16,383 research outputs found
Simulating terahertz quantum cascade lasers: Trends from samples from different labs
We present a systematic comparison of the results from our non-equilibrium
Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum
cascade lasers previously published in the literature. Employing identical
material and simulation parameters for all samples, we observe that
discrepancies between measured and calculated peak currents are similar for
samples from a given group. This suggests that the differences between
experiment and theory are partly due to a lacking reproducibility for devices
fabricated at different laboratories. Varying the interface roughness height
for different devices, we find that the peak current under lasing operation
hardly changes, so that differences in interface quality appear not to be the
sole reason for the lacking reproducibility.Comment: 9 pages, 6 figures; section VI with 2 figures added in v2; accepted
for publication in J. Appl. Phy
Superlattice gain in positive differential conductivity region
We analyze theoretically a superlattice structure proposed by A. Andronov et
al. [JETP Lett 102, 207 (2015)] to give Terahertz gain for an operation point
with positive differential conductivity. Here we confirm the existence of gain
and show that an optimized structure displays gain above 20 cm at low
temperatures, so that lasing may be observable. Comparing a variety of
simulations, this gain is found to be strongly affected by elastic scattering.
It is shown that the dephasing modifies the nature of the relevant states, so
that the common analysis based on Wannier-Stark states is not reliable for a
quantitative description of the gain in structures with extremely diagonal
transitions.Comment: 4 pages, 5 figure
Mathematical Models and Biological Meaning: Taking Trees Seriously
We compare three basic kinds of discrete mathematical models used to portray
phylogenetic relationships among species and higher taxa: phylogenetic trees,
Hennig trees and Nelson cladograms. All three models are trees, as that term is
commonly used in mathematics; the difference between them lies in the
biological interpretation of their vertices and edges. Phylogenetic trees and
Hennig trees carry exactly the same information, and translation between these
two kinds of trees can be accomplished by a simple algorithm. On the other
hand, evolutionary concepts such as monophyly are represented as different
mathematical substructures are represented differently in the two models. For
each phylogenetic or Hennig tree, there is a Nelson cladogram carrying the same
information, but the requirement that all taxa be represented by leaves
necessarily makes the representation less efficient. Moreover, we claim that it
is necessary to give some interpretation to the edges and internal vertices of
a Nelson cladogram in order to make it useful as a biological model. One
possibility is to interpret internal vertices as sets of characters and the
edges as statements of inclusion; however, this interpretation carries little
more than incomplete phenetic information. We assert that from the standpoint
of phylogenetics, one is forced to regard each internal vertex of a Nelson
cladogram as an actual (albeit unsampled) species simply to justify the use of
synapomorphies rather than symplesiomorphies.Comment: 15 pages including 6 figures [5 pdf, 1 jpg]. Converted from original
MS Word manuscript to PDFLaTe
Nonlinear response of quantum cascade structures
The gain spectrum of a terahertz quantum cascade laser is analysed by a non
equilibrium Green's functions approach. Higher harmonics of the response
function were retrievable, providing a way to approach nonlinear phenomena in
quantum cascade lasers theoretically. Gain is simulated under operation
conditions and results are presented both for linear response and strong laser
fields. An iterative way of reconstructing the field strength inside the laser
cavity at lasing conditions is described using a measured value of the level of
the losses of the studied system. Comparison with recent experimental data from
time-domain-spectroscopy indicates that the experimental situation is beyond
linear response.Comment: 4 pages, 3 figures included in text, to appear in Applied Physics
Letter
Theoretical properties of quasi-stationary Monte Carlo methods
This paper gives foundational results for the application of
quasi-stationarity to Monte Carlo inference problems. We prove natural
sufficient conditions for the quasi-limiting distribution of a killed diffusion
to coincide with a target density of interest. We also quantify the rate of
convergence to quasi-stationarity by relating the killed diffusion to an
appropriate Langevin diffusion. As an example, we consider in detail a killed
Ornstein--Uhlenbeck process with Gaussian quasi-stationary distribution.Comment: 27 pages, 1 figure. Final version of accepted paper. Minor typos
correcte
Identifying capacitive and inductive loss in lumped element superconducting hybrid titanium nitride/aluminum resonators
We present a method to systematically locate and extract capacitive and
inductive losses in superconducting resonators at microwave frequencies by use
of mixed-material, lumped element devices. In these devices, ultra-low loss
titanium nitride was progressively replaced with aluminum in the
inter-digitated capacitor and meandered inductor elements. By measuring the
power dependent loss at 50 mK as the Al-TiN fraction in each element is
increased, we find that at low electric field, i.e. in the single photon limit,
the loss is two level system in nature and is correlated with the amount of Al
capacitance rather than the Al inductance. In the high electric field limit,
the remaining loss is linearly related to the product of the Al area times its
inductance and is likely due to quasiparticles generated by stray radiation. At
elevated temperature, additional loss is correlated with the amount of Al in
the inductance, with a power independent TiN-Al interface loss term that
exponentially decreases as the temperature is reduced. The TiN-Al interface
loss is vanishingly small at the 50 mK base temperature.Comment: 10 pages, 5 figure
Coherence in a transmon qubit with epitaxial tunnel junctions
We developed transmon qubits based on epitaxial tunnel junctions and
interdigitated capacitors. This multileveled qubit, patterned by use of
all-optical lithography, is a step towards scalable qubits with a high
integration density. The relaxation time T1 is .72-.86mu sec and the ensemble
dephasing time T2 is slightly larger than T1. The dephasing time T2 (1.36mu
sec) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker
level splitting than observed in qubits with amorphous barriers in
equivalent-size junctions. The qubit's inferred microwave loss closely matches
the weighted losses of the individual elements (junction, wiring dielectric,
and interdigitated capacitor), determined by independent resonator
measurements
Evidence for Ubiquitous Collimated Galactic-Scale Outflows along the Star-Forming Sequence at z~0.5
We present an analysis of the MgII 2796, 2803 and FeII 2586, 2600 absorption
line profiles in individual spectra of 105 galaxies at 0.3<z<1.4. The galaxies,
drawn from redshift surveys of the GOODS fields and the Extended Groth Strip,
fully sample the range in star formation rates (SFRs) occupied by the
star-forming sequence with stellar masses log M_*/M_sun > 9.5 at 0.3<z<0.7.
Using the Doppler shifts of the MgII and FeII absorption lines as tracers of
cool gas kinematics, we detect large-scale winds in 66+/-5% of the galaxies.
HST/ACS imaging and our spectral analysis indicate that the outflow detection
rate depends primarily on galaxy orientation: winds are detected in ~89% of
galaxies having inclinations (i) <30 degrees (face-on), while the wind
detection rate is only ~45% in objects having i>50 degrees (edge-on). Combined
with the comparatively weak dependence of the wind detection rate on intrinsic
galaxy properties, this suggests that biconical outflows are ubiquitous in
normal, star-forming galaxies at z~0.5. We find that the wind velocity is
correlated with host galaxy M_* at 3.4-sigma significance, while the equivalent
width of the flow is correlated with host galaxy SFR at 3.5-sigma significance,
suggesting that hosts with higher SFR may launch more material into outflows
and/or generate a larger velocity spread for the absorbing clouds. Assuming
that the gas is launched into dark matter halos with simple, isothermal density
profiles, the wind velocities measured for the bulk of the cool material
(~200-400 km/s) are sufficient to enable escape from the halo potentials only
for the lowest-M_* systems in the sample. However, the outflows typically carry
sufficient energy to reach distances of >50 kpc, and may therefore be a viable
source of cool material for the massive circumgalactic medium observed around
bright galaxies at z~0. [abridged]Comment: Submitted to ApJ. 61 pages, 25 figures, 4 tables, 4 appendices. Uses
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