24,157 research outputs found
Pilot decision making in a computer-aided flight management situation
An experimental representation of a computer-aided multi-task flight management situation has been developed. A computer aiding program was implemented to serve as a back-up decision maker. An experiment was conducted with a balanced design of several subject runs for different workload levels. This was achieved using three levels of subsystem event arrival rates, three levels of control task involvement, and three levels of availability of computer aiding. Experimental results compared quite favorably with those from a computer simulation which employed a queueing model. It was shown that the aiding had enhanced system performance as well as subjective ratings, and that the adaptive aiding policy further reduced subsystem delay
Large Magneto-Dielectric Effects in Orthorhombic HoMnO3 and YMnO3
We have found a remarkable increase (up to 60 %) of the dielectric constant
with the onset of magnetic order at 42 K in the metastable orthorhombic
structures of YMnO3 and HoMnO3 that proves the existence of a strong
magneto-dielectric coupling in the compounds. Magnetic, dielectric, and
thermodynamic properties show distinct anomalies at the onset of the
incommensurate magnetic order and thermal hysteresis effects are observed
around the lock-in transition temperature at which the incommensurate magnetic
order locks into a temperature independent wave vector. The orders of Mn3+
spins and Ho3+ moments both contribute to the magneto-dielectric coupling. A
large magneto-dielectric effect was observed in HoMnO3 at low temperature where
the dielectric constant can be tuned by an external magnetic field resulting in
a decrease of up to 8 % at 7 Tesla. By comparing data for YMnO3 and HoMnO3 the
contributions to the coupling between the dielectric response and Mn and Ho
magnetic orders are separated.Comment: revised manuscrip
Pressure-Temperature Phase Diagram of Multiferroic
The pressure-temperature phase diagram of multiferroic is
investigated for hydrostatic pressures up to 2 GPa. The stability range of the
ferroelectric phase associated with the incommensurate helical spin order is
reduced by pressure and ferroelectricity is completely suppressed at the
critical pressure of 1.64 GPa at 6.2 K. Thermal expansion measurements at
ambient pressure show strong step-like anomalies of the lattice parameters
associated with the lock-in transition into the commensurate paraelectric
phase. The expansion anomalies are highly anisotropic, the related volume
change is consistent with the high-pressure phase diagram
Ferroelectricity in perovskite and
Ferroelectricity is observed in orthorhombic and at the
magnetic lock-in transitions into an E-type structure or an incommensurate
phase with a temperature independent wave vector, respectively. In
the ferroelectric polarization strongly depends on the external magnetic field
indicating the involvement of the rare earth moment order in this compound. The
results are discussed within the framework of recent theoretical models, in
particular the double exchange driven polar displacements predicted for E-type
magnetic structures.Comment: 5 pages, 3 figure
Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2
We report specific-heat and resistivity experiments performed in parallel in
a Bridgman-type of pressure cell in order to investigate the nature of
pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The
presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of
the superconducting state. The thermodynamic transition temperature differs
dramatically from the onset of the resistive transition. Our data indicates
that superconductivity occurs in the vicinity of a crystallographic phase
transition. We discuss the discrepancy between the two methods as caused by
strain-induced superconducting precursors formed above the bulk thermodynamic
transition due to the vicinity of the structural instability
Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects
The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are
discussed based on structural, transport, and high pressure experiments. The
positive Seebeck coefficient and its linear temperature dependence for Tc<T<160
K provide evidence that the low-temperature transport in MgB2 is due to
hole-like metallic carriers. Structural and transport data show the important
role of defects as indicated by the correlation of Tc, the residual resistance
ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc
with hydrostatic pressure is well explained by the strong-coupling BCS theory.
The large scatter of the pressure coefficients of Tc for different MgB2
samples, however, cannot be explained within this theory. We speculate that
pressure may increase the defect density, particularly in samples with large
initial defect concentration.Comment: Presented at NATO Advanced Research Workshop "New Trends in
Superconductivity", Yalta (Ukraine) 16-20 September, 200
- …