Activities

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/ 

Transformation of a Fermi surface into Luttinger arcs: A novel analytical insight
I 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.

[1] P. Worm, M. Reitner, K. Held, and A. Toschi, Phys. Rev. Lett. 133, 166501 (2024).

[2] Y. Hatsugai and M. Kohmoto, J. Phys. Soc. Jpn. 61, 2056 (1992).

[3] A. Toschi, A.A. Katanin, and K. Held, Phys. Rev. B 75, 045118

(2007); G. Rohringer et al., Rev. Mod. Phys. 90, 025003 (2018).

Registration: https://thomasyoungcentre.org/event/tyc-highlight-seminar-prof-hannes-jonsson-university-of-iceland/

Calculations 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.

Several 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].

 [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).

TYC Seminar: Ab Initio Modeling of Exciton-Phonon Interactions in Emerging Materials: Applications and Recent Developments - THOMAS YOUNG CENTRE

Ab Initio Modeling of Exciton-Phonon Interactions in Emerging Materials: Applications and Recent Developments - Jonah Haber, Standford University

Excitons — 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.

In 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.

Motivated 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.

Venkat Kapil from UCL will give an overview of ML force fields – their generation, use and software which can enable this (inc. its use on HPC/Young).

Future talks aim to include commonly codes used on Young, such as Quantum ESPRESSO and Casino and include some emerging technologies such as machine learning with Keras, Tensorflow and Torch

Join Zoom Meeting:

Meeting ID: 991 6854 2304
Passcode: TYCSWS

The physical-chemistry of the graphene/aqueous–electrolyte interface underpins the operational conditions of a wide range of devices. Despite its importance, this interface is poorly understood due to the challenges faced in its experimental characterization and the difficulty of developing models that encompass its full physics. In this talk I’ll present the simulation methods we have developed to model such interface also under confinement and how modelling can aid the full characterization of this interface.
 
https://thomasyoungcentre.org/event/tyc-highlight-seminar-prof-paola-carbonne-university-of-manchester/

https://thomasyoungcentre.org/event/tyc-postgraduate-student-day-2025/
 
We invite all TYC students to submit abstracts to present a poster of their research, and for final year students to submit abstracts for talks.  ~12 talks will be selected (12 minute presentations and 2 minute Q&A), and all of the posters from across the four London TYC colleges, plus LSBU and Brunel, will be on display at the poster presentation during lunch and at a drinks reception at the end of the day.
 
Abstract submission deadline: Sunday 18th May 2025