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Gerhard Wellein

Publications and source records attributed to Gerhard Wellein.

99 records · Page 6Linked to original sources
arXiv2008

Data access optimizations for highly threaded multi-core CPUs with multiple memory controllers

Processor and system architectures that feature multiple memory controllers are prone to show bottlenecks and erratic performance numbers on codes with regular access patterns. Although such effects are well known in the form of cache thrashing and aliasing conflicts, they become more severe when memory access is involved. Using the new Sun UltraSPARC T2 processor as a prototypical multi-core design, we analyze performance patterns in low-level and application benchmarks and show ways to circumvent bottlenecks by careful data layout and padding.

cs.DC↗
arXiv2007

RZBENCH: Performance evaluation of current HPC architectures using low-level and application benchmarks

RZBENCH is a benchmark suite that was specifically developed to reflect the requirements of scientific supercomputer users at the University of Erlangen-Nuremberg (FAU). It comprises a number of application and low-level codes under a common build infrastructure that fosters maintainability and expandability. This paper reviews the structure of the suite and briefly introduces the most relevant benchmarks. In addition, some widely known standard benchmark codes are reviewed in order to emphasize the need for a critical review of often-cited performance results. Benchmark data is presented for the HLRB-II at LRZ Munich and a local InfiniBand Woodcrest cluster as well as two uncommon system architectures: A bandwidth-optimized InfiniBand cluster based on single socket nodes ("Port Townsend") and an early version of Sun's highly threaded T2 architecture ("Niagara 2").

cs.DC↗
arXiv2006

Carrier-density effects in many-polaron systems

Many-polaron systems with finite charge-carrier density are often encountered experimentally. However, until recently, no satisfactory theoretical description of these systems was available even in the framework of simple models such as the one-dimensional spinless Holstein model considered here. In this work, previous results obtained using numerical as well as analytical approaches are reviewed from a unified perspective, focussing on spectral properties which reveal the nature of the quasiparticles in the system. In the adiabatic regime and for intermediate electron-phonon coupling, a carrier-density driven crossover from a polaronic to a rather metallic system takes place. Further insight into the effects due to changes in density is gained by calculating the phonon spectral function, and the fermion-fermion and fermion-lattice correlation functions. Finally, we provide strong evidence against the possibility of phase separation.

cond-mat.str-el↗
arXiv2006

The spin-Peierls chain revisited

We extend previous analytical studies of the ground-state phase diagram of a one-dimensional Heisenberg spin chain coupled to optical phonons, which for increasing spin-lattice coupling undergoes a quantum phase transition from a gap-less to a gaped phase with finite lattice dimerisation. We check the analytical results against established four-block and new two-block density matrix renormalisation group (DMRG) calculations. Different finite-size scaling behaviour of the spin excitation gaps is found in the adiabatic and anti-adiabatic regimes.

cond-mat.str-el↗
arXiv2006

The Kernel Polynomial Method

Efficient and stable algorithms for the calculation of spectral quantities and correlation functions are some of the key tools in computational condensed matter physics. In this article we review basic properties and recent developments of Chebyshev expansion based algorithms and the Kernel Polynomial Method. Characterized by a resource consumption that scales linearly with the problem dimension these methods enjoyed growing popularity over the last decade and found broad application not only in physics. Representative examples from the fields of disordered systems, strongly correlated electrons, electron-phonon interaction, and quantum spin systems we discuss in detail. In addition, we illustrate how the Kernel Polynomial Method is successfully embedded into other numerical techniques, such as Cluster Perturbation Theory or Monte Carlo simulation.

cond-mat.other↗
arXiv2006

Many-polaron problem by cluster perturbation theory

The carrier-density dependence of the photoemission spectrum of the Holstein many-polaron model is studied using cluster perturbation theory combined with an improved cluster diagonalization by Chebychev expansion.

cond-mat.str-el↗
arXiv2005

Comparative numerical study of Anderson localization in disordered electron systems

Taking into account that a proper description of disordered systems should focus on distribution functions, the authors develop a powerful numerical scheme for the determination of the probability distribution of the local density of states (LDOS), which is based on a Chebyshev expansion with kernel polynomial refinement and allows the study of large finite clusters (up to $100^3$). For the three-dimensional Anderson model it is demonstrated that the distribution of the LDOS shows a significant change at the disorder induced delocalisation-localisation transition. Consequently, the so-called typical density of states, defined as the geometric mean of the LDOS, emerges as a natural order parameter. The calculation of the phase diagram of the Anderson model proves the efficiency and reliability of the proposed approach in comparison to other localisation criteria, which rely, e.g., on the decay of the wavefunction or the inverse participation number.

cond-mat.dis-nn↗