted to DPG spring meeting at Regensburg,
March 200
"I try and smile, I try and be cheery, I try not to be pushy. I try to say âIâm here for helpâ but I leave feeling⊠worriedââ: A qualitative study of perceptions of interactions with health professionals by community-based older adults with chronic pain
Background: Over 50% of community-dwelling older adults experience chronic pain, which threatens their quality of life. Of importance to their pain management is older peopleâs interaction with health professionals that, if unsatisfactory, may impair the outcome.
Aims: To add to the limited research specific to older people living with chronic pain in the community, we explored how they perceive their experiences of interacting with health professionals, seeking factors that might optimise these interactions.
Methods: Purposive sampling was used to recruit men and women .65 years with self-reported musculoskeletal chronic pain. Qualitative individual interviews and one group interview were undertaken with 23 participants. Data were transcribed verbatim and underwent Framework Analysis.
Results: Three themes were identified. Seeking help illustrates issues around why older people in the community may or may not seek help for chronic pain, and highlights the potential involvement of social comparison. Importance of diagnosis illustrates the desire for professional validation of their condition and an aversion to vague explanations based on the personâs age. Being listened to and being heard illustrates the importance of empathic communication and understanding expectations, with due respect for the personâs age.
Conclusions: In common with people of all ages, an effective partnership between an older person in pain and health professionals is essential if pain is to be reported, appropriately assessed and managed, because of the subjective nature of pain and its treatment responses. For older people with pain, perception about their age, by both parties in the partnership, is an additional factor that can unnecessarily interfere with the effectiveness of this partnership. Health professionals should engage with older adults to clarify their expectations about pain and its management, which may be influenced by perceptions about age; and to encourage expression of their concerns, which may also be affected by perceptions about age
Effect of age, sex and gender on pain sensitivity: A narrative review
© 2017 Eltumi And Tashani. Introduction: An increasing body of literature on sex and gender differences in pain sensitivity has been accumulated in recent years. There is also evidence from epidemiological research that painful conditions are more prevalent in older people. The aim of this narrative review is to critically appraise the relevant literature investigating the presence of age and sex differences in clinical and experimental pain conditions. Methods: A scoping search of the literature identifying relevant peer reviewed articles was conducted on May 2016. Information and evidence from the key articles were narratively described and data was quantitatively synthesised to identify gaps of knowledge in the research literature concerning age and sex differences in pain responses. Results: This critical appraisal of the literature suggests that the results of the experimental and clinical studies regarding age and sex differences in pain contain some contradictions as far as age differences in pain are concerned. While data from the clinical studies are more consistent and seem to point towards the fact that chronic pain prevalence increases in the elderly findings from the experimental studies on the other hand were inconsistent, with pain threshold increasing with age in some studies and decreasing with age in others. Conclusion: There is a need for further research using the latest advanced quantitative sensory testing protocols to measure the function of small nerve fibres that are involved in nociception and pain sensitivity across the human life span. Implications: Findings from these studies should feed into and inform evidence emerging from other types of studies (e.g. brain imaging technique and psychometrics) suggesting that pain in the older humans may have unique characteristics that affect how old patients respond to intervention
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