Archiv 2017

Gastgeber: Angel Rubio

Franco Bonafé - Time-dependent electron-nuclear dynamics in DFTB+: theory and applications

MPSD Seminar
This seminar will be focused on an implementation of electron-nuclear real-time dynamics within the Density Functional Tight-Binding (DFTB) formalism in the DFTB+ package[1], as a result of a collaboration between the Quantum Dynamics Group (University of Córdoba) and the BCCMS (University of Bremen). Some theory details will be presented as well as its application to explain the launching of mechanical oscillations in metal nanoparticles under plasmon-resonant laser illumination[2]. [mehr]

Eric Heller - Graphene Spectroscopy and Ultrafast Pump-Probe Experiments and Theory

MPSD Seminar
Graphene is a key reduced dimensionality solid with many promising applications. Its spectroscopy is vital to understanding the quantum physics of graphene and to evaluate some of the potential uses of graphene. We have found that new and very different interpretations of graphene spectroscopy and ultrafast pump-probe experiments are required. This will be explained. They are very informative about the role of phonon assisted processes in solids and the role of the electronic transition matrix elements. [mehr]

Daniele Fausti - Optical control and quasiparticle witnessing in strongly correlated electron systems

MPSD Seminar
The prospect of “forcing” the formation of quantum coherent states in matter, by means of pulsed electromagnetic fields, discloses a new regime of physics where thermodynamic limits can be bridged and quantum effects can, in principle, appear at ambient temperatures. In this presentation I will introduce the field of optical control of correlated electron systems. I will focus on the possibility of coherently driving low-lying excitations of quantum many body systems making light-based control of quantum phases in real materials feasible. I will review the recent results in archetypal strongly correlated cuprate superconductors and introduce our new approach to go beyond mean photon number observables. I will show that quantum features of light can provide a richer statistical information than standard linear and non-linear optical spectroscopies. This will potentially uncover with unprecedented detail the evolution and properties of light-induced transient states of matter.ReferencesScience 331 (6014), 189 (2011)Nature Comm. 6, 10249 (2015)Nature Comm. 5, 5112 (2014)New J. Phys. 16 043004 (2014)Nature materials 12 (10), 882-886(2013) [mehr]

Binghai Yan - Nonlinear optical responses of Weyl semimetal materials

MPSD Seminar
In the band structure of a Weyl semimetal (WSM), the conduction and valence bands cross each linearly through Weyl points that are usually treated as “monopoles” of the Berry curvature. As a second-order response, WSMs were very recently demonstrated to show strong nonlinear optical effects including an exotic nonlinear Hall effect. This is caused by the non-equilibrium distribution of the Berry curvature, described as the “dipole” of the Berry curvature. In this talk, I will talk about our recent computational results on nonlinear response for representative WSM materials TaAs and MoTe2. [mehr]

Michael Schüler: Nonequilibrium topological states traced by transient spectroscopies

MPSD Seminar
Chern insulators exhibit fascinating properties which originate from the topologically nontrivial state characterized by the Chern number. How these properties are affected and manifest in the presence time-dependent perturbations is still a sparsely explored field of research. This applies,in particular, to quantum quenches between topologically distinct phases. Identifying robust measures of topology which are applicable in nonequilibrium scenarios — ideally also for correlated materials or dissipative systems — is thus a nontrivial task. [mehr]

Lukas Müchler - Exploring topological phenomena in Molecules

MPSD Seminar
The use of topological methods has revolutionized the field of condensed matter physics both theoretically and experimentally. Many new exotic states of matter with unremovable surface states that can carry dissipationless currents have been predicted and quickly been verified experimentally. Moreover, topological techniques that characterize periodic Hamiltonians according to their spatial and non-spatial symmetries have lead to an almost complete classification of all possible non-magnetic states of matter that can be realized in crystals. [mehr]

Ryan Requist - Reduced formula for the macroscopic polarization including quantum Fluctuations

MPSD Seminar
The macroscopic polarization of a solid is an fundamental quantity from which permittivity and piezoelectric tensors can be derived. The Berry phase formula of King-Smith and Vanderbilt expresses the macroscopic polarization in terms of the Bloch states of a mean-field band structure, almost invariably taken from density functional theory. Although this procedure has been successful for many materials, quantum fluctuations cause it to break down in strongly correlated systems. [mehr]

Philipp Strasberg - Strong coupling thermodynamics and Markovian embedding strategies

Whenever a small-scale system is weakly coupled to macroscopic and Markovian reservoirs, it is possible to establish a consistent thermodynamic framework -- even for systems far away from equilibrium and even at the level of single, fluctuating trajectories. But outside the Markovian and weak coupling regime already the definition of basic quantities such as internal energy or heat becomes problematic. After reviewing the phenomenology of non-equilibrium thermodynamics, I will discuss a technique which allows to map a strongly coupled, non-Markovian system to a weakly coupled, Markovian one by appropriately including environmental degrees of freedom in the description of the system. Thus, by redefining the system-environment partition, it is possible to carry over a consistent thermodynamic framework to the strong coupling situation. [mehr]

Michael Lubasch - Density Functional and Tensor Network Theory

MPSD Seminar
In my talk I will discuss this question: How can density functional and tensor network theory be combined in such a way that they benefit from each other. In particular I will present our publication [1] in which we developed a systematic procedure for the approximation of density functionals in density functional theory that consists of two parts. In the first part, for the efficient approximation of a general density functional, we introduced an efficient ansatz whose non-locality can be increased systematically. In the second part, we presented a fitting strategy that is based on systematically increasing a reasonably chosen set of training densities. I will present our results from reference [1] for strongly correlated fermions on a one-dimensional lattice. In this context we focused on the exchange-correlation energy and demonstrated how an efficient approximation can be found that includes and systematically improves beyond the local density approximation. Remarkably, we could show this systematic improvement for target densities that are quite different from the training densities. [1] M. Lubasch, J. I. Fuks, H. Appel, A. Rubio, J. I. Cirac, and M.-C. Banuls, New Journal of Physics 18, 083039 (2016)." [mehr]

Yang Peng - Boundary Green functions of topological insulators and Superconductors

MPSD Seminar
Topological insulators and superconductors are characterized by their gapless boundary modes. In this talk, we develop a recursive approach to the boundary Green function which encodes this nontrivial boundary physics. Our approach describes the various topologically trivial and nontrivial phases as fixed points of a recursion and provides direct access to the phase diagram, the localization properties of the edge modes, as well as topological indices. We illustrate our approach in the context of various familiar models such as the Su-Schrieffer-Heeger model, the Kitaev chain, and a model for a Chern insulator. We also show that the method provides an intuitive approach to understand recently introduced topological phases which exhibit gapless corner states. Ref: Yang Peng, Yimu Bao, Felix von Oppen, arXiv:1704.05862 (2017) [mehr]

Dante Kennes - Transient superconductivity from electronic squeezing of optically pumped phonons

MPSD Seminar
Advances in light sources and time resolved spectroscopy have made it possible to excite specific atomic vibrations in solids and to observe the resulting changes in electronic properties but the mechanism by which phonon excitation causes qualitative changes in electronic properties, has remained unclear. Here we show that the dominant symmetry-allowed coupling between electron density and dipole active modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction, independent of the sign of the bare electron-phonon coupling and with a magnitude proportional to the degree of laser-induced phonon excitation. Reasonable excitation amplitudes lead to non-negligible attractive interactions that may cause significant transient changes in electronic properties including superconductivity. The mechanism is generically applicable to a wide range of systems, offering a promising route to manipulating and controlling electronic phase behavior in novel materials. [mehr]

Jorge Jover - Open quantum systems: a geometric description

MPSD Seminar
In this talk, I will present some of the results in my PhD dissertation, whose main topic is the geometric description of open quantum systems. Differential geometry allows for an intrinsic formulation of mathematical models, thus achieving a better characterisation of their properties. I will analyse from a geometric perspective the manifold of pure and mixed states of quantum systems and its properties, such as its stratification in terms of the rank of states [1]. The algebraic properties of observables allow to define a Poisson and a symmetric tensor fields on the manifold, which are necessary in order to describe features such as dissipation and Markovian dynamics in an intrinsic way. Applications to Molecular Dynamics, in particular the Hamiltonian description of the Ehrenfest model, will also be discussed [2,3]. [1] Grabowski, Kus, Marmo. Symmetries, group actions and entanglement. Open Syst. Inf. Dyn. 13, 343–362 (2006) [2] Alonso et al. Statistics and Nosé formalism for Ehrenfest dynamics. J. Phys. A Math. Theor. 44 395004 (2011) [3] Alonso et al. Ehrenfest dynamics is purity non-preserving: a necessary ingredient for decoherence. J. Chem. Phys. 137 54106 (2012) [mehr]

Sven Ahrens - Relativistic quantum dynamics and the electron spin in standing light waves

MPSD Seminar
Strong laser beams allow for the coherent control of electrons, as for example electron diffraction in standing light waves [1]. In my talk I will discuss the possible manipulation of the electron spin in X-ray diffraction, in which the interacting standing wave of light can be formed from intensive and coherent X-ray beams from free-electron lasers [2]. The quantum dynamics of this process is theoretically modeled by inserting a plane wave ansatz for the electro-magnetic field and the electron wave function into the Dirac equation [3]. Based on this relativistic quantum description and substantiated by numeric and analytic solutions I will talk about the necessary kinematic conditions for generating spin effects in light-matter interaction. To the end I will present spin dynamics in circularly polarized laser fields [4,5] and discuss how circular polarization can be used for generating spin-dependent diffraction and spin polarization [6]. [1] D. L. Freimund, K. Aflatooni, H. Batelaan, Nature 413, 142 (2001). [2] S. Ahrens, H. Bauke, C. H. Keitel, and C. Müller, Phys. Rev. Lett. 109, 043601 (2012). [3] S. Ahrens, H. Bauke, C. H. Keitel, and C. Müller, Phys. Rev. A 88, 012115 (2013). [4] H. Bauke, S. Ahrens, C. H Keitel, R. Grobe, New J. Phys. 16, 103028 (2014). [5] H. Bauke, S. Ahrens, R. Grobe Phys. Rev. A 90, 052101 (2014). [6] S. Ahrens, arXiv:1604.06201 [quant-ph] (2016). [mehr]

Peter Deák - How to choose the correct hybrid functional for defect calculations

MPSD Seminar
The electronic and optical properties of a material critically depend on its defects, and understanding that requires substantial and accurate input from theory. Defect calculations in traditional semiconductors have relied on the local and semi/local approximations of density functional theory, which in wide band gap materials may lead to fatal errors. Since first-principle total energy methods beyond these approximations cannot yet be carried out with sufficient accuracy for the supercells needed in defect calculations, nowadays semi-empirical hybrid functionals are often applied instead. In my talk I will analyze the performance of the HSE06 screened hybrid functional on defects in Group-IV semiconductors and in TiO2, and show that its success is the result of error compensation between semi-local and non-local exchange, resulting in a proper derivative discontinuity (reproduction of the band gap) and a total energy which is a linear function of the fractional occupation numbers (removing most of the electron self-interaction). As it is well known, however, HSE06 does not work equally well for all materials. On the example of Ga2O3, I will show that tuning both the mixing and the screening parameter of HSE for the given material allows to ensure the same error compensation. Unless the electronic screening is strongly direction- or orbital-dependent (as in ZnO), the optimized HSE hybrid is nearly self-interaction free and provides a band structure on par with GW. Since the total energy can also be calculated, the real equilibrium structure of a defect can be found and the levels are in good agreement with experimental observations. [mehr]

Noejung Park - Ab initio study of ultrafast dynamics of spin-valley polarized states in TMDC with a particular focus on the role of phonon

MPSD Seminar
The valley degree of freedom and the possibility of spin-valley coupling of solid materials have attracted growing interest, and the relaxation dynamics of spin- and valley-polarized states has become an important focus of recent studies. In spin-orbit-coupled inversion-asymmetric two-dimensional materials, such as MoS2, it has been found that the spin randomization is characteristically faster than the time scales for inter- and intra-valley scatterings. In this talk, I present our recent study of the ultrafast non-collinear spin dynamics of the electron in a valley of monolayer MoS2 by using real-time propagation time-dependent density functional theory. We found that the spin precession is sharply selectively coupled only with the particular optical phonon that lifts the in-plane mirror symmetry. We suggest that the observed spin randomization can be attributed to this spin-phonon interaction. Further, our results imply that flipping of spins in a spin-orbit-coupled system can be achieved by the control over phonons. In a later part of the talk, I would also describe the feature of the computational package we have developed and have used for the spin-phonon dynamics, which is based on the plane-wave basis set and various types of pseudopotentials. [mehr]

Vahid Sandoghdar - Nano-Quantum-Optics: from single photons and emitters to cooperative effects

MPSD Seminar
I plan to start this presentation with an overview of our work over the past decade on the efficient coupling of light and single quantum emitters, leading to the single-photon communication of two individual molecules at long distances [1]. In this context, we will also discuss new results on a high-efficiency triggered source of single photons [2] and coherent nonlinear optical phenomena, which let a single organic molecule act as an efficient switch for weak beams of light [3]. The long-term goal of these projects is to establish a platform for nano-quantum-optical operations and cooperative interactions in a mesoscopic system of photons and quantum emitters [4, 5]. In order to achieve this, we have developed novel microcavity [6] and chip-based nanoguide circuitry [7] for use at cryogenic conditions.References:[1] Y. Rezus, et al., Phys. Rev. Lett. 108, 093601 (2012). [2] X-L. Chu, et al., Nature Photonics, 11, 58 (2017).[3] A. Maser, et al., Nature Photonics 10, 450 (2016).[4] S. Faez, et al., Phys. Rev. Lett. 113, 213601 (2014). [5] H. Haakh, et al., Phys. Rev. A, 94, 053840 (2016).[6] D. Wang, et al., Phys. Rev. X, under review.[7] P. Türschmann, et al., submitted. [mehr]

Michael Lorke - Carrier dynamics in MoS2

MPSD Seminar
In the context of the current interest in transition-metal dichalcogenides, we study the optical generation and relaxation of excited carriers and their influence on optical properties. [mehr]

Claudius Hubig - Time Evolution with a Krylov Variant for MPS Time Evolution with a Krylov Variant for MPS

MPSD Seminar
I will briefly motivate the need for time-evolution methods on matrix-product states and provide an overview over the available methods. Focus will then be placed on the Krylov subspace method to evaluate exp(-tH)|ψ❭, starting from a naive translation of the standard method suitable for dense matrix-vector arithmetic to matrix-product arithmetic. Key improvements that can be made to the algorithm when applied to matrix-product arithmetics will then be illustrated, resulting in increased precision and reduced computational effort. [mehr]

Tong Zhou - Quantum Spin-quantum Anomalous Hall Effect with Tunable Edge States in Sb Monolayer-based Materials

MPSD Seminar
The quantum anomalous Hall (QAH) effect, quantum spin Hall (QSH) effect and (quantum) valley Hall ((Q)VH) effect have attracted considerable attention in condensed matter physics and material science. Generally, only one of the QAH, QSH, and QVH effects can be realized in a specific system. It would be very interesting if these three effects can be achieved in one single system. In this talk, I shall represent this interesting imagination may be realized in Sb monolayer-based materials, where the QAH state occurs at one valley and the QSH state occurs at the other valley, called quantum spin-quantum anomalous Hall (QSQAH) effect. [mehr]

Michael Sentef - Light-enhanced electron-phonon coupling from nonlinear electron-phonon coupling

MPSD Seminar
In light of recent experiments suggesting light-induced superconductivity [1] as well as light-enhanced electron-lattice coupling [2] for strongly driven IR phonons, it is natural to ask for a minimal and generic theoretical model that predicts such enhancement effects of important couplings in different material classes. One idea that comes to mind is nonlinear electron-phonon coupling [3,4]. A quadratic coupling term of the form " g2 nel x2IR " is generically the lowest order symmetry-allowed direct coupling of an IR-active phonon coordinate xIR to the electronic density nel in systems with inversion symmetry. In this talk I will present model evidence for light-enhanced electron-phonon coupling and light-induced effective attraction between electrons based on nonlinear electron-phonon coupling [3], the latter of which was already discussed in a similar context in [4]. [1] M. Mitrano et al., Nature 530, 461 (2016) [2] E. Pomarico et al., Phys. Rev. B 95, 024304 (2017) [3] M. A. Sentef, arXiv: 1702.00952 [4] D. M. Kennes et al., Nature Physics (2017) , doi:10.1038/nphys4024, arXiv:1609.03802 [mehr]

Markus Kowalewski - Coherent Signatures of Conical Intersections in Ultrafast X-Ray Spectroscopy

MPSD Seminar
The rates and outcomes of virtually all photochemical and photobiological processes are dominated by conical intersections (CIs), which provide a fast sub-100-femtosecond nonradiative pathway back to the ground state. At a CI, the electronic and nuclear degrees of freedom frequencies are comparable and strongly mix due to the breakdown of the Born-Oppenheimer approximation. [mehr]

Klaas Giesbertz - One-body reduced density matrix to potential mapping in finite basis sets at elevated temperatures One-body reduced density matrix to potential mapping in finite basis sets at elevated temperatures

MPSD Seminar
Density functional theory in finite basis sets tends to degenerate to one-body reduced (1RDM) functional theory. As all calculations are done in finite basis sets, a rigorous foundation of 1RDM functional theory is desirable. To avoid uniqueness problems in the potential to 1RDM mapping, I will discuss the foundations of 1RDM functional theory in finite basis sets at finite temperatures, both for fermions and bosons. The fermionic case turns out to be relatively straightforward, but the bosonic case requires more care. The main result is that we can rigorously proof v-representability and functional differentiability in this setting. [mehr]

Niko Säkkinen - Application of Time-Dependent Many-Body Perturbation Theory to Excitation Spectra of Selected Finite Model Systems

MPSD Seminar
Many-Body Perturbation Theory (MBPT) is a methodology routinely employed in computational spectroscopy to calculate photoemission and absorption spectra. However, usually these computational experiments are only possible for real nanostructures, solids, etc. by resorting to simple approximations in which, e.g., self-consistency is neglected. [mehr]

Matteo Puviani - Strongly correlated Floquet systems: Cluster Perturbation Theory approach

CFEL Theory Seminar
Under the influence of periodic fields quantum systems may reach regimes inaccessible under equilibrium conditions and new phases may be engineered by a tunable control [1]. The coexistence of periodic driving forces and electron-electron correlation is particularly interesting for two main reasons: on one side the external driving effectively modulates the inter-site hopping enhancing the effects of the e-e repulsion and the tendency to an insulating behaviour. On the other hand, irradiation itself is responsible for a photo-doping consisting in an electronic energy dressing that may turn a Mott insulator into a metal. Due to these competing effects, novel phenomena are expected when strongly correlated quantum systems are exposed to time-dependent fields. [mehr]

Enrico Ronca - Theoretical Methods for Excited State Properties in Extended Systems

MPSD Seminar
Light matter interaction is involved in several fundamental processes in Chemistry, Physics and Biology and is at the basis of Spectroscopy, experimental technique representing probably the most important source of information about properties of matter. Due to the significant improvements introduced in the last decades, spectroscopic techniques became very accurate and the interpretation of spectra, just by observation of the experimental results, became sometimes extremely complicated. For this reason the need of accurate theoretical methods able to reproduce and interpret the experimental results is becoming more and more pressing. The accurate theoretical simulation of spectra is therefore a very a challenging task, in particular for extended systems for which only approximated methods can be used. [mehr]
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