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Signatures of coherent electronic quasiparticles in the paramagnetic Mott insulator
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 23, p. 235129-Article in journal (Refereed) Published
Abstract [en]

We study the Mott insulating state of the half-filled paramagnetic Hubbard model within dynamical mean field theory using a recently formulated stochastic and nonperturbative quantum impurity solver. The method is based on calculating the impurity self energy as a sample average over a representative distribution of impurity models solved by exact diagonalization. Due to the natural parallelization of the method, millions of poles are readily generated for the self energy which allows us to work with very small pole-broadening eta. Solutions at small and large eta are qualitatively different; solutions at large eta show featureless Hubbard bands whereas solutions at small eta <= 0.001 (in units of half bare band width) show a band of electronic quasiparticles with very small quasiparticle weight at the inner edge of the Hubbard bands. The validity of the results are supported by agreement within statistical error sigma(QMC) similar to 10(-4) on the imaginary frequency axis with calculations using a continuous time quantum Monte Carlo solver. Nevertheless, convergence with respect to finite size of the stochastic exact diagonalization solver remains to be rigourously established.

Place, publisher, year, edition, pages
2014. Vol. 90, no 23, p. 235129-
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-241956DOI: 10.1103/PhysRevB.90.235129ISI: 000346614800002OAI: oai:DiVA.org:uu-241956DiVA, id: diva2:782134
Available from: 2015-01-20 Created: 2015-01-19 Last updated: 2018-10-10Bibliographically approved
In thesis
1. Theoretical and Computational Studies of Strongly Correlated Electron Systems: Dynamical Mean Field Theory, X-ray Absorption Spectroscopy and Analytical Continuation
Open this publication in new window or tab >>Theoretical and Computational Studies of Strongly Correlated Electron Systems: Dynamical Mean Field Theory, X-ray Absorption Spectroscopy and Analytical Continuation
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis encompasses theoretical and computational studies of strongly correlated elec-tron systems. Understanding how electrons in solids interact with each other is of great im-portance for future technology and other applications. From a fundamental point of view, the Coulomb interaction in a solid leads to a very challenging many-body problem, encapsulating many physical phenomena, e.g. magnetism. Treating this interaction, with a focus on local contributions, is the subject of this thesis. Both models and materials have been investigated, to obtain insight on the mechanisms determining the macroscopic properties of matter. This thesis is divided in four parts, each corresponding to a different project or topic.

In the first project a many body method called dynamical mean field theory (DMFT) is used to study the paramagnetic phase of the Hubbard model. A stochastic version of the exact di-agonalization technique is developed for solving the effective impurity model arising in DMFT and generating real frequency spectral functions. In the next project, by combining density functional theory (DFT) with a static solution of the DMFT equations (DFT+U), magnetic ex-change interactions in transition metal oxides (TMOs) are investigated. The spin dependence of the functional is shown to be important for mapping magnetic excitations form the quantum mechanical system to a classical model.

The next topic in this thesis concerns the x-ray absorption spectroscopy of TMOs. Spectral functions, in good agreement with experimental data, are calculated by combining DFT with multiplet ligand field theory (MLFT). The effects of the presence of a core-hole are studied in detail for NiO, as well as double counting issues related to higher order terms of the multiple ex-pansion of the Coulomb interaction. A strained induced linearly polarized spectrum is obtained for CaTiO3. Lastly, charge disproportionation is seen in Mo doped LaFeO3.

Finally, a critical step in DMFT, called analytical continuation, to obtain physical observ-ables of interest is investigated. Analytical continuation means a transformation of a function in the complex plane. Several methods for performing this transformation are explained, and in particular steps for improving the robustness and accuracy of the Padé approximant method are described.

Place, publisher, year, edition, pages
Uppala: Acta Universitatis Upsaliensis, 2018. p. 112
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1729
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-362834 (URN)978-91-513-0471-7 (ISBN)
Public defence
2018-11-30, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Note

the opponents university is University of Bremen

Available from: 2018-11-09 Created: 2018-10-10 Last updated: 2018-11-19

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