3,002 research outputs found
Effect of Adiabatic Phonons on Striped and Homogeneous Ground States
The effects of adiabatic phonons on a spin-fermion model for high T_c
cuprates are studied using numerical simulations. In the absence of
electron-phonon interactions (EPI), stripes in the ground state are observed
for certain dopings while homogeneous states are stabilized in other regions of
parameter space. Different modes of adiabatic phonons are added to the
Hamiltonian:breathing, shear and half-breathing modes. Diagonal and
off-diagonal electron-phonon couplings are considered. It is observed that
strong diagonal EPI generate stripes in previously homogeneous states, while in
striped ground states an increase in the diagonal couplings tends to stabilize
the stripes, inducing a gap in the density of states (DOS) and rendering the
ground state insulating. The off-diagonal terms, on the other hand, destabilize
the stripes creating inhomogeneous ground states with a pseudogap at the
chemical potential in the DOS. The breathing mode stabilizes static diagonal
stripes; while the half-breathing (shear) modes stabilize dynamical (localized)
vertical and horizontal stripes. The EPI induces decoherence of the
quasi-particle peaks in the spectral functions.Comment: latex, 9 pages,13 figure
Complementary Pair Density Wave and d-wave Checkerboard Order in High Temperature Superconductors
The competing orders in the particle-particle (P-P) channel and the
particle-hole (P-H) channel have been proposed separately to explain the
pseudogap physics in cuprates. By solving the Bogoliubov-deGennes equation
self-consistently, we show that there is a general complementary connection
between the d-wave checkerboard order (DWCB) in the particle-hole (P-H) channel
and the pair density wave order (PDW) in the particle-particle (P-P) channel. A
small pair density localization generates DWCB and PDW orders simultaneously.
The result suggests that suppressing superconductivity locally or globally
through phase fluctuation should induce both orders in underdoped cuprates. The
presence of both DWCB and PDW orders with periodicity can
explain the checkerboard modulation observed in FT-STS from STM and the
puzzling dichotomy between the nodal and antinodal regions as well as the
characteristic features such as non-dispersive Fermi arc in the pseudogap
state
The Antarctic krill fishery : a techno-economic investigation
From a review of recent developments in the harvesting and processing of krill, an analysis is made of the market potential for the main products of a krill fishery, namely whole krill, tail meats, mince (particularly surimi) and meal.
A techno-economic analysis of various catching - processing - product systems is made, in terms of costs and prices applicable in Western countries in 1977. The study finds that in practice the rapid
spoilage rate of- krill after capture effectively determines that it should be processed on board freezer or factory trawlers. However, unless krill is marketed essentially as a crustacean product (even
though in minced form) it is most unlikely to generate sufficient revenue to justify the high costs of its exploitation.
Consideration is also given to the management issues raised by the
exploitation of Antarctic krill in the context of the Southern Ocean.
It is concluded that the quantities of krill likely to be taken by Western countries in the foreseeable future will be comparatively low.
However. should the Soviets continue to expand their activities then even comparatively modest catch levels (of the order of a few million tonnes per year) could have a serious effect on Southern Ocean stocks, if not on krill, then on other species dependent upon this resource
Applying Automated Theorem Proving to Computer Security
While more and more data is stored and accessed electronically, better access control methods need to be implemented for computer security. Formal modelling and analysis have been successfully used in certain areas of computer systems, such as verifying the security properties of cryptographic and authentication protocols. However, formal models for computer systems in cyberspace, like networks, have hardly advanced. A highly regarded graduate textbook cites the Take-Grant model created in 1977 as one of the \current examples of security modelling and analysis techniques. This model is rarely used in practice though. This research implements the Take-Grant Protection model\u27s four de jure rules and Can Share predicate in the Prototype Verification System (PVS) which automates model checking and theorem proving. This facilitates the ability to test a given TakeGrant model against many systems which are modelled using digraphs. Two models, one with error checking and one without, are created to implement take-grant rules. The first model that does not have error checking incorporated requires manual error checking. The second model uses recursion to allow for the error checking. The Can Share theorem requires further development
Inhomogeneous charge textures stabilized by electron-phonon interactions in the t-J model
We study the effect of diagonal and off-diagonal electron-phonon coupling in
the ground state properties of the t-J model. Adiabatic and quantum phonons are
considered using Lanczos techniques. Charge tiles and stripe phases with mobile
holes (localized holes) are observed at intermediate (large) values of the
diagonal electron-phonon coupling. The stripes are stabilized by half-breathing
modes, while the tiles arise due to the development of extended breathing
modes. Off-diagonal terms destabilize the charge inhomogeneous structures with
mobile holes by renormalizing the diagonal coupling but do not produce new
phases. Buckling modes are also studied and they seem to induce a gradual phase
separation between hole rich and hole poor regions. The pairing correlations
are strongly suppressed when the holes are localized. However, in charge
inhomogeneous states with mobile holes no dramatic changes, compared with the
uniform state, are observed in the pairing correlations indicating that D-wave
pairing and moderate electron-phonon interactions can coexist.Comment: minor changes; to appear in Physical Review
Paramagnon-induced dispersion anomalies in the cuprates
We report the self-energy associated with RPA magnetic susceptibility in the
hole-doped Bi_2Sr_2CuO_6 (Bi2201) and the electron-doped Nd_{2-x}Ce_xCuO_4
(NCCO) in the overdoped regime within the framework of a one-band Hubbard
model. Strong weight is found in the magnetic spectrum around (pi, 0) at about
360 meV in Bi2201 and 640 meV in NCCO, which yields dispersion anomalies in
accord with the recently observed `waterfall' effects in the cuprates.Comment: Submitted to PRL, Dec. 21, 2006; 4 eps figures, revte
Signatures of modulated pair interaction in cuprate superconductors
Recent low-temperature scanning tunnelling spectroscopy experiments on the
surface of BSCCO-2212 have revealed a strong positive correlation between the
position of localized resonances at -960 meV identified with interstitial
oxygen dopants and the size of the local spectral gap. We review efforts to
understand these correlations within a model where the dopants modulate the
pair interaction on an atomic scale. We provide further evidence for this model
by comparing the correlations between the dopants and the local density of
states with experimental results.Comment: 4 pages, 2 figures, submitted to M2S-HTSC VIII, Dresden 200
The temperature dependence of the local tunnelling conductance in cuprate superconductors with competing AF order
Based on the model with proper chosen parameters for describing
the cuprate superconductors, it is found that near the optimal doping at low
temperature (), only the pure d-wave superconductivity (SC) prevails and
the antiferromagnetic (AF) order is completely suppressed. At higher , the
AF order with stripe modulation and the accompanying charge order may emerge,
and they could exist above the SC transition temperature. We calculate the
local differential tunnelling conductance (LDTC) from the local density of
states (LDOS) and show that their energy variations are rather different from
each other as increases. Although the calculated modulation periodicity in
the LDTC/LDOS and bias energy dependence of the Fourier amplitude of LDTC in
the "pseudogap" region are in good agreement with the recent STM experiment
[Vershinin , Science {\bf 303}, 1995 (2004)], we point out that some of
the energy dependent features in the LDTC do not represent the intrinsic
characteristics of the sample
Anti-phase Modulation of Electron- and Hole-like States in Vortex Core of Bi2Sr2CaCu2Ox Probed by Scanning Tunneling Spectroscopy
In the vortex core of slightly overdoped Bi2Sr2CaCu2Ox, the electron-like and
hole-like states have been found to exhibit spatial modulations in anti-phase
with each other along the Cu-O bonding direction. Some kind of
one-dimensionality has been observed in the vortex core, and it is more clearly
seen in differential conductance maps at lower biases below +-9 mV
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