39,208 research outputs found
Universality of soft and collinear factors in hard-scattering factorization
Universality in QCD factorization of parton densities, fragmentation
functions, and soft factors is endangered by the process dependence of the
directions of Wilson lines in their definitions. We find a choice of directions
that is consistent with factorization and that gives universality between
e^+e^- annihilation, semi-inclusive deep-inelastic scattering, and the
Drell-Yan process. Universality is only modified by a time-reversal
transformation of the soft function and parton densities between Drell-Yan and
the other processes, whose only effect is the known reversal of sign for T-odd
parton densities like the Sivers function. The modifications of the definitions
needed to remove rapidity divergences with light-like Wilson lines do not
affect the results.Comment: 4 pages. Extra references. Text and references as in published
versio
Electronic properties of deep levels in p‐type CdTe
DLTS and associated electrical measurements were made on unintentionally doped CdTe crystals obtained from several vendors, on Cu‐doped CdTe, and on Te‐annealed CdTe. All of the crystals were p‐type. Four majority carrier deep levels were observed in the temperature range from 100–300 K with activation energies relative to the valence band of 0.2, 0.41, 0.45, and 0.65 eV. Two of these levels were specific to certain crystals while the other two were seen in every sample and are attributed to common impurities or native defects. Fluctuations in the concentrations of levels across samples and as a result of modest sample heating (400 K) were also observed
Prospects for the future of narrow bandgap materials
Recently there has been greatly expended interest in narrow bandgap materials. Modern epitaxial techniques and the growing interest in nanostructures have provided areas of application for some of the unique properties of the narrow bandgap material. As always, one of the primary sources of interest is the small bandgap which makes them the material of choice for many applications in the infrared. However, in recent years their other unique properties have been the basis for a broader set of interests in narrow bandgap semiconductors. The type II band offsets (InAs/GaSb) have been the basis for novel tunnel devices and infrared superlattices. The very small effective masses inherent in small bandgap materials make them the obvious candidates in which to observe quantum confinement effects at larger dimensions than in materials of larger effective mass and wider gap. The ease of making electrical contact to some of the materials (ohmic contact to n-InAs) has made them the material of choice for electrical nanostructures. The ability to put in large amounts of magnetic ions to make magnetic semiconductors has led to a number of novel properties. The technical importance of a narrow bandgap and the unique applications promised by some of the other properties of these materials bode well for substantial research in narrow bandgap semiconductors well into the next decade
Mapping of AlxGa1–xAs band edges by ballistic electron emission spectroscopy
We have employed ballistic electron emission microscopy (BEEM) to study the energy positions in the conduction band of AlxGa1 – xAs. Epilayers of undoped AlxGa1 – xAs were grown by molecular beam epitaxy on conductive GaAs substrates. The Al composition x took on values of 0, 0.11, 0.19, 0.25, 0.50, 0.80 and 1 so that the material was examined in both the direct and indirect band gap regime. The AlxGa1 – xAs layer thickness was varied from 100 to 500 Å to ensure probing of bulk energy levels. Different capping layers and surface treatments were explored to prevent surface oxidation and examine Fermi level pinning at the cap layer/AlxGa1 – xAs interface. All samples were metallized ex situ with a 100 Å Au layer so that the final BEEM structure is of the form Au/capping layer/AlxGa1 – xAs/bulk GaAs. Notably we have measured the Schottky barrier height for Au on AlxGa1 – xAs. We have also probed the higher lying band edges such as the X point at low Al concentrations and the L point at high Al concentrations. Variations of these critical energy positions with Al composition x were mapped out in detail and compared with findings from other studies. Local variations of these energy positions were also examined and found to be on the order of 30–50 meV. The results of this study suggest that BEEM can provide accurate positions for multiple energy levels in a single semiconductor structure
Factorization theorems for exclusive heavy-quarkonium production
We outline the proofs of the factorization theorems for exclusive two-body
charmonium production in B-meson decay and e^+e^- annihilation to all orders in
perturbation theory in quantum chromodynamics. We find that factorized
expressions hold up to corrections of order m_c/m_b in B-meson decay and
corrections of order m_c^2/s in e^+e^- annihilation, where m_c is the
charm-quark mass, m_b is the bottom-quark mass, and root-s is the e^+e^-
center-of-momentum energy.Comment: 4 pages, 2 figure
Factorization of low-energy gluons in exclusive processes
We outline a proof of factorization in exclusive processes, taking into
account the presence of soft and collinear modes of arbitrarily low energy,
which arise when the external lines of the process are taken on shell.
Specifically, we examine the process of e^+e^- annihilation through a virtual
photon into two light mesons. In an intermediate step, we establish a
factorized form that contains a soft function that is free of collinear
divergences. In contrast, in soft-collinear effective theory, the low-energy
collinear modes factor most straightforwardly into the soft function. We point
out that the cancellation of the soft function, which relies on the
color-singlet nature of the external hadrons, fails when the soft function
contains low-energy collinear modes.Comment: 18 pages, 10 figures, 2 tables, version published in Physical Review
Reflection high-energy electron diffraction studies of the growth of lnAs/Ga_(1-x)In_xSb strained-layer superlattices
We have used reflection high‐energy electron diffraction to study the surface periodicity of the growth front of InAs/GaInSb strained‐layer superlattices (SLSs). We found that the apparent surface lattice spacing reproducibly changed during layers which subsequent x‐ray measurements indicated were coherently strained. Abrupt changes in the measured streak spacings were found to be correlated to changes in the growth flux. The profile of the dynamic streak spacing was found to be reproducible when comparing consecutive periods of a SLSs or different SLSs employing the same shuttering scheme at the InAs/GaInSb interface. Finally, when the interface shuttering scheme was changed, it was found that the dynamic streak separation profile also changed. Large changes in the shuttering scheme led to dramatic differences in the streak separation profile, and small changes in the shuttering scheme led to minor changes in the profile. In both cases, the differences in the surface periodicity profile occurred during the parts of the growth where the incident fluxes differed
QCD Factorization for Semi-Inclusive Deep-Inelastic Scattering at Low Transverse Momentum
We demonstrate a factorization formula for semi-inclusive deep-inelastic
scattering with hadrons in the current fragmentation region detected at low
transverse momentum. To facilitate the factorization, we introduce the
transverse-momentum dependent parton distributions and fragmentation functions
with gauge links slightly off the light-cone, and with soft-gluon radiations
subtracted. We verify the factorization to one-loop order in perturbative
quantum chromodynamics and argue that it is valid to all orders in perturbation
theory.Comment: 28 pages, figures include
Factorization in exclusive quarkonium production
We present factorization theorems for two exclusive heavy-quarkonium
production processes: production of two quarkonia in e^+e^- annihilation and
production of a quarkonium and a light meson in B-meson decays. We describe the
general proofs of factorization and supplement them with explicit one-loop
analyses, which illustrate some of the features of the soft-gluon
cancellations. We find that violations of factorization are generally
suppressed relative to the factorized contributions by a factor v^2m_c/Q for
each S-wave charmonium and a factor m_c/Q for each L-wave charmonium with L>0.
Here, v is the velocity of the heavy quark or antiquark in the quarkonium rest
frame, Q=sqrt{s} for e^+e^- annihilation, Q=m_B for B-meson decays, sqrt{s} is
the e^+e^- center-of-momentum energy, m_c is the charm-quark mass, and m_B is
the B-meson mass. There are modifications to the suppression factors if
quantum-number restrictions apply for the specific process.Comment: 69 pages, 12 figures, 2 tables. v2: Version published in Physical
Review
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