84,178 research outputs found
Family of exactly solvable models with an ultimative quantum paramagnetic ground state
We present a family of two-dimensional frustrated quantum magnets solely
based on pure nearest-neighbor Heisenberg interactions which can be solved
quasi-exactly. All lattices are constructed in terms of frustrated quantum
cages containing a chiral degree of freedom protected by frustration. The
ground states of these models are dubbed ultimate quantum paramagnets and
exhibit an extensive entropy at zero temperature. We discuss the unusual and
extensively degenerate excitations in such phases. Implications for
thermodynamic properties as well as for decoherence free quantum computation
are discussed
Disorder by disorder and flat bands in the kagome transverse field Ising model
We study the transverse field Ising model on a kagome and a triangular
lattice using high-order series expansions about the high-field limit. For the
triangular lattice our results confirm a second-order quantum phase transition
in the 3d XY universality class. Our findings for the kagome lattice indicate a
notable instance of a disorder by disorder scenario in two dimensions. The
latter follows from a combined analysis of the elementary gap in the high- and
low-field limit which is shown to stay finite for all fields h. Furthermore,
the lowest one-particle dispersion for the kagome lattice is extremely flat
acquiring a dispersion only from order eight in the 1/h limit. This behaviour
can be traced back to the existence of local modes and their breakdown which is
understood intuitively via the linked cluster expansion.Comment: 11 pages, 11 figrue
Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors
Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25.Comment: 4 pages, 4 figures included updated version taking new data into
account; factor in spectral weight corrected; Figs. 2 and 4 change
Aluminium oxide in the optical spectrum of VY Canis Majoris
We report the first identification of the optical bands of the B-X system of
AlO in the red supergiant VY CMa. In addition to TiO, VO, ScO, and YO, which
were recognized in the optical spectrum of the star long time ago, AlO is
another refractory molecule which displays strong emission bands in this
peculiar star. Simulating the bands of AlO, we derive a rotational temperature
of the circumstellar gas of Trot=700K. By resolving individual rotational
components of the bands, we derive the kinematical characteristics of the gas,
finding that the emission is centered at the stellar radial velocity and its
intrinsic width is 13.5 km/s (full width at half maximum). It is the narrowest
emission among all (thermal) features observed in VY CMa so far. The
temperature and line widths suggest that the emission arises in gas located
within ~20 stellar radii, where the outflow is still being accelerated. This
result contradicts equilibrium-chemistry models which predict substantial AlO
abundances only to within a few stellar radii. We argue that non-equilibrium
models involving propagation of shocks are needed to explain the observations.Comment: to appear in A&
Pilot-optimal multivariable control synthesis by output feedback
A control system design approach for optimal stability augmentation, systems, using limited state feedback theory with the specific inclusion of the human pilot in the loop is presented. The methodology is especially suitable for application to flight vehicles exhibiting nonconventional dynamic characteristics and for which quantitative handling qualities specifications are not available. The design is based on a correlation between pilot ratings and objective function of the optimal control model of the human pilot. Simultaneous optimization for augmentation and pilot gains are required
Optimal cooperative control synthesis applied to a control-configured aircraft
A multivariable control augmentation synthesis method is presented that is intended to enable the designer to directly optimize pilot opinion rating of the augmented system. The approach involves the simultaneous solution for the augmentation and predicted pilot's compensation via optimal control techniques. The methodology is applied to the control law synthesis for a vehicle similar to the AFTI F16 control-configured aircraft. The resulting dynamics, expressed in terms of eigenstructure and time/frequency responses, are presented with analytical predictions of closed loop tracking performance, pilot compensation, and other predictors of pilot acceptance
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