495 research outputs found
Spin-Wave Lifetimes Throughout the Brillouin Zone
We use a neutron spin-echo method with eV resolution to determine the
lifetimes of spin waves in the prototypical antiferromagnet MnF over the
entire Brillouin zone. A theory based on the interaction of magnons with
longitudinal spin fluctuations provides an excellent, parameter-free
description of the data, except at the lowest momenta and temperatures. This is
surprising, given the prominence of alternative theories based on magnon-magnon
interactions in the literature. The results and technique open up a new avenue
for the investigation of fundamental concepts in magnetism. The technique also
allows measurement of the lifetimes of other elementary excitations (such as
lattice vibrations) throughout the Brillouin zone.Comment: 12 pages, 4 figure
Energy Gaps and Kohn Anomalies in Elemental Superconductors
The momentum and temperature dependence of the lifetimes of acoustic phonons
in the elemental superconductors Pb and Nb was determined by resonant spin-echo
spectroscopy with neutrons. In both elements, the superconducting energy gap
extracted from these measurements was found to converge with sharp anomalies
originating from Fermi-surface nesting (Kohn anomalies) at low temperatures.
The results indicate electron many-body correlations beyond the standard
theoretical framework for conventional superconductivity. A possible mechanism
is the interplay between superconductivity and spin- or charge-density-wave
fluctuations, which may induce dynamical nesting of the Fermi surface
Electrochemical De-intercalation, Oxygen Non-stoichiometry, and Crystal Growth of NaxCoO2-d
We report a detailed study of de-intercalation of Na from the compound
NaxCoO2-d using an electrochemical technique. We find evidence for stable
phases with Na contents near the fractions ~1/3, 1/2, 5/8, 2/3, and 3/4.
Details regarding the floating-zone crystal growth of Na0.75CoO2 single
crystals are discussed as well as results from magnetic susceptibility
measurements. We observe the presence of significant oxygen deficiencies in
powder samples of Na0.75CoO2-d prepared in air, but not in single crystal
samples prepared in an oxygen atmosphere. The oxygen deficiencies in a
Na0.75CoO2-d sample with d ~ 0.08 remain even after electrochemically
de-intercalating to Na0.3CoO2-d.Comment: 6 pages, 5 figure
Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics
A high-resolution neutron spectroscopic technique is used to measure
momentum-resolved magnon lifetimes in the prototypical two- and
three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin
zone and a wide range of temperatures. We rederived theories of the lifetime
resulting from magnon-magnon scattering, thereby broadening their applicability
beyond asymptotically small regions of wavevector and temperature.
Corresponding computations, combined with a small contribution reflecting
collisions with domain boundaries, yield excellent quantitative agreement with
the data.Comment: 5 pages, 4 figure
Large enhancement of the thermopower in NaCoO at high Na doping
Research on the oxide perovskites has uncovered electronic properties that
are strikingly enhanced compared with those in conventional metals. Examples
are the high critical temperatures of the cuprate superconductors and the
colossal magnetoresistance in the manganites. The conducting layered cobaltate
displays several interesting electronic phases as is varied
including water-induced superconductivity and an insulating state that is
destroyed by field. Initial measurements showed that, in the as-grown
composition, displays moderately large thermopower and
conductivity . However, the prospects for thermoelectric cooling
applications faded when the figure of merit was found to be small at this
composition (0.60.7). Here we report that, in the poorly-explored
high-doping region 0.75, undergoes an even steeper enhancement. At the
critical doping 0.85, (at 80 K) reaches values 40 times
larger than in the as-grown crystals. We discuss prospects for low-temperature
thermoelectric applications.Comment: 6 pages, 7 figure
Synthesis of Asymmetrical Macrocyclic Ligands and their Metal Complexes
Our motive behind the synthesis of this type of asymmetrical macrocyclic ligands was to examine their possible applications in cation recognition processes, as homo or heteronuclear complexes can be synthesized from alkali and transition metal cations and these complexes may also serve as models of relevance to bioinorganic chemistry such as metalloenzymes. Even though the enormous number of Schiff base macrocycles and their complexes have already been described, many more interesting systems of this type surely await discovery. These structures are found to be powerful tools used to define interesting features relating to general chemistry and application. In this study, we present the preparation of the asymmetrical acyclic and cyclic compartmental Schiff bases containing two adjacent chambers to obtain the mono and dinuclear complexes with the appropriate alkaline earth and transition metal center and their related soft and hard complexes. We used 2,3-dihydroxy benzaldehyde as a starting material and obtained the macrocyclic ligands by the reaction of 2,3-dihydroxy benzaldehyde and bis(2-aminopropyl) amine in the presence of Ba(ClO4)2 as template agent, then its appropriate transition metal complex as with Ni+2 in CH3OH was prepared. All of the compounds were characterized by elemental analysis, IR, 1H NMR and MS spectrophotometer techniques
Study of intercalation and deintercalation of Na_xCoO_2 yH_2O single crystals
Single crystals of NaxCoO2 with beta-phase (x=0.55, 0.60 and 0.65),
alpha'-phase (x=0.75) and alpha-phase (x=0.9, 1.0) have been grown by the
floating zone technique. The Na-extraction and hydration were carried out for
the alpha'-sample to get superconducting phase of NaxCoO2.yH2O (x~0.3, y~1.3).
Hydrated single crystals exhibit cracked layers perpendicular to the c-axis due
to a large expansion when the water is inserted into the structure. A study of
intercalation/deintercalation was performed to determine the stability of the
hydrated phase and effects of hydration on the structure of the compound. X-ray
diffraction and Thermogravimetric experiments are used to monitor the process
of water molecules accommodated in and removed from the crystal lattice. The
initial intercalation process takes place with two-water molecules
corresponding to y=0.6) inserted in a formula unit, followed by a group of four
(y=1.3) to form a cluster of Na(H2O)4. Thermogravimetric analysis suggests that
the deintercalation occurs with the removal of the water molecules one by one
from the hydrated cluster at elevated temperatures of approximately 50, 100,
200 and 300 C, respectively. Our investigations reveal that the hydration
process is dynamic and that water molecule inter- and deintercalation follow
different reaction paths in an irreversible way.Comment: 15 pages, 6 figures, figures with higher resolution by email request
from the corresponding autho
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