"Modeling of Complex Systems" Seminar
2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025
2024-11-14 (Thursday)
room 1.40, Pasteura 5 at 15:15 
Dr. Thomas Brumme (Theoretical Chemistry, TU Dresden, Germany)The whole is more than the sum of its parts: electron-phonon and exciton-phonon coupling in layered materials
Scientists are uncovering fascinating ways in which light interacts with electrons and vibrations in ultra-thin layers of special materials called transition metal dichalcogenides. When exposed to light, electrons in these materials get "excited" and begin to transfer energy to the atoms around them. This process, involving vibrations known as phonons, is key to controlling how these materials conduct heat and electricity, which could make them valuable for next-gen electronics. Combining femtosecond electron diffraction experiments with our theoretical calculations, we studied in WSe2 how excited electrons distribute their energy across different phonons. We found that this energy transfer is not uniform; it varies with momentum and initially creates an uneven, “non-thermal” phonon distribution. Over time, however, this distribution evens out, leading to a stable, thermal state.Going further, we studied more recently the interaction between WSe2 and MoSe2 when stacked together in a layered structure. Here, we observed a strong interlayer connection: phonons in one layer influence excitations (excitons) in the other. Using Raman scattering techniques together with density-functional calculations, we show that MoSe2 vibrations couple stronger to WSe2 excitons than vice versa, highlighting a unique coupling effect that cannot be understood by looking at either layer alone.Instead, it is the interaction between layers that creates new electronic and vibrational behaviors. These findings might help to develop finely-tuned, light-responsive materials with applications in optoelectronics and quantum technologies.
The seminar is held in hybrid mode:Join Zoom Meetinghttps://uw-edu-pl.zoom.us/j/97084466352?pwd=REoPAygK6p2JyEJevuxObOry69lc6r.1Meeting ID: 970 8446 6352Passcode: 982002
2024-10-24 (Thursday)
room 1.40, Pasteura 5 at 15:15
Stanisław Żukowski (IFT UW)Breakthrough-Induced Loop Formation in Evolving Transport Networks
ransport networks, such as jellyfish canal network or river networks, provide key functions in organisms and the environment. They usually contain loops whose significance for the stability and robustness of the network is well documented. However, the dynamics of their formation is usually not considered. Such structures often grow in response to the gradient of an external field. During evolution, extending branches compete for the available flux of the field, which leads to effective repulsion between them and screening of the shorter ones. Yet, in remarkably diverse processes, from the canal system of jellyfish to unstable fluid flows, loops suddenly form near the breakthrough when the longest branch reaches the boundary of the system. We provide a physical explanation for this universal behavior. Using a 1D model, we explain that the appearance of effective attractive forces results from the field drop inside the leading finger as it approaches the outlet. Furthermore, we numerically study the interactions between two fingers, including screening in the system and its disappearance near the breakthrough. Finally, we perform simulations of the temporal evolution of the fingers to show how revival and attraction to the longest finger leads to dynamic loop formation. We compare the simulations to the experiments and find that the dynamics of the shorter finger are well reproduced. Our results demonstrate that reconnection is a prevalent phenomenon in systems driven by diffusive fluxes, occurring both when the ratio of the mobility inside the growing structure to the mobility outside is low and near the breakthrough.
The seminar will be held in hybrid mode, join Zoom Meeting, https://uw-edu-pl.zoom.us/j/97084466352?pwd=REoPAygK6p2JyEJevuxObOry69lc6r.1Meeting ID: 970 8446 6352 Passcode: 982002
2024-10-17 (Thursday)
room 1.40, Pasteura 5 at 15:15
Carmine Autieri (MagTop IF PAN)Spin-orbital properties of the altermagnets
The Dzyaloshinskii-Moriya interaction (DMI) has explained successfully the weak ferromagnetism in some centrosymmetric antiferromagnets. However, in the last years, it was generally claimed that the DMI is not effective in such systems. We reconciled these views by separating the conventionalantiferromagnets from altermagnets. Altermagnets are collinear magnets having zero magnetization preserved by crystal symmetries in the non-relativistic limit. The spin-up and spin-down sublattices are connected by rotation (proper or improper and symmorphic or nonsymmorphic). Consequently, the system shows even-parity wave spin order in the k-space lifting the Kramer’s degeneracy in the non-relativistic band structure leading to unconventional magnetism. The staggered DMI is one ofthe mechanisms that can create weak ferromagnetism or weak ferrimagnetism in centrosymmetric and noncentrosymmetric altermagnets while it is not effective in conventional antiferromagnets.Once the spin-orbit coupling is included in an altermagnetic system (where the time-reversal symmetry is broken) with staggered DMI, the components of spin moments of the two sublattices alongthe N´eel vector are antiparallel but the other two spin components orthogonal to the N´eel vector can be null, parallel or antiparallel. In cases where we have different bands showing parallel and antiparallel spin components at the same time, the magnetic order results in weak ferrimagnetism. The altermagnetic compounds can host weak ferromagnetism, weak ferrimagnetism or zero magnetization. Restricted to the altermagnet with 2 atoms and staggered DMI, the Hall vector is orthogonalto the N´eel vector in the case of weak ferromagnetism and weak ferrimagnetism with a magnetic component proportional to the DMI. We find a sign change of the magnetization, and possibly of theanomalous Hall effect, as a function of the band filling and N´eel vector. We describe the dependence of the weak ferromagnetism on the charge doping.
The seminar is held in hybrid mode:Join Zoom Meetinghttps://uw-edu-pl.zoom.us/j/97084466352?pwd=REoPAygK6p2JyEJevuxObOry69lc6r.1Meeting ID: 970 8446 6352Passcode: 982002
2024-10-10 (Thursday)
room 1.40, Pasteura 5 at 15:15
Tomasz Woźniak (IFT UW)Excitonic g-factors in van der Waals structures
The shifts of optical peaks energies under external magnetic field, quantified by effective g-factors, provide a deep insight into electronic and excitonic structures of two-dimensional materials. A recently developed first-principles-based method for calculation of g-factors, including bands-summation formula, yields excellent agreement with experiments for intralayer excitons in monolayer (1L) transition metal dichalcogenides (TMDCs), interlayer excitons in TMDCs heterobilayers, as well as larger excitonic complexes in doped 1L TMDCs [1,2]. The last case corroborates the accuracy of the method for evaluation of single bands g-factors. Here we present more advanced cases which are beyond the possibilities of simplistic models with spin, atomic orbital and valley contributions. We explain the reduction of g-factors measured in MoSe2/WS2 moiré heterobilayer by inclusion of exciton g-factor’s dispersion and spatial confinement in moiré potential [3]. Inclusion of excitonic wavefunctions calculated by model Bethe-Salpeter equation enables us to understand the measured excitonic state dependence of g-factors in 1L and homobilayer TMDCs [4,5]. We analyze the influence of biaxial strain on 1L TMDs, finding a large strain dependence of excitonic g-factors, with significant spin-mixing effects [6]. The calculated trends of direct and indirect excitons g-factors in WS2 micro-bubbles allow us to explain the strain-induced exciton hybridization in WS2 monolayers unveiled by magnetooptical measurements [7]. We investigate a new class of hexagonal 2D materials with formula MSi2Z4 (M: Mo, W; Z: N, P, As, Sb), which are isosymmetric to 1L TMDCs. We find a new set of circularly polarized excitonic transitions with high binding energies and large positive g-factors [8].[1] Phys. Rev. B 101, 235408 (2020).[2] Nano Lett. 21, 2519 (2021).[3] Nano Lett. 22, 8641 (2022).[4] Nano Lett. 19, 2464 (2019).[5] 2D Mater. 10, 025014 (2023).[6] New J. Phys. 24, 083004 (2022).[7] Phys. Rev. Lett. 129, 067402 (2022).[8] Small 19 2206444 (2023).
The seminar will be held in hybrid mode: in room 1.40 (Faculty of Physics, UW). Join Zoom Meeting https://uw-edu-pl.zoom.us/j/97084466352?pwd=REoPAygK6p2JyEJevuxObOry69lc6r.1 Meeting ID: 970 8446 6352 Passcode: 982002