Die Gewinner der SPG Preise 2018

Facets of Gravity

Numerous observations support the remarkably simple cosmological standard model, which is based on General Relativity (GR) together with two symmetry assumptions. Albeit conceptually simple, this model requires the majority of matter in the universe to be of an unknown form, and the vast majority of the energy content of the universe to be contributed by an equally unknown dark energy.
These awkward conclusions may be bypassed by replacing GR by another, further generalised theory of gravity. Under the assumptions of Lorentz symmetry, unitarity, locality and a pseudo- Riemannian manifold, any attempt at generalising the theory of gravity inevitably leads to new dynamical degrees of freedom, which can be scalar, vector, or tensor fields. The gauge fields in the Standard Model of Particle Physics motivate investigating the role that bosonic vector fields may play for the evolution of the universe. An abelian vector field with U(1) gauge symmetry does not admit a homogeneous and isotropic background. If one explicitly breaks its U(1) symmetry, the resulting generalised Proca theories are the most general vector-tensor theories with second-order equations of motion for the vector and the tensor fields alike [1].
In a further step, tensor theories can be combined with both additional scalar and vector fields into a single scalar-vector-tensor theory. This unification and the interactions it implies depend sensitively on whether the vector field has a gauge symmetry or not. The resulting theories, with or without gauge invariance, will have rich applications in cosmology and astrophysics.
We have grown accustomed to attributing gravity to the curvature of space-time. This perception has masked the fact that differential geometry provides much wider classes of objects to represent the geometrical properties of manifolds. Besides the curvature, these are torsion and non-metricity. In Einstein’s theory, both non-metricity and torsion vanish. An equivalent representation of GR can however be constructed based on a flat space-time with a metric, but asymmetric connection: This teleparallel description assigns gravity entirely to torsion. Perhaps surprisingly, a third equivalent and simpler representation can be constructed on an equally flat space-time without torsion, in which gravity is purely ascribed to non-metricity. By a suitable gauge choice, the connection then vanishes completely. This representation of GR has the advantage of depriving gravity from any inertial character, and the resulting action is purged from the boundary term [2].

[1] L. Heisenberg. Generalization of the Proca Action. JCAP, 5:015, May 2014.
[2] L. Heisenberg. Cosmology with new gravitational degrees of freedom. Physics Reports, in press, 2018.

Quantum Computation and Many-Body Physics with Trapped Ionsl

Over the past three decades, quantum information processing has seen incredible progress in theory and experiments. Today, we move closer to the realization of Feynman’s vision of designing a fully controllable quantum device capable of simulating classically intractable problems. In this PhD work, linearly trapped ions have been used to encode spin information into two electronic states. Laser light fields have been applied to coherently manipulate these spin states, to engineer tunable spin-spin interactions and to measure the spin information.
Besides having addressed various topics of quantum information processing in our work, two experiments with regards to quantum simulations of interacting many-body systems are highlighted in particular.
The first experiment addresses a fundamental question in interacting systems [1]: how fast can information propagate in such systems? We show that a local perturbation generates entanglement, which then propagates through the entire system. Additionally, we investigate the velocity of correlation spreading for different interaction lengths, showing that the picture of a lightcone-like propagation becomes invalid for long-range interactions.
In the second experiment, we report on the first observation of a dynamical quantum phase transition, i.e., non-analytical points (kinks) in the time evolution of quenched systems [2]. We show that these phase transitions are indeed robust against deformations of the underlying Hamiltonian, i.e., changes in the interaction parameters, and uncover a previously unknown relation between these special points in time and entanglement growth.

[1] Quasiparticle engineering and entanglement propagation in a quantum many-body system, P. Jurcevic, B. P. Lanyon, P. Hauke, C. Hempel, P. Zoller, R. Blatt & C. F. Roos Nature 511, 202–205, (2014)
[2] Direct Observation of Dynamical Quantum Phase Transitions in an Interacting Many-Body System, P. Jurcevic, H. Shen, P. Hauke, C. Maier, T. Brydges, C. Hempel, B. P. Lanyon, M. Heyl, R. Blatt, & C. F. Roos, PRL 119, 080501 (2017)

Understanding Perovskite Solar Cells

Inorganic-organic lead-halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap of 1.6 eV – in particular considering that it is solution processed. On the other hand, perovskite solar cells suffer from instabilities on different timescales. These instabilities due to slow processes are the origins of a scan-rate dependent hysteresis in the current-voltage curve, of light-soaking effects, and of reversible degradation on the long term.

This work on the devices physics of perovskite solar cells sheds some light on these peculiar phenomena. Using electroluminescence and further spectroscopic measurements, high luminescence yields and sharp absorption onset are identified as reasons for the high photovoltage [1]. Loss mechanisms such as recombination of charge carriers at interfaces are investigated using the temperature and light-intensity dependence of the diode ideality factor. Furthermore, defects formed by metals from the electrodes migrating into the perovskite are identified to form tail states responsible for permanent degradation.

The reversible effects are attributed to the interplay between electronic and ionic conduction in the perovskite crystal. Using various transient characterization techniques and device modeling, it is found that displaced ions change the electric field in the device and modify recombination rates [2]. Ions accumulated at interfaces to the contacts also modify charge carrier injection properties, which helps to explain the high photomultiplication effects observed in perovskite photodetectors.

These findings contribute to a better understanding of the operation principles of perovskite solar cells and pave the way towards a targeted optimization of optoelectronic perovskite devices.

[1] Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv. 2, e1501170 (2016).
[2] Tress, W. et al. Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy Environ. Sci. 8, 995–1004 (2015).

2D/3D Hybrid Perovskites for Stable and Efficient Solar Cells

Three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >22%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor stability (= device lifetime), mainly due to material decomposition upon contact with water, is now the bottleneck for the widespread of this technology. Diverse technological approaches have been proposed delivering appreciable improvements, but still failing by far the market requirements demanding 25-years lifetime. In this talk, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid by interface engineering 2D/3D heterostructures will be discussed as a mean to boost device efficiency and stability together. The 2D/3D composite self-assembles into an exceptional gradually organized structure where the 2D perovskite anchors on the TiO₂ substrate, templating the growth of a highly ordered 3D perovskite on top. This results in mesoporous solar cells leading to 12.9% PCE [3]. Aiming at the up-scaling of this technology, we realize 10x10 cm² large-area solar modules using a fully printable, hole conductor free device configuration (i.e. where a carbon electrode is used to replace the organic hole transporter and Gold). The module delivers 11.2% efficiency stable for more than 10,000 hours with no degradation under accelerated testing conditions, leading to a record one-year stability. On the other side a 3D/2D interface will be also presented, where 2D layer lies on top of the 3D as a mean to protect the 3D underneath while also blocking the electron hole recombination at the perovskite/hole transporter interface. This results in enhanced stability without compromising the efficiency, leading to PCE = 20% stable for 1000 h [4].

[1] http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
[2] Correa-Baena, J.-P. et al. Promises and challenges of perovskite solar cells. Science358, 739–744 (2017).
[3] Grancini, G. et al. One-Year stable perovskite solar cells by 2D/3D interface engineering. Nat. Commun. 8, ncomms15684 (2017).
[4] Taek Cho, K. et al. Selective growth of layered perovskites for stable and efficient photovoltaics. Energy Environ. Sci. (2018). doi:10.1039/C7EE03513F

Thermodynamics at the level of a single electron

Traditional thermodynamics, which is a phenomenological theory, is built upon the so-called thermodynamic limit where the number of particles comprising a system reaches the limit of infinitely many. However, in the trend towards minimizing the system sizes, such as reducing the size of computer chips, or studying single particles as candidates for quantum bits, the thermodynamic limit is not reached. For small systems sizes, fluctuations are observed, as individual particles may have values of observables which are very different from the expectation value of the whole ensemble. Fluctuation theorems, as described below, have enhanced the understanding of these deviations. In particular, the Jarzinsky equality [1] has lead to the insight, that fluctuations of work done on a system driven out of equilibrium from an equilibrated state are explicitly related to an equilibrium parameter, namely, the free energy difference between the initial and the final state of the driven system. This equality has been tested experimentally and enables the evaluation of the free energy in systems where calculations thereof are difficult. Here, we consider the definition and measurement of heat and work in a system consisting of a QD coupled to a reservoir, where a single electron in the QD is driven up and down in energy. We analyse the fluctuations by measuring and calculating the distribution of produced heat and work obtained in single repetitions of driving the electron with the same drive protocol. We show, how the violation of the second law of thermodynamics in single repetitions of the experiment can be interpreted as a blurring in the arrow of time, and finally, we use use the distribution of work provide a test of the Jarzinsky equality.

[1] Jarzynski, C. (1997), "Nonequilibrium equality for free energy differences", Phys. Rev. Lett., 78, 2690