12,378 research outputs found
An analysis of astronaut performance capability in the lunar environment. Volume 1 - Performance problems and requirements for additional research
Analyzing data on expected astronaut performance in lunar environmen
An Analysis of Astronaut Performance Capability in the Lunar Environment. Volume 2 - Performance Capability Support Data
Astronaut performance capability in lunar environmen
Effects of specimen resonances on acoustic-ultrasonic testing
The effects of specimen resonances on acoustic ultrasonic (AU) nondestructive testing were investigated. Selected resonant frequencies and the corresponding normal mode nodal patterns of the aluminum block are measured up to 75.64 kHz. Prominent peaks in the pencil lead fracture and sphere impact spectra from the two transducer locations corresponded exactly to resonant frequencies of the block. It is established that the resonant frequencies of the block dominated the spectral content of the output signal. The spectral content of the output signals is further influenced by the transducer location relative to the resonant frequency nodal lines. Implications of the results are discussed in relation to AU parameters and measurements
Magnetic phase diagram of a frustrated ferrimagnetic ladder: Relation to the one-dimensional boson Hubbard model
We study the magnetic phase diagram of two coupled mixed-spin
Heisenberg chains as a function of the frustration parameter related to
diagonal exchange couplings. The analysis is performed by using spin-wave
series and exact numerical diagonalization techniques. The obtained phase
diagram--containing the Luttinger liquid phase, the plateau phase with a
magnetization per rung , and the fully polarized phase--is closely
related to the generic phase diagram of the one-dimensional boson
Hubbard model.Comment: 4 pages, 2 figure
Electronic structure of the molecule based magnet Cu PM(NO3)2 (H2O)2
We present density functional calculations on the molecule based S=1/2
antiferromagnetic chain compound Cu PM(NO3)2 (H2O)2; PM = pyrimidine. The
properties of the ferro- and antiferromagnetic state are investigated at the
level of the local density approximation and with the hybrid functional B3LYP.
Spin density maps illustrate the exchange path via the pyrimidine molecule
which mediates the magnetism in the one-dimensional chain. The computed
exchange coupling is antiferromagnetic and in reasonable agreement with the
experiment. It is suggested that the antiferromagnetic coupling is due to the
possibility of stronger delocalization of the charges on the nitrogen atoms,
compared to the ferromagnetic case. In addition, computed isotropic and
anisotropic hyperfine interaction parameters are compared with recent NMR
experiments
Orlov's Equivalence and Maximal Cohen-Macaulay Modules over the Cone of an Elliptic Curve
We describe a method for doing computations with Orlov's equivalence between
the bounded derived category of certain hypersurfaces and the stable category
of graded matrix factorisations of the polynomials describing these
hypersurfaces. In the case of a smooth elliptic curve over an algebraically
closed field we describe the indecomposable graded matrix factorisations of
rank one. Since every indecomposable Maximal Cohen-Macaulay module over the
completion of a smooth cubic curve is gradable, we obtain explicit descriptions
of all indecomposable rank one matrix factorisations of such potentials.
Finally, we explain how to compute all indecomposable matrix factorisations of
higher rank with the help of a computer algebra system.Comment: 26 page
A DMRG Study of Low-Energy Excitations and Low-Temperature Properties of Alternating Spin Systems
We use the density matrix renormalization group (DMRG) method to study the
ground and low-lying excited states of three kinds of uniform and dimerized
alternating spin chains. The DMRG procedure is also employed to obtain
low-temperature thermodynamic properties of these systems. We consider a 2N
site system with spins and alternating from site to site and
interacting via a Heisenberg antiferromagnetic exchange. The three systems
studied correspond to being equal to and
; all of them have very similar properties. The ground state is found
to be ferrimagnetic with total spin . We find that there is
a gapless excitation to a state with spin , and a gapped excitation to
a state with spin . Surprisingly, the correlation length in the ground
state is found to be very small for this gapless system. The DMRG analysis
shows that the chain is susceptible to a conditional spin-Peierls instability.
Furthermore, our studies of the magnetization, magnetic susceptibility
and specific heat show strong magnetic-field dependences. The product
shows a minimum as a function of temperature T at low magnetic fields; the
minimum vanishes at high magnetic fields. This low-field behavior is in
agreement with earlier experimental observations. The specific heat shows a
maximum as a function of temperature, and the height of the maximum increases
sharply at high magnetic fields. Although all the three systems show
qualitatively similar behavior, there are some notable quantitative differences
between the systems in which the site spin difference, , is large
and small respectively.Comment: 16 LaTeX pages, 13 postscript figure
Boston's Education Pipeline: A Report Card
Assesses the city's progress in creating an effective and equitable education pipeline from early childhood through college or postsecondary training. Examines demographic and outcome data and risk and prevention factors, and notes areas for improvement
Exchange Interactions and High-Energy Spin States in Mn_12-acetate
We perform inelastic neutron scattering measurements on the molecular
nanomagnet Mn_12-acetate to measure the excitation spectrum up to 45meV (500K).
We isolate magnetic excitations in two groups at 5-6.5meV (60-75K) and
8-10.5meV (95-120K), with higher levels appearing only at 27meV (310K) and
31meV (360K). From a detailed characterization of the transition peaks we show
that all of the low-energy modes appear to be separate S = 9 excitations above
the S = 10 ground state, with the peak at 27meV (310K) corresponding to the
first S = 11 excitation. We consider a general model for the four exchange
interaction parameters of the molecule. The static susceptibility is computed
by high-temperature series expansion and the energy spectrum, matrix elements
and ground-state spin configuration by exact diagonalization. The theoretical
results are matched with experimental observation by inclusion of cluster
anisotropy parameters, revealing strong constraints on possible parameter sets.
We conclude that only a model with dominant exchange couplings J_1 ~ J_2 ~
5.5meV (65K) and small couplings J_3 ~ J_4 ~ 0.6meV (7K) is consistent with the
experimental data.Comment: 17 pages, 12 figure
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