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Thomas B. Schrøder

Bio: Thomas B. Schrøder is an academic researcher from Roskilde University. The author has contributed to research in topics: Virial theorem & Radial distribution function. The author has an hindex of 43, co-authored 129 publications receiving 7213 citations. Previous affiliations of Thomas B. Schrøder include National Institute of Standards and Technology & University of California, Berkeley.


Papers
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TL;DR: In this paper, it is argued that ac universality reflects an underlying percolation determining dc as well as ac conductivity in the extreme disorder limit, i.e., when the local randomly varying mobilities cover many orders of magnitude.
Abstract: The striking similarity of ac conduction in quite different disordered solids is discussed in terms of experimental results, modeling, and computer simulations. After giving an overview of experiment, a macroscopic and a microscopic model are reviewed. For both models the normalized ac conductivity as a function of a suitably scaled frequency becomes independent of details of the disorder in the extreme disorder limit, i.e., when the local randomly varying mobilities cover many orders of magnitude. The two universal ac conductivities are similar, but not identical; both are examples of unusual non-power-law universalities. It is argued that ac universality reflects an underlying percolation determining dc as well as ac conductivity in the extreme disorder limit. Three analytical approximations to the universal ac conductivities are presented and compared to computer simulations. Finally, model predictions are briefly compared to experiment.

1,143 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of the polymer/nanoparticle interactions, surface-to-volume ratio, and boundary conditions on both the structure and dynamics of a bead-spring polymer melt surrounding a nanoscopic particle was explored.
Abstract: We perform molecular dynamics simulations of a bead−spring polymer melt surrounding a nanoscopic particle. We explore the effect of the polymer/nanoparticle interactions, surface-to-volume ratio, and boundary conditions on both the structure and dynamics of the polymer melt. We find that the chains near the nanoparticle surface are elongated and flattened and that this effect is independent of the interaction for the range of interactions we study. We show that the glass transition temperature Tg of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and nanoparticle. A gradual change of the polymer dynamics approaching the nanoparticle surface causes the change in the glass transition. The magnitude of the shift is exaggerated by increasing fraction of surface monomers in the system. These behaviors support a “many-layer”-based interpretation of the dynamics. Our findings appear applicable to systems in which surface interactions dominate, including b...

472 citations

Journal ArticleDOI
TL;DR: In this paper, a four-point, time-dependent density correlation function g4(r,t) and corresponding structure factor S4(q,t), which measures the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity is presented.
Abstract: Relaxation in supercooled liquids above their glass transition and below the onset temperature of “slow” dynamics involves the correlated motion of neighboring particles This correlated motion results in the appearance of spatially heterogeneous dynamics or “dynamical heterogeneity” Traditional two-point time-dependent density correlation functions, while providing information about the transient “caging” of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity Here, we study a four-point, time-dependent density correlation function g4(r,t) and corresponding “structure factor” S4(q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity We study g4(r,t) and S4(q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above We find that the correl

391 citations

Journal ArticleDOI
TL;DR: In this paper, an equilibrated model glass-forming liquid is studied by mapping successive configurations produced by molecular dynamics simulation onto a time series of inherent structures (local minima in the potential energy).
Abstract: An equilibrated model glass-forming liquid is studied by mapping successive configurations produced by molecular dynamics simulation onto a time series of inherent structures (local minima in the potential energy). Using this “inherent dynamics” approach we find direct numerical evidence for the long held view that below a crossover temperature, Tx, the liquid’s dynamics can be separated into (i) vibrations around inherent structures and (ii) transitions between inherent structures [M. Goldstein, J. Chem. Phys. 51, 3728 (1969)], i.e., the dynamics become “dominated” by the potential energy landscape. In agreement with previous proposals, we find that Tx is within the vicinity of the mode-coupling critical temperature Tc. We further find that near Tx, transitions between inherent structures occur via cooperative, stringlike rearrangements of groups of particles moving distances substantially smaller than the average interparticle distance.

275 citations

Journal ArticleDOI
TL;DR: It is shown that the glass transition temperature T(g) of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and filler by manipulating the dynamics of the polymer dynamics approaching the filler surface.
Abstract: ~Received 19 July 2000; revised manuscript received 9 January 2001; published 27 July 2001! We perform molecular dynamics simulations of an idealized polymer melt surrounding a nanoscopic filler particle. We show that the glass transition temperature Tg of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and filler. A gradual change of the polymer dynamics approaching the filler surface causes the change in the glass transition. We also find that polymers close to the surface tend to be elongated and flattened. Our findings show a strong similarity to those obtained for ultrathin polymer films.

257 citations


Cited by
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08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

29,323 citations

Reference EntryDOI
31 Oct 2001
TL;DR: The American Society for Testing and Materials (ASTM) as mentioned in this paper is an independent organization devoted to the development of standards for testing and materials, and is a member of IEEE 802.11.
Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

3,792 citations

Journal ArticleDOI
TL;DR: Van Kampen as mentioned in this paper provides an extensive graduate-level introduction which is clear, cautious, interesting and readable, and could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes.
Abstract: N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

3,647 citations

Journal ArticleDOI
TL;DR: Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methamphetamine) composites.
Abstract: Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

3,245 citations