256 research outputs found
Atomistic Simulations of the Efficiencies of Ge and Pt Ion Implantation into Graphene
Recent success in the direct implantation of 74Ge+ ion, the heaviest atomic impurity to date, into monolayer graphene presents a general question of the efficiency of low-energy ion implantation technique for heavy atoms. A comparative computational study, using classical molecular dynamics, of low-energy Ge and Pt ions implantation into single- and double-layer graphene is presented. It confirms that the highest probability for the perfect substitutional doping of single-layer graphene, i.e., direct implanting of ion into monovacancy, can be achieved 80 eV and it reaches the value of 64% for Ge ions directed at 45° angle to graphene plane and 21% for Pt ion beam perpendicular to graphene. Implantation efficiency is strongly dependent on the angle of ion beam. The sputtering yield of carbon atoms is found to be lower for double layer of graphene, which has better protective properties against low-energy ion irradiation damage than a single graphene layer. In double-layer graphene, incident ions traveling in the direction perpendicular to graphene can be trapped between the layers with the highest efficiency above or equal to 80% in the energy range of 40–90 eV for Ge ions and above 90% in the energy range of 40–70 eV for Pt ions. The energy range corresponding to the efficient trapping of ions in double-layer graphene is shifted toward higher energies upon tilting of the angle of incident ion beam
Construction of Non-Perturbative, Unitary Particle-Antiparticle Amplitudes for Finite Particle Number Scattering Formalisms
Starting from a unitary, Lorentz invariant two-particle scattering amplitude
, we show how to use an identification and replacement process to construct a
unique, unitary particle-antiparticle amplitude. This process differs from
conventional on-shell Mandelstam s,t,u crossing in that the input and
constructed amplitudes can be off-diagonal and off-energy shell. Further,
amplitudes are constructed using the invariant parameters which are appropriate
to use as driving terms in the multi-particle, multichannel non-perturbative,
cluster decomposable, relativistic scattering equations of the Faddeev-type
integral equations recently presented by Alfred, Kwizera, Lindesay and Noyes.
It is therefore anticipated that when so employed, the resulting multi-channel
solutions will also be unitary. The process preserves the usual
particle-antiparticle symmetries. To illustrate this process, we construct a
J=0 scattering length model chosen for simplicity. We also exhibit a class of
physical models which contain a finite quantum mass parameter and are Lorentz
invariant. These are constructed to reduce in the appropriate limits, and with
the proper choice of value and sign of the interaction parameter, to the
asymptotic solution of the non-relativistic Coulomb problem, including the
forward scattering singularity, the essential singularity in the phase, and the
Bohr bound-state spectrum
The Dynamics of Zeroth-Order Ultrasensitivity: A Critical Phenomenon in Cell Biology
It is well known since the pioneering work of Goldbeter and Koshland [Proc.
Natl. Acad. Sci. USA, vol. 78, pp. 6840-6844 (1981)] that cellular
phosphorylation- dephosphorylation cycle (PdPC), catalyzed by kinase and
phosphatase under saturated condition with zeroth order enzyme kinetics,
exhibits ultrasensitivity, sharp transition. We analyse the dynamics aspects of
the zeroth order PdPC kinetics and show a critical slowdown akin to the phase
transition in condensed matter physics. We demonstrate that an extremely
simple, though somewhat mathematically "singular" model is a faithful
representation of the ultrasentivity phenomenon. The simplified mathematical
model will be valuable, as a component, in developing complex cellular
signaling network theory as well as having a pedagogic value.Comment: 8 pages, 3 figure
Multiphoton Transitions in a Spin System Driven by Strong Bichromatic Field
EPR transient nutation spectroscopy is used to measure the effective field
(Rabi frequency) for multiphoton transitions in a two-level spin system
bichromatically driven by a transverse microwave (MW) field and a longitudinal
radio-frequency (RF) field. The behavior of the effective field amplitude is
examined in the case of a relatively strong MW field, when the derivation of
the effective Hamiltonian cannot be reduced to first-order perturbation theory
in w_{1} / w_{rf} (w_{1} is the microwave Rabi frequency, w_{rf} is the RF
frequency). Experimental results are consistently interpreted by taking into
account the contributions of second and third order in w_{1} / w_{rf} evaluated
by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously
broadened EPR line, the third-order correction modifies the nutation frequency,
while the second-order correction gives rise to a change in the nutation
amplitude due to a Bloch-Siegert shift.Comment: 7 pages, 6 figure
Energy state distributions of the P(b) centers at the (100), (110), and (111) Si/SiO(2) interfaces investigated by Laplace deep level transient spectroscopy
The energy distribution of the P(b) centers at the Si/SiO(2) interface has been determined using isothermal laplace deep level transient spectroscopy. For the (111) and (110) interface orientations, the distributions are similar and centered at 0.38 eV below the silicon conduction band. This is consistent with only P(b0) states being present. For the (100) orientation, two types of the interface states are observed: one similar to the (111) and (110) orientations while the other has a negative-U character in which the emission rate versus surface potential dependence is qualitatively different from that observed for P(b0) and is presumed to be P(b1). (C) 2008 American Institute of Physics. (DOI: 10.1063/1.2939001
Zn doping into InP induced by Nd : YAG continuous wave laser
Nd:YAG continuous wave (CW) laser (lambda = 1.06 mum) was used to irradiate Zn film deposited by evaporation deposition on the n-type InP substrates. The PN junction was obtained. The depth of the junctions and the doping distribution dependence upon the irradiation time and the irradiation power density were investigated. The relation between the depths of the junctions and the thickness of Zn deposition is linear. The hole concentration of uniform distribution, shallow junction, and heavy doping concentration are attained (10(19)-10(20) cm(-3)). The primary mechanism of Zn doping is considered that alloy junctions form after irradiated. (C) 2002 Elsevier Science Ltd. All rights reserved
Tunable local polariton modes in semiconductors
We study the local states within the polariton bandgap that arise due to deep
defect centers with strong electron-phonon coupling. Electron transitions
involving deep levels may result in alteration of local elastic constants. In
this case, substantial reversible transformations of the impurity polariton
density of states occur, which include the appearance/disappearance of the
polariton impurity band, its shift and/or the modification of its shape. These
changes can be induced by thermo- and photo-excitation of the localized
electron states or by trapping of injected charge carriers. We develop a simple
model, which is applied to the center in . Further possible
experimental realizations of the effect are discussed.Comment: 7 pages, 3 figure
Effective Field and the Bloch-Siegert Shift at Bihromatic Excitation of Multiphoton EPR
The dynamics of multiphoton transitions in a two-level spin system excited by
transverse microwave and longitudinal RF fields with the frequencies w_{mw} and
w_{rf}, respectively, is analyzed. The effective time-independent Hamiltonian
describing the "dressed" spin states of the "spin + bichromatic field" system
is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The
direct detection of the time behavior of the spin system by the method of
nonstationary nutations makes it possible to identify the multiphoton
transitions for resonances w_{0} = w_{mw} + rw_{rf} (w_{0} is the central
frequency of the EPR line, r = 1, 2), to measure the amplitudes of the
effective fields of these transitions, and to determine the features generated
by the inhomogeneous broadening of the EPR line. It is shown that the
Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening
of spectral lines reduce only the nutation amplitude but do not change their
frequencies.Comment: 6 pages, 5 figure
Singularly Perturbed Monotone Systems and an Application to Double Phosphorylation Cycles
The theory of monotone dynamical systems has been found very useful in the
modeling of some gene, protein, and signaling networks. In monotone systems,
every net feedback loop is positive. On the other hand, negative feedback loops
are important features of many systems, since they are required for adaptation
and precision. This paper shows that, provided that these negative loops act at
a comparatively fast time scale, the main dynamical property of (strongly)
monotone systems, convergence to steady states, is still valid. An application
is worked out to a double-phosphorylation ``futile cycle'' motif which plays a
central role in eukaryotic cell signaling.Comment: 21 pages, 3 figures, corrected typos, references remove
Deep-level Transient Spectroscopy of GaAs/AlGaAs Multi-Quantum Wells Grown on (100) and (311)B GaAs Substrates
Si-doped GaAs/AlGaAs multi-quantum wells structures grown by molecular beam epitaxy on (100) and (311)B GaAs substrates have been studied by using conventional deep-level transient spectroscopy (DLTS) and high-resolution Laplace DLTS techniques. One dominant electron-emitting level is observed in the quantum wells structure grown on (100) plane whose activation energy varies from 0.47 to 1.3 eV as junction electric field varies from zero field (edge of the depletion region) to 4.7 × 106 V/m. Two defect states with activation energies of 0.24 and 0.80 eV are detected in the structures grown on (311)B plane. The Ec-0.24 eV trap shows that its capture cross-section is strongly temperature dependent, whilst the other two traps show no such dependence. The value of the capture barrier energy of the trap at Ec-0.24 eV is 0.39 eV
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