34,078 research outputs found
Structure and optical properties of Cd substituted ZnO (Zn1-xCdxO) nanostructures synthesized by high pressure solution route
We report synthesis of Cd substituted ZnO nanostructures (Zn1-xCdxO with x
upto \approx .09) by high pressure solution growth method. The synthesized
nanostructures comprise of nanocrystals that are both particles (~ 10-15 nm)
and rods which grow along (002) direction as established by Transmission
electron microscope (TEM) and X-ray diffraction (XRD) analysis. Rietveld
analysis of the XRD data shows monotonous increase of the unit cell volume with
the increase of Cd concentration. The optical absorption as well as the
photoluminescence (PL) shows red shift on Cd substitution. The line width of
the PL spectrum is related to the strain inhomogenity and it peaks in the
region where the CdO phase separates from the Zn1-xCdxO nanostructures. The
time resolved photoemission showed a long lived (~10ns) component. We propose
that the PL behavior of the Zn1-xCdxO is dominated by strain in the sample with
the redshift of the PL linked to the expansion of the unit cell volume on Cd
substitution
Ground state properties of the bond alternating spin- anisotropic Heisenberg chain
Ground state properties, dispersion relations and scaling behaviour of spin
gap of a bond alternating spin- anisotropic Heisenberg chain have
been studied where the exchange interactions on alternate bonds are
ferromagnetic (FM) and antiferromagnetic (AFM) in two separate cases. The
resulting models separately represent nearest neighbour (NN) AFM-AFM and AFM-FM
bond alternating chains. Ground state energy has been estimated analytically by
using both bond operator and Jordan-Wigner representations and numerically by
using exact diagonalization. Dispersion relations, spin gap and several ground
state orders have been obtained. Dimer order and string orders are found to
coexist in the ground state. Spin gap is found to develop as soon as the
non-uniformity in alternating bond strength is introduced in the AFM-AFM chain
which further remains non-zero for the AFM-FM chain. This spin gap along with
the string orders attribute to the Haldane phase. The Haldane phase is found to
exist in most of the anisotropic region similar to the isotropic point.Comment: 16 pages, 6 figures, 1 tabl
Electric field induced reversible control of visible photoluminescence from ZnO nanoparticles
Reversible control of the photoluminescence of ZnO occurring in the visible
range, has been achieved by application of a few volts (< 5V) to a device
consisting of nanostructured ZnO film sandwiched between Indium Tin Oxide
electrode and polyethylene oxide-lithium perchlorate, a solid polymer
electrolyte. The photoluminescence intensity shows nearly 100% modulation with
a response time less than 30 seconds, when the bias is applied at the
electrolyte-electrode. A model is proposed for the observed effect that is
based on defect states of ZnO and the band bending at the ZnO-electrolyte
interface that can be changed by the applied bias
Voltage bias induced modification of all oxide Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 junctions
In this paper we report what happens to a pristine oxide junction
Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 (PCMO/Nb:STO), when it is subjected to cycling
of voltage bias of moderate value ({\pm}4V). It is found that the initial
cycling leads to formation of a permanent state of lower resistance where the
lower resistance arises predominantly due to development of a shunt across the
device film (PCMO). On successive voltage cycling with increasing magnitude,
this state transforms into states of successive lower resistance that can be
transformed back to the initial stable state on cycling to below a certain
bias. A simple model based on p-n junction with shunt has been used to obtain
information on the change of the junction on voltage cycling. It has been shown
that the observation can be explained if the voltage cycling leads to lowering
of barrier at the interface and also reduction in series resistance. It is
suggested that this lowering can be related to the migration of oxygen ions and
vacancies at the junction region. Cross-sectional imaging of the junction shows
formation of permanent filamentary bridges across the thickness of the PCMO
after the pristine p-n junction is first taken through a voltage cycle, which
would explain appearance of a finite shunt across the p-n junction.Comment: 12 pages,7figure
Structural and Optical properties of Zn(1-x)MgxO nanocrystals obtained by low temperature method
In this paper we report structural and optical properties of Magnesium
substituted Zinc Oxide (Zn1-xMgxO) nanocrystals (~10-12nm) synthesized by low
temperature route. In the low temperature synthesis route it was possible to
reach x = 0.17 without segregation of Mg rich phase. The exact chemical
composition has been established by quantitative analysis. Rietveld analysis of
the XRD data confirms the Wurzite structure and a continuous compaction of the
lattice (in particular the c-axis parameter) as x increases. There is an
enhancement of the strain in the lattice as the Mg is substituted. The bandgap
also gets enhanced as x is increased and reaches a value of 4eV for x = 0.17.
From the TEM and the XRD data it has been concluded that when there is a phase
segregation for x > 0.17, there is a shell of Mg(OH)2 on the ZnO. The
absorption also shows persistence of the excitoinc absorption on Mg
substitution. The nanocrystals show near band edge photo luminescence (PL) at
room temperature which shows blue shift on Mg incorporation. In addition to the
near band edge emission the ZnO and Zn1-xMg xO alloy nanocrystals show
considerable emission in the blue-green region at wavelength of ~550 nm. We
find that the relative intensity of the green emission increases with the Mg
concentration for very low x (upto x = 0.05) and on further increase of the Mg
concentration there is a sharp decrease of relative intensity of the green
emission eventually leading to a complete quenching of blue emission. It is
concluded that due to phase segregation (for x \geq 0.20), the formation of the
shell of Mg(OH)2 on the ZnO leads to quenching of the green emission .However,
this shell formation does not have much effect on the near band edge PL
Three Dimensional Gauge Theory with Topological and Non-topological Mass: Hamiltonian and Lagrangian Analysis
Three dimensional (abelian) gauged massive Thirring model is bosonized in the
large fermion mass limit. A further integration of the gauge field results in a
non-local theory. A truncated version of that is the Maxwell Chern Simons (MCS)
theory with a conventional mass term or MCS Proca theory. This gauge invariant
theory is completely solved in the Hamiltonian and Lagrangian formalism, with
the spectra of the modes determined. Since the vector field constituting the
model is identified (via bosonization) to the fermion current, the charge
current algebra, including the Schwinger term is also computed in the MCS Proca
model.Comment: Eight pages, Latex, No figures
Electrode erosion in low power arcjet thrusters
Low power arcjet thrusters (0.5 to 3 kW) are currently being considered for North-South station keeping of geosynchronous communication satellites. The erosion mechanisms of cathodes in these thrusters are not well understood. The experimental set-up to produce a steady state electric arc in a gas flow is described and some preliminary theoretical results on cathode erosion are presented
- …