У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 17. децембра 2020. године у 14 часова путем Zoom платформе, др Вељко Јанковић (Лабораторија за примену рачунара у науци, Институт за физику у Београду) одржаће предавање:

Relevance of incoherent light-induced coherences for photosynthetic energy transfer

САЖЕТАК:

The interpretation of oscillatory features in experimental signals from photosynthetic pigment-protein complexes excited by laser pulses [1] has been motivating vigorous interest in quantum effects in photosynthetic energy transfer (ET) [2]. However, electronic dynamics triggered by natural light, which is stationary and incoherent, is generally substantially different from the one observed in pulsed laser experiments. It has been suggested that the physically correct picture of photosynthetic ET should be in terms of a nonequilibrium steady state (NESS) [3], which is formed when a photosynthetic complex is continuously photoexcited and continuously delivers the excitation energy to the reaction center (RC), in which charge separation takes place.

In this talk we will consider ET in a molecular aggregate that is driven by weak incoherent radiation and coupled to its immediate environment and the RC. We will combine a second-order treatment of the photoexcitation with an exact treatment of the excitation-environment coupling and formulate the hierarchy of equations of motion (HEOM) that explicitly takes into account the photoexcitation process [4]. We will develop an efficient numerical scheme that respects the continuity equation for the excitation fluxes to compute the NESS of the aggregate [5]. The NESS is most conveniently described in the so-called preferred basis, in which the excitonic density matrix is diagonal. Having established the proper NESS description, we will critically reassess the involvement of stationary coherences in the photosynthetic ET and claims that stationary coherences may enhance the ET efficiency. Focusing on a model photosynthetic dimer, we will also examine the properties of the NESS and establish a general relationship between NESS picture and time-dependent description of an incoherently driven, but unloaded system.

[1] G. S. Engel et al., Nature 446, 782 (2007).
[2] J. Cao et al., Sci. Adv. 6, eaaz4888 (2020).
[3] P. Brumer, J. Phys. Chem. Lett. 9, 2946 (2018).
[4] V. Janković and T. Mančal, arXiv: 2001.07180 (2020).
[5] V. Janković and T. Mančal, arXiv: 2008.13395 (2020).

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Meeting ID: 899 2935 0836
Passcode: 104769

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 10. децембра 2020. године у 14 часова путем Zoom платформе, Милан Јоцић (Лабораторија за примену рачунара у науци, Институт за физику у Београду) одржаће предавање:

Construction of symmetry-adapted k.p Hamiltonians for semiconductor nanostructures

САЖЕТАК:

Since semiconductor materials are at the heart of almost all electronic devices, it is paramount to use the ones that have the best performances at the lowest cost. This is, in particular, important for solar cells, that convert clean solar energy to electricity. In recent years, metal-halide perovskites have drawn great attention in literature, since the power conversion efficiency of solar cells based on these materials has increased significantly and has reached more than 20% with the potential for improving even further. Understanding and modeling the electronic properties of these materials and their various nanostructures is of great importance.

Using ab initio methods like DFT for nanostructures is computationally very expensive, even with modern supercomputers. However, in this talk we will show that an accurate quantitative picture can be obtained with a k.p method by starting with Kohn-Sham (KS) states obtained from ab initio calculations for bulk structure. We will demonstrate this by comparing k.p with DFT calculations for the case of CdSe quantum wells [1]. We will obtain the analytical form and numerical parameters of well-studied 4×4 and 8×8 k.p Hamiltonians found in literature [2], for the case where spin-orbit coupling is omitted and included, respectively. Also, we will demonstrate an improvement over 4×4 and 8×8 Hamiltonians, by expanding the number of states from 4 (8) to 13 (26), which yields more accurate excited states. Another improvement can be made, by using the GW approximation within the many-body perturbation theory. This method can give more accurate bulk band gaps, which in turn yields improved results for nanostructures. At the end of the talk, we will present our latest results, for the inorganic halide-perovskite CsPbBr3 cubic quantum dot, and discuss further direction of our work.

[1] M. Jocić and N. Vukmirović, Phys. Rev. B 102, 085121 (2020).
[2] L. C. Lew Yan Voon and M. Willatzen, The k.p Method: Electronic Properties of Semiconductors (Springer-Verlag, Berlin, 2009).

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Meeting ID: 847 8071 9484
Passcode: 543566

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 3. децембра 2020. године у 14 часова путем Zoom платформе, Марија Шиндик (Лабораторија за примену рачунара у науци, Институт за физику у Београду) одржаће предавање:

Quantum droplets in dipolar ring-shaped Bose-Einstein condensates

САЖЕТАК:

Bose-Einstein condensation of atoms with a permanent magnetic dipole moment was experimentally first achieved in 2005. This opened up a possibility of exploring the effects of dipole-dipole interactions that are long-ranged, in contrast to the short-ranged contact interaction, a consequence of scattering in the system. Due to the competition between these two interactions, one repulsive and one of the varying type, the effects of quantum fluctuations become more prominent, and can lead to the emergence of quantum droplets as a new state of matter.

In this talk, we will discuss the results of numerical investigation of the regime in which a quantum droplet can be formed in a dipolar condensate. The system is described by the Gross-Pitaevskii equation, including the dipolar analogue of the Lee-Huang-Yang correction term, and the condensate depletion due to quantum fluctuations is taken into account. A condensate is initially prepared in a stable ground state of the system, and droplet formation is triggered by a sudden quench of the contact interaction. We will discuss how the number of the obtained droplets depend on the total number of atoms in the system, as well as on the strength of the contact and the dipole-dipole interaction.

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Meeting ID: 862 2807 6758
Passcode: 682709

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у четвртак, 26. новембра 2020. године у 14 часова путем Zoom платформе, др Јакша Вучићевић (Лабораторија за примену рачунара у науци, Институт за физику у Београду) одржаће предавање:

Controlled solutions of the Hubbard model

САЖЕТАК:

Hubbard model is one of the most studied models in condensed matter theory. It is believed to capture the generic phase diagram of strongly correlated materials, such as kappa-organics and cuprate superconductors. Despite the decades of intense study and an immense progress that has already been made, solving the Hubbard model still presents a major challenge. In this talk, we will give an overview of some of the recent developments in the field. We will focus on controlled approaches, i.e., the numerical methods that can be systematically tuned towards an exact theory, and can thus be used to converge numerically exact results. In particular, we will discuss various quntum Monte Carlo techniques and embedded cluster theories, and present our recent results for the superconducting phase diagram, as well as the transport, thermodynamic and spectral properties of the Hubbard model.

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Meeting ID: 815 3051 1843
Passcode: 929121