11,820 research outputs found
Colossal electroresistance in ferromagnetic insulating state of single crystal NdPbMnO
Colossal electroresistance (CER) has been observed in the ferromagnetic
insulating (FMI) state of a manganite. Notably, the CER in the FMI state occurs
in the absence of magnetoresistance (MR). Measurements of electroresistance
(ER) and current induced resistivity switching have been performed in the
ferromagnetic insulating state of a single crystal manganite of composition
NdPbMnO (NPMO30). The sample has a paramagnetic to
ferromagnetic (Curie) transition temperature, Tc = 150 K and the ferromagnetic
insulating state is realized for temperatures, T <~ 130 K. The colossal
electroresistance, arising from a strongly nonlinear dependence of resistivity
() on current density (j), attains a large value () in the
ferromagnetic insulating state. The severity of this nonlinear behavior of
resistivity at high current densities is progressively enhanced with decreasing
temperature, resulting ultimately, in a regime of negative differential
resistivity (NDR, d/dj < 0) for temperatures <~ 25 K. Concomitant with
the build-up of the ER however, is a collapse of the MR to a small value (<
20%) even in magnetic field, H = 7 T. This demonstrates that the mechanisms
that give rise to ER and MR are effectively decoupled in the ferromagnetic
insulating phase of manganites. We establish that, the behavior of
ferromagnetic insulating phase is distinct from the ferromagnetic metallic
(FMM) phase as well as the charge ordered insulating (COI) phase, which are the
two commonly realized ground state phases of manganites.Comment: 24 pages (RevTeX4 preprint), 8 figures, submitted to PR
The effect of network structure on phase transitions in queuing networks
Recently, De Martino et al have presented a general framework for the study
of transportation phenomena on complex networks. One of their most significant
achievements was a deeper understanding of the phase transition from the
uncongested to the congested phase at a critical traffic load. In this paper,
we also study phase transition in transportation networks using a discrete time
random walk model. Our aim is to establish a direct connection between the
structure of the graph and the value of the critical traffic load. Applying
spectral graph theory, we show that the original results of De Martino et al
showing that the critical loading depends only on the degree sequence of the
graph -- suggesting that different graphs with the same degree sequence have
the same critical loading if all other circumstances are fixed -- is valid only
if the graph is dense enough. For sparse graphs, higher order corrections,
related to the local structure of the network, appear.Comment: 12 pages, 7 figure
Critical properties of the double exchange ferromagnet Nd0.4Pb0.4MnO3
Results of a study of dc-magnetization M(T, H), performed on a Nd0.6Pb0.4MnO3
single crystal in the temperature range around T_C (Curie temperature) which
embraces the critical region | epsilon | = |T -T_C |/T_C <= 0.05 are reported.
The magnetic data analyzed in the critical region using the Kouvel-Fisher
method give the values for the T_C =156.47 +/- 0.06 K and the critical
exponents, beta = 0.374 +/- 0.006 (from the temperature dependence of
magnetization), and gamma = 1.329 +/- 0.003 (from the temperature dependence of
initial susceptibility). The critical isotherm M(T_C, H) gives delta = 4.547
+/- 0.1. Thus the scaling law gamma+beta=delta beta is fulfilled. The critical
exponents obey the single scaling-equation of state M(H, epsilon) = epsilon^b
f_+/- (H/epsilon^(beta + gamma)) where, f_+ for T > T_C and f_- for T< T_C. The
exponent values are very close to those expected for the universality class of
3D Heisenberg ferromagnets with short-range interactions.Comment: 19 pages, including 6 figure
Growth Stress Induced Tunability of Dielectric Constant in Thin Films
It is demonstrated here that growth stress has a substantial effect on the
dielectric constant of zirconia thin films. The correct combination of
parameters - phase, texture and stress - is shown to yield films with high
dielectric constant and best reported equivalent oxide thickness of 0.8 nm. The
stress effect on dielectric constant is twofold, firstly, by the effect on
phase transitions and secondly by the effect on interatomic distances. We
discuss and explain the physical mechanisms involved in the interplay between
the stress, phase changes and the dielectric constant in detail.Comment: 11 pages, 5 figure
Non-resonant microwave absorption studies of superconducting MgB_2
Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a
frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported.
The NRMA results indicate near absence of intergranular weak links. A linear
temperature dependence of the lower critical field H_c1 is observed indicating
a non s-wave superconductivity. However, the phase reversal of the NRMA signal
which could suggest d-wave symmetry is also not observed.Comment: 8 pages, 2 figure
Evidence for long-term variability in the ultra high energy photon flux from Cygnus X-3
A time-correlation analysis of atmospheric Cerenkov pulses by a wide-angle photomultiplier system was previously shown to have present in it a nonrandom component which seemed associated with the Right Ascension (RA) range approx. 20+or-04h. A recent examination of multi-muon events recorded by a photon-decay detector shows a similar time-dependent effect, closely matching the previous results, supporting the suggestion that the effect is of cosmic origin. However, even though Cyg. X-3 lies well inside the region of peak intensity, it does not seem possible to ascribe to it the whole effect, for the implied photon flux appears too large to be reconciled to various gamma-ray measurements of Cyg. X-3. The original data were subjected to a phase-histogram analysis and it as found that only 2.5% of overall recorded data are compatible with a phase-dependent emission from Cyg. X-3. Assuming these events to be gamma rays yields a detected flux of (2.6 + or - 0.3) x 10 to the minus 12th power gamma cm -2s-1 above 5 x 10 to the 14th power eV. Comparing this value with more recent ultra high energy (UHE) photon data from the same source, it is suggested that the available data generally favor a long-term reduction in the Cyg. X-3 inferred luminosity ( 10 to the 13th power eV) by a factor of (1.8 + or - 0.3) per year
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