201 research outputs found
Unveiling First Order CMR Transitions in the Two-Orbital Model for Manganites
Large scale Monte Carlo simulation results for the two-orbital model for
manganites, including Jahn-Teller lattice distortions, are here presented. At
hole density x=1/4 and in the vicinity of the region of competition between the
ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the
colossal magnetoresistance (CMR) phenomenon is observed with a
magnetoresistance ratio ~10,000% Our main result is that this CMR transition is
found to be of first order in some portions of the phase diagram, in agreement
with early results from neutron scattering, specific heat, and magnetization,
thus solving a notorious discrepancy between experiments and previous
theoretical studies. The first-order characteristics of the transition survive,
and are actually enhanced, when weak quenched disorder is introduced.Comment: 4 pages, 4 figure
Ferroelectricity in the Magnetic E-Phase of Orthorhombic Perovskites
We show that the symmetry of the spin zigzag chain E phase of the
orthorhombic perovskite manganites and nickelates allows for the existence of a
finite ferroelectric polarization. The proposed microscopic mechanism is
independent of spin-orbit coupling. We predict that the polarization induced by
the E-type magnetic order can potentially be enhanced by up to two orders of
magnitude with respect to that in the spiral magnetic phases of TbMnO3 and
similar multiferroic compounds.Comment: 4 pages, 2 figures, somewhat changed emphases, accepted to PR
Competing Ferromagnetic and Charge-Ordered States in Models for Manganites: the Origin of the CMR Effect
The one-orbital model for manganites with cooperative phonons and
superexchange coupling has been investigated via large-scale Monte
Carlo (MC) simulations. Results for two-orbitals are also briefly discussed.
Focusing on electronic density =0.75, a regime of competition between
ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was
identified. In the vicinity of the associated bicritical point, colossal
magnetoresistance (CMR) effects were observed. The CMR is associated with the
development of short-distance correlations among polarons, above the spin
ordering temperatures, resembling the charge arrangement of the low-temperature
CO state.Comment: 4 pages, 4 figures, accepted for publication in the Phys. Rev. Let
Mean-field renormalization group theory of the t-J model
Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 2002.Thesis (Master's) -- Bilkent University, 2002.Includes bibliographical references leaves 54.The quantum nature of the high temperature superconductivity models makes
analytical approaches to these systems almost impossible to implement. In this
thesis, a computational study of the one and two dimensional t − J models that
combines mean-field treatments with renormalization group techniques will be
presented. This allows one to deal with the noncommutations of the operators
at two consecutive sites of the lattices on which these models are defined. The
resulting phase diagram for the 1D t − J model reveals an antiferromagnetic
ground state, which may, upon doping with increasing temperature, show striped
formation that is seen in the high-Tc cuprates. The qualitative features of the
phase diagram of the 2D case is also presented, which reveals a phase transition
between the disordered and antiferomagnetically ordered phases.Şen, CengizM.S
Study of Short-distance Spin and Charge Correlations and Local Density-of-States in the CMR regime of the One-Orbital Model for Manganites
The metal-insulator transition, and the associated magnetic transition, in
the colossal magnetoresistance (CMR) regime of the one-orbital model for
manganites is here studied using Monte Carlo (MC) techniques. Both cooperative
oxygen lattice distortions and a finite superexchange coupling among the
spins are included in our investigations. Charge and spin
correlations are studied. In the CMR regime, a strong competition between the
ferromagnetic metallic and antiferromagnetic charge-ordered insulating states
is observed. This competition is shown to be important to understand the
resistivity peak that appears near the critical temperature. Moreover, it is
argued that the system is dynamically inhomogeneous, with short-range charge
and spin correlations that slowly evolve with MC time, producing the glassy
characteristics of the CMR state. The local density-of-states (LDOS) is also
investigated, and a pseudogap (PG) is found to exist in the CMR temperature
range. The width of the PG in the LDOS is calculated and directly compared with
recent scanning-tunneling-spectroscopy (STS) experimental results. The
agreement between our calculation and the experiment suggests that the
depletion of the conductance at low bias observed experimentally is a
reflection on the existence of a PG in the LDOS spectra, as opposed to a hard
gap. The apparent homogeneity observed via STS techniques could be caused by
the slow time characteristics of this probe. Faster experimental methods should
unveil a rather inhomogeneous state in the CMR regime, as already observed in
neutron scattering experiments.Comment: 13 pages, 10 figure
Military jet pilots have higher p-wave dispersions compared to the transport aircraft aircrew
Objectives: For the purpose of flight safety military aircrew must be healthy. P-wave dispersion (PWD) is the p-wave length difference in an electrocardiographic (ECG) examination and represents the risk of developing atrial fibrillation. In the study we aimed at investigating PWD in healthy military aircrew who reported for periodical examinations. Material and Methods: Seventy-five asymptomatic military aircrew were enrolled in the study. All the subjects underwent physical, radiologic and biochemical examinations, and a 12-lead electrocardiography. P-wave dispersions were calculated. Results: The mean age of the study participants was 36.15±8.97 years and the mean p-wave duration was 100.8±12 ms in the whole group. Forty-seven subjects were non-pilot aircrew, and 28 were pilots. Thirteen study subjects were serving in jets, 49 in helicopters, and 13 were transport aircraft pilots. Thirty-six of the helicopter and 11 of the transport aircraft aircrew were non-pilot aircrew. P-wave dispersion was the lowest in the transport aircraft aircrew, and the highest in jet pilots. P-wave dispersions were similar in the pilots and non-pilot aircrew. Twenty-three study subjects were overweight, 19 had thyroiditis, 26 had hepatosteatosis, 4 had hyperbilirubinemia, 2 had hypertension, and 5 had hyperlipidemia. The PWD was significantly associated with thyroid-stimulating hormone (TSH) levels. Serum uric acid levels were associated with p-wave durations. Serum TSH levels were the most important predictor of PWD. Conclusions: When TSH levels were associated with PWD, uric acid levels were associated with p-wave duration in the military aircrew. The jet pilots had higher PWDs. These findings reveal that military jet pilots may have a higher risk of developing atrial fibrillation, and PWD should be recorded during periodical examinations
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