6,728 research outputs found
Super-slippery Carbon Nanotubes: Symmetry Breaking breaks friction
The friction between the walls of multi-wall carbon nanotubes is shown to be
extremely low in general, with important details related to the specific choice
of the walls. This is governed by a simple expression revealing that the
phenomenon is a profound consequence of the specific symmetry breaking:
super-slippery sliding of the incommensurate walls is a Goldstone mode. Three
universal principles of tribology, offering a recipe for the lubricant
selection are emphasized.Comment: 4 pages, 2 figures, 1 table; pdf available from:
http://www.ff.bg.ac.yu/qmf/qsg_e.ht
Symmetry Based Properties of the Transition Metal Dichalcogenide Nanotubes
The full geometrical symmetry groups (the line groups) of the monolayered,
2Hb and 3R polytypes of the inorganic MoS2 and WS2 micro- and nanotubes of
arbitrary chirality are found. This is used to find the coordinates of the
representative atoms sufficient to determine completely geometrical structure
of tubes. Then some physical properties which can be deduced from the symmetry
are discussed: electron band degeneracies, selection rules, general forms of
the second rank tensors and potentials, phonon spectra.Comment: 6 pages 1 figur
Performance Improvement of QPSK Signal Predetection EGC Diversity Receiver
This paper proposes a modification of quadrature phase-shift-keying (QPSK) signal diversity reception with predetection equal gain combiner (EGC). The EGC combining is realized by using the constant modulus algorithm (CMA). Carrier synchronization is performed by the phase locked loop (PLL). Comparative analysis of the modified and ordinary diversity receiver in the presence of carrier frequency offset in the additive white Gaussian noise (AWGN) channel, as well as in Rician fading channel is shown. The proposed diversity receiver allows significant frequency offset compared to the diversity receiver that uses only PLL, and the error probability of the proposed receiver is very close to the error probability of the receiver with only PLL and zero frequency offset. The functionality of the proposed diversity receiver, as well as its properties is experimentally verified on a system based on universal software radio peripheral (USRP) hardware. The performed comparison confirms the expected behavior of the system
Surgery, selection or biology in lung metastasectomy for colorectal lung metastasectomy? PulMiCC may not provide all the answers but it does narrow down the question
Irreducible Representations of Diperiodic Groups
The irreducible representations of all of the 80 diperiodic groups, being the
symmetries of the systems translationally periodical in two directions, are
calculated. To this end, each of these groups is factorized as the product of a
generalized translational group and an axial point group. The results are
presented in the form of the tables, containing the matrices of the irreducible
representations of the generators of the groups. General properties and some
physical applications (degeneracy and topology of the energy bands, selection
rules, etc.) are discussed.Comment: 30 pages, 5 figures, 28 tables, 18 refs, LaTex2.0
PulMiCC and data from other randomized controlled trials challenge the usefulness of metastasectomy
Effects of Imperfect Reference Signal Recovery on Performance of SC and SSC Receivers over Generalized Fading Channels
This paper presents the study of the effects of imperfect reference signal recovery on the bit error rate (BER) performance of dual-branch switch-and-stay combining (SSC) and multibranch selection combining (SC) receivers in a generalized - fading channel. The average BER of binary and quaternary phase shift keying (BPSK and QPSK) is derived under the assumption that the reference carrier signal is extracted from the received modulated signal. For SSC receiver the optimal switching threshold (in a minimum BER sense) is numerically evaluated. Hereby we determine and discuss the simultaneous influence of the average signal-to-noise ratio (SNR) per bit, fading severity, product phase-locked loop (PLL) bandwidthbit duration (B_LT_b), switching threshold of SSC and diversity order of SC on BER performance. The influence of B_LT_b in different channel conditions and modulation formats is pointed out. The numerical results are confirmed by computer simulations
Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies
The generation of harmonics by atoms or ions in a two-color, coplanar field
configuration with commensurate frequencies is investigated through both, an
analytical calculation based on the Lewenstein model and the numerical ab
initio solution of the time-dependent Schroedinger equation of a
two-dimensional model ion. Through the analytical model, selection rules for
the harmonic orders in this field configuration, a generalized cut-off for the
harmonic spectra, and an integral expression for the harmonic dipole strength
is provided. The numerical results are employed to test the predictions of the
analytical model. The scaling of the cut-off as a function of both, one of the
laser intensities and frequency ratio , as well as entire spectra for
different and laser intensities are presented and analyzed. The
theoretical cut-off is found to be an upper limit for the numerical results.
Other discrepancies between analytical model and numerical results are
clarified by taking into account the probabilities of the absorption processes
involved.Comment: 8 figure
Five-year survival of patients in control groups of randomized controlled trials is much higher than that assumed in observational study reports
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