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Polaron stability in molecular crystals
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2012 (English)In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 376, no 22, 1807-1811 p.Article in journal (Refereed) Published
Abstract [en]

A semi-empirical Peierls-Holstein model is applied to studies of the stability of polarons in two-dimensional molecular crystal systems. Calculations for a broad range of intra- and inter-molecular parameters within this model were performed in order to obtain detailed knowledge concerning the stability of the polaron solution with respect to a rigid lattice band solution. For realistic values of the parameters the polaron solution is stable with a polaron energy in the range 50-100 meV. A metastable polaron solution is also identified. The polarons that result from our model are highly localized and it is questionable if adiabatic polaron transport can occur in the system.

Place, publisher, year, edition, pages
Elsevier , 2012. Vol. 376, no 22, 1807-1811 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-78584DOI: 10.1016/j.physleta.2012.04.007ISI: 000304338100010OAI: oai:DiVA.org:liu-78584DiVA: diva2:534144
Note
Funding Agencies|Swedish Research Council (VR)||Available from: 2012-06-15 Created: 2012-06-15 Last updated: 2017-12-07
In thesis
1. A Theoretical Study of Charge Transport in Molecular Crystals
Open this publication in new window or tab >>A Theoretical Study of Charge Transport in Molecular Crystals
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The main objective of this thesis is to provide a deeper understanding of the charge transport phenomena occuring in molecular crystals. The focus is on the stability and the dynamics of the polaron as the charge carrier.

To achieve this goal, a series of numerical calculations are performed using the semi-emprical "Holstien-Peierls" model. The model considers both intra- (Holstein) and inter- (Peierls) molecular interactions, in particular the electron-phonon interactions.

First, the stability of the polaron in an ordered two dimensional molecular lattice with an excess charge is studied using Resilient backPropagation, RPROP, algorithm. The stability is defined by the "polaron formation energy". This formation energy is obtained for a wide range of parameter sets including both intra- and inter-molecular electron-phonon coupling strengths and their vibrational frequencies, transfer intergral and electric field. We found that the polaron formation energies lying in the range of 50-100 meV are more interesting for our studies.

The second step to cover is the dynamical behaviour of the polaron. Using the stable polaron solutions acheived in the first step, an electric field is applied as an external force, pushing the charge to move. We observed that the polaron remains stable and moves with a constant velocity for only a limited range of parameter sets. Finally, the impact of disorder and temperature on the charge dynamics is considered. Adding disorder to the system will result in a more restricted parameter set space for which the polaron is dynamically stable and mobile.

Temperature is included in the Newtonian equations of motion via a random force. We observed that the polaron remains localized and moves with a diffusive behaviour up to a certain temperature. If the temperature increases to values above this critical temperature, the localized polaron becomes delocalized.

All this research work is coded in MATLAB software , allowing us to run the calculations, test and validate our results.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 41 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1560
Keyword
Molecular Crystals, Charge transport, Polaron, Holstein model, Peierls coupling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-88111 (URN)LIU-TEK-LIC-2012:45 (Local ID)978-91-7519-731-9 (ISBN)LIU-TEK-LIC-2012:45 (Archive number)LIU-TEK-LIC-2012:45 (OAI)
Presentation
2013-02-04, Schrödinger, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2013-01-30 Created: 2013-01-30 Last updated: 2013-01-30Bibliographically approved
2. Theoretical Description of the Electron-Lattice Interaction in Molecular and Magnetic Crystals
Open this publication in new window or tab >>Theoretical Description of the Electron-Lattice Interaction in Molecular and Magnetic Crystals
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electron-lattice interactions are often considered not to play a major role in material's properties as they are assumed to be small, the second-order effects. However, this study shows the importance of taking these effects into account in the simulations. My results demonstrate the impact of the electron-lattice interaction on the physics of the material and our understanding from it. One way to study these effects is to add them as perturbations to the unperturbed Hamiltonians in numerical simulations. The main objective of this thesis is to study electron-lattice interactions in molecular and magnetic crystals. It is devoted to developing numerical techniques considering model Hamiltonians and first-principles calculations to include the effect of lattice vibrations in the simulations of the above mentioned classes of materials.

In particular, I study the effect of adding the non-local electron-phonon coupling on top of the Holstein Hamiltonian to study the polaron stability and polaron dynamics in molecular crystals. The numerical calculations are based on the semi-empirical Holstein-Peierls model in which both intra (Holstein) and inter (Peierls) molecular electron-phonon interactions are taken into account. I study the effect of different parameters including intra and intermolecular electron-phonon coupling strengths and their vibrational frequencies, the transfer integral and the electric field on polaron stability. I found that in an ordered two dimensional molecular lattice the polaron is stable for only a limited range of parameter sets with the polaron formation energies lying in the range between 50 to 100 meV. Using the stable polaron solutions, I applied an electric field to the system and I observed that the polaron is dynamically stable and mobile for only a limited set of parameters. Adding disorder to the system will result in even more restricted parameter set space for which the polaron is stable and moves adiabatically with a constant velocity. In order to study the effect of temperature on polaron dynamics, I include a random force in Newtonian equations of motion in a one dimensional molecular lattice. I found that there is a critical temperature above which the polaron destabilizes and becomes delocalized.

Moreover, I study the role of lattice vibrations coupled to magnetic degrees of freedom in finite temperature paramagnetic state of magnetic materials. Calculating the properties of paramagnetic materials at elevated temperatures is a cumbersome task. In this thesis, I present a new method which allows us to couple lattice vibrations and magnetic disorder above the magnetic transition temperature and treat them on the same footing. The method is based on the combination of disordered local moments model and ab initio molecular dynamics (DLM-MD). I employ the method to study different physical properties of some model systems such as CrN and NiO in which the interaction between the magnetic and lattice degrees of freedom is very strong making them very good candidates for such a study.

I calculate the formation energies and study the effect of nitrogen defects on the electronic structure of paramagnetic CrN at high temperatures. Using this method I also study the temperature dependent elastic properties of paramagnetic CrN. The results highlight the importance of taking into account the magnetic excitations and lattice vibrations in the studies of magnetic materials at finite temperatures. A combination of DLM-MD with another numerical technique namely temperature dependent effective potential (TDEP) method is used to study the vibrational free energy and phase stability of CrN. We found that the combination of magnetic and vibrational contributions to the free energy shifts down the phase boundary between the cubic paramagnetic and orthorhombic antiferromagnetic phases of CrN towards the experimental value.

I used the stress-strain relation to study the temperature-dependent elastic properties of paramagnetic materials within DLM-MD with CrN as my model system. The results from a combinimation of DLM-MD with another newly developed method, symmetry imposed force constants (SIFC) in conjunction with TDEP is also presented as comparison to DLM-MD results.I also apply DLM-MD method to study the electronic structure of NiO in its paramagnetic state at finite temperatures. I found that lattice vibrations have a prominent impact on the electronic structure of paramagnetic NiO at high temperatures and should be included for the proper description of the density of states.

In summary, I believe that the proposed techniques give reliable results and allow us to include the effects from electron-lattice interaction in simulations of materials.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 85 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1766
Keyword
Molecular crystals, Charge transport, Polaron, Magnetic materials, Paramagnetic state, Molecular dynamics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-130517 (URN)10.3384/diss.diva-130517 (DOI)9789176857625 (ISBN)
Public defence
2016-09-16, Plank, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-08-23 Created: 2016-08-11 Last updated: 2016-08-23Bibliographically approved

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Mozafari, ElhamStafström, Sven

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