4,788 research outputs found
Phonons in intrinsic Josephson systems with parallel magnetic field
Subgap resonances in the I-V curves of layered superconductors are explained
by the coupling between Josephson oscillations and phonons with dispersion in
c-direction. In the presence of a magnetic field applied parallel to the layers
additional structures due to fluxon motion appear. Their coupling with phonons
is investigated theoretically and a shift of the phonon resonances in strong
magnetic fields is predicted.Comment: Invited Paper to the "2nd International Symposium on Intrinsic
Josephson Effects and Plasma Oscillations in High-Tc Superconductors", 22-24
August 2000, Sendai, Japan, to be published in Physica
The design and construction of the CAD-1 airship
The background history, design philosophy and Computer application as related to the design of the envelope shape, stress calculations and flight trajectories of the CAD-1 airship, now under construction by Canadian Airship Development Corporation are reported. A three-phase proposal for future development of larger cargo carrying airships is included
Subgap structures in the current-voltage characteristic of the intrinsic Josephson effect due to phonons
A modified RSJ-model for the coupling of intrinsic Josephson oscillations and
c-axis phonons in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta}
and Bi_2Sr_2CaCu_2O_{8+\delta} is deveoped. This provides a very good
explanation for recently reported subgap structures in the I-V-characteristic
of the c-axis transport. It turns out that the voltages of these structures
coincide with the eigenfrequencies of longitudinal optical phonons, providing a
new measurement technique for this quantity. The significantly enhanced
microwave emission at the subgap structures in both the GHz and THz region is
discussed.Comment: correction of minor misprints, revtex, 3 pages, two postscript
figures, aps, epsf, Contributed Paper to the "International Symposion on the
Intrinsic Josphson effect and THz Plasma Oscillations", 22-25 February 1997,
Sendai, Japan; to be published in Physica
Microscopic theory of the coupling of intrinsic Josephson oscillations and phonons
A microscopic theory for the coupling of intrinsic Josephson oscillations and
dispersive phonon branches in layered superconductors is developed. Thereby the
effect of phonons on the electronic c-axis transport enters through an
effective longitudinal dielectric function. This coupling provides an
explanation of recently observed subgap resonances in the --
curve of anisotropic cuprate superconductors forming a stack of short Josephson
junctions. Due to the finite dispersion these resonances can appear at
van-Hove-singularities of both optical and acoustical phonon branches,
explaining low-voltage structures in the I-V-characteristic, which are not
understood in phonon models without dispersion. In long junctions the
dispersion of collective electron-phonon modes parallel to the layers is
investigated.Comment: 4 pages, 3 figures, 1 table, espcrc2.sty, invited contribution to
"Materials and Mechanisms of Superconductivity and High Temperature
Superconductors VI - M2S-HTSC-VI", Houston, Texas, 20-25 Feb 2000, to appear
in Physica
Coupling of intrinsic Josephson oscillations in layered superconductors by charge fluctuations
The coupling of Josephson oscillations in layered superconductors is studied
with help of a tunneling Hamiltonian formalism. The general form of the current
density across the barriers between the superconducting layers is derived. The
induced charge fluctuations on the superconducting layers lead to a coupling of
the Josephson oscillations in different junctions. A simplified set of
equations is then used to study the non-linear dynamics of the system. In
particular the influence of the coupling on the current-voltage characteristics
is investigated and upper limits for the coupling strength are estimated from a
comparison with experiments on cuprate superconductors.Comment: To be published in proceedings of SPIE conference San Diego 199
Improving wellbore cement performance through the application of nano-materials for oil and gas integrity
It is the ultimate goal that the application of nano-materials in wellbore cement enhance not only the integrity of the cement, but also the public view of the petroleum industry. Wellbore integrity should be a paramount concern of all petroleum companies due to the potential severity of the consequences if integrity is not maintained. Ecosystems, animal and plant life, and even human life are at great risk if wellbore integrity fails. It is this fact that the motivation for this thesis is based upon. Nano silica is the nano-particle under investigation for this thesis. Furthermore, wellbore integrity will be improved upon by the addition of an elastomer particle. A new blend specifically designed for use in wellbore cement will be formulated. The mixture of nano silica and an elastomer particle will be used in combination to increase the cements resilience to failure. This thesis is an experimental investigation rather that purely hypothetical.;There have been proven results of the incorporation of nano silica into cement and concrete with the construction industry as the intended use. Furthermore, crumb rubber has also been tested and provided beneficial results once more for use in the construction industry. However, the incorporation of both these particles, in the concentrations chosen in class G cement for wellbore integrity has not been proposed.;There will be a series of tests performed by Halliburton with our oversight to properly test and analyze all key properties to determine if the proposed blend will be beneficial as a wellbore integrity means. The tests will include quantitative and API tests as well as non-API and qualitative tests. The ultimate defining parameter to draw conclusions from is the compressive strength test. However, an increase in strength along with a decrease in migration pathway formation potential is the primary goal of the proposed blend.;This research found that with the incorporation of 2% nano silica and 2% crumb rubber into class G cement, all critical properties were enhanced. These properties include an increase in compressive strength of 3.5% after 48 hours, 0% free fluid where the base case had .52%, a 35% decrease in thickening time (to 100 BC), a decrease in fluid loss by 50%, and a decrease in transition time (to 500 psi) of 15%. The combination of 2% nano silica and 2% crumb rubber has thus been confirmed to provide beneficial qualities to class G cement for wellbore integrity
Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors
The recently reported subgap structures observed in the current-voltage
characteristic of intrinsic Josephson junctions in the high-T_c superconductors
Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by
the coupling between c-axis phonons and Josephson oscillations. A model is
developed where c-axis lattice vibrations between adjacent superconducting
multilayers are excited by the Josephson oscillations in a resistive junction.
The voltages of the lowest structures correspond well to the frequencies of
longitudinal c-axis phonons with large oscillator strength in the two
materials, providing a new measurement technique for this quantity.Comment: 4 pages, 3 figures, revtex, aps, epsf, psfig. submitted to Physical
Review Letters, second version improved in detai
Static Charge Coupling of Intrinsic Josephson Junction
A microscopic theory for the coupling of intrinsic Josephson oscillations due
to charge fluctuations on the quasi two-dimensional superconducting layers is
presented. Thereby in close analogy to the normal state the effect of the
scalar potential on the transport current is taken into account consistently.
The dispersion of collective modes is derived and an estimate of the coupling
constant is given. It is shown that the correct treatment of the quasiparticle
current is essential in order to get the correct position of Shapiro steps. In
this case the influence of the coupling on dc-properties like the -curve
is negligible.Comment: 6 pages latex, 5 figures, espcrc2.sty, Invited Contribution to "2nd
International Symposiom on Intrinsic Josephson Effects and Plasma
Oscillations in High-TC Superconductors", 22-24 August, Sendai, Japan, to be
published in Physica
NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition
We demonstrate the operation of low-noise nano superconducting quantum
interference devices (SQUIDs) based on the high critical field and high
critical temperature superconductor YBaCuO (YBCO) as
ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The
nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have
been patterned by focused ion beam milling. This allows to precisely define the
lateral dimensions of the SQUIDs so as to achieve large magnetic coupling
between the nanoloop and individual MNPs. By means of focused electron beam
induced deposition, cobalt MNPs with typical size of several tens of nm have
been grown directly on the surface of the sensors with nanometric spatial
resolution. Remarkably, the nanoSQUIDs are operative over extremely broad
ranges of applied magnetic field (-1 T 1 T) and temperature (0.3
K 80 K). All these features together have allowed us to perform
magnetization measurements under different ambient conditions and to detect the
magnetization reversal of individual Co MNPs with magnetic moments (1 - 30)
. Depending on the dimensions and shape of the
particles we have distinguished between two different magnetic states yielding
different reversal mechanisms. The magnetization reversal is thermally
activated over an energy barrier, which has been quantified for the (quasi)
single-domain particles. Our measurements serve to show not only the high
sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these
sensors are exceptional magnetometers for the investigation of the properties
of individual nanomagnets
Voltage-flux-characteristics of asymmetric dc SQUIDs
We present a detailed analysis of voltage-flux V(Phi)-characteristics for
asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry
alpha_I in the critical currents of the two Josephson junctions, the minima in
the V(Phi)-characteristics for bias currents of opposite polarity are shifted
along the flux axis by Delta_Phi = (alpha_I)*(beta_L) relative to each other;
beta_L is the screening parameter. This simple relation allows the
determination of alpha_I in our experiments on YBa_2Cu_3O_(7-x} dc SQUIDs and
comparison with theory. Extensive numerical simulations within a wide range of
beta_L and noise parameter Gamma reveal a systematic dependence of the transfer
function V_Phi on alpha_I and alpha_R (junction resistance asymmetry). As for
the symmetric dc SQUID, V_Phi factorizes into
g(Gamma*beta_L)*f(alpha_I,beta_L), where now f also depends on alpha_I. For
\beta_L below five we find mostly a decrease of V_Phi with increasing alpha_I,
which however can only partially account for the frequently observed
discrepancy in V_Phi between theory and experiment for high-T_c dc SQUIDs.Comment: 4 pages, 7 figures, Applied Superconductivity Conference 2000, to be
published in IEEE Trans. Appl. Supercon
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