118,561 research outputs found
Recent Studies in Superconductivity at Extreme Pressures
Studies of the effect of high pressure on superconductivity began in 1925
with the seminal work of Sizoo and Onnes on Sn to 0.03 GPa and have continued
up to the present day to pressures in the 200 - 300 GPa range. Such enormous
pressures cause profound changes in all condensed matter properties, including
superconductivity. In high pressure experiments metallic elements, Tc values
have been elevated to temperatures as high as 20 K for Y at 115 GPa and 25 K
for Ca at 160 GPa. These pressures are sufficient to turn many insulators into
metals and magnetics into superconductors. The changes will be particularly
dramatic when the pressure is sufficient to break up one or more atomic shells.
Recent results in superconductivity to Mbar pressures wll be discussed which
exemplify the progress made in this field over the past 82 years.Comment: Proceedings of the 21st AIRAPT and 45th EHPRG International
Conference on High Pressure Science and Technology, Catania, Italy, Sept.
17-21, 200
The sign problem in full configuration interaction quantum Monte Carlo: Linear and sub-linear representation regimes for the exact wave function
We investigate the sign problem for full configuration interaction quantum
Monte Carlo (FCIQMC), a stochastic algorithm for finding the ground state
solution of the Schr\"odinger equation with substantially reduced computational
cost compared with exact diagonalisation. We find -space Hubbard models for
which the solution is yielded with storage that grows sub-linearly in the size
of the many-body Hilbert space, in spite of using a wave function that is
simply linear combination of states. The FCIQMC algorithm is able to find this
sub-linear scaling regime without bias and with only a choice of Hamiltonian
basis. By means of a demonstration we solve for the energy of a 70-site
half-filled system (with a space of determinants) in 250 core hours,
substantially quicker than the 10 core hours that would be
required by exact diagonalisation. This is the largest space that has been
sampled in an unbiased fashion. The challenge for the recently-developed FCIQMC
method is made clear: expand the sub-linear scaling regime whilst retaining
exact on average accuracy. This result rationalizes the success of the
initiator adaptation (i-FCIQMC) and offers clues to improve it. We argue that
our results changes the landscape for development of FCIQMC and related
methods.Comment: 6 pages, 4 figures. The mentioned supplementary material is included
as "Ancillary files". Comments welcom
Mining electron density for functionally relevant protein polysterism in crystal structures.
This review focuses on conceptual and methodological advances in our understanding and characterization of the conformational heterogeneity of proteins. Focusing on X-ray crystallography, we describe how polysterism, the interconversion of pre-existing conformational substates, has traditionally been analyzed by comparing independent crystal structures or multiple chains within a single crystal asymmetric unit. In contrast, recent studies have focused on mining electron density maps to reveal previously 'hidden' minor conformational substates. Functional tests of the importance of minor states suggest that evolutionary selection shapes the entire conformational landscape, including uniquely configured conformational substates, the relative distribution of these substates, and the speed at which the protein can interconvert between them. An increased focus on polysterism may shape the way protein structure and function is studied in the coming years
Slisp: A Flexible Software Toolkit for Hybrid, Embedded and Distributed Applications
We describe Slisp (pronounced ‘Ess-Lisp’), a hybrid Lisp–C programming toolkit for the development of scriptable and distributed applications. Computationally expensive operations implemented as separate C-coded modules are selectively compiled into a small Xlisp interpreter, then called as Lisp functions in a Lisp-coded program. The resulting hybrid program may run in several modes: as a stand-alone executable, embedded in a different C program, as a networked server accessed from another Slisp client, or as a
networked server accessed from a C-coded client. Five years of experience with Slisp, as well experience with other scripting languages such as Tcl and Perl, are summarized. These experiences suggest that Slisp will be most useful for mid-sized applications in which the kinds of scripting and embeddability features provided by Tcl and Perl can be extended in an efficient manner to larger applications, while maintaining a
well-defined standard (Common Lisp) for these extensions. In addition, the generality of Lisp makes Lisp a good candidate for an application-level communication language in distributed environments
Optimal realizations of floating-point implemented digital controllers with finite word length considerations.
The closed-loop stability issue of finite word length (FWL) realizations is
investigated for digital controllers implemented in floating-point arithmetic.
Unlike the existing methods which only address the effect of the mantissa bits
in floating-point implementation to the sensitivity of closed-loop stability,
the sensitivity of closed-loop stability is analysed with respect to both the
mantissa and exponent bits of floating-point implementation. A computationally
tractable FWL closed-loop stability measure is then defined, and the method of
computing the value of this measure is given. The optimal controller realization
problem is posed as searching for a floating-point realization that maximizes
the proposed FWL closed-loop stability measure, and a numerical optimization
technique is adopted to solve for the resulting optimization problem. Simulation
results show that the proposed design procedure yields computationally efficient
controller realizations with enhanced FWL closed-loop stability performance
Advanced propulsion for LEO and GEO platforms
Mission requirements and mass savings applicable to specific low earth orbit and geostationary earth orbit platforms using three highly developed propulsion systems are described. Advanced hypergolic bipropellant thrusters and hydrazine arcjets can provide about 11 percent additional instrument payload to 14,000 kg LEO platforms. By using electric propulsion on a 8,000 kg class GEO platform, mass savings in excess of 15 percent of the beginning-of-life platform mass are obtained. Effects of large, advanced technology solar arrays and antennas on platform propulsion requirements are also discussed
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