1,685 research outputs found
Optimally Convex Controller and Model Reduction for a Dynamic System
This paper presents analysis and design of a family of controllers based on numerical convex optimization for an aircraft pitch control system. A design method is proposed here to solve control system design problems in which a set of multiple closed loop performance specifications are simultaneously satisfied. The transfer matrix of the system is determined through the convex combination of the transfer matrices of the plant and the controllers. The present system with optimal convex controller has been tested for stability using Kharitonovâs Stability Criteria. The simulation deals here withthe problem of pitch control system of a BRAVO fighter aircraft which results in higher order close loop transfer function. So the order of the higher order transfer function is reduced to minimize the complexity of the system
Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR
We report bulk superconductivity (SC) in
EuSr(FeCo)As single crystals by
means of electrical resistivity, magnetic susceptibility, and specific heat
measurements with 20 K with an antiferromagnetic
(AFM) ordering of Eu moments at 2.0 K in
zero field. As NMR experiments have been performed in the two external
field directions (H) and (H). As-NMR spectra are analyzed in
terms of first order quadrupolar interaction. Spin-lattice relaxation rates
(1/) follow a law in the temperature range 4.2-15 K. There is no
signature of Hebel-Slichter coherence peak just below the SC transition
indicating a non s-wave or s type of superconductivity. The increase of
1/ with lowering the temperature in the range 160-18 K following
law reflecting 2D AFM spin fluctuations
Field tuned critical fluctuations in YFe2Al10: Evidence from magnetization, 27Al (NMR, NQR) investigations
We report magnetization, specific heat, and NMR investigations on YFe2Al10
over a wide range in temperature and magnetic field and zero field (NQR)
measurements. Magnetic susceptibility, specific heat and spin-lattice
relaxation rate divided by T (1/T1T) follow a weak power law (T^-0.4)
temperature dependence, which is a signature of critical fluctuations of Fe
moments. The value of the Sommerfeld-Wilson ratio and linear relation between
1/T1T and chi(T) suggest the existence of ferromagnetic correlations in this
system. No magnetic ordering down to 50 mK in Cp(T) and the unusual temperature
and field scaling of the bulk and NMR data are associated with a magnetic
instability which drives the system to quantum criticality. The magnetic
properties of the system are tuned by field wherein ferromagnetic fluctuations
are suppressed and a crossover from quantum critical to FL behavior is observed
with increasing magnetic field
Local magnetism and spin dynamics of the frustrated honeycomb rhodate Li2RhO3
We reportmagnetization, heat capacity, 7Li nuclear magnetic resonance (NMR), and muon-spin rotation (ÎŒSR) measurements on the honeycomb 4d5 spin liquid candidate Li2RhO3. The magnetization in small magnetic fields provides evidence of the partial spin-freezing of a small fraction of Rh4+ moments at 6 K, whereas the Curie-Weiss behavior above 100 K suggests a pseudo-spin-1/2 paramagnet with a moment of about 2.2ÎŒB. The magnetic specific heat (Cm) exhibits no field dependence and demonstrates the absence of long-range magnetic order down to 0.35 K. Cm/T passes through a broad maximum at about 10 K and Cm â T 2 at low temperatures. Measurements of the spin-lattice relaxation rate (1/T1) reveal a gapless slowing-down of spin fluctuations upon cooling with 1/T1 ⌠T 2.2. The results from NMR and ÎŒSR are consistent with a scenario in which a minority of Rh4+ moments are in a short-range correlated frozen state and coexist with a majority of moments in a liquid-like state that continue to fluctuate at low temperatures
Magnetism and field-induced effect in a spin-orbit entangled Jeff = 1/2 honeycomb lattice
The interplay between spin-orbit coupling, frustration-induced anisotropic
magnetic interaction, and spin correlations can lead to novel states with
exotic excitations in rare-earth-based quantum magnets. Herein, we present the
crystal structure, magnetization, electron spin resonance (ESR), specific heat,
and nuclear magnetic resonance (NMR) experiments on the polycrystalline samples
of Ba9Yb2Si6O24 in which Yb3+ ions form a perfect honeycomb lattice without
detectable anti-site disorder. Magnetization data reveal antiferromagnetically
coupled spin-orbit entangled Jeff = 1/2 degrees of freedom of Yb3+ ions in the
Kramers doublet state where the Curie-Weiss temperature is - 2.97 K, as
obtained from the low-temperature magnetic susceptibility data. The ESR
measurements reveal that the first excited Kramers doublet is 32.3(7) meV above
the ground state. The specific heat results suggest the presence of an
antiferromagnetic phase transition at 2.26 K. The long-range antiferromagnetic
order is completely suppressed upon the application of magnetic field and a
field-induced disordered state is observed in an applied magnetic field of 2.5
T, which is also confirmed by NMR measurements. Furthermore, the NMR
spin-lattice relaxation rate reveals the presence of a field-induced gap that
is attributed to the Zeeman splitting of Kramers doublet state in this quantum
material. Our experiments suggest the presence of a phase transition and
short-range spin correlations appearing well above the antiferromagnetic phase
transition temperature and a field-induced disordered state in this spin-orbit
entangled Jeff =1/2 rare-earth magnet on a honeycomb lattice
Magnetic properties and spin dynamics in the spin-orbit driven Jeff = 1 2 triangular lattice antiferromagnet Ba 6 Yb 2 Ti 4 O 17
119Sn solid state NMR and M\"ossbauer spectroscopic studies of the intermediate-valent stannide CeRuSn
The ternary stannide CeRuSn is a static mixed-valent cerium compound with an
or-dering of trivalent and intermediate-valent cerium on two distinct
crystallographic sites. 119Sn M\"ossbauer spectra showed two electronically
almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K)
two tin sites can clearly be distinguished. 119Sn solid state NMR experiments
are performed to probe the local hyperfine fields at the two different Sn
sites. 119Sn NMR powder spectra are nicely fitted with two Sn sites with nearly
the same magnetic anisotropy, but with different absolute shift values. Both Sn
sites are strongly affected by crossover-like transitions between 100 and 280
K. This local-site study confirms the superstructure modulations found in
previous investiga-tions. Towards lower temperatures the powder spectra are
broadened giving strong evidence for the antiferromagnetically ordered ground
state
Experimental signatures of quantum and topological states in frustrated magnetism
Frustration in magnetic materials arising from competing exchange
interactions can prevent the system from adopting long-range magnetic order and
can instead lead to a diverse range of novel quantum and topological states
with exotic quasiparticle excitations. Here, we review prominent examples of
such emergent phenomena, including magnetically-disordered and extensively
degenerate spin ices, which feature emergent magnetic monopole excitations,
highly-entangled quantum spin liquids with fractional spinon excitations,
topological order and emergent gauge fields, as well as complex particle-like
topological spin textures known as skyrmions. We provide an overview of recent
advances in the search for magnetically-disordered candidate materials on the
three-dimensional pyrochlore lattice and two-dimensional triangular, kagome and
honeycomb lattices, the latter with bond-dependent Kitaev interactions, and on
lattices supporting topological magnetism. We highlight experimental signatures
of these often elusive phenomena and single out the most suitable experimental
techniques that can be used to detect them. Our review also aims at providing a
comprehensive guide for designing and investigating novel frustrated magnetic
materials, with the potential of addressing some important open questions in
contemporary condensed matter physics
- âŠ