Konfigurations-Pfadintegral-Monte-Carlo:Ab-initio-Simulationen von Fermionen im Bereich warmer, dichter Materie

Recent advances in warm dense matter physics, e.g. laser compressed matter, lead to an increasing interest in the description of correlated, degenerate electrons at finite temperatures. Path integral Monte Carlo (PIMC) methods cannot correctly describe weakly to moderately coupled and strongly degenerate Fermi systems due to the so-called fermion sign problem. The Configuration Path Integral Monte Carlo (CPIMC) approach greatly reduces the sign problem and allows for the exact computation of thermodynamic properties in this regime. In addition, the first successful implementation of the diagrammatic worm algorithm for a general Hamiltonian in Fock space with arbitrary pair interactions gives direct access to the Matsubara Green function. This thesis demonstrates the capabilities of the CPIMC approach for a model system of Coulomb interacting fermions in a two-dimensional harmonic trap. The correctness of the CPIMC implementation is verified by rigorous comparisons with an exact diagonalization method. Benchmark results are presented, which reveal large errors of the Hartree-Fock approximation in open shell configurations even for weak coupling and a significant deviation of multi-level blocking PIMC data in the complete basis set limit. The application of the CPIMC method to the warm dense homogeneous electron gas (HEG) quantifies the accuracy of recently published restricted PIMC (RPIMC) results, which have been the basis for the construction of exchange-correlation free energy functionals to be used in finite-temperature density functional theory calculations of warm dense matter. It is shown that the errors of the RPIMC data exceed 10 % at intermediate densities. Additionally, highly accurate data for the exchange-correlation energy at high densities, which are inaccessible by the RPIMC method, are provided. These results are needed to significantly increase the quality of future exchange-correlation functionals to be used in finite-temperature applications.Auf Grund großer Fortschritte im Bereich warmer, dichter Materie, wie z.B. Laser-Kompression, ist die genaue Beschreibung korrelierter, entarteter Elektronen bei endlichen Temperaturen von wachsender Bedeutung. Pfadintegral-Monte-Carlo (PIMC) Methoden können schwach bis moderat gekoppelte und stark entartete Fermionen auf Grund des sogenannten fermionischen Vorzeichenproblems nicht korrekt beschreiben. Der Konfigurations-Pfadintegral-Monte-Carlo (CPIMC) Ansatz reduziert das Vorzeichenproblem und erlaubt die exakte Berechnung thermodynamischer Eigenschaften in diesem Bereich. Die erste erfolgreiche Implementierung des diagrammatischen Wurmalgorithmus für einen allgemeinen Hamiltonoperator im Fockraum mit beliebiger Paarwechselwirkung ermöglicht den direkten Zugriff auf die Matsubara-Green-Funktion. Diese Arbeit demonstriert die Fähigkeiten des CPIMC Ansatzes für ein Modellsystem aus Coulomb-wechselwirkenden Fermionen in einer 2D harmonischen Falle. Die Korrektheit der CPIMC-Implementierung wird durch Vergleiche mit einer exakten Diagonalisierungsmethode verifiziert. Die präsentierten Benchmark-Ergebnisse belegen große Fehler der Hartree-Fock-Näherung für Systeme mit offenen Schalen schon bei geringen Wechselwirkungsstärken und eine signifikante Abweichung von Multi-Level-Blocking-PIMC-Daten im Limes einer vollständigen Basis. Die Anwendung der CPIMC-Methode auf das warme, dichte homogene Elektronengas (HEG) erbringt den Nachweis von systematischen Ungenauigkeiten von vor Kurzem veröffentlichen Restricted-PIMC (RPIMC) Ergebnissen. Die relativen Abweichungen der RPIMC-Daten zu den exakten CPIMC-Resultaten bei mittleren Dichten übersteigen 10%. Die in dieser Arbeit erzeugten, äußerst genauen Ergebnisse für die Austauschkorrelationsenergie bei hohen Dichten, welche unzugänglich für die RPIMC-Methode sind, können helfen, die Genauigkeit von zukünftigen Austauschkorrelationsfunktionalen für die Dichtefunktionaltheorie bei endlichen Temperaturen signifikant zu erhöhen

Momentum distribution function and short-range correlations of the warm dense electron gas -- ab initio quantum Monte Carlo results

In a classical plasma the momentum distribution, $n(k)$, decays exponentially, for large $k$, and the same is observed for an ideal Fermi gas. However, when quantum and correlation effects are relevant simultaneously, an algebraic decay, $n_\infty(k)\sim k^{-8}$ has been predicted. This is of relevance for cross sections and threshold processes in dense plasmas that depend on the number of energetic particles. Here we present extensive \textit{ab initio} results for the momentum distribution of the nonideal uniform electron gas at warm dense matter conditions. Our results are based on first principle fermionic path integral Monte Carlo (CPIMC) simulations and clearly confirm the $k^{-8}$ asymptotic. This asymptotic behavior is directly linked to short-range correlations which are analyzed via the on-top pair distribution function (on-top PDF), i.e. the PDF of electrons with opposite spin. We present extensive results for the density and temperature dependence of the on-top PDF and for the momentum distribution in the entire momentum range

Sensitivity of Pine Island Glacier to observed ocean forcing

We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered a few decades ago by increased ocean-induced thinning, which may have initiated marine ice-sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold-ocean period, the evolving glacier-bed/ice-shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin-wide thinning signal did not change. Thus, as predicted by theory, once marine ice-sheet instability is underway, a single transient high-amplitude ocean cooling has only a relatively minor effect on ice flow. The long-term effects of ocean-temperature variability on ice flow, however, are not yet known

Multitasking with typical use of hearing aid noise reduction in older listeners

Objective: Older hearing-impaired adults typically experience difficulties understanding speech in noise. Most hearing aids address this issue using digital noise reduction. While noise reduction does not necessarily improve speech recognition, it may reduce the resources required to process the speech signal. Those available resources may, in turn, aid the ability to perform another task while listening to speech (i.e., multitasking). This study examined to what extent changing the strength of digital noise reduction in hearing aids affects the ability to multitask. Design: Multitasking was measured using a dual-task paradigm, combining a speech recognition task and a visual monitoring task. The speech recognition task involved sentence recognition in the presence of six-talker babble at signal-to-noise ratios (SNRs) of 2 and 7 dB. Participants were fit with commercially-available hearing aids programmed under three noise reduction settings: off, mild, strong. Study sample: 18 hearing-impaired older adults. Results: There were no effects of noise reduction on the ability to multitask, or on the ability to recognize speech in noise. Conclusions: Adjustment of noise reduction settings in the clinic may not invariably improve performance for some tasks

Momentum distribution function and short-range correlations of the warm dense electron gas: Ab initio quantum Monte Carlo results

In a classical plasma the momentum distribution, $n(k)$, decays exponentially, for large k, and the same is observed for an ideal Fermi gas. However, when quantum and correlation effects are relevant simultaneously, an algebraic decay, $n_∞ (k)∼$ $^{k−}8$ has been predicted. This is of relevance for cross sections and threshold processes in dense plasmas that depend on the number of energetic particles. Here we present extensive ab initio results for the momentum distribution of the nonideal uniform electron gas at warm dense matter conditions. Our results are based on first principle fermionic path integral Monte Carlo (CPIMC) simulations and clearly confirm the $k^{−8}$ asymptotic. This asymptotic behavior is directly linked to short-range correlations which are analyzed via the on-top pair distribution function (on-top PDF), i.e., the PDF of electrons with opposite spin. We present extensive results for the density and temperature dependence of the on-top PDF and for the momentum distribution in the entire momentum range

Switching streams across ears to evaluate informational masking of speech-on-speech

info:eu-repo/semantics/publishe