4,456 research outputs found
Excitons and charged excitons in semiconductor quantum wells
A variational calculation of the ground-state energy of neutral excitons and
of positively and negatively charged excitons (trions) confined in a
single-quantum well is presented. We study the dependence of the correlation
energy and of the binding energy on the well width and on the hole mass. The
conditional probability distribution for positively and negatively charged
excitons is obtained, providing information on the correlation and the charge
distribution in the system. A comparison is made with available experimental
data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well
structures, which indicates that trions become localized with decreasing
quantum well width.Comment: 9 pages, 11 figure
Active shape correction of a thin glass/plastic X-ray mirror
Optics for future X-ray telescopes will be characterized by very large
aperture and focal length, and will be made of lightweight materials like glass
or plastic in order to keep the total mass within acceptable limits. Optics
based on thin slumped glass foils are currently in use in the NuSTAR telescope
and are being developed at various institutes like INAF/OAB, aiming at
improving the angular resolution to a few arcsec HEW. Another possibility would
be the use of thin plastic foils, being developed at SAO and the Palermo
University. Even if relevant progresses in the achieved angular resolution were
recently made, a viable possibility to further improve the mirror figure would
be the application of piezoelectric actuators onto the non-optical side of the
mirrors. In fact, thin mirrors are prone to deform, so they require a careful
integration to avoid deformations and even correct forming errors. This however
offers the possibility to actively correct the residual deformation. Even if
other groups are already at work on this idea, we are pursuing the concept of
active integration of thin glass or plastic foils with piezoelectric patches,
fed by voltages driven by the feedback provided by X-rays, in intra-focal setup
at the XACT facility at INAF/OAPA. In this work, we show the preliminary
simulations and the first steps taken in this project
Dependence of effective properties upon regular perturbations
In this survey, we present some results on the behavior of effective properties in presence of perturbations of the geometric and physical parameters. We first consider the case of a Newtonian fluid flowing at low Reynolds numbers around a periodic array of cylinders. We show the results of [43], where it is proven that the average longitudinal flow depends real analytically upon perturbations of the periodicity structure and the cross section of the cylinders. Next, we turn to the effective conductivity of a periodic two-phase composite with ideal contact at the interface. The composite is obtained by introducing a periodic set of inclusions into an infinite homogeneous matrix made of a different material. We show a result of [41] on the real analytic dependence of the effective conductivity upon perturbations of the shape of the inclusions, the periodicity structure, and the conductivity of each material. In the last part of the chapter, we extend the result of [41] to the case of a periodic two-phase composite with imperfect contact at the interface
Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells
A variational calculation of the spin-singlet and spin-triplet state of a
negatively charged exciton (trion) confined to a single quantum well and in the
presence of a perpendicular magnetic field is presented. We calculated the
probability density and the pair correlation function of the singlet and
triplet trion states. The dependence of the energy levels and of the binding
energy on the well width and on the magnetic field strength was investigated.
We compared our results with the available experimental data on GaAs/AlGaAs
quantum wells and find that in the low magnetic field region (B<18 T) the
observed transition are those of the singlet and the dark triplet trion (with
angular momentum ), while for high magnetic fields (B>25 T) the dark
trion becomes optically inactive and possibly a transition to a bright triplet
trion (angular momentum ) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.
Evaluating the performance of five different chemical ionization techniques for detecting gaseous oxygenated organic species
The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOCs), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from alpha-Pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new "vocus" PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim of determining which oxygenated VOCs (OVOCs) the different methods were sensitive to and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOCs, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies, the PTR instruments were the only ones able to measure the precursor VOC, the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing nonlinear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOCs in the atmosphere. While specifically shown for the case of alpha-Pinene ozonolysis, we expect our general findings to also be valid for a wide range of other VOC-oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.Peer reviewe
Conformal invariance and its breaking in a stochastic model of a fluctuating interface
Using Monte-Carlo simulations on large lattices, we study the effects of
changing the parameter (the ratio of the adsorption and desorption rates)
of the raise and peel model. This is a nonlocal stochastic model of a
fluctuating interface. We show that for the system is massive, for
it is massless and conformal invariant. For the conformal
invariance is broken. The system is in a scale invariant but not conformal
invariant phase. As far as we know it is the first example of a system which
shows such a behavior. Moreover in the broken phase, the critical exponents
vary continuously with the parameter . This stays true also for the critical
exponent which characterizes the probability distribution function of
avalanches (the critical exponent staying unchanged).Comment: 22 pages and 20 figure
Non-Abelian adiabatic statistics and Hall viscosity in quantum Hall states and p_x+ip_y paired superfluids
Many trial wavefunctions for fractional quantum Hall states in a single
Landau level are given by functions called conformal blocks, taken from some
conformal field theory. Also, wavefunctions for certain paired states of
fermions in two dimensions, such as p_x+ip_y states, reduce to such a form at
long distances. Here we investigate the adiabatic transport of such
many-particle trial wavefunctions using methods from two-dimensional field
theory. One context for this is to calculate the statistics of widely-separated
quasiholes, which has been predicted to be non-Abelian in a variety of cases.
The Berry phase or matrix (holonomy) resulting from adiabatic transport around
a closed loop in parameter space is the same as the effect of analytic
continuation around the same loop with the particle coordinates held fixed
(monodromy), provided the trial functions are orthonormal and holomorphic in
the parameters so that the Berry vector potential (or connection) vanishes. We
show that this is the case (up to a simple area term) for paired states
(including the Moore-Read quantum Hall state), and present general conditions
for it to hold for other trial states (such as the Read-Rezayi series). We
argue that trial states based on a non-unitary conformal field theory do not
describe a gapped topological phase, at least in many cases. By considering
adiabatic variation of the aspect ratio of the torus, we calculate the Hall
viscosity, a non-dissipative viscosity coefficient analogous to Hall
conductivity, for paired states, Laughlin states, and more general quantum Hall
states. Hall viscosity is an invariant within a topological phase, and is
generally proportional to the "conformal spin density" in the ground state.Comment: 44 pages, RevTeX; v2 minor changes; v3 typos corrected, three small
addition
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