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Trends in Magnetism: From Strong Correlations to “-onics” Technology
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Despite of enormous progress in experimental nanophysics theoretical studies of low-dimensional electron systems still remains a challenging task. Indeed, most of the structures are strongly correlated, so that an effective perturbative treatment is impossible due to the lack of a small parameter. The problem can be partly solved within the dynamical mean-field theory (DMFT) paradigm, nevertheless the correlations in physically relevant high-temperature superconductors are of purely non-local nature. The recently developed dual fermion approximation, combining field-theoretical diagram technique and numerical methods, allows for explicit account of spatial correlations. The approximation was shown to be of fastest convergence compared with standard DMFT extensions, and along with renormalization group is used here to study Fermi condensation on a triangular lattice near van Hove singularities. The still debated phenomenon of Fermi condensation is believed to be a precursor to strongly correlated low-temperature instability and is found in this thesis to be robust even at high temperature, making its experimental verification feasible. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localized clot of magnetic energy. Consistent study of spin, which essentially is a quantum mechanical entity, led to the emergence of spintronics (spin-based electronics) and magnonics (photonics with spin waves), in the meanwhile topologically protected magnetic solitons and skyrmions might potentially be applied for data processing and information storage in next generation of electronic technology (rapidly advancing solitonics and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics" technology. It is shown here that spins on a kagome lattice, interacting via Heisenberg exchange and Dzyaloshinskii-Moriya coupling, allow the formation of topologically protected edge states through which a skyrmion can propagate. Not only can chemical methods be used to design novel functionality, but also geometric structuring. It is demonstrated that for graphene sandwiched between two insulating media external circularly-polarized light serves as an effective magnetic field. The direct practical implication permits to control light polarization and induce spin-waves propagating on the surface of e.g. a topological insulator. The newly discovered Dirac materials, graphene and three-dimensional topological insulators, are not easy to handle. In fact, the quasiparticle band function is gapless preventing them from being used in integrated circuits, nevertheless the problem is shown here to be partially relaxed by placing a vacancy on top of it. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 109 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1214
Keyword [en]
Strongly interacting electron systems, Spin dynamics, Topological matter
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-238177ISBN: 978-91-554-9130-7 (print)OAI: oai:DiVA.org:uu-238177DiVA: diva2:770727
Public defence
2015-02-13, Hall IX, Main University Building, Biskopsgatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2015-01-22 Created: 2014-12-10 Last updated: 2015-03-09
List of papers
1. Fermi Condensation Near van Hove Singularities Within the Hubbard Model on the Triangular Lattice
Open this publication in new window or tab >>Fermi Condensation Near van Hove Singularities Within the Hubbard Model on the Triangular Lattice
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2014 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 7, 070403- p.Article in journal (Refereed) Published
Abstract [en]

The proximity of the Fermi surface to van Hove singularities drastically enhances interaction effects and leads to essentially new physics. In this work we address the formation of flat bands ("Fermi condensation") within the Hubbard model on the triangular lattice and provide a detailed analysis from an analytical and numerical perspective. To describe the effect we consider both weak-coupling and strong-coupling approaches, namely the renormalization group and dual fermion methods. It is shown that the band flattening is driven by correlations and is well pronounced even at sufficiently high temperatures, of the order of 0.1-0.2 of the hopping parameter. The effect can therefore be probed in experiments with ultracold fermions in optical lattices.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-220984 (URN)10.1103/PhysRevLett.112.070403 (DOI)000331953300003 ()
Available from: 2014-03-25 Created: 2014-03-24 Last updated: 2017-12-05Bibliographically approved
2. Topological excitations in a kagome magnet
Open this publication in new window or tab >>Topological excitations in a kagome magnet
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2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, 4815- p.Article in journal (Refereed) Published
Abstract [en]

Chirality-that is, left or right handedness-is present in many scientific areas, and particularly in condensed matter physics. Inversion symmetry breaking relates chirality with skyrmions, which are protected field configurations with particle-like and topological properties. Here we show that a kagome magnet, with Heisenberg and Dzyaloshinskii-Moriya interactions, causes non-trivial topological and chiral magnetic properties. We also find that under special circumstances, skyrmions emerge as excitations, having stability even at room temperature. Chiral magnonic edge states of a kagome magnet offer, in addition, a promising way to create, control and manipulate skyrmions. This has potential for applications in spintronics, that is, for information storage or as logic devices. Collisions between these particle-like excitations are found to be elastic at very low temperature in the skyrmion-skyrmion channel, albeit without mass-conservation. Skyrmion-antiskyrmion collisions are found to be more complex, where annihilation and creation of these objects have a distinct non-local nature.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-236091 (URN)10.1038/ncomms5815 (DOI)000342929000005 ()
Available from: 2014-11-14 Created: 2014-11-12 Last updated: 2017-12-05
3. Spontaneous gap generation on the surface of weakly interacting topological insulators using nonmagnetic impurities
Open this publication in new window or tab >>Spontaneous gap generation on the surface of weakly interacting topological insulators using nonmagnetic impurities
2014 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 90, no 16, 161413- p.Article in journal (Refereed) Published
Abstract [en]

Strong nonmagnetic impurities on the surface of three-dimensional topological insulators (TIs) generate localized resonance peaks close to the Dirac point. We show that this results in a strongly reduced critical Coulomb interaction strength to reach a magnetic surface state, following a Stoner-like criterion. Thus even weakly interacting TIs host a finite (local) magnetization around strong nonmagnetic impurities. The local magnetization gives rise to a global energy gap, linearly dependent on the maximum value of the magnetization but decreasing with reduced impurity concentration.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-238174 (URN)10.1103/PhysRevB.90.161413 (DOI)000344032300002 ()
Available from: 2014-12-10 Created: 2014-12-10 Last updated: 2017-12-05Bibliographically approved
4. Dynamics of quasiparticles in graphene under intense circularly polarized light
Open this publication in new window or tab >>Dynamics of quasiparticles in graphene under intense circularly polarized light
2015 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 91, no 7, 075419Article in journal (Refereed) Published
Abstract [en]

A monolayer of graphene irradiated with circularly polarized light suggests a unique platform for surface electromagnetic wave (plasmon-polariton) manipulation. In fact, the time periodicity of the Hamiltonian leads to a geometric Aharonov-Anandan phase and results in a photovoltaic Hall effect in graphene, creating off-diagonal components of the conductivity tensor. The latter drastically changes the dispersion relation of surface plasmon-polaritons, leading to hybrid wave generation. In this paper we present a systematic and self-contained analysis of the hybrid surface waves obtained from Maxwell equations based on a microscopic formula for the conductivity. We consider a practical example of graphene sandwiched between two dielectric media and show that in the one-photon approximation there is formation of propagating hybrid surface waves. From this analysis emerges the possibility of a reliable experimental realization to study Zitterbewegung of charge carriers of graphene.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-238176 (URN)10.1103/PhysRevB.91.075419 (DOI)000350252000005 ()
Funder
Swedish Research Council
Available from: 2014-12-10 Created: 2014-12-10 Last updated: 2017-12-05Bibliographically approved

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