14,948 research outputs found
Exact Solutions to Sourceless Charged Massive Scalar Field Equation on Kerr-Newman Background
The separated radial part of a sourceless massive complex scalar field
equation on the Kerr-Newman black hole background is shown to be a generalized
spin-weighted spheroidal wave equation of imaginary number order. While the
separated angular part is an ordinary spheroidal wave equation. General exact
solutions in integral forms and in power series expansion as well as several
special solutions with physical interest are given for the radial equation in
the non-extreme case. In the extreme case, power series solution to the radial
equation is briefly studied. Recurrence relations between coefficients in power
series expansion of general solutions and connection between the radial
equation are discussed in both cases.Comment: 22 Pages, in LaTex, no figure, to appear in J. Math. Phy
Differential space-time block-coded OFDMA for frequency-selective fading channels
Combining differential Alamouti space-time block code (DASTBC) with orthogonal frequency-division multiple access (OFDMA), this paper introduces a multiuser/multirate transmission scheme, which allows full-rate and full-diversity noncoherent communications using two transmit antennas over frequency-selective fading channels. Compared with the existing differential space-time coded OFDM designs, our scheme imposes 10 restrictions on signal constellations, and thus can improve the spectral efficiency by exploiting efficient modulation techniques such as QAM, APSK etc. The main principles of our design are s follows: OFDMA eliminates multiuser interference, and converts multiuser environments to single-user ones; Space-time coding achieves performance improvement by exploiting space diversity available with multiple antennas, no matter whether channel state information is known to the receiver. System performance is evaluated both analytically and with simulations
Sudden stoppage of rotor in a thermally driven rotary motor made from double-walled carbon nanotubes
In a thermally driven rotary motor made from double-walled carbon nanotubes, the rotor (inner tube) can be actuated to rotate within the stator (outer tube) when the environmental temperature is high enough. A sudden stoppage of the rotor can occur when the inner tube has been actuated to rotate at a stable high speed. To find the mechanisms of such sudden stoppages, eight motor models with the same rotor but different stators are built and simulated in the canonical NVT ensembles. Numerical results demonstrate that the sudden stoppage of the rotor occurs when the difference between radii is near 0.34 nm at a high environmental temperature. A smaller difference between radii does not imply easier activation of the sudden rotor stoppage. During rotation, the positions and electron density distribution of atoms at the ends of the motor show that a sp(1) bonded atom on the rotor is attracted by the sp(1) atom with the biggest deviation of radial position on the stator, after which they become two sp(2) atoms. The strong bond interaction between the two atoms leads to the loss of rotational speed of the rotor within 1 ps. Hence, the sudden stoppage is attributed to two factors: the deviation of radial position of atoms at the stator's ends and the drastic thermal vibration of atoms on the rotor in rotation. For a stable motor, sudden stoppage could be avoided by reducing deviation of the radial position of atoms at the stator's ends. A nanobrake can be, thus, achieved by adjusting a sp(1) atom at the ends of stator to stop the rotation of rotor quickly.The authors are grateful for financial support from the National Natural-Science-Foundation of China (Grant Nos. 50908190, 11372100)
Non-existence of New Quantum Ergosphere Effect of a Vaidya-type Black Hole
Hawking evaporation of Dirac particles and scalar fields in a Vaidya-type
black hole is investigated by the method of generalized tortoise coordinate
transformation. It is shown that Hawking radiation of Dirac particles does not
exist for components but for components in any
Vaidya-type black holes. Both the location and the temperature of the event
horizon change with time. The thermal radiation spectrum of Dirac particles is
the same as that of Klein-Gordon particles. We demonstrates that there is no
new quantum ergosphere effect in the thermal radiation of Dirac particles in
any spherically symmetry black holes.Comment: Latex, 9 pages, no figure, submitted to Mod. Phys. Lett.
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