Papers by Clemens Adolphs
Single-particle dispersion in tetragonal CuO
Improved Bounds for Discrete Diffusive Load Balancing
2012 IEEE 26th International Parallel and Distributed Processing Symposium, 2012
ABSTRACT In this paper we consider load balancing in a static and discrete setting where a fixed ... more ABSTRACT In this paper we consider load balancing in a static and discrete setting where a fixed number of indivisible tasks have to be allocated to processors. We assume uniform tasks but the processors may have different speeds. The load of a processor is the number of tasks assigned to it divided by its speed. We consider diffusion load balancing which works in rounds. In every round the processors are allowed to compare their own load with the load of their neighbors and to balance the load with the neighbors, using their local information only. The question is how many rounds does it take until the whole processor network is balanced, meaning the load discrepancy (difference between maximum load and m/n) is minimized. Our balancing algorithm is deterministic and extends the algorithm studied in [1] from the case of uniform speeds to non-uniform speeds. We use a potential function argument to show that a better load balance can be obtained when the algorithm is allowed to run longer compared to the algorithm of [1].
Proceedings of the 2012 ACM symposium on Principles of distributed computing - PODC '12, 2012
In this paper we consider neighborhood load balancing in the context of selfish clients. We assum... more In this paper we consider neighborhood load balancing in the context of selfish clients. We assume that a network of n processors and m tasks is given. The processors may have different speeds and the tasks may have different weights. Every task is controlled by a selfish user. The objective of the user is to allocate his/her task to a processor with minimum load.
Journal of Physics A Mathematical and Theoretical
In two recent papers we have shown how one-particle and few-particle lattice Green's functions ca... more In two recent papers we have shown how one-particle and few-particle lattice Green's functions can be calculated efficiently for models with only nearestneighbor hopping, using continued fractions. Here, we show that a similar type of solution is possible for models with longer (but finite) range hopping.
In this paper we consider neighborhood load balancing in the context of selfish clients. We assum... more In this paper we consider neighborhood load balancing in the context of selfish clients. We assume that a network of n processors and m tasks is given. The processors may have different speeds and the tasks may have different weights. Every task is controlled by a selfish user. The objective of the user is to allocate his/her task to a processor with minimum load. We revisit the concurrent probabilistic protocol introduced in [6], which works in sequential rounds. In each round every task is allowed to query the load of one randomly chosen neighboring processor. If that load is smaller the task will migrate to that processor with a suitably chosen probability. Using techniques from spectral graph theory we obtain upper bounds on the expected convergence time towards approximate and exact Nash equilibria that are significantly better than the previous results in [6]. We show results for uniform tasks on non-uniform processors and the general case where the tasks have different weight...
Physical Review B, 2014
We use the Momentum Average approximation (MA) to study the ground-state properties of strongly b... more We use the Momentum Average approximation (MA) to study the ground-state properties of strongly bound bipolarons in the double-well electron-phonon (el-ph) coupling model, which describes certain intercalated lattices where the linear term in the el-ph coupling vanishes due to symmetry. We show that this model predicts the existence of strongly bound yet lightweight bipolarons in some regions of the parameter space. This provides a novel mechanism for the appearance of such bipolarons, in addition to long-range el-ph coupling and special lattice geometries.
Failure of the Holstein model to describe strong electron-phonon coupling
ABSTRACT The coupling of electrons to phonons and the properties of the resulting quasiparticle, ... more ABSTRACT The coupling of electrons to phonons and the properties of the resulting quasiparticle, the polaron, are important for understanding many materials, including strongly correlated systems like the cuprates and the manganites. For some materials, the effective electron-phonon (el-ph) coupling λ is well known. For others, like the cuprates, estimates range from very small (λ˜0.3) to extremely large (λ˜5). Here, we point out an inconsistency in the widely used theoretical models. Both the Holstein and the Fröhlich model assume that lattice distortions are sufficiently small to allow treating the el-ph coupling as linear. At strong coupling, however, it is well established that a small polaron forms, with potentially considerable lattice distortions, invalidating the original assumption. We use the momentum average approximation to study the effect of higher-order coupling terms in the Holstein model. We show that they have drastic consequences on the properties of the polaron when compared to the linear model and that these effects cannot be captured by a linear model with renormalized parameters. Since linear models fail to describe strongly coupled systems, estimates of λ based on those models have to be reevaluated.
Journal of Modern Physics, 2014
We simulate ultra-cold interacting Bosons in quasi-one-dimensional, incommensurate optical lattic... more We simulate ultra-cold interacting Bosons in quasi-one-dimensional, incommensurate optical lattices. In the tight-binding limit, these lattices have pseudo-random on-site energies and thus can potentially lead to Anderson localization. We explore the parameter regimes that lead to Anderson localization and investigate the role of repulsive interactions, harmonic confinement and finite temperature. We find that interactions can obscure the exponential localization characteristic of Anderson localization, thus impeding the direct observation of this phenomenon when interactions are present.
Physical Review B, 2014
We show that in crystals where light ions are symmetrically intercalated between heavy ions, the ... more We show that in crystals where light ions are symmetrically intercalated between heavy ions, the electron-phonon coupling for carriers located at the light sites cannot be described by a Holstein model. We introduce the double-well electron-phonon coupling model to describe the most interesting parameter regime in such systems, and study it in the single carrier limit using the momentum average approximation. For sufficiently strong coupling, a small polaron with a robust phonon cloud appears at low energies. While some of its properties are similar to those of a Holstein polaron, we highlight some crucial differences. These prove that the physics of the double-well electron-phonon coupling model cannot be reproduced with a linear Holstein model.
EPL (Europhysics Letters), 2013
Polarons and electron-phonon interactions PACS 72.10.Di -Scattering by phonons, magnons, and othe... more Polarons and electron-phonon interactions PACS 72.10.Di -Scattering by phonons, magnons, and other nonlocalized excitations PACS 63.20.kd -Phonon-electron interactions
Journal of Physics A: Mathematical and Theoretical, 2012
In two recent papers, we have shown how one-particle and few-particle lattice Green functions can... more In two recent papers, we have shown how one-particle and few-particle lattice Green functions can be calculated efficiently for models with only nearestneighbor hopping, using continued fractions. Here, we show that a similar type of solution is possible for models with longer (but finite) range hopping.
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Papers by Clemens Adolphs