# Talks

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

## Vorträge in chronologischer Reihenfolge

22 Jul 2019
16:30

### Colloquium: Light-matter-life interactions at the micron scale

Prof. Roberto Di Leonardo (Sapienza Università di Roma, Italy)

In 1676, using candle light for illumination and a small glass sphere as the lens, Antonie van Leeuwenhoek revealed to the world the beauty and complexity of a microscopic universe densely populated by living microorganisms. Today, using lasers, spatial light modulators, digital cameras and computers, we investigate the statistical and fluid mechanics of microswimmers in ways that were unimaginable only 50 years ago. With light we can image bacteria as they swim rapidly through 3D environments, apply gentle and controllable force fields or sculpt the 3D structure of their environment. In addition to shaping the physical world outside cells we can also use light to control the internal biological state of microorganisms that are genetically engineered to respond to light signals. I will review our recent work on the interactions between light, matter and bacteria, from the fundamental problems of rectification and non-equilibrium steady states in active matter to the use of genetically modified bacteria as propellers for micro-machines or as a "living" paint that can be controlled by light.

Seminarroom 1+2 iCal Event
29 Jul 2019
16:30

### Colloquium: Quantum Materials: Full of Frustration and Correlations

Prof. Dr. Martin Dressel (Universität Stuttgart)

The pioneering work on geometrical frustration dates back to the 1920s, when Linus Pauling realized that the hydrogen bonds between H$_2$O molecules in ice can be allocated in multiple ways. A given oxygen atom in water ice is situated at the vertex of a diamond lattice and has four nearest-neighbor oxygen atoms, each connected via an intermediate proton according to the ice rule “two-in two-out”. Although these considerations used electric dipoles, Phil Anderson mapped them to a spin model possessing an extensive degeneracy of states. Quantum spin liquids attract great interest due to their exceptional magnetic properties characterized by the absence of long-range order down to low temperatures despite the strong magnetic interaction. Commonly these compounds are strongly correlated electrons systems, and their electrodynamic response is governed by the Mott gap in the excitation spectrum. Here we will summarize and discuss the optical properties of several two-dimensional quantum spin liquid candidates with different degrees of effective correlations. Placing organic molecules on a triangular lattice, a spin liquid ground state can be realized which allows us to investigate the genuine Mott state in the absence of magnetic order. Combining our optical data with pressure-dependent transport studies and theoretical calculations, we can construct a universal phase diagram of the correlation-controlled Mott insulator. But how important is the coupling of the fluctuating magnetic moments? How important is disorder for the electronic properties? If this resembles a quantum phase transition, is there a superconducting phase found in the vicinity and what is the superconducting glue? Can our findings be generalized, when going to a kagome or hexagonal lattice, realized in Herbert-smithites or $\alpha$-RuCl$_3$ for instance? Reference: A. Pustogow et al., Nature Materials 17, 773 (2018); Phys. Rev. Lett. 121, 056402 (2018). M. Dressel et al., J. Phys. Cond. Matter 30, 203001 (2018)

Seminarroom 1+2 iCal Event
05 Aug 2019
16:30

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

Prof. Jörg Schmalian (Institut für Theorie der Kondensierten Materie (TKM), Karlsruhe)

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.

Seminarroom 1+2+3 iCal Event
12 Aug 2019
16:30

### Colloquium: tba

Prof. Ariel Amir (Harvard University, John A. Paulson School, USA)

Seminarroom 1+2 iCal Event
26 Aug 2019
16:30

### Colloquium: tba.

Prof. David Quéré (ESPCI & Ecole polytechnique)

Seminarroom 1+2 iCal Event
16 Sep 2019
16:30

### Colloquium: New Phases of Quantum Magnets in and out of Equilibrium

Prof. Dr. Christian Rüegg (Paul Scherrer Institute, Villigen, Schweiz)

In quantum magnets spins form well-defined lattices and serve as model systems to study many-body quantum phases such as interacting quantum-dimer qubits, spin Luttinger-liquids, Bose glasses, or magnon Bose-Einstein condensates. Neutron, muon and photon sources are unique tools for high-precision studies of such phases, and of their correlations and excitations in as well as out of equilibrium. An overview of current frontiers in the field will be presented with special focus on recent developments in computational physics and exciting new opportunities that free electron lasers like SwissFEL and European XFEL will offer to study the time-dependence and out-of-equilibrium quantum mechanics of such systems.

Seminarroom 1+2+3 iCal Event
30 Sep 2019
16:30

### Colloquium: tba.

Prof. Jean Dalibard (College de France, Laboratoire Kastler Brossel, Paris, France)

Seminarroom 1+2 iCal Event
07 Okt 2019
16:30

### Colloquium: tba.

Seminarroom 1+2 iCal Event
21 Okt 2019
16:30

### Colloquium: tba.

Seminarroom 1+2 iCal Event
28 Okt 2019
16:30

### Colloquium: tba

Prof. Jonathan Simon (Chicago University, USA)

Seminarroom 1+2 iCal Event