4,018 research outputs found
Ab-initio study of disorder effects on the electronic and magnetic structures of SrFeMoO
We have investigated the electronic structure of ordered and disordered
SrFeMoO using {\it ab-initio} band structure methods. The effect of
disorder was simulated within super-cell calculations to realize several
configurations with mis-site disorders. It is found that such disorder effects
destroy the half-metallic ferro-magnetic state of the ordered compound. It also
leads to a substantial reduction of the magnetic moments at the Fe sites in the
disordered configurations. Most interestingly, it is found for the disordered
configurations, that the magnetic coupling within the Fe sub-lattice as well as
that within the Mo sub-lattice always remain ferro-magnetic, while the two
sub-lattices couple anti-ferromagnetically, in close analogy to the magnetic
structure of the ordered compound, but in contrast to recent suggestions.Comment: 7 pages, 3 figure
Vortex Phase Diagram of weakly pinned YBaCuO for H c
Vortex phase diagram in a weakly pinned crystal of YBCO for H c
is reviewed in the light of a recent elucidation of the process of `inverse
melting' in a Bismuth cuprate system and the imaging of an interface between
the ordered and the disordered regions across the peak effect in 2H-NbSe.
In the given YBCO crystal, a clear distinction can be made between the second
magnetization peak (SMP) and the peak effect (PE) between 65 K and 75 K. The
field region between the peak fields of the SMP (H) and the onset
fields of the PE (H)is not only continuously connected to the Bragg
glass phase at lower fields but it is also sandwiched between the higher
temperature vortex liquid phase and the lower temperature vortex glass phase.
Thus, an ordered vortex state between H and H can get
transformed to the (disordered) vortex liquid state on heating as well as to
the (disordered) vortex glass state on cooling, a situation analogous to the
thermal melting and the inverse melting phenomenon seen in a Bismuth cuprate.Comment: Presented in IWCC-200
Analysis of the resistance in p-SiGe over a wide temperature range
The temperature dependence of a system exhibiting a `metal-insulator
transition in two dimensions at zero magnetic field' (MIT) is studied up to
90K. Using a classical scattering model we are able to simulate the
non-monotonic temperature dependence of the resistivity in the metallic high
density regime. We show that the temperature dependence arises from a complex
interplay of metallic and insulating contributions contained in the calculation
of the scattering rate 1/\td(E,T), each dominating in a limited temperature
range.Comment: 4 pages with 5 figure
Dilepton asymmetries at factories in search of transitions
In order to detect the possible presence of
amplitudes in neutral meson decays, we consider the measurement of decay
time asymmetries involving like-sign dilepton events at the factories.Comment: 5 pages, latex, no fig
Raman Scattering Spectra of Elementary Electronic Excitations in Coupled Double-Quantum Well Structures
Using the time-dependent-local-density-approximation (TDLDA) within a
self-consistent linear response theory, we calculate the elementary excitation
energies and the associated inelastic light-scattering spectra of a strongly
coupled two-component plasma in a double-quantum well system with electron
occupation of symmetric and antisymmetric subbands. We find, consistent with
the results of a recent experimental Raman scattering study, that the
intersubband spin density excitations tend to merge with the single particle
excitations (i.e. the excitonic shift decreases monotonically) as the Fermi
energy increases beyond the symmetric-antisymmetric energy gap
. However, our TDLDA calculation does not show the abrupt
suppresion of the excitonic shift seen experimentally at a finite value of the
subband occupancy parameter .Comment: 9 pages, RevTeX, 5 figures available upon request, PIT-SDS-00
Plasmons in coupled bilayer structures
We calculate the collective charge density excitation dispersion and spectral
weight in bilayer semiconductor structures {\it including effects of interlayer
tunneling}. The out-of-phase plasmon mode (the ``acoustic'' plasmon) develops a
long wavelength gap in the presence of tunneling with the gap being
proportional to the square root (linear power) of the tunneling amplitude in
the weak (strong) tunneling limit. The in-phase plasmon mode is qualitatively
unaffected by tunneling. The predicted plasmon gap should be a useful tool for
studying many-body effects.Comment: 10 pages, 6 figures. to appear in Phys. Rev. Let
Energy relaxation of an excited electron gas in quantum wires: many-body electron LO-phonon coupling
We theoretically study energy relaxation via LO-phonon emission in an excited
one-dimensional electron gas confined in a GaAs quantum wire structure. We find
that the inclusion of phonon renormalization effects in the theory extends the
LO-phonon dominated loss regime down to substantially lower temperatures. We
show that a simple plasmon-pole approximation works well for this problem, and
discuss implications of our results for low temperature electron heating
experiments in quantum wires.Comment: 10 pages, RevTex, 4 figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng
X-ray photoemission study of NiS_{2-x}Se_x (x = 0.0 - 1.2)
Electronic structure of NiS_{2-x}Se_x system has been investigated for
various compositions (x) using x-ray photoemission spectroscopy. An analysis of
the core level as well as the valence band spectra of NiS_2 in conjunction with
many-body cluster calculations provides a quantitative description of the
electronic structure of this compound. With increasing Se content, the on-site
Coulomb correlation strength (U) does not change, while the band width W of the
system increases, driving the system from a covalent insulating state to a
pd-metallic state.Comment: 19 pages, 6 figures, To appear in Phys. Rev. B, 200
Dissipationless transport in low density bilayer systems
In a bilayer electronic system the layer index may be viewed as the
z-component of an isospin-1/2. An XY isospin-ordered ferromagnetic phase was
observed in quantum Hall systems and is predicted to exist at zero magnetic
field at low density. This phase is a superfluid for opposite currents in the
two layers. At B=0 the system is gapless but superfluidity is not destroyed by
weak disorder. In the quantum Hall case, weak disorder generates a random gauge
field which probably does not destroy superfluidity. Experimental signatures
include Coulomb drag and collective mode measurements.Comment: 4 pages, no figures, submitted to Phys. Rev. Let
Summarizing and measuring development activity
Software developers pursue a wide range of activities as part of their work, and making sense of what they did in a given time frame is far from trivial as evidenced by the large number of awareness and coordination tools that have been developed in recent years. To inform tool design for making sense of the information available about a developer's activity, we conducted an empirical study with 156 GitHub users to investigate what information they would expect in a summary of development activity, how they would measure development activity, and what factors in uence how such activity can be condensed into textual summaries or numbers. We found that unexpected events are as important as expected events in summaries of what a developer did, and that many developers do not believe in measuring development activity. Among the factors that in uence summarization and measurement of development activity, we identified development experience and programming languages.Christoph Treude, Fernando Figueira Filho, Uirá Kulesz
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