Projects / Programmes
Investigation of strongly interacting electron systems by a computational study of a model for organic superconductors
| Code |
Science |
Field |
Subfield |
| 1.02.02 |
Natural sciences and mathematics |
Physics |
Theoretical physics |
| Code |
Science |
Field |
| P260 |
Natural sciences and mathematics |
Condensed matter: electronic structure, electrical, magnetic and optical properties, supraconductors, magnetic resonance, relaxation, spectroscopy |
| Code |
Science |
Field |
| 1.03 |
Natural Sciences |
Physical sciences |
Solid state theory, correlated electrons, organic superconductors, Hubbard model, frustrated systems, phase diagram, Mott insulator, spin liquid, thermodynamic properties, transport properties, finite temperatures, numerical methods
Researchers (1)
| no. |
Code |
Name and surname |
Research area |
Role |
Period |
No. of publicationsNo. of publications |
| 1. |
26458 |
PhD Jure Kokalj |
Physics |
Head |
2013 - 2015 |
0 |
Organisations (1)
| no. |
Code |
Research organisation |
City |
Registration number |
No. of publicationsNo. of publications |
| 1. |
0106 |
Jožef Stefan Institute |
Ljubljana |
5051606000 |
18 |
Abstract
Organic superconductors have opened a new avenue to investigate strongly correlated electron systems, mainly by offering the possibility of experimentally changing the extent of frustration and the strength of interaction. This is not possible in the more intensively studied cuprate high-temperature superconducors, and has led to new discoveries, e.g., a new phase of matter called the spin liquid, and opened new fundamental questions, e.g., the universality class of the Mott metal-insulator transition. Although both families of materials can tentatively be described with similar Hubbard models (cuprates on the square lattice and organics on the anisotropic triangular lattice) understanding these materials is difficult due to the strong correlations, many-body quantum effects, competing phases, and low dimensionality.
One of the few reliable approaches to study these systems is by computer simulations using state-of-the-art numerical methods. In this project, methods based on exact diagonalization, e.g., the finite temperature Lanczos method, will be used to extensively study the Hubbard model on the anisotropic triangular lattice. These methods have already proven successful in investigations of the t-J and Hubbard models for the cuprates, but are even better suited for the study of organic superconductors.
Fundamental questions that will be addressed are the differences and similarities of destroying the Mott insulator by doping (realized in cuprates), or by increasing frustration or decreasing interaction strength (as realized in organics). Similarities were experimentally found, e.g., in anomalous linear-in-temperature resistivity, close to the Mott metal-insulator transition, which also exhibits a connection to high-temperature superconductivity, while differences were observed in the quasi-particle mass renormalization. These still await proper microscopic explanation.
Furthermore, study of the model in different regimes will give information on the two most competitive theories of high temperature superconductivity, namely the resonating valence bond (RVB) and spin fluctuation theory. The universality class of the Mott metal-insulator transition will also be addressed, for which a new and not yet understood set of critical exponents has been observed and is a subject of ongoing controversy.
Furthermore, a large part of this project will be more experimentally orientated and the following question will be addressed: “How well can electronic properties of organics, which are rather complicated molecular crystals, be described with the Hubbard model on the anisotropic triangular lattice?”. This question demands a resolution, since it will give a firm foundation for the future research. The phase diagram of the model will be calculated and compared to the experimental phase diagram of organics, which will be assisted by the quantitative comparison of results with many experimental studies on static and dynamic quantities. Understanding the behaviors of some of these quantities represent challenges on their own. These include the non-monotonic temperature dependence of transport quantities (DC conductivity, Hall coefficient and thermopower), suppression of the NMR relaxation rate at low temperature (existence of a pseudogap) and existence of the reentrant phase (insulator-metal-insulator transitions appearing by just lowering the temperature). All these quantities show intriguing temperature dependence and need to be addressed with calculations for finite temperatures.
Methods used in this project allow for the calculation of temperature, momentum and frequency dependencies of many physical quantities, which are hard, if not impossible, to calculate with other methods. This project will therefore represent a comprehensive study of organic superconductors, and in addition offer a necessary complementary study to the few existing dynamical mean-field theory results.
Significance for science
Within this project we studied the strongly interacting electrons, which present the central challenge for the theory of condensed matter. By using the novel and exact approach from the strongly spin polarized system, we showed that the main mechanism for the Mott transition is the binding and unbinding of holon-doublon pairs. This strongly advances our understanding of the transition and gives a foundation for further understanding of the transition and its properties, including its critical behaviour. By calculating a number of physical quantities we determined the phase diagram of the Hubbard model on the anisotropic triangular lattice and the properties of various phases. This considerably improves the theoretical understanding and description of the phase diagram and phases within, which is important for the further and more precise investigation of each phase, their properties and for the comparison with experimental data. Among other phases we also determined the regimes of more controversial spin liquid and pseudogap phase. Further on we compared within the model-calculated quantities with the experimental data for the organic superconductors and mostly obtained very good quantitative agreement. This gives a firm support to the description of the organic superconductors with the used microscopic model, which presents firm foundation for the future theoretical studies of these materials. On the other hand, we stressed that for the description and understanding of certain properties, the model needs to be upgraded, e.g., for the proper description of thermoelectricity one needs to include the dimerization of transfer integrals on the triangular lattice. In addition we showed that the thermoelectric effect is enhanced at the transition from the Fermi liquid to the bad-metallic phase, which is interesting also for the applications for conversion between electric voltage and temperature gradient. By calculations of optical conductivity, charge susceptibility and diffusion constant close to a Mott transition, we show that in the bad-metallic phase and at high temperatures the conductivity can be understood by the behaviour of the charge susceptibility. Via this simpler and more fundamental quantity one can understand the strong dependence of the resistivity on doping, as well as its linearity in temperature and the violation of the Mott-Ioffe-Regel limit. We also showed that the spin correlations increase the charge susceptibility, which leads to decreased resistivity and suggest this as a possible explanation of pseudogap signatures in the resistivity. Suggested picture considerably improves our understanding of bad-metallic behaviour via simpler quantities and we hope, it will stimulate further theoretical, and even more, more precise experimental verifications. Relevance of the scientific results is already reflected by the publications in the respected international journals, international response and larger number of invited talks.
Significance for the country
The knowledge and better understanding of the materials with strongly interacting electrons obtained within this project helps towards potential application of these materials, e.g., as thermoelectric or high-temperature superconductors. The project established firm collaborations with foreign researches working at the University of Queensland, Brisbane (Australia), Yukawa institute for theoretical physics, Kyoto (Japan), RIKEN Advanced Institute for Computational Science, Kobe (Japan), Tokyo University of Science, Tokyo (Japan), RIKEN, Wako (Japan), University of Crete, Heraklion (Greece) and ETH Zürich (Switzerland). This makes the foreign knowledge and expertise accessible and allows for the better results via combination of local and foreign knowledge. In addition we established within this project also the collaborations with two experimental groups, one at the ETH Zürich (Switzerland) and the other at the Institute Jožef Stefan, which allows for the advances in understanding by cooperation between the theory and the experiment. All this considerably improves the visibility and recognition of Slovenian science, which is further supported by publications in international journals, citations and presentations abroad. This increases the possibility of acquiring international (e.g. European) projects and makes Slovenia more attractive for foreign researches and students.
Most important scientific results
Annual report
2013,
2014,
final report,
complete report on dLib.si
Most important socioeconomically and culturally relevant results
Annual report
2013,
2014,
final report,
complete report on dLib.si
