coffee, tea, cookies at 16:00 in the main hall

Monday 16:30-17:30

Seminar room 1+2

- monthly seminars -

Wednesday 15:30 - 17:00

Seminar room 4

- weekly seminars -

Monday 11:00-12:00

Room 1D1

Wednesday 16:30 - 17:30

Seminar room 1D1

Thursday 14:00-15:00

Seminar room 4

26 Feb 2020

15:30

15:30

Complex carbohydrates such as glycosaminoglycans and the glycan moieties of glycoproteins are a functionally very important group of molecules. Yet, very little is known about the 3D structure of this class of molecules. Traditional structural biology methods, predominantly X-ray diffraction and NMR, fail to deliver high resolution structural information on complex carbohydrates, why alternative methods are needed in order to study these. Raman optical activity (ROA), a solution-phase chiroptical spectroscopic method, has experimentally proven to deliver a high level of information content from the spectral data of carbohydrates [1,2]. But as in any other optical spectroscopic method, for true validation of the experimental data, comparison with modelling at ab initio/DFT level is required. Carbohydrates, especially larger oligosaccharides, have until recently been out of the scope of DFT level calculations of the ROA property tensors, as the early algorithms relied heavily on cumbersome numerical iterations. Advances in theory, developed over the last two decades [3,4], have allowed for the boundaries of theoretical ROA calculations to be pushed beyond any perceivable limits, permitting the computational chemistry community to attempt calculations on ever larger model compounds. In this presentation, I will outline the current state-of-the-art of experimental and computational ROA, focusing on the efforts made to understand the complex world of carbohydrates. [1] Bell, A.F.; Hecht, L.; Barron, L.D., J. Am. Chem. Soc. 1994, 116, 5155-5161. [2] Johannessen, C.; Pendrill, R.; Widmalm, G.; Hecht, L.; Barron, L.D. Angew. Chem. Int. Ed. 2011, 123, 5461-5463. [3] Quinet, O.; Liégeois, V.; Champagne, B. J. Chem. Theory Comput., 2005, 1, 444-452. [4] Liégeois, V.; Ruud, K.; Champagne, B. J. Chem. Phys., 2007, 127, 204105

Seminarroom 4
iCal Event

26 Feb 2020

16:30

16:30

Surfaces are known to strongly in uence rates and mechanisms of chemical reactions. The most prominent example is heterogeneous catalysis, where the surface catalyzes the reaction. For dynamical simulations of chemical processes at surfaces, the coupling of the adsorbate to the degrees of freedom of the surface, i.e., electronhole pairs and phonons, is especially challenging. However, these couplings, which can lead to largely reduced lifetimes of electronically or vibrationally excited adsorbate states, are often essential. For instance, photo-induced reactions on metal surfaces are typically surface mediated, and proceed rather through couplings to so-called hot electrons, instead of a direct coupling to the laser field. Also, STMinduced reactions on surfaces, which can be either triggered by the electric field in the tunneling junction or by so called inelastic electron tunneling (IET), are strongly in uenced by the surface. In addition, reactions can also be induced by the STM non-locally, which means several nm away from the tip. The surface is even more important in such cases. In this presentation the following examples for the theoretical simulation of laser- and STM-driven processes at surfaces will be presented: i) the femtosecond laser induced desorption of H2 (D2) from Ru(0001)1, ii) the STM-switching of a 1,5-cyclooctadiene molecule (COD) on Si(100)2 and iii) the non-local STM-induced desorption of Chlorobenzene on Si(111)-7x73. For all these systems, potential energy surfaces are computed with density functional theory, either on the y or prior to the time propagations. On these surfaces we compute either classical dynamics, i.e., Born-Oppenheimer molecular dynamics or the solution of the Langevin equation, or quantum dynamics for open systems, i.e., the solution of the Liouville{von Neumann equation. References: 1 G. Füchsel, J.C. Tremblay, T. Klamroth, P. Saalfrank, Quantum Dynamical Simulations of the Femtosecond-Laser-Induced Ultrafast Desorption of H2 and D2 from Ru(0001), ChemPhysChem 14, 1471 (2013); G. Füchsel, J.C. Tremblay, T. Klamroth, P. Saalfrank, and C. Frischkorn, Phys. Rev. Lett. 109, 098303 (2012). 2 K. Zenichowski, J. Dokic, T. Klamroth, and P. Saalfrank, J. Chem. Phys. 136, 094705 (2012) ; K. Zenichowski, Ch. Nacci, S. Fölsch, J. Dokic, T. Klamroth, and P. Saalfrank, J. Phys.: Condens. Matter 24, 394009 (2012);S. Fölsch, K. Zenichowski, J. Dokic, T. Klamroth, and P. Saalfrank, Nano Lett. 9, 2996 (2009). 3 M. Utecht, T. Pan, T. Klamroth, R.E. Palmer, J. Phys. Chem. A 118, 6699 (2014); M. Utecht, R. E. Palmer, T. Klamroth, Phys. Rev. Materials 1, 026001 (2017); M. Utecht, T. Klamroth, Mol. Phys. 116, 1687 (2018); M. Utecht, T. Gaebel, T. Klamroth, J. Comp. Chem. 39, 2517 (2018).

Room 1D1
iCal Event

26 Feb 2020

16:45

16:45

Sugars play a key role in all organisms with functions spanning from structural, such as cellulose or chitin, to storage, like starch or glycogen. As compared to proteins, where established 3D structure probing techniques provide atomistic details, the microscopic structure of saccharides in solution is much more complex and elusive. Traditional techniques are not suitable for these highly mobile, hydrophilic molecules. Therefore, methods which could tackle such molecules in their functional environment, are of a great importance. In our work we developed a method, which allows us to gain knowledge about the microscopic structure of such molecules. Our approach is to couple computational techniques, together with Raman/ROA/NMR spectroscopies, which in turn enables us to interpret the experimental data and fully assign the 3D structural ensemble for the molecule. The developed program iteratively searches through the phase space of the molecule and calculates its spectra using ab initio methods. It is able to quickly find structural regions that are required to reproduce all experimental data. An important factor is that all experimental data is being fitted at the same time, therefore, increasing the confidence of the 3D structure predictions. The developed program is modular, meaning that any experimental data that is structure related and can be calculated/measured, e.g. IR/VCD/ECD, can be plugged in and used to make even more precise predictions. To this end, the program is able to accurately interpret Raman/ROA/NMR experimental data and predict which structural regions are needed to reproduce the experimental data. In the future we plan to use the gained knowledge to develop even better computer models (force fields) which would generate ensembles that provide spectroscopic data that agree well with experiment.

Seminarroom 4
iCal Event

27 Feb 2020

14:00

14:00

Physical systems differing in their microscopic details often display strikingly similar behaviour when probed at macroscopic scales. Those universal properties, largely determining their physical characteristics, are revealed by the renormalization group (RG) procedure, which systematically retains ‘slow’ degrees of freedom and integrates out the rest. We demonstrate a machine-learning algorithm based on a model-independent, information-theoretic characterization of real-space RG, capable of identifying the relevant degrees of freedom and executing RG steps iteratively without any prior knowledge about the system. We prove results about optimality of the procedure, in particular we show a connection between the complexity of the resulting effective theory and the information-theoretic quantities optimized. We discuss applications to disordered systems.

Seminarroom 4
iCal Event

02 Mär 2020

16:30

16:30

Seminarroom 1+2
iCal Event

05 Mär 2020

14:00

14:00

I will discuss the anomalous transport properties of one-dimensional, disordered Hubbard chain with spin-1/2 fermions. For sufficiently strong disorder, the SU(2)-invariant model exhibits partial many-body localization, in that localized charge degrees of freedom coexist with delocalized spin excitations. It will be shown that the spin excitations spread subdiffusively, due to singular distribution of effective exchange interactions. The exponent relevant for the subdiffusion is determined by the Anderson localization length and the density of electrons. Surprisingly, the decay of local energy correlations is suppressed and is at least marginally nonergodic, being qualitatively different from the subdiffusive dynamics and relaxation of local spins. It will be demonstrated that even weak spin-asymmetry localizes spins and restores full many-body localization. Finally, I will also discuss transport properties of selected strongly disordered two-dimensional systems with many-body interactions.

Seminarroom 4
iCal Event

06 Mär 2020

11:00

11:00

This presentation shows the ability of the Lorenz’s (2005) chaotic model to simulate predictability curve of the ECMWF model calculated from data over the 1986 to 2011 period and it demonstrates similarity of the predictability curves for the Lorenz’s model with N = 90 variables. Approximations of predictability curves and their differentials are also presented and discussed, aiming to correct the ECMWF model estimated parameters and thus allow for estimation of the largest Lyapunov exponent, model error and limit value of the predictability curve. The correction is based on comparing the parameters estimated for the Lorenz’s and ECMWF and on comparison with the largest Lyapunov exponent (λ=0.35 day -1 ) and limit value of the predictability curve (E ∞ =8.2) of the Lorenz’s model. Parameters are calculated from approximations made by the Quadratic hypothesis with and without model error, as well as by Logarithmic and General hypotheses and by hyperbolic tangent employing corrections with and without model error. Average value of the largest Lyapunov exponent is estimated to be in the range <0.32; 0.41> day -1 for the ECMWF model, limit values of the predictability curves are estimated with lower theoretically derived values and new approach of calculation of model error based on comparison of models is presented.

Seminarroom 4
iCal Event

06 Mär 2020

11:00

11:00

Room 1D1
iCal Event

09 Mär 2020

16:30

16:30

Tensor network methods have become workhorses in the study of strongly correlated systems in condensed matter physics and increasingly also in the context of molecular systems. While their variational character provides results of remarkable accuracy, the capture quantum states featuring low entanglement well, a set of states often referred to as the 'physical corner'. While this physical describes ground states of local Hamiltonian problems from the condensed matter context well, the same cannot be said for problems involving molecules in quantum chemistry or systems undergoing time evolution. In this talk I will provide an overview over tensor network methods augmented by fermionic mode transformations in the context of molecular problems [1], condensed-matter simulations [2], and time evolution [3]. If time allows, I might briefly mention two new applications of tensor networks in being a design principle for building quantum devices [4] and in machine learning [5,6]. [1] Fermionic orbital optimisation in tensor network states C. Krumnow, L. Veis, Ö. Legeza, J. Eisert Phys. Rev. Lett. 117, 210402 (2016) [2] Towards overcoming the entanglement barrier when simulating long-time evolution C. Krumnow, J. Eisert, Ö. Legeza arXiv:1904.11999 [3] Dimension reduction with mode transformations: Simulating two-dimensional fermionic condensed matter systems C. Krumnow, L. Veis, J. Eisert, Ö. Legeza arXiv:1906.00205 [4] Simulating topological tensor networks with Majorana qubits C. Wille, R. Egger, J. Eisert, A. Altland Phys. Rev. B 99, 115117 (2019) [5] Expressive power of tensor-network factorizations for probabilistic modeling, with applications from hidden Markov models to quantum machine learning I. Glasser, R. Sweke, N. Pancotti, J. Eisert, J. I. Cirac arXiv:1907.03741, NeurIPS (2019) [6] Tensor network approaches for learning non-linear dynamical laws A. Goessmann, M. Goette, I. Roth, G. Kutyniok, J. Eisert, R. Sweke Submitted to ICML2020 (2020)

Seminarroom 1+2
iCal Event

11 Mär 2020

11:00

11:00

Room 1D1
iCal Event

26 Mär 2020

14:00

14:00

Developing a theory of activated dynamics is one of the most challenging problems of disordered systems. Activated glassy dynamics is central in many different contexts both in physics and beyond, e.g. in computer science and biology. In this talk, after a general introduction, I will describe recent research works aimed at characterising the activated dynamics of mean-field glassy systems. In particular I will discuss numerical results on the random energy model and variants, and analytical results on the organization of barriers in the p-spin spherical model.

Seminarroom 4
iCal Event

30 Mär 2020

16:30

16:30

Seminarroom 1+2
iCal Event

22 Apr 2020

15:30

15:30

Ultracold atoms in optical lattices constitute a versatile platform to study the fascinating phenomena of gauge fields and topological matter. Periodic driving can induce topological band structures with non-trivial Chern number of the effective Floquet Hamiltonian and paradigmatic models, such as the Haldane model on the honeycomb latticce, can be directly engineered. In this talk, I will report on our recent experiments, in which we realized new approaches for measuring the Chern number in this system and map out the Haldane phase diagram. This includes time-resolved Bloch-state tomography allowing for the observation of a dynamical linking number after a quench as well as the application of machine learning techniques to analyse experimental data. In the future, the combination of gauge fields with a quantum gas microscope will allow accessing new regimes such as fractional Chern insulators.

Seminarroom 4
iCal Event

22 Apr 2020

16:45

16:45

While the engineering of static and uniform artificial magnetic fields in ultracold atomic systems has drawn tremendous attention, we show that tailoring artificial gauge potentials in both space and time is also of big interest. One example is the implementation of a local magnetic flux insertion, which plays an important role in gedanken experiments of quantum Hall physics. By combining Floquet engineering of artificial magnetic fields with the ability of single-site addressing in quantum gas microscopes, we propose a scheme for the realization of such local solenoid-type magnetic fields in optical lattices. We show that it can be employed to manipulate and probe elementary excitations of a topological Chern insulator. Secondly, we investigate protocols for adiabatic state preparation based on ramping up the vector potential. Taking an interacting bosonic flux ladder as a minimal model, we find that the time required for adiabatic state preparation dramatically depends on which spatial pattern of Peierls phases is used. This can be related to shortcuts to adiabaticity via counterdiabatic driving and opens up new possibilities for adiabatic preparation in quantum simulators with artificial gauge fields.

Seminarroom 4
iCal Event

11 Mai 2020

16:30

16:30

Seminarroom 1+2
iCal Event

18 Mai 2020

16:30

16:30

Building on the discovery that a Weyl superconductor in a magnetic field supports chiral Landau level motion along the vortex lines, we have investigated its transport properties out of equilibrium. The chirality of the Weyl fermions protects the zeroth Landau level by means of a topological index theorem. As a result, the heat conductance parallel to the magnetic field through an area enclosing N vortices is quantized at N/2 times the thermal conductance quantum. The electrical conductance is renormalized by an energy-dependent effective quasiparticle charge, as a consequence of which a nonzero thermo-electric coefficient appears even in the absence of energy-dependent scattering processes.

Seminarroom 1+2
iCal Event