Talks

Symmetry-breaking terms in the Hamiltonian and charge transport in mesoscopic bilayer graphene

It is well-established since first studies on monolayer graphene in early 2000s that this material shows universal quantum value of the conductivity accompanied by sub-poissonian shot noise (quantified by the Fano factor F=1/3). These are the measurable consequences of the relativistic character of the effective Hamiltonian for low-energy excitations. In a case of bilayer graphene the situation is much less clear, since several symmetry-breaking terms (such as the trigonal warping), even if inifinitisemal, are expected to have finite impacts on transport characteristics in the mesoscopic limit. In the talk, I will describe the effects of such terms on the minimal conductivity and basic thermoelectric properties. References: [1] G.Rut and A.Rycerz, arXiv:1407.1684; arXiv:1511.04705. [2] D.Suszalski, G.Rut, A.Rycerz, arXiv:1712.05857; arXiv:1810.02280.

Hydrodynamics of Active Lévy Matter

Collective motion emerges spontaneously in many biological systems such as bird flocks, insect swarms and tissue under dynamic reorganization. This phenomenon is often modeled within the framework of active fluids, where the constituent active particles, when interactions with other particles are switched off, perform normal diffusion at long times. However, single-particle superdiffusion and fat-tailed displacement statistics are also widespread in biology as, e.g., they can represent optimal search strategies for living organisms. The collective properties of interacting systems exhibiting such anomalous diffusive dynamics – which we call active L ́evy matter – cannot be captured by current active fluid theories. Here, we formulate a hydrodynamic theory of active L ́evy matter by coarse-graining a microscopic model of aligning polar active particles performing superdiffusion manifest as L ́evy flights. Using a linear stability analysis, our theory predicts spatially homogeneous disordered and ordered phases at high and low orientational noise levels, respectively, as in ordinary active fluids; however, in contrast to its conventional counterpart, the order-disorder transition can become critical. We further support our analytical predictions with simulation results. This work not only highlights the need for more realistic models of active matter integrating both anomalous diffusive motility and inter-particle interactions, but also potentially enriches the universal properties of active systems.

Colloquium: Quantum Atom Optics: Ocean & Universality in Quantum Dynamics

The experimental platform of atoms manipulated by light offers answers to a broad spectrum of open questions. With two explicit and very different examples I will give you a glimpse how broad this spectrum is. I will start with a fundamental question in oceanography: At what time has the deep water in the ocean been in exchange with the atmosphere? Quantum atom optics offers the experimental possibility to detect the very rare Argon 39 atoms one by one and with that allows the dating of water samples as small as ten liters [1]. A very different question in physics is about the existence of universal behavior. Specifically in respect to time dynamics this has only recently been discussed theoretically in the context of the early phase of a heavy ion collision. 'Universal' means in this context, that the evolution does not depend on the initial condition and follows the scaling hypothesis in time and space. I will introduce the concept and present the first observation of this phenomenon in highly controlled ultracold Bose gases [2]. [1] Ar-39 dating with small samples provides new key constraints on ocean ventilation, Nature Comm. 9, 5046 (2018). [2] Observation of universal dynamics in a spinor Bose gas far from equilibrium, Nature 563, 217 (2018).

On the thermodynamics of lipid mixtures on curved substrates

It has been known since long that thermodynamic stability of two-dimensional lipid mixtures is influenced by curvature. Inspired by recent experimental results on lipid membranes coated on colloidal scaffolds, we propose a simple and general framework for interpreting the phase diagram of inhomogeneous closed systems. When the critical behaviour of a system is affected by its shape, we further show how new thermodynamic states can arise in the phase diagram, where lipids are in a mixed phase yet exhibit strong lateral segregation. We prove how different types of inhomogeneous couplings must induce qualitatively different phenomena - such as a curvature dependent line tension - and thus provide a way to distinguish different types of curvature interactions.

Colloquium: Exploiting the Malleability of Disorder to Design Biologically-Inspired Function

Physicists have been thinking for a century about ordered solids. But ordered systems can’t compete with what biological systems such as proteins, which aren’t ordered, can do. We’ve discovered that relaxing the constraint of order increases design flexibility enormously, so that systems can be tuned to perform complex biologically-inspired tasks. Allostery in a protein is a phenomenon in which a molecule binding locally to one site affects the ability of another molecule to bind at a second distant site. Inspired by the long-range coupled conformational changes that constitute allosteric function in proteins, we tune in “allostery” into disordered mechanical networks derived from granular packings by modifying the network architecture to control the local strain at one point in such a network in response to a strain applied elsewhere in the system. In another biological example, the vascular network in the brain contracts and dilates blood vessels in order to direct enhanced blood flow to multiple specific regions of the brain as it performs multiple tasks at once. We have designed flow networks to accomplish similar complex tasks. In both examples, we address a key question: what, precisely, are we tuning into the structure in order to accomplish the function? Surprisingly, we find that the structure/function relationship is not geometrical, but topological in nature.

IMPRS Seminar: TBA

IMPRS Seminar: Tuning the Multi Davydov Ansatz for Spin-Bath Problems

The multi Davydov Ansatz, a powerful coherent state based technique for real-time evolution of bosonic many-body systems, has recently been successfully applied to a wide variety of systems [1]. Despite its success it is widely known to suffer from numerical instabilities which restrict its reliability to short times or good first approximations. In coexistence with the multi Davydov Ansatz, Gaussian-based techniques in the MCTDH community [2] surprisingly suffer from some different but also some similar issues. In my talk I will present a method called apoptosis of coherent states, with whose help these issues may be overcome and the Davydov-Ansatz can be made a reliable high-precision tool. In an open quantum system setting, I will present converged long-time results obtained with numbers of coherent states far beyond previous limits. --- [1] R. Hartmann et al., J. Chem. Phys. 150, 234105 (2019) [2] G. W. Richings et al., Int. Rev. in Phys. Chem. 34, 269 (2015).

IMPRS Journal Club: Weak measurements, Superkicks and Superoscillations

This talk starts by revisiting the classic Stern-Gerlach experiment, by noticing that it sensu stricto does not measure atomic spin, but a deflection proportional which is dependent on the atomic spin. The atomic spin is then inferred from the observed deflection. By performing a $\it{weak}$ $\it{measurement}$, where the deflection is much smaller than the momentum spread of the incoming beam and then removing a large part of the outgoing beam with a second, strong, Stern-Gerlach experiment, it is possible to obtain deflections far bigger than expected from the magnitude of the atomic spin. A similar effect occurs for momentum. It is shown that an atom, when absorbing one photon, can experience a recoil larger than the momentum of the absorbed photon, a $\it{superkick}$. This is done by placing the atom in a region where the light field is $\it{superoscillatory}$, which means it changes on shorter length scales than its wavelength. As an example I will use an atom near a vortex core. Suggested reading: Y. Aharonov, D. Z. Albert & L. Vaidman, how the result of a measurement of a component of the spin of a spin 1/2 particle can turn out to be 100, Phys. Rev. Lett. 60 1351 S. M. Barnett & M. V. Berry, superweak momentum transfer near optical vortices, J. Opt. 15 125701

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Colloquium: Critical and incoherent electron systems and superconductivity within the Sachdev-Ye-Kitaev approach

The established and highly successful description of metals is based on Landau’s Fermi liquid theory. The relevant phase space for electron-electron collisions is determined by the Pauli blocking of a degenerate Fermi gas due to its Fermi surface. In some strongly correlated systems narrow bands form and the energetics of the system at elevated energies or temperatures becomes dominated by strong interactions and is no longer restricted by Fermi-surface phase-space effects. To develop a well-controlled approach of this regime is an important question in the theory of strongly correlated electrons. In this talk I will give an overview over the description of incoherent and critical electronic systems using the Sachdev-Ye-Kitaev approach. We consider versions of the model where electrons interact with each other, with boson collective modes, and even with phonons. We also comment on the very direct relation to holographic approaches to strongly coupled quantum theories. Finally we address the emergence of superconductivity in such an incoherent metal and show that pairing and superconductivity of a fully incoherent electronic system is allowed and leads to a pairing state with high transition temperature but low condensation energy.

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