16,420 research outputs found
Effect of electron-phonon interaction on spectroscopies in graphene
We calculate the effect of the electron-phonon interaction on the electronic
density of states (DOS), the quasiparticle properties and on the optical
conductivity of graphene. In metals with DOS constant on the scale of phonon
energies, the electron-phonon renormalizations drop out of the dressed DOS,
however, due to the Dirac nature of the electron dynamics in graphene, the band
DOS is linear in energy and phonon structures remain, which can be emphasized
by taking an energy derivative. There is a shift in the chemical potential and
in the position in energy of the Dirac point. Also, the DOS can be changed from
a linear dependence out of value zero at the Dirac point to quadratic out of a
finite value. The optical scattering rate sets the energy scale for
the rise of the optical conductivity from its universal DC value
(expected in the simplest theory when chemical potential and temperature are
both ) to its universal AC background value . As in ordinary metals the DC conductivity remains unrenormalized
while its AC value is changed. The optical spectral weight under the intraband
Drude is reduced by a mass renormalization factor as is the effective
scattering rate. Optical weight is transferred to an Holstein phonon-assisted
side band. Due to Pauli blocking the interband transitions are sharply
suppressed, but also nearly constant, below twice the value of renormalized
chemical potential and also exhibit a phonon-assisted contribution. The
universal background conductivity is reduced below at large
energies.Comment: 22 pages, 19 figures, submitted to PR
Identification and verification of frequency-domain models for XV-15 tilt-rotor aircraft dynamics
Frequency-domain methods are used to extract the open-loop dynamics of the XV-15 tilt-rotor aircraft from flight test data for the cruise condition (V = 170 knots). The frequency responses are numerically fitted with transfer-function forms to identify equivalent model characteristics. The associated handling quality parameters meet or exceed Level 2, Category A, requirements for fixed-wing military aircraft. Step response matching is used to verify the time-domain fidelity of the transfer-function models for the cruise and hover flight conditions. The transient responses of the model and aircraft are in close agreement in all cases, except for the normal acceleration response to elevator deflection in cruise. This discrepancy is probably due to the unmodeled rotor rpm dynamics. The utility of the frequency-domain approach for dynamics identification and analysis is clearly demonstrated
Central Charge and the Andrews-Bailey Construction
From the equivalence of the bosonic and fermionic representations of
finitized characters in conformal field theory, one can extract mathematical
objects known as Bailey pairs. Recently Berkovich, McCoy and Schilling have
constructed a `generalized' character formula depending on two parameters \ra
and , using the Bailey pairs of the unitary model . By taking
appropriate limits of these parameters, they were able to obtain the characters
of model , model , and the unitary model with
central charge . In this letter we computed the effective
central charge associated with this `generalized' character formula using a
saddle point method. The result is a simple expression in dilogarithms which
interpolates between the central charges of these unitary models.Comment: Latex2e, requires cite.sty package, 13 pages. Additional footnote,
citation and reference
Electric field assisted annealing and formation of prominent deep-level defect in ion-implanted n-type 4H-SiC
High-purity and low-doped n -type epitaxial layers of 4H-SiC have been implanted with N and C ions by using energies in the MeV range and doses from 2×10⁸to1×10⁹cm⁻² . Postimplant annealing was performed at 1100°C prior to sample analysis by deep-level transient spectroscopy (DLTS). A drastic and irreversible instability of the prominent EH7 deep-level defect occurs during the first DLTS temperature scan because of the electric field applied during the measurements. Depending on the implanted species, EH7 can decrease (N implants) as well as increase (C implants) in strength and the effect is attributed to charge-state controlled annealing and formation processes of EH7. The origin of EH7 is discussed and the experimental data support a model invoking interstitial C atoms.The authors thank the Australian Research Council and
Norwegian Research Council
Singular behaviour of the electromagnetic field
The singularities of the electromagnetic field are derived to include all the
point-like multipoles representing an electric charge and current distribution.
Firstly derived in the static case, the result is generalized to the dynamic
one. We establish a simple procedure for passing from the first, to the second
case.Comment: Latex, 21.pages, no figure
Eigenvector Expansion and Petermann Factor for Ohmically Damped Oscillators
Correlation functions in ohmically damped
systems such as coupled harmonic oscillators or optical resonators can be
expressed as a single sum over modes (which are not power-orthogonal), with
each term multiplied by the Petermann factor (PF) , leading to "excess
noise" when . It is shown that is common rather than
exceptional, that can be large even for weak damping, and that the PF
appears in other processes as well: for example, a time-independent
perturbation \sim\ep leads to a frequency shift \sim \ep C_j. The
coalescence of () eigenvectors gives rise to a critical point, which
exhibits "giant excess noise" (). At critical points, the
divergent parts of contributions to cancel, while time-independent
perturbations lead to non-analytic shifts \sim \ep^{1/J}.Comment: REVTeX4, 14 pages, 4 figures. v2: final, 20 single-col. pages, 2
figures. Streamlined with emphasis on physics over formalism; rewrote Section
V E so that it refers to time-dependent (instead of non-equilibrium) effect
Quasi-Normal Mode Expansion for Linearized Waves in Gravitational Systems
The quasinormal modes (QNM's) of gravitational systems modeled by the
Klein-Gordon equation with effective potentials are studied in analogy to the
QNM's of optical cavities. Conditions are given for the QNM's to form a
complete set, i.e., for the Green's function to be expressible as a sum over
QNM's, answering a conjecture by Price and Husain [Phys. Rev. Lett. {\bf 68},
1973 (1992)]. In the cases where the QNM sum is divergent, procedures for
regularization are given. The crucial condition for completeness is the
existence of spatial discontinuities in the system, e.g., the discontinuity at
the stellar surface in the model of Price and Husain.Comment: 12 pages, WUGRAV-94-
Quantum authentication with unitary coding sets
A general class of authentication schemes for arbitrary quantum messages is
proposed. The class is based on the use of sets of unitary quantum operations
in both transmission and reception, and on appending a quantum tag to the
quantum message used in transmission. The previous secret between partners
required for any authentication is a classical key. We obtain the minimal
requirements on the unitary operations that lead to a probability of failure of
the scheme less than one. This failure may be caused by someone performing a
unitary operation on the message in the channel between the communicating
partners, or by a potential forger impersonating the transmitter.Comment: RevTeX4, 10 page
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