251 research outputs found
State-space distribution and dynamical flow for closed and open quantum systems
We present a general formalism for studying the effects of dynamical
heterogeneity in open quantum systems. We develop this formalism in the state
space of density operators, on which ensembles of quantum states can be
conveniently represented by probability distributions. We describe how this
representation reduces ambiguity in the definition of quantum ensembles by
providing the ability to explicitly separate classical and quantum sources of
probabilistic uncertainty. We then derive explicit equations of motion for
state space distributions of both open and closed quantum systems and
demonstrate that resulting dynamics take a fluid mechanical form analogous to a
classical probability fluid on Hamiltonian phase space, thus enabling a
straightforward quantum generalization of Liouville's theorem. We illustrate
the utility of our formalism by analyzing the dynamics of an open two-level
system using the state-space formalism that are shown to be consistent with the
derived analytical results
Interpretation of the Veiling of the Photospheric Spectrum for T Tauri Stars in Terms of an Accretion Model
The problem on heating the atmospheres of T Tauri stars by radiation from an
accretion shock has been solved. The structure and radiation spectrum of the
emerging so-called hot spot have been calculated in the LTE approximation. The
emission not only in continuum but also in lines has been taken into account
for the first time when calculating the spot spectrum. Comparison with
observations has shown that the strongest of these lines manifest themselves as
narrow components of helium and metal emission lines, while the weaker ones
decrease significantly the depth of photospheric absorption lines, although
until now, this effect has been thought to be due to the emission continuum
alone. The veiling by lines changes the depth of different photospheric lines
to a very different degree even within a narrow spectral range. Therefore, the
nonmonotonic wavelength dependence of the degree of veiling r found for some
CTTS does not suggest a nontrivial spectral energy distribution of the veiling
continuum. In general, it makes sense to specify the degree of veiling r only
by providing the set of photospheric lines from which this quantity was
determined. We show that taking into account the contribution of lines to the
veiling of the photospheric spectrum can cause the existing estimates of the
accretion rate onto T Tauri stars to decrease by several times, with this being
also true for stars with a comparatively weakly veiled spectrum. Neglecting the
contribution of lines to the veiling can also lead to appreciable errors in
determining the effective temperature, interstellar extinction, radial
velocity, and vsin(i)
Negative-Mass Instability in Nonlinear Plasma Waves
The negative-mass instability (NMI), previously found in ion traps, appears as a distinct regime of the sideband instability in nonlinear plasma waves with trapped particles. As the bounce frequency of these particles decreases with the bounce action, bunching can occur if the action distribution is inverted in trapping islands. In contrast to existing theories that also infer instabilities from the anharmonicity of bounce oscillations, spatial periodicity of the islands turns out to be unimportant, and the particle distribution can be unstable even if it is at at the resonance. An analytical model is proposed which describes both single traps and periodic nonlinear waves and concisely generalizes the conventional description of the sideband instability in plasma waves. The theoretical results are supported by particle-in-cell simulations carried out for a regime accentuating the NMI effect
The Post-AGB Star IRAS 07253-2001: Pulsations, Long-Term Brightness Variability and Spectral Peculiarities
The observations and comprehensive study of intermediate initial mass stars
at the late stages of evolution, and after the asymptotic giant branch (AGB) in
particular, are of crucial importance to identify the common properties for the
stars of given group and to reveal binaries among them. This work aims to
investigate photometric and spectral peculiarities of a poorly studied post-AGB
candidate and infrared source IRAS 07253-2001. We present the new multicolour
photometry obtained with the telescopes of the Caucasian
mountain observatory and analyse it together with the data acquired by the All
Sky Automated Survey for SuperNovae. We report on the detection of multiperiod
brightness variability caused by pulsations. A beating of close periods, the
main one of 73 days and additional ones of 68 and 70 days, leads to amplitude
variations. We have also detected a long-term sine trend in brightness with a
period of nearly 1800 days. We suppose it to be orbital and IRAS 07253-2001 to
be binary. Based on new low-resolution spectroscopic data obtained with the
2.5-m telescope of the Caucasian mountain observatory in 2020 and 2023 in the
3500-7500 wavelength range we have identified spectral lines and
compiled a spectral atlas. We have found the [N II], [Ni II] and [S II]
forbidden emission lines in the spectrum and discuss their origin. The
H line has a variable double-peaked emission component. We have derived
preliminary estimates of the star's parameters and detected a variation of
radial velocity with a peak-to-peak amplitude of about 30 km s.Comment: 19 pages, 11 figures, 6 tables, Table 2 is only available in
electronic form, accepted to Astrophysical Bulleti
Fractional and unquantized dc voltage generation in THz-driven semiconductor superlattices
We consider the spontaneous creation of a dc voltage across a strongly
coupled semiconductor superlattice subjected to THz radiation. We show that the
dc voltage may be approximately proportional either to an integer or to a half-
integer multiple of the frequency of the applied ac field, depending on the
ratio of the characteristic scattering rates of conducting electrons. For the
case of an ac field frequency less than the characteristic scattering rates, we
demonstrate the generation of an unquantized dc voltage.Comment: 6 pages, 3 figures, RevTEX, EPSF. Revised version v3: corrected typo
Axiomatic geometrical optics, Abraham-Minkowski controversy, and photon properties derived classically
By restating geometrical optics within the field-theoretical approach, the
classical concept of a photon (and, more generally, any elementary excitation)
in arbitrary dispersive medium is introduced, and photon properties are
calculated unambiguously. In particular, the canonical and kinetic momenta
carried by a photon, as well as the two corresponding energy-momentum tensors
of a wave, are derived from first principles of Lagrangian mechanics. As an
example application of this formalism, the Abraham-Minkowski controversy
pertaining to the definitions of these quantities is resolved for linear waves
of arbitrary nature, and corrections to the traditional formulas for the photon
kinetic energy-momentum are found. Several other applications of axiomatic
geometrical optics to electromagnetic waves are also presented
Development of a Curved Layer LOM Process for Monolithic Ceramics and Ceramic Matrix Composites
A novel rapid prototyping technology incorporating a curved layer building style was developed. The new process, based on Laminated Object Manufacturing (LOM), was designed for efficient fabrication of curved layer structures made from ceramics and fiber reinforced composites. A new LOM machine was created, referred to as Curved Layer LOM. This new machine uses ceramic tapes and fiber prepregs as feedstocks and fabricates curved structures on a curved-layer by curved-layer basis. The output of the process is a three dimensional green ceramic that is capable of being processed to a seamless, fully dense ceramic using traditional techniques. A detailed description is made of the necessary software and hardware for this new process. Also reviewed is the development of ceramic preforms and accompanying process technology for net shape ceramic fabrication. Monolithic ceramic (SiC) and ceramic matrix composite (SiC/SiC) articles were fabricated using both the flat layer and curved layer LOM processes. For making curved layer objects, the curved process afforded the advantages of eliminated stair step effect, increased build speed, reduced waste, reduced need for decubing, and maintenance of continuous fibers in the direction of curvature
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