49,733 research outputs found
Evidence for two spin-glass transitions with magnetoelastic and magnetoelectric couplings in the multiferroic (BiBa)(FeTi)O system
For disordered Heisenberg systems with small single ion anisotropy, two spin
glass transitions below the long range ordered phase transition temperature has
been predicted theoretically for compositions close to the percolation
threshold. Experimental verification of these predictions is still
controversial for conventional spin glasses. We show that multiferroic spin
glass systems can provide a unique platform for verifying these theoretical
predictions via a study of change in magnetoelastic and magnetoelectric
couplings, obtained from an analysis of diffraction data, at the spin glass
transition temperatures. Results of macroscopic and microscopic (x-ray and
neutron scattering) measurements are presented on disordered BiFeO3, a
canonical Heisenberg system with small single ion anisotropy, which reveal
appearance of two spin glass phases SG1 and SG2 in coexistence with the LRO
phase below the A-T and G-T lines. It is shown that the temperature dependence
of the integrated intensity of the antiferromagnetic peak shows dips with
respect to the Brillouin function behaviour around the SG1 and SG2 transition
temperatures. The ferroelectric polarisation changes significantly at the two
spin glass transition temperatures. These results, obtained using microscopic
techniques, clearly demonstrate that the SG1 and SG2 transitions occur on the
same magnetic sublattice and are intrinsic to the system. We also construct a
phase diagram showing all the magnetic phases in BF-xBT system. While our
results on the two spin glass transitions support the theoretical predictions,
it also raises several open questions which need to be addressed by revisiting
the existing theories of spin glass transitions by taking into account the
effect of magnetoelastic and magnetoelectric couplings as well as
electromagnons.Comment: 59 pages 21 figure
Integrated design optimization research and development in an industrial environment
An overview is given of a design optimization project that is in progress at the GE Research and Development Center for the past few years. The objective of this project is to develop a methodology and a software system for design automation and optimization of structural/mechanical components and systems. The effort focuses on research and development issues and also on optimization applications that can be related to real-life industrial design problems. The overall technical approach is based on integration of numerical optimization techniques, finite element methods, CAE and software engineering, and artificial intelligence/expert systems (AI/ES) concepts. The role of each of these engineering technologies in the development of a unified design methodology is illustrated. A software system DESIGN-OPT has been developed for both size and shape optimization of structural components subjected to static as well as dynamic loadings. By integrating this software with an automatic mesh generator, a geometric modeler and an attribute specification computer code, a software module SHAPE-OPT has been developed for shape optimization. Details of these software packages together with their applications to some 2- and 3-dimensional design problems are described
Anomalous Raman scattering from phonons and electrons of superconducting FeSe
We report interesting anomalies in the temperature dependent Raman spectra of
FeSe measured from 3K to 300K in the spectral range from 60 to 1800
cm and determine their origin using complementary first-principles
density functional calculations. A phonon mode near 100 cm exhibits a
sharp increase by 5% in frequency below a temperature T ( 100
K) attributed to strong spin-phonon coupling and onset of short-range
antiferromagnetic order. In addition, two high frequency modes are observed at
1350 cm and 1600 cm, attributed to electronic Raman scattering
from ()to / -orbitals of Fe.Comment: 19 pages, 4 figures, 1 tabl
Canonical representation for electrons and its application to the Hubbard model
A new representation for electrons is introduced, in which the electron
operators are written in terms of a spinless fermion and the Pauli operators.
This representation is canonical, invertible and constraint-free. Importantly,
it simplifies the Hubbard interaction. On a bipartite lattice, the Hubbard
model is reduced to a form in which the exchange interaction emerges simply by
decoupling the Pauli subsystem from the spinless fermion bath. This exchange
correctly reproduces the large superexchange. Also derived, for
, is the Hamiltonian to study Nagaoka ferromagnetism. In this
representation, the infinite- Hubbard problem becomes elegant and easier to
handle. Interestingly, the ferromagnetism in Hubbard model is found to be
related to the gauge invariance of the spinless fermions. Generalization of
this representation for the multicomponent fermions, a new representation for
bosons, the notion of a `soft-core' fermion, and some interesting unitary
transformations are introduced and discussed in the appendices.Comment: 10+ pages, 3 Figure
Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet.Comment: Accepted for Publication in JPC
Metallic monoclinic phase in VO induced by electrochemical gating: in-situ Raman study
We report in-situ Raman scattering studies of electrochemically top gated
VO thin film to address metal-insulator transition (MIT) under gating. The
room temperature monoclinic insulating phase goes to metallic state at a gate
voltage of 2.6 V. However, the number of Raman modes do not change with
electrolyte gating showing that the metallic phase is still monoclinic. The
high frequency Raman mode A(7) near 616 cm ascribed to V-O vibration
of bond length 2.06 \AA~ in VO octahedra hardens with increasing gate
voltage and the B(3) mode near 654 cm softens. This shows that the
distortion of the VO octahedra in the monoclinic phase decreases with
gating. The time dependent Raman data at fixed gate voltages of 1 V (for 50
minute, showing enhancement of conductivity by a factor of 50) and 2 V (for 130
minute, showing further increase in conductivity by a factor of 5) show similar
changes in high frequency Raman modes A(7) and B(3) as observed in
gating. This slow change in conductance together with Raman frequency changes
show that the governing mechanism for metalization is more likely to the
diffusion controlled oxygen vacancy formation due to the applied electric
field.Comment: 5 pages, 6 figure
Identification of blast resistance expression in rice genotypes using molecular markers (RAPD & SCAR)
Rice is the second most important cereal crop of developing countries and the staple food of about 65% of the world’s population. In this endeavor, it is important to identify resistant gene(s) with the help of markers. Once a gene is tagged with a molecular marker, it can be transferred selectively into different genetic backgrounds by marker assisted selection. For this purpose, 48 elite Indian and exotic rice genotypes were evaluated for resistance to blast disease under induced epiphytotic conditions obtained in the field. The disease severity (%) and AUDPC was less than 45% and 1000, respectively, in all the resistant genotypes, while it was around 85% and higher than 2000 in the case of susceptible genotypes, respectively. Substantial variability was present among rice genotypes for resistance toMagnaporthe grisea. Ten random amplified polymorphic DNAs (RAPD) and two sequence characterized amplified region (SCAR) primers were used to identify blast resistant genes. Markers OPA-05, OPF-06, OPF-09, OPF-17, OPG-17, OPG-18, OPG-19, OPH-18, OPK-12, P-265-550 and P-286-350 found linked to blast resistance in most of the resistant genotypes could be considered as potent molecular markers in the selection of blast resistant genotypes. Amplification with RAPD and SCAR primers revealed a nonallelic relationship among resistant genotypes and thus, there is a good possibility of obtaining enhanced resistance through gene pyramiding
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