26,542 research outputs found
Complementarity of perturbations driving insulator-to-metal transition in a charge ordered manganite
Modulation of charge carrier dynamics and hence electrical conductivity of
solids by photoexcitation has been a rich field of research with numerous
applications. Similarly, electric and magnetic field assisted enhancement of
conductivity are of fundamental importance and technological use. Hole doped
manganites of the type (ABMnO, where A and B are rare and
alkaline earth metals respectively have the distinction of showing all three
effects. Here we establish the complementarity of the electric, magnetic and
photon fields in driving an insulator-metal transition in epitaxial thin films
of LaPrCaMnO whose electrical ground state
is insulating. Both pulsed and CW lasers cause a giant photon flux dependent
enhancement of conductivity. It is further observed that electric and magnetic
fields trigger the persistent enhancement of conductivity whose magnitude can
be accentuated by application of these fields in parallel.Comment: 17 pages 6 figure
Magnetic and electron transport properties of the rare-earth cobaltates, La0.7-xLnxCa0.3CoO3 (Ln = Pr, Nd, Gd and Dy) : A case of phase separation
Magnetic and electrical properties of four series of rare earth cobaltates of
the formula La0.7-xLnxCa0.3CoO3 with Ln = Pr, Nd, Gd and Dy have been
investigated. Compositions close to x = 0.0 contain large ferromagnetic
clusters or domains, and show Brillouin-like behaviour of the field-cooled DC
magnetization data with fairly high ferromagnetic Tc values, besides low
electrical resistivities with near-zero temperature coefficients. The
zero-field-cooled data generally show a non-monotonic behaviour with a peak at
a temperatures slightly lower than Tc. The near x = 0.0 compositions show a
prominent peak corresponding to the Tc in the AC-susceptibility data. The
ferromagnetic Tc varies linearly with x or the average radius of the A-site
cations, (rA). With increase in x or decrease in (rA), the magnetization value
at any given temperature decreases markedly and the AC-susceptibility
measurements show a prominent transition arising from small magnetic clusters
with some characteristics of a spin-glass. Electrical resistivity increases
with increase in x, showed a significant increase around a critical value of x
or (rA), at which composition the small clusters also begin to dominate. These
properties can be understood in terms of a phase separation scenario wherein
large magnetic clusters give way to smaller ones with increase in x, with both
types of clusters being present in certain compositions. The changes in
magnetic and electrical properties occur parallely since the large
ferromagnetic clusters are hole-rich and the small clusters are hole-poor.
Variable-range hopping seems to occur at low temperatures in these cobaltates.Comment: 23 pages including figure
Electronic phase separation in the rare earth manganates, (La1-xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y)
All the three series of manganates showsaturation magnetization
characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with
increasing in x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2
respectively for Nd, Gd, Y). For x > xc, the magnetic moments are considerably
smaller showing a small increase around TM, the value of TM decreasing slightly
with increase in x or decrease in . The ferromagnetic compositions (x xc)
show insulator-metal (IM) transitions, while the compositions with x > xc are
insulating. The magnetic and electrical resistivity behavior of these
manganates is consistent with the occurrence of phase separation in the
compositions around xc, corresponding to a critical average radius of the
A-site cation, , of 1.18 A. Both Tc and TIM increase linearly when < rA
> > or x xc as expected of a homogenous ferromagnetic phase. Both Tc
and TM decrease linearly with the A-site cation size disorder at the A-site as
measured by the variance s2. Thus, an increase in s2 favors the insulating AFM
state. Percolative conduction is observed in the compositions with > <
rAc >. Electron transport properties in the insulating regime for x > xc
conforms to the variable range hopping mechanism. More interestingly, when x >
xc, the real part of dielectric constant (e') reaches a high value (104-106) at
ordinary temperatures dropping to a very small (~500) value below a certain
temperature, the value of which decreases with decreasing frequency.Comment: 27 pages; 11 figures, Submitted to J.Phys:Condens Matte
Absence of ferromagnetism in Mn- and Co-doped ZnO
Following the theoretical predictions of ferromagnetism in Mn- and Co-doped
ZnO, several workers reported ferromagnetism in thin films as well as in bulk
samples of these materials. While some observe room-temperature ferromagnetism,
others find magnetization at low temperatures. Some of the reports, however,
cast considerable doubt on the magnetism of Mn- and Co-doped ZnO. In order to
conclusively establish the properties of Mn- and Co-doped ZnO, samples with 6
percent and 2 percent dopant concentrations, have been prepared by the
low-temperature decomposition of acetate solid solutions. The samples have been
characterized by x-ray diffraction, EDAX and spectroscopic methods to ensure
that the dopants are substitutional. All the Mn- and Co-doped ZnO samples
(prepared at 400 deg C and 500 deg C) fail to show ferromagnetism. Instead,
their magnetic properties are best described by a Curie-Weiss type behavior. It
appears unlikely that these materials would be useful for spintronics, unless
additional carriers are introduced by some means.Comment: 23 pages, 9 figures. submitted to J. Mater. Chem 200
Do Hydrogen-Deficient Carbon Stars Have Winds?
We present high resolution spectra of the five known hydrogen-deficient carbon (HdC) stars in the vicinity of the 10830 angstrom line of neutral helium. In R Coronae Borealis (RCB) stars the He I line is known to be strong and broad, often with a P Cygni profile, and must be formed in the powerful winds of those stars. RCB stars have similar chemical abundances as HdC stars and also share greatly enhanced O-18 abundances with them, indicating a common origin for these two classes of stars, which has been suggested to be white dwarf mergers. A narrow He I absorption line may be present in the hotter HdC stars, but no line is seen in the cooler stars, and no evidence for a wind is found in any of them. The presence of wind lines in the RCB stars is strongly correlated with dust formation episodes so the absence of wind lines in the HdC stars, which do not make dust, is as expected.NSFScience and Technology FacilitiesCouncilthe National Research CouncilCONICYTAustralian Research CouncilMinistᄡerio da Ciᄡencia e TecnologiaSECYTMcDonald Observator
Current-induced phase control in charged-ordered Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 crystals
Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced
insulator-metal transitions at low temperatures. In addition, the
charge-ordering transition temperature decreases with increasing current. The
electroresistive ratio, defined as r0.5/rI where r0.5 is the resistivity at a
current of 0.5 mA and rI the resistivity at a given applied current, I, varies
markedly with temperature and the value of I. Thermal hysteresis observed in
Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the insulator-metal transition indicates
that the transition is first-order. The current-induced changes are comparable
to those induced by magnetic fields, and the insulator-metal transition in
Pr0.6Ca0.4MnO3 is accordingly associated with a larger drop in resistivity.Comment: 12 pages, 3 figures, first submitted to submitted to J. Phys. D;
applied physics on 18th march 200
Changes in the electronic structure and properties of graphene induced by molecular charge-transfer
Interaction with electron donor and acceptor molecules such as aniline and
nitrobenzene brings about marked changes in the Raman spectrum and the
electronic structure of graphene, prepared by the exfoliation of graphitic
oxide.Comment: 13 pages, 4 figure
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