913 research outputs found
Performance of a Large Diameter Tunnel in Weak Rocks
The correlation of tunnel movement versus rock mass quality was investigated using actual monitored data as well as theoretical studies. Results revealed that meaningful empirical correlation between the commonly used rock mass rating system and tunnel deformation can be obtained only if geological structure and in-situ stresses are taken into account. In this respect, the commonly used rock mass rating system is not very suitable for such purpose. A new parameter using rock mass strength normalized by in-situ stress level appears to be more suitable for establishing the relationship between tunnel deformation and rock mass quality
In Situ deposition of YBCO high-T(sub c) superconducting thin films by MOCVD and PE-MOCVD
Metalorganic Chemical Vapor Deposition (MOCVD) offers the advantages of a high degree of compositional control, adaptability for large scale production, and the potential for low temperature fabrication. The capability of operating at high oxygen partial pressure is particularly suitable for in situ formation of high temperature superconducting (HTSC) films. Yttrium barium copper oxide (YBCO) thin films having a sharp zero-resistance transition with T( sub c) greater than 90 K and Jc approx. 10 to the 4th power A on YSZ have been prepared, in situ, at a substrate temperature of about 800 C. Moreover, the ability to form oxide films at low temperature is very desirable for device applications of HTSC materials. Such a process would permit the deposition of high quality HTSC films with a smooth surface on a variety of substrates. Highly c-axis oriented, dense, scratch resistant, superconducting YBCO thin films with mirror-like surfaces have been prepared, in situ, at a reduced substrate temperature as low as 570 C by a remote microwave-plasma enhanced metalorganic chemical vapor deposition (PE-MOCVD) process. Nitrous oxide was used as a reactant gas to generate active oxidizing species. This process, for the first time, allows the formation of YBCO thin films with the orthorhombic superconducting phase in the as-deposited state. The as-deposited films grown by PE-MOCVD show attainment of zero resistance at 72 K with a transition width of about 5 K. MOCVD was carried out in a commercial production scale reactor with the capability of uniform deposition over 100 sq cm per growth run. Preliminary results indicate that PE-MOCVD is a very attractive thin film deposition process for superconducting device technology
Propagating Coherent Acoustic Phonon Wavepackets in InMnAs/GaSb
We observe pronounced oscillations in the differential reflectivity of a
ferromagnetic InMnAs/GaSb heterostructure using two-color pump-probe
spectroscopy. Although originally thought to be associated with the
ferromagnetism, our studies show that the oscillations instead result from
changes in the position and frequency-dependent dielectric function due to the
generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and
their subsequent propagation into the GaSb. Our theory accurately predicts the
experimentally measured oscillation period and decay time as a function of
probe wavelength.Comment: 4 pages, 4 figure
In-situ deposition of YBCO high-Tc superconducting thin films by MOCVD and PE-MOCVD
Metal-Organic Chemical Vapor Deposition (MOCVD) offers the advantages of a high degree of compositional control, adaptability for large scale production, and the potential for low temperature fabrication. The capability of operating at high oxygen partial pressure is particularly suitable for in situ formation of high temperature superconducting (HTSC) films. Yttrium barium copper oxide (YBCO) thin films having a sharp zero-resistance transition with T(sub c) greater than 90 K and J(sub c) of approximately 10(exp 4) A on YSZ have been prepared, in situ, at a substrate temperature of about 800 C. Moreover, the ability to form oxide films at low temperature is very desirable for device applications of HTSC materials. Such a process would permit the deposition of high quality HTSC films with a smooth surface on a variety of substrates. Highly c-axis oriented, dense, scratch resistant, superconducting YBCO thin films with mirror-like surfaces have been prepared, in situ, at a reduced substrate temperature as low as 570 C by a remote microwave-plasma enhanced metal-organic chemical vapor deposition (PE-MOCVD) process. Nitrous oxide was used as a reactant gas to generate active oxidizing species. This process, for the first time, allows the formation of YBCO thin films with the orthorhombic superconducting phase in the as-deposited state. The as-deposited films grown by PE-MOCVD show attainment of zero resistance at 72 K with a transition width of about 5 K. MOCVD was carried out in a commercial production scale reactor with the capability of uniform deposition over 100 sq cm per growth run. Preliminary results indicate that PE-MOCVD is a very attractive thin film deposition process for superconducting device technology
Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes
Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one
can generate and detect coherent phonons in chirality-specific semiconducting
single-walled carbon nanotubes. The signals are resonantly enhanced when the
pump photon energy coincides with an interband exciton resonance, and analysis
of such data provides a wealth of information on the chirality-dependence of
light absorption, phonon generation, and phonon-induced band structure
modulations. To explain our experimental results, we have developed a
microscopic theory for the generation and detection of coherent phonons in
single-walled carbon nanotubes using a tight-binding model for the electronic
states and a valence force field model for the phonons. We find that the
coherent phonon amplitudes satisfy a driven oscillator equation with the
driving term depending on photoexcited carrier density. We compared our
theoretical results with experimental results on mod 2 nanotubes and found that
our model provides satisfactory overall trends in the relative strengths of the
coherent phonon signal both within and between different mod 2 families. We
also find that the coherent phonon intensities are considerably weaker in mod 1
nanotubes in comparison with mod~2 nanotubes, which is also in excellent
agreement with experiment.Comment: 21 pages, 22 figure
Anti-self-dual Riemannian metrics without Killing vectors, can they be realized on K3?
Explicit Riemannian metrics with Euclidean signature and anti-self dual
curvature that do not admit any Killing vectors are presented. The metric and
the Riemann curvature scalars are homogenous functions of degree zero in a
single real potential and its derivatives. The solution for the potential is a
sum of exponential functions which suggests that for the choice of a suitable
domain of coordinates and parameters it can be the metric on a compact
manifold. Then, by the theorem of Hitchin, it could be a class of metrics on
, or on surfaces whose universal covering is .Comment: Misprints in eqs.(9-11) corrected. Submitted to Classical and Quantum
Gravit
Selfsimilar solutions in a sector for a quasilinear parabolic equation
We study a two-point free boundary problem in a sector for a quasilinear
parabolic equation. The boundary conditions are assumed to be spatially and
temporally "self-similar" in a special way. We prove the existence, uniqueness
and asymptotic stability of an expanding solution which is self-similar at
discrete times. We also study the existence and uniqueness of a shrinking
solution which is self-similar at discrete times.Comment: 23 page
Spinor formulation of topologically massive gravity
In the framework of real 2-component spinors in three dimensional space-time
we present a description of topologically massive gravity (TMG) in terms of
differential forms with triad scalar coefficients. This is essentially a real
version of the Newman-Penrose formalism in general relativity. A triad
formulation of TMG was considered earlier by Hall, Morgan and Perjes, however,
due to an unfortunate choice of signature some of the spinors underlying the
Hall-Morgan-Perjes formalism are real, while others are pure imaginary. We
obtain the basic geometrical identities as well as the TMG field equations
including a cosmological constant for the appropriate signature. As an
application of this formalism we discuss the Bianchi Type exact
solutions of TMG and point out that they are parallelizable manifolds. We also
consider various re-identifications of these homogeneous spaces that result in
black hole solutions of TMG.Comment: An expanded version of paper published in Classical and Quantum
Gravity 12 (1995) 291
Classification and nondegeneracy of Toda system with singular sources
We consider the following Toda system \Delta u_i + \D \sum_{j = 1}^n
a_{ij}e^{u_j} = 4\pi\gamma_{i}\delta_{0} \text{in}\mathbb R^2, \int_{\mathbb
R^2}e^{u_i} dx -1\delta_0a_{ij}\gamma_i=0\forall \;1\leq i\leq n\gamma_i+\gamma_{i+1}+...+\gamma_j \notin \mathbb Z1\leq i\leq
j\leq nu_i$ is \textit{radially symmetric} w.r.t. 0.
(iii) We prove that the linearized equation at any solution is
\textit{non-degenerate}. These are fundamental results in order to understand
the bubbling behavior of the Toda system.Comment: 28 page
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