2,225 research outputs found
Do correlations create an energy gap in electronic bilayers? Critical analysis of different approaches
This paper investigates the effect of correlations in electronic bilayers on
the longitudinal collective mode structure. We employ the dielectric
permeability constructed by means of the classical theory of moments. It is
shown that the neglection of damping processes overestimates the role of
correlations. We conclude that the correct account of damping processes leads
to an absence of an energy gap.Comment: 4 page
Polarization simulations of stellar wind bow shock nebulae. II. The case of dust scattering
We study the polarization produced by scattering from dust in a bow
shock-shaped region of enhanced density surrounding a stellar source, using the
Monte Carlo radiative transfer code SLIP. Bow shocks are structures formed by
the interaction of the winds of fast-moving stars with the interstellar medium.
Our previous study focused on the polarization produced in these structures by
electron scattering; we showed that polarization is highly dependent on
inclination angle and that multiple scattering changes the shape and degree of
polarization. In contrast to electron scattering, dust scattering is
wavelength-dependent, which changes the polarization behaviour. Here we explore
different dust particle sizes and compositions and generate polarized spectral
energy distributions for each case. We find that the polarization SED behaviour
depends on the dust composition and grain size. Including dust emission leads
to polarization changes with temperature at higher optical depth in ways that
are sensitive to the orientation of the bow shock. In various scenarios and
under certain assumptions, our simulations can constrain the optical depth and
dust properties of resolved and unresolved bow shock-shaped scattering
regions.Constraints on optical depth can provide estimates of local ISM density
for observed bow shocks. We also study the impact of dust grains filling the
region between the star and bow shock. We see that as the density of dust
between the star and bow shock increases, the resulting polarization is
suppressed for all the optical depth regimes.Comment: 21 pages, accepted for publication in MNRA
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Development of a Multi Megawatt Circulator for X Band
Research is in progress on a TeV-scale linear collider that will operate at 5-10 times the energy of present-generation accelerators. This will require development of high power RF sources generating of 50-100 MW per source. Transmission of power at this level requires overmoded waveguide to avoid breakdown. In particular, the TE{sub 01} circular waveguide mode is currently the mode of choice for waveguide transmission at Stanford Linear Accelerator Center (SLAC) in the Multimode Delay Line Distribution System (MDLDS). A common device for protecting an RF source from reflected power is the waveguide circulator. A circulator is typically a three-port device that allows low loss power transmission from the source to the load, but diverts power coming from the load (reflected power) to a third terminated port. To achieve a low loss, matched, three port junction requires nonreciprocal behavior. This is achieved using ferrites in a static magnetic field which introduces a propagation constant dependent on RF field direction relative to the static magnetic field. Circulators are currently available at X-Band for power levels up to 1 MW in fundamental rectangular waveguide; however, the next generation of RF sources for TeV-level accelerators will require circulators in the 50-100 MW range. Clearly, conventional technology is not capable of reaching the power level required. In this paper, we discuss the development of an X-Band circulator operating at multi-megawatt power levels in overmoded waveguide. The circulator will employ an innovative coaxial geometry using the TE{sub 01} mode. Difficulties in maintaining mode purity in oversized rectangular guide preclude increasing guide area to allow increasing the power level to the desired 50-100 MW range. The TE{sub 01} mode in circular waveguide is very robust mode for transmission of high power in overmoded waveguide. The mode is ideal for transmission of high power microwaves because of its low-losses, zero tangential field on the guide (which minimizes arcing problems) and reduced propensity for mode conversion compared to non-asymmetric circular waveguide modes. Unfortunately, no current designs exist for circulators using the circular TE{sub 01} mode. The basic building block for all low-loss circulators and isolators is a nonreciprocal element with a phase shift dependent on the propagation direction in the guide. Such an element can be constructed by placement of a hollow ferrite rod in a cylindrical waveguide. An inner conductor placed inside the ferrite rod conducts a current pulse that induces an azimuthal magnetic field inside the ferrite. This configuration is depicted in Figure 1a. An alternate configuration using permanent magnets is shown in Figure 1b. Either of these configurations will create a different phase shift for waves propagating in opposite directions along the waveguide axis. This feature can be used to develop a high power circulator. We are currently testing a TE{sub 01} nonreciprocal phase shifter in a 50 MW test stand. This device is in the configuration shown in Figure 1a. The induced differential phase shift and loss will be measured and compared to calculations
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Design and Test of a 100MW X Band TE01 Window
Research at Stanford Linear Accelerator Center (SLAC) is in progress on a TeV-scale linear collider that will operate at 5-10 times the energy of present generation accelerators. This will require development of high power X-Band sources generating 50-100 MW per source. Conventional pillbox window designs are capable of transmitting peak rf powers up to about 30 MW, well below the desired level required for the use of a single window per tube. SLAC has developed a 75 MW TE{sub 01} window [1] that uses a 'traveling wave' design to minimize fields at the window face. Irises match to the dielectric window impedance, resulting in a pure traveling wave in the ceramic and minimum fields on the window face. The use of the TE{sub 01} mode also has zero electric field on the braze fillet. Unfortunately, in-band resonances prevented this window design from achieving the desired 75MW power level. It was believed the resonances resulted from sudden steps in the circular guide to match the 38mm input diameter to the overmoded (TE{sub 01} and TE{sub 02} mode propagating) 65 mm diameter of the window ceramic. Calabazas Creek Research Inc. is currently developing a traveling wave window using compact, numerically optimized, parabolic tapers to match the input diameter of 38mm to the window ceramic diameter of 76mm (Figure 1). The design is projected to handle 100 MW of pulse power with a peak field at the window face of 3.6 MV/m. Cold test of the window has shown the return loss to be better than -25 dB over a 100 MHz bandwidth and to be resonance free (Figure 2). The window is scheduled for high-power testing in July 2003 at the SLAC
Many-body correlations probed by plasmon-enhanced drag measurements in double quantum well structures
Electron drag measurements of electron-electron scattering rates performed
close to the Fermi temperature are reported. While evidence of an enhancement
due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys.
Rev. Lett. 73, 3572 (1994)], is found, important differences with the
random-phase approximation based calculations are observed. Although static
correlation effects likely account for part of this difference, it is argued
that correlation-induced multiparticle excitations must be included to account
for the magnitude of the rates and observed density dependences.Comment: 4 pages, 3 figures, revtex Accepted in Phys. Rev.
Phases in Strongly Coupled Electronic Bilayer Liquids
The strongly correlated liquid state of a bilayer of charged particles has
been studied via the HNC calculation of the two-body functions. We report the
first time emergence of a series of structural phases, identified through the
behavior of the two-body functions.Comment: 5 pages, RevTEX 3.0, 4 ps figures; Submitted to Phys. Rev. Let
Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers
We study the occurrence of excitonic superfluidity in electron-hole bilayers
at zero temperature. We not only identify the crossover in the phase diagram
from the BCS limit of overlapping pairs to the BEC limit of non-overlapping
tightly-bound pairs but also, by varying the electron and hole densities
independently, we can analyze a number of phases that occur mainly in the
crossover region. With different electron and hole effective masses, the phase
diagram is asymmetric with respect to excess electron or hole densities. We
propose as the criterion for the onset of superfluidity, the jump of the
electron and hole chemical potentials when their densities cross.Comment: 4 pages, 3 figure
Dynamic correlations in symmetric electron-electron and electron-hole bilayers
The ground-state behavior of the symmetric electron-electron and
electron-hole bilayers is studied by including dynamic correlation effects
within the quantum version of Singwi, Tosi, Land, and Sjolander (qSTLS) theory.
The static pair-correlation functions, the local-field correction factors, and
the ground-state energy are calculated over a wide range of carrier density and
layer spacing. The possibility of a phase transition into a density-modulated
ground state is also investigated. Results for both the electron-electron and
electron-hole bilayers are compared with those of recent diffusion Monte Carlo
(DMC) simulation studies. We find that the qSTLS results differ markedly from
those of the conventional STLS approach and compare in the overall more
favorably with the DMC predictions. An important result is that the qSTLS
theory signals a phase transition from the liquid to the coupled Wigner crystal
ground state, in both the electron-electron and electron-hole bilayers, below a
critical density and in the close proximity of layers (d <~ r_sa_0^*), in
qualitative agreement with the findings of the DMC simulations.Comment: 13 pages, 11 figures, 2 table
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