8,168 research outputs found
Entropy-induced Microphase Separation in Hard Diblock Copolymers
Whereas entropy can induce phase behavior that is as rich as seen in
energetic systems, microphase separation remains a very rare phenomenon in
entropic systems. In this paper, we present a density functional approach to
study the possibility of entropy-driven microphase separation in diblock
copolymers. Our model system consists of copolymers composed of freely-jointed
slender hard rods. The two types of monomeric segments have comparable lengths,
but a significantly different diameter, the latter difference providing the
driving force for the phase separation. At the same time these systems can also
exhibit liquid crystalline phases. We treat this system in the appropriate
generalization of the Onsager approximation to chain-like particles. Using a
linear stability (bifurcation) analysis, we analytically determine the onset of
the microseparated and the nematic phases for long chains. We find that for
very long chains the microseparated phase always preempts the nematic. In the
limit of infinitely long chains, the correlations within the chain become
Gaussian and the approach becomes exact. This allows us to define a Gaussian
limit in which the theory strongly simplifies and the competition between
microphase separation and liquid crystal formation can be studied essentially
analytically. Our main results are phase diagrams as a function of the
effective diameter difference, the segment composition and the length ratio of
the segments. We also determine the amplitude of the positional order as a
function of position along the chain at the onset of the microphase separation
instability. Finally, we give suggestions as to how this type of
entropy-induced microphase separation could be observed experimentally.Comment: 16 pages, 7 figure
Preservation of equilibrium in orthograde and inverted body positions
The mechanism for regulation of the vertical pose with retention of equilibrium in the inverted body position was investigated
Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments
An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool
Weak localization, Aharonov-Bohm oscillations and decoherence in arrays of quantum dots
Combining scattering matrix theory with non-linear -model and Keldysh
technique we develop a unified theoretical approach enabling one to
non-perturbatively study the effect of electron-electron interactions on weak
localization and Aharonov-Bohm oscillations in arbitrary arrays of quantum
dots. Our model embraces (i) weakly disordered conductors (ii) strongly
disordered conductors and (iii) metallic quantum dots. In all these cases at the electron decoherence time is found to saturate to a finite value
determined by the universal formula which agrees quantitatively with numerous
experimental results. Our analysis provides overwhelming evidence in favor of
electron-electron interactions as a universal mechanism for zero temperature
electron decoherence in disordered conductors.Comment: 19 pages, 13 figures, invited paper, published in a special issue of
Fiz. Nizk. Temp. (Kharkov) dedicated to Prof. Igor Kuli
Efficiency of thermal relaxation by radiative processes in protoplanetary discs: constraints on hydrodynamic turbulence
Hydrodynamic, non-magnetic instabilities can provide turbulent stress in the
regions of protoplanetary discs, where the MRI can not develop. The induced
motions influence the grain growth, from which formation of planetesimals
begins. Thermal relaxation of the gas constrains origins of the identified
hydrodynamic sources of turbulence in discs.
We estimate the radiative relaxation timescale of temperature perturbations
and study the dependence of this timescale on the perturbation wavelength, the
location within the disc, the disc mass, and the dust-to-gas mass ratio. We
then apply thermal relaxation criteria to localise modes of the convective
overstability, the vertical shear instability, and the zombie vortex
instability.
Our calculations employed the latest tabulated dust and gas mean opacities
and we account for the collisional coupling to the emitting species.
The relaxation criterion defines the bulk of a typical T Tauri disc as
unstable to the development of linear hydrodynamic instabilities. The midplane
is unstable to the convective overstability from at most 2\mbox{ au} and up
to 40\mbox{ au}, as well as beyond 140\mbox{ au}. The vertical shear
instability can develop between 15\mbox{ au} and 180\mbox{ au}. The
successive generation of (zombie) vortices from a seeded noise can work within
the inner 0{.}8\mbox{ au}.
Dynamic disc modelling with the evolution of dust and gas opacities is
required to clearly localise the hydrodynamic turbulence, and especially its
non-linear phase.Comment: 13 pages, 8 figure
Thermal budget of superconducting digital circuits at sub-kelvin temperatures
Superconducting single-flux-quantum (SFQ) circuits have so far been developed
and optimized for operation at or above helium temperatures. The SFQ approach,
however, should also provide potentially viable and scalable control and
read-out circuits for Josephson-junction qubits and other applications with
much lower, milli-kelvin, operating temperatures. This paper analyzes the
overheating problem which becomes important in this new temperature range. We
suggest a thermal model of the SFQ circuits at sub-kelvin temperatures and
present experimental results on overheating of electrons and silicon substrate
which support this model. The model establishes quantitative limitations on the
dissipated power both for "local" electron overheating in resistors and
"global" overheating due to ballistic phonon propagation along the substrate.
Possible changes in the thermal design of SFQ circuits in view of the
overheating problem are also discussed.Comment: 10 pages, 8 figures, submitted to J. Appl. Phy
A Wire Position Monitor System for the 1.3 GHZ Tesla-Style Cryomodule at the Fermilab New-Muon-Lab Accelerator
The first cryomodule for the beam test facility at the Fermilab New-Muon-Lab
building is currently under RF commissioning. Among other diagnostics systems,
the transverse position of the helium gas return pipe with the connected 1.3
GHz SRF accelerating cavities is measured along the ~15 m long module using a
stretched-wire position monitoring system. An overview of the wire position
monitor system technology is given, along with preliminary results taken at the
initial module cool down, and during further testing. As the measurement system
offers a high resolution, we also discuss options for use as a vibration
detector.Comment: 4 pp. 15th International Conference on RF Superconductivity
(SRF2011). 25-29 Jul 2011. Chicago, Illinois, US
Determination of quantum-noise parameters of realistic cavities
A procedure is developed which allows one to measure all the parameters
occurring in a complete model [A.A. Semenov et al., Phys. Rev. A 74, 033803
(2006); quant-ph/0603043] of realistic leaky cavities with unwanted noise. The
method is based on the reflection of properly chosen test pulses by the cavity.Comment: 5 pages, 2 figure
Persistent current noise and electron-electron interactions
We analyze fluctuations of persistent current (PC) produced by a charged
quantum particle moving in a ring and interacting with a dissipative
environment formed by diffusive electron gas. We demonstrate that in the
presence of interactions such PC fluctuations persist down to zero temperature.
In the case of weak interactions and/or sufficiently small values of the ring
radius PC noise remains coherent and can be tuned by external magnetic flux
piercing the ring. In the opposite limit of strong interactions and/or
large values of fluctuations in the electronic bath strongly suppress
quantum coherence of the particle down to and induce incoherent
-independent current noise in the ring which persists even at
when the average PC is absent.Comment: 12 pages, 8 figure
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