164 research outputs found
Studies of structural, magnetic, electrical and photoconducting properties of BiCaMnO epitaxial thin films
The dynamics of the charge ordered (CO) state under non-equilibrium
conditions created by strong dc-electric field (~106 V/cm) and
photo-illumination with short (~ 6 ns) laser pulses is investigated in
Bi1-xCaxMnO3 (x > 0.5) epitaxial films. A pulsed laser deposition method was
used to synthesize films on (100) LaAlO3 (LAO) and (100) SrTiO3 (STO)
substrates. The crystallographic structure, temperature dependence of
electrical resistivity and magnetization of the samples of different
composition prepared under different oxygen partial pressure (pO2) and
deposition temperature (TD) are studied. For the x = 0.6 sample grown on LAO, a
clear signature of charge ordering at ~275 K is seen in the magnetization and
at ~ 260 K in the resistivity data. The same sample grown on STO revealed a
complex behavior, which entails charge ordering at ~300 K, a Neel order at ~150
K and finally a weak ferromagnetic phase below 50 K. A strong correlation
between charge ordering temperature (TCO) and the c-axis lattice parameter (c)
of the type (dTCO/dc ~-350 K/A) imerges from measurements on films deposited
under different growth conditions. Since the out of plane lattice parameter (c)
increases with in plane compressive strain, this effect directly show a
compressive strain induced suppression of the TCO. The current (I)- voltage (V)
characteristics of the samples at T < TCO show hysteresis due to a compound
effect of Joule heating and collapse of the CO state. Transient changes in
conductivity of lifetime ranging from nano to microseconds are seen at T < TCO
on illumination with pulsed UV (355 nm) radiation. These observations are
explained on the basis of the topological and electronic changes in the charge
ordered phase.Comment: 19 figures, 34 page
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
Scaled frequency-dependent transport in the mesoscopically phase-separated colossal magnetoresistive manganite La_{0.625-y}Pr_yCa_{0.375}MnO_3
We address the issue of massive phase separation (PS) in manganite family of
doped Mott insulators through ac conductivity measurements on
LaPrCaMnO (0.375 y 0.275), and
establish applicability of the scaling theory of percolation in the critical
regime of the PS. Measurements of dc resistivity, magnetization (M(T)) and
electron diffraction show incomplete growth of a ferromagnetic (FM) metallic
component on cooling the high temperature charge ordered (CO) phase well below
Curie temperature. The impedance Z(T,f) measured over a frequency
(f) range of 10 Hz to 10 MHz in the critical regime follows a universal scaling
of the form R(T,0)g(f) with 0.86
and the normalized correlation length varying from 1 to 4, suggesting anomalous
diffusion of holes in percolating FM clusters.Comment: 12 pages and 5 figure
Robust coupling of superconducting order parameter in a mesoscale NbN-Fe-NbN epitaxial structure
We report an unconventional and promising route to self-assemble distributed
superconductor-ferromagnet-superconductor (S-F-S) Josephson Junctions on single
crystal [100] MgO. These structures consist of [110] epitaxial nano-plaquettes
of Fe covered with superconducting NbN films of varying thickness. The S-F-S
structures are characterized by strong magnetoresistance (MR) anisotropy for
the in-plane and out-of-plane magnetic fields. The stronger in-plane MR
suggests decoherence of S-F-S junctions whose critical current follows a
(1-T/Tc) and (1-T/Tc)1/2 dependence for T Tc and T<<Tc respectively, in
accordance with the theory of supercurrent transport in such junctions.Comment: 9 pages, 4 figure
Evidence of mobile carriers with Charge Ordering gap in Epitaxial PrCaMnO Thin Films
Epitaxial thin films of charge-ordered PrCaMnO have
been studied using variable temperature Scanning tunneling microscopy and
spectroscopy (STM/STS). The as grown films were found to be granular while the
annealed films show atomic terraces at all temperatures and are found to be
electronically homogeneous in 78-300K temperature range. At high temperatures
(TT 230 K) the local tunnel spectra of the annealed films show
a depression in the density of states (DOS) near Fermi energy implying a
pseudogap with a significant DOS at E. The gap feature becomes more robust
with cooling with a sharp jump in DOS at E at T and with a gap value
of 0.3 eV at 78K. At low temperatures we find a small but finite DOS at
E indicative of some delocalized carriers in the CO phase together with an
energy gap. This is consistent with bulk transport, which shows weakening of
the activation gap with cooling below 200K, and indicates the presence of two
types of carriers at low temperatures.Comment: 4 pages, 4 figure
Gapped tunneling spectra in the normal state of PrCeCuO
We present tunneling data in the normal state of the electron doped cuprate
superconductor PrCeCuO for three different values of the doping
. The normal state is obtained by applying a magnetic field greater than the
upper critical field, for . We observe an anomalous normal
state gap near the Fermi level. From our analysis of the tunneling data we
conclude that this is a feature of the normal state density of states. We
discuss possible reasons for the formation of this gap and its implications for
the nature of the charge carriers in the normal and the superconducting states
of cuprate superconductors.Comment: 7 pages ReVTeX, 11 figures files included, submitted to PR
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
Tunneling Study of the Charge-Ordering Gap on the Surface of LaPrCaMnO Thin Films
Variable temperature scanning tunneling microscopy/spectroscopy studies on
(110) oriented epitaxial thin films of
LaPrCaMnO are reported in the temperature
range of 77 to 340 K. The films, grown on lattice matched NdGaO substrates,
show a hysteretic metal-insulator transition in resistivity at 170 K. The
topographic STM images show step-terrace morphology while the conductance
images display a nearly homogeneous surface. The normalized conductance spectra
at low temperatures (T150 K) show an energy gap of 0.5 eV while for
T180 K a gap of 0.16 eV is found from the activated behavior of the zero
bias conductance. The presence of energy gap and the absence of phase
separation on the surface over more than 2 m2 m area
contradicts the metallic behavior seen in resistivity measurements at low
temperatures. We discuss the measured energy gap in terms of the stabilization
of the insulating CO phase at the film surface.Comment: 5 pages, 5 figures To appear in Phys. Rev.
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