2,772 research outputs found
Stabilization of A-type layered antiferromagnetic phase in LaMnO_3 by cooperative Jahn-Teller deformations
It is shown that the layered antiferromagnetic order in stoechiometric
LaMnO_3 cannot be understood purely from electronic interactions. On the
contrary, it mainly results from strong cooperative Jahn-Teller deformations.
Those involve a compression of the Mn-O octahedron along the c-axis (mode Q_3 <
0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q_2).
These deformations stabilize a certain type of orbital ordering. The resulting
superexchange couplings are calculated by exact diagonalization, taking into
account both e_g and t_{2g} orbitals. The main result is that antiferromagnetic
(ferromagnetic) coupling along the c-direction (ab-planes) can be understood
only if the Jahn-Teller energy is much larger than the superexchange couplings,
which is consistent with experiments. This mechanism contrasts with that based
on weak Jahn-Teller coupling which instead predicts elongation along the c-axis
(Q_3 > 0). The crucial role of the deformation anisotropy Q_2/Q_3 is also
emphasized.Comment: 8 pages, 6 figure
Performance of twin two-dimensional wedge nozzles including thrust vectoring and reversing effects at speeds up to Mach 2.20
Transonic tunnel and supersonic pressure tunnel tests were reformed to determine the performance characteristics of twin nonaxisymmetric or two-dimensional nozzles with fixed shrouds and variable-geometry wedges. The effects of thrust vectoring, reversing, and installation of various tails were also studied. The investigation was conducted statically and at flight speeds up to a Mach number of 2.20. The total pressure ratio of the simulated jet exhaust was varied up to approximately 26 depending on Mach number. The Reynolds number per meter varied up to 13.20 x 1 million. An analytical study was made to determine the effect on calculated wave drag by varying the mathematical model used to simulate nozzle jet-exhaust plume
Effect of thrust vectoring and wing maneuver devices on transonic aeropropulsive characteristics of a supersonic fighter
The aeropropulsive characteristics of an advanced fighter designed for supersonic cruise were determined in the Langley 16-Foot Transonic Tunnel. The objectives of this investigation were to evaluate the interactive effects of thrust vectoring and wing maneuver devices on lift and drag and to determine trim characteristics. The wing maneuver devices consisted of a drooped leading edge and a trailing-edge flap. Thrust vectoring was accomplished with two dimensional (nonaxisymmetric) convergent-divergent nozzles located below the wing in two single-engine podded nacelles. A canard was utilized for trim. Thrust vector angles of 0 deg, 15 deg, and 30 deg were tested in combination with a drooped wing leading edge and with wing trailing-edge flap deflections up to 30 deg. This investigation was conducted at Mach numbers from 0.60 to 1.20, at angles of attack from 0 deg to 20 deg, and at nozzle pressure ratios from about 1 (jet off) to 10. Reynolds number based on mean aerodynamic chord varied from 9.24 x 10 to the 6th to 10.56 x 10 to the 6th
Aerodynamic Characteristics of a Supersonic Fighter Aircraft Model at Mach 0.40 to 2.47
The aerodynamic characteristics of an advanced twin-engine fighter aircraft designed for supersonic cruise have been studied in the Langley 16-Foot Transonic Tunnel and the Lewis 10- by 10-Foot Supersonic Tunnel. The objective of this investigation was to establish an aerodynamic data base for the configuration with flow-through nacelles and representative inlets. The use of a canard for trim and the effects of fairing over the inlets were assessed. Comparisons between experimental and theoretical results were also made. The theoretical results were determined by using a potential vortex lift code for subsonic speeds and a linear aerodynamic code for supersonic speeds. This investigation was conducted at Mach numbers from 0.40 to 2.47, at angles of attack from 0 deg to about 20 deg, and at inlet capture ratios of about 0.5 to 1.4
Rotationally-invariant slave-bosons for Strongly Correlated Superconductors
We extend the rotationally invariant formulation of the slave-boson method to
superconducting states. This generalization, building on the recent work by
Lechermann et al. [Phys. Rev. B {\bf 76}, 155102 (2007)], allows to study
superconductivity in strongly correlated systems. We apply the formalism to a
specific case of strongly correlated superconductivity, as that found in a
multi-orbital Hubbard model for alkali-doped fullerides, where the
superconducting pairing has phonic origin, yet it has been shown to be favored
by strong correlation owing to the symmetry of the interaction. The method
allows to treat on the same footing the strong correlation effects and the
interorbital interactions driving superconductivity, and to capture the physics
of strongly correlated superconductivity, in which the proximity to a Mott
transition favors the superconducting phenomenon.Comment: 18 pages, 7 figure
Visual materials as tools in the teaching of geography
Thesis (Ed.M.)--Boston University, 1947. This item was digitized by the Internet Archive
Limiting the valence: advancements and new perspectives on patchy colloids, soft functionalized nanoparticles and biomolecules
Limited bonding valence, usually accompanied by well-defined directional
interactions and selective bonding mechanisms, is nowadays considered among the
key ingredients to create complex structures with tailored properties: even
though isotropically interacting units already guarantee access to a vast range
of functional materials, anisotropic interactions can provide extra
instructions to steer the assembly of specific architectures. The anisotropy of
effective interactions gives rise to a wealth of self-assembled structures both
in the realm of suitably synthesized nano- and micro-sized building blocks and
in nature, where the isotropy of interactions is often a zero-th order
description of the complicated reality. In this review, we span a vast range of
systems characterized by limited bonding valence, from patchy colloids of new
generation to polymer-based functionalized nanoparticles, DNA-based systems and
proteins, and describe how the interaction patterns of the single building
blocks can be designed to tailor the properties of the target final structures
Exciton Mott transition revisited
The dissociation of excitons into a liquid of holes and electrons in photoexcited semiconductors, despite being one of the first recognized examples of a Mott transition, still defies a complete understanding, especially regarding the nature of the transition, which is found to be continuous in some cases and discontinuous in others. Here we consider an idealized model of photoexcited semiconductors that can be mapped onto a spin-polarized half-filled Hubbard model, whose phase diagram reproduces most of the phenomenology of those systems and uncovers the key role of the exciton binding energy in determining the nature of the exciton Mott transition. We find indeed that the transition changes from discontinuous to continuous as the binding energy increases. Moreover, we uncover a rather anomalous electron-hole liquid phase next to the transition, which still sustains excitonic excitations despite being a degenerate Fermi liquid of heavy mass quasiparticles
Resonating bipolarons
Electrons coupled to local lattice deformations end up in selftrapped
localized molecular states involving their binding into bipolarons when the
coupling is stronger than a certain critical value. Below that value they exist
as essentially itinerant electrons. We propose that the abrupt crossover
between the two regimes can be described by resonant pairing similar to the
Feshbach resonance in binary atomic collision processes. Given the
intrinsically local nature of the exchange of pairs of itinerant electrons and
localized bipolarons, we demonstrate the occurrence of such a resonance on a
finite-size cluster made out of metallic atoms surrounding a polaronic ligand
center.Comment: 7 pages, 4 figures, to be published in Europhysics Letter
Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator
We study the superconducting state of the hole-doped two-dimensional Hubbard
model using Cellular Dynamical Mean Field Theory, with the Lanczos method as
impurity solver. In the under-doped regime, we find a natural decomposition of
the one-particle (photoemission) energy-gap into two components. The gap in the
nodal regions, stemming from the anomalous self-energy, decreases with
decreasing doping. The antinodal gap has an additional contribution from the
normal component of the self-energy, inherited from the normal-state pseudogap,
and it increases as the Mott insulating phase is approached.Comment: Corrected typos, 4.5 pages, 4 figure
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