1,291 research outputs found
Phase stability analysis in Fe-Pt and Co-Pt alloy systems: An augmented space study
We have studied the problem of phase stability in Fe-Pt and Co-Pt alloy
systems. We have used the orbital peeling technique in the conjunction of
augmented space recursion based on the tight binding linear orbital method as
the method for the calculation of pair interaction energies. In particular, we
have generalized our earlier technique to take into account of magnetic effects
for the cases where the magnetic transition is higher than the order disorder
chemical transition temperature as in the case of CoPt. Our theoretical
results obtained within this framework successfully reproduce the
experimentally observed trends.Comment: 17 pages, 9 Figures. Accepted for publication in Journal of Physics :
Condensed Matte
Phase shift experiments identifying Kramers doublets in a chaotic superconducting microwave billiard of threefold symmetry
The spectral properties of a two-dimensional microwave billiard showing
threefold symmetry have been studied with a new experimental technique. This
method is based on the behavior of the eigenmodes under variation of a phase
shift between two input channels, which strongly depends on the symmetries of
the eigenfunctions. Thereby a complete set of 108 Kramers doublets has been
identified by a simple and purely experimental method. This set clearly shows
Gaussian unitary ensemble statistics, although the system is time-reversal
invariant.Comment: RevTex 4, 5 figure
Experimental Test of a Trace Formula for a Chaotic Three Dimensional Microwave Cavity
We have measured resonance spectra in a superconducting microwave cavity with
the shape of a three-dimensional generalized Bunimovich stadium billiard and
analyzed their spectral fluctuation properties. The experimental length
spectrum exhibits contributions from periodic orbits of non-generic modes and
from unstable periodic orbit of the underlying classical system. It is well
reproduced by our theoretical calculations based on the trace formula derived
by Balian and Duplantier for chaotic electromagnetic cavities.Comment: 4 pages, 5 figures (reduced quality
Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8
We present powder and single-crystal neutron diffraction and bulk
measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to
8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This
system contains two types of Ni ions, which we call "spine" and "cross-tie".
Our neutron measurements can be described with the paramagnetic space group
Cmca for T < 15K and each observed magnetically ordered phase is characterized
by the appropriate irreducible representation(s). Our zero-field measurements
show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order
which is dominated by a longitudinally-modulated structure with the spine spins
mainly parallel to the a-axis. Upon further cooling, a transition is induced at
T_HL=6.3K to an elliptically polarized incommensurate structure with both spine
and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a
first-order phase transition, below which the magnetic structure is a
commensurate antiferromagnet with the staggered magnetization primarily along
the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat
peak at T_CC'=2.3K indicates an additional transition, which we were however
not able to relate to a change of the magnetic structure. Neutron, specific
heat, and magnetization measurements produce a comprehensive temperature-field
phase diagram. The symmetries of the two incommensurate magnetic phases are
consistent with the observation that only one phase has a spontaneous
ferroelectric polarization. All the observed magnetic structures are explained
theoretically using a simplified model Hamiltonian, involving competing
nearest- and next-nearest-neighbor exchange interactions, spin anisotropy,
Dzyaloshinskii-Moriya and pseudo-dipolar interactions.Comment: 25 pages, 19 figure
Properties of lightly doped t-J two-leg ladders
We have numerically investigated the doped t-J ladder using exact
diagonalization. We have studied both the limit of strong inter-chain coupling
and isotropic coupling. The ladder scales to the Luther-Emery liquid regime in
the strong inter-chain coupling limit. In this strong coupling limit there is a
simple picture of the excitation spectrum that can be continued to explain the
behavior at isotropic coupling. At J=0 we have indications of a ferromagnetic
ground state. At a large the ladder is phase separated into holes and a
Heisenberg ladder. At intermediate coupling the ground state shows hole pairing
with a modified d-wave symmetry. The excitation spectrum separates into a
limited number of quasiparticles which carry charge and spin and a triplet magnon mode. At half-filling the former vanish but the latter
evolves continuously into the magnon band of the spin liquid. At low doping the
quasiparticles form a dilute Fermi gas with a strong attraction but
simultaneously the Fermi wave vector, as would be measured in photoemission, is
large. The dynamical structure factors are calculated and are found to be very
similar to calculations on 2D clusters
Transition Radiation Spectroscopy with Prototypes of the ALICE TRD
We present measurements of the transition radiation (TR) spectrum produced in
an irregular radiator at different electron momenta. The data are compared to
simulations of TR from a regular radiator.Comment: 4 pages, 5 Figures, Proceedings for "TRDs for the 3rd millennium"
(Sept. 4-7, 2003, Bari, Italy
Effective Interactions and Volume Energies in Charged Colloids: Linear Response Theory
Interparticle interactions in charge-stabilized colloidal suspensions, of
arbitrary salt concentration, are described at the level of effective
interactions in an equivalent one-component system. Integrating out from the
partition function the degrees of freedom of all microions, and assuming linear
response to the macroion charges, general expressions are obtained for both an
effective electrostatic pair interaction and an associated microion volume
energy. For macroions with hard-sphere cores, the effective interaction is of
the DLVO screened-Coulomb form, but with a modified screening constant that
incorporates excluded volume effects. The volume energy -- a natural
consequence of the one-component reduction -- contributes to the total free
energy and can significantly influence thermodynamic properties in the limit of
low-salt concentration. As illustrations, the osmotic pressure and bulk modulus
are computed and compared with recent experimental measurements for deionized
suspensions. For macroions of sufficient charge and concentration, it is shown
that the counterions can act to soften or destabilize colloidal crystals.Comment: 14 pages, including 3 figure
Microsystem Technology for Ambient Assisted Living (AAL)
AbstractAAL is certainly an application area with sensor as well as actuator needs. Some of the requirements can be fulfilled by state of the art technology; some areas however still need a lot of R&D efforts for potential applications in homes. The contribution describes two areas of interest and actual development: One is the topic of robust fire detection; the other domain is fall detection. For both application areas one has to understand both the state of the art and the drawbacks of the current solutions. One can state clearly that there is a huge potential for the development of new microsystems. Still one has to keep in mind that usage in elderly homes also requires consent and cooperation of the users which is the focus of the user centered design principle
Microscopic dynamics in liquid metals: the experimental point of view
The experimental results relevant for the understanding of the microscopic
dynamics in liquid metals are reviewed, with special regards to the ones
achieved in the last two decades. Inelastic Neutron Scattering played a major
role since the development of neutron facilities in the sixties. The last ten
years, however, saw the development of third generation radiation sources,
which opened the possibility of performing Inelastic Scattering with X rays,
thus disclosing previously unaccessible energy-momentum regions. The purely
coherent response of X rays, moreover, combined with the mixed
coherent/incoherent response typical of neutron scattering, provides enormous
potentialities to disentangle aspects related to the collectivity of motion
from the single particle dynamics.
If the last twenty years saw major experimental developments, on the
theoretical side fresh ideas came up to the side of the most traditional and
established theories. Beside the raw experimental results, therefore, we review
models and theoretical approaches for the description of microscopic dynamics
over different length-scales, from the hydrodynamic region down to the single
particle regime, walking the perilous and sometimes uncharted path of the
generalized hydrodynamics extension. Approaches peculiar of conductive systems,
based on the ionic plasma theory, are also considered, as well as kinetic and
mode coupling theory applied to hard sphere systems, which turn out to mimic
with remarkable detail the atomic dynamics of liquid metals. Finally, cutting
edges issues and open problems, such as the ultimate origin of the anomalous
acoustic dispersion or the relevance of transport properties of a conductive
systems in ruling the ionic dynamic structure factor are discussed.Comment: 53 pages, 41 figures, to appear in "The Review of Modern Physics".
Tentatively scheduled for July issu
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