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DTSTART;TZID=Europe/London:20250507T150000
DTEND;TZID=Europe/London:20250507T163000
DTSTAMP:20260411T050212
CREATED:20250402T125159Z
LAST-MODIFIED:20250402T133323Z
UID:6603-1746630000-1746635400@thomasyoungcentre.org
SUMMARY:TYC Seminar: Transformation of a Fermi surface into Luttinger arcs: A novel analytical insight - Alessandro Toschi\, TU Wien\, Austria
DESCRIPTION:TYC Seminar: Transformation of a Fermi surface into Luttinger arcs: A novel analytical insight – Alessandro Toschi\, TU Wien\, Austria Share on X\n\n\n\n\n \n\n\n\n\nRegister here\n\n\n\n\n\n\n\n\nAlessandro Toschi\, Institute of Solid State Physics\, TU Wien\, Austria \n\n\n\nTransformation of a Fermi surface into Luttinger arcs: A novel analytical insightI will present [1] an analytically solvable model for correlated electrons\, which is able to capture the major Fermi surface modifications occurring in both hole- and electron-doped cuprates as a function of doping. The proposed Hamiltonian\, which represents an extension of the Hatsugai-Kohmoto model [2]\, qualitatively reproduces the results of numerically demanding many-body calculations\, here obtained using the dynamical vertex approximation [3] in its ladder implementation. Our analytical theory provides a transparent description of a precise mechanism\, capable of driving the formation of disconnected segments along the Fermi surface (the highly debated “Fermi arcs”)\, as well as the opening of a pseudogap in hole and electron doping. This occurs through a specific mechanism: The electronic states on the Fermi arcs remain intact\, while the Fermi surface part where the gap opens transforms into a Luttinger arc. \n\n\n\n[1] P. Worm\, M. Reitner\, K. Held\, and A. Toschi\, Phys. Rev. Lett. 133\, 166501 (2024). \n\n\n\n[2] Y. Hatsugai and M. Kohmoto\, J. Phys. Soc. Jpn. 61\, 2056 (1992). \n\n\n\n[3] A. Toschi\, A.A. Katanin\, and K. Held\, Phys. Rev. B 75\, 045118 \n\n\n\n(2007); G. Rohringer et al.\, Rev. Mod. Phys. 90\, 025003 (2018).
URL:https://thomasyoungcentre.org/event/transformation-of-a-fermi-surface-into-luttinger-arcs-a-novel-analytical-insight-alessandro-toschi-institute-of-solid-state-physics-tu-wien-austria/
LOCATION:S0.03\, King’s College London\, Strand Campus\, Strand Building\, WC2R 2LS
CATEGORIES:Main event
ORGANIZER;CN="Jan Tomczak":MAILTO:jan.tomczak@kcl.ac.uk
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BEGIN:VEVENT
DTSTART;TZID=Europe/London:20250514T130000
DTEND;TZID=Europe/London:20250514T150000
DTSTAMP:20260411T050212
CREATED:20250221T141856Z
LAST-MODIFIED:20250506T132735Z
UID:6389-1747227600-1747234800@thomasyoungcentre.org
SUMMARY:TYC Masterclass: Mean field description of electronic structure: From Hartree-Fock to DFT and beyond
DESCRIPTION:Prof. Hannes Jonsson\, University of Iceland \n\n\n\n\n\n\n\n\n\n\n\n\nTYC Masterclass: Mean field description of electronic structure: From Hartree-Fock to DFT and beyond Share on X\n\n\n\n\nThe simplest picture we have for describing systems of electrons is to assume that each electron is only subject to the average influence of the other electrons. This is the basis of ‘mean field’ approximations. A function describing a single electron in such a mean field is referred to as an ‘orbital’ and the probability distribution for the location of the electron is the ‘orbital density’. While Hartree-Fock (HF) theory appears to be the optimal mean field description it turns out not to be in part because of the infinite range of Fock exchange. Today\, most calculations in chemistry and condensed matter physics are carried out using density functional theory (DFT) with some approximate functional of the Kohn-Sham (KS) form where the quantum mechanical aspects of the interaction between electrons is of finite range. This so-called ‘nearsightedness’ of the electrons is\, for example\, manifested in the chemical concept of functional groups. But\, the goal of Kohn-Sham theory is to describe electronic systems with only the total electron density\, thereby abandoning in principle the concept of orbitals. Orbitals are\, however\, introduced in KS-DFT only to obtain accurate enough approximation of the kinetic energy of the electrons\, but are not used in the estimation of the classical Coulomb interaction\, thereby introducing a self-interaction error (SIE). Even if the system consists of just one electron\, the KS estimate of the Coulomb interaction gives a non-zero value. The SIE is the primary source of many of the shortcomings of practical implementations of KS-DFT\, such as the tendency to overly delocalize electrons\, and the incorrect long range form of the potential. By making use of the concept of orbitals and the associated orbital density\, the self-interaction in the classical Coulomb interaction can be avoided\, but this brings in additional complexity in the calculations since the functional is then orbital density dependent. Over 40 years ago\, Perdew and Zunger proposed an orbital based self-interaction correction to KS functionals\, but it has not become commonly used for several reasons\, one being the added complexity of the numerical calculations. Several examples of such calculations will be given in the lecture\, especially for systems where commonly used KS functionals give poor estimates or even fail to give qualitatively correct results. The application of a self-interaction correction to a KS functional is\, however\, just a small step in the direction of an optimal mean field description. The development of an optimal orbital density dependent and self-interaction free functional remains a future task.
URL:https://thomasyoungcentre.org/event/tyc-masterclass-towards-an-optimal-mean-field-description-of-electronic-structure-in-molecules-and-condensed-matter-with-an-explicit-orbital-based-self-interaction-correction-to-kohn-sham-density-fun/
LOCATION:UCL Physics A1/3\, Physics Building\, Gower Street\, London\, WC1E 6BT\, United Kingdom
CATEGORIES:Main event
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BEGIN:VEVENT
DTSTART;TZID=Europe/London:20250515T150000
DTEND;TZID=Europe/London:20250515T170000
DTSTAMP:20260411T050212
CREATED:20250204T153703Z
LAST-MODIFIED:20250410T104451Z
UID:6319-1747321200-1747328400@thomasyoungcentre.org
SUMMARY:TYC Highlight Seminar: Calculations of excited electronic states by converging on saddle points on the electronic energy surface
DESCRIPTION:Prof. Hannes Jonsson\, University of Iceland \n\n\n\n\n\nFollowed by a reception in UCL Physics E7 \n\n\n\n\n\n\n\n\n\n\nTYC Highlight Seminar: Calculations of excited electronic states by converging on saddle points on the electronic energy surface Share on X\n\n\n\n\nCalculations of excited electronic states are important in various contexts such as light harvesting\, photocatalysis and molecular motors. They are challenging as commonly used optimization algorithms are based on minimization and converge on the ground state. As a result\, a time-dependent formulation of density functional theory (DFT) is frequently used\, TD-DFT\, especially within the linear response and adiabatic approximations. This approximate approach\, however\, has several limitations especially when significant charge transfer occurs during the excitation and when states are close in energy. Within configuration interaction (CI) theory\, it is evident that excited states correspond to saddle points on the electronic energy surface\, with the saddle point order increasing with the excitation level. While CI calculations can be accelerated greatly by using neural networks [1]\, they are much too computationally demanding for most systems of interest. DFT is used in most electronic structure calculations carried out today\, and by using an algorithm for converging on saddle points on the electronic energy surface\, the orbitals can be optimised for excited states to provide higher energy solutions to the underlying Kohn-Sham equations [2\,3]. This gives more robust estimates of the excitation energy than TD-DFT with computational effort similar to that of ground state calculations. \n\n\n\nSeveral applications of this approach with commonly used density functionals will be presented\, as well as calculations using a self-interaction corrected functional that gives improved results. In particular\, various excited states of the ethylene molecule\, including twisting of the C=C double bond\, the active element of various molecular motors\, and high energy Rydberg states\, have been analysed [4]. In a solid state application\, the various states relevant for the optical preparation of a pure spin state in nitrogen/vacancy defect in diamond\, a system used in various types of quantum technologies\, have been calculated. The results show close agreement with computationally demanding\, high-level calculations as well as experiments [5]. \n\n\n\n [1] Y.L.A. Schmerwitz et al. 19\, 3634 (2025). [2] G. Levi\, A.V. Ivanov and H. Jónsson\, J. Chem. Theo. Comput. 16\, 6968 (2020). [3] Y.L.A. Schmerwitz\, G. Levi and H. Jónsson\, J. Chem. Theory and Comput. 19\, 3634 (2023). [4] A.E. Sigurdarson\, Y.L.A. Schmerwitz\, D.K.V. Tveiten\, G. Levi and H. Jónsson\, J. Chem. Phys. 159\, 214109 (2023). [5] A.V. Ivanov\, Y.L.A. Schmerwitz\, G. Levi and H. Jónsson\, SciPost Physics 15\, 009 (2023). \n\n\n\n\nRegister here
URL:https://thomasyoungcentre.org/event/tyc-highlight-seminar-prof-hannes-jonsson-university-of-iceland/
LOCATION:Harrie Massey Lecture Theatre\, UCL\, 25 Gordon Street\, London\, WC1H 0AY
CATEGORIES:Main event
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DTSTART;TZID=Europe/London:20250520T160000
DTEND;TZID=Europe/London:20250520T170000
DTSTAMP:20260411T050212
CREATED:20250411T090359Z
LAST-MODIFIED:20250411T090726Z
UID:6628-1747756800-1747760400@thomasyoungcentre.org
SUMMARY:TYC Seminar: Ab Initio Modeling of Exciton-Phonon Interactions in Emerging Materials: Applications and Recent Developments
DESCRIPTION:TYC Seminar: Ab Initio Modeling of Exciton-Phonon Interactions in Emerging Materials: Applications and Recent Developments Share on X\n\n\n\n\n\n\n\n\n\nRegister\n\n\n\n\n\n\n\n\nJonah Haber\, Stanford University\n\n\n\nExcitons — correlated electron-hole pairs generated upon photoexcitation — provide a fundamental framework for describing low-energy optical excitations in semiconductors and insulators. Understanding how these quasiparticles interact with their environment\, particularly their coupling to atomic lattice vibrations (phonons)\, is key to optimizing materials for next-generation optoelectronic devices\, including photovoltaics\, LEDs\, and quantum emitters. \n\n\n\nIn this seminar\, I will present our recent efforts to develop and apply ab initio methods\, grounded in many-body perturbation theory\, to study exciton-phonon interactions in complex materials. I will begin by discussing various ways in which phonons can couple to excitons\, including how phonons renormalize exciton binding energies in halide perovskites and influence exciton line shapes in two-dimensional transition metal dichalcogenides. \n\n\n\nMotivated by the inherent complexity of modeling coupled exciton–phonon systems\, the second part of the talk will introduce our recent work on Maximally Localized Exciton Wannier Functions (MLXWFs). This new formalism provides a compact\, real-space representation of exciton states\, offering  insights into exciton band dispersion and  topology\, and paving the way for scalable modeling of exciton dynamics. I will demonstrate the utility of this framework through a detailed case study on how lattice vibrations influence exciton transport in organic semiconductors—highlighting how MLXWFs open new avenues for understanding exciton behavior at the microscopic level.
URL:https://thomasyoungcentre.org/event/tyc-seminar-ab-initio-modeling-of-exciton-phonon-interactions-in-emerging-materials-applications-and-recent-developments-jonah-haber-stanford-university/
LOCATION:Room 131\, Imperial College London\, Royal School of Mines\, Prince Consort Road\, South Kensington\, SW7 2AZ\, United Kingdom
CATEGORIES:Main event
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