# Talks

Colloquium
coffee, tea, cookies at 16:15 in the main hall
Monday 16:30-17:30
Seminar room 1+2
- monthly seminars -
Wednesday 16:30 - 18:00
Seminar room 1+2
- weekly seminars -
Monday 11:00-12:00
Room 1D1
Wednesday 16:30 - 17:30
Seminar room 1D1
Thursday 14:00-15:00
Seminar room 3

## Talks in chronological order

iCal All Events
27 Mar 2017
04:30 PM

### Magnons, spinons, Majorana fermions and quantum spin liquids

Prof. Stephen Nagler (Oak Ridge National Laboratory)

In recent years many physicists researching quantum materials have become very excited about an unusual state of matter: the quantum spin liquid (QSL). In this talk I will explore the difference between “conventional” ordered magnetic systems and QSLs. To understand these systems the way physicists do one needs to appreciate the importance of collective elementary excitations. In ordinary magnets these are called spin-waves, or, in the quantum version, magnons. These will be explained and discussed along with other excitations that are important in quantum magnetism sometime called “triplons” and spinons. I will discuss a major technique for characterizing these excitations: inelastic neutron scattering. The last part of the talk will examine one material of particular recent interest: $\alpha-RuCl_3$. This material orders magnetically, but it is close to a special kind of quantum spin liquid called the “Kitaev quantum spin liquid”, and exhibits unusual excitations that might be related to particles known as Majorana Fermions. Finally, I will discuss how the magnetic order can be suppressed by either site-dilution of the magnetic $Ru^{3+}$ ions, or the application of a magnetic field. The resultant consequences on the magnetic excitation spectrum are seen to be of particular interest.

Seminarroom 1+2+3 iCal Event
03 Apr 2017
04:30 PM

### Topological sound and odd viscosity in chiral active matter

Prof. Vincenzo Vitelli (Leiden University)

Abstract: Active materials are composed of interacting particles individually powered by motors. In this talk, we focus on chiral active materials that violate parity and time reversal symmetry. First, we show how to generate topological sound in fluids of self-propelled particles exhibiting a spontaneous chiral active flow under confinement. These topological sound modes propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Next, we discuss an exotic transport coefficient characteristic of quantum Hall fluids, called odd viscosity, which controls the hydrodynamics of classical fluids composed of active rotors. This odd viscosity couples pressure to vorticity leading to transverse flow in piston compression experiments. We envision that such transverse response may be exploited to design self-assembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active rotors fluids.

Seminarroom 1+2+3 iCal Event
10 Apr 2017
01:30 PM

### Absorbing state phase transitions in driven dissipative spin systems

Michael Buchhold (Institute for Theoretical Physics, University of Cologne)

Phase transitions to absorbing states are among the simplest examples of critical phenomena out of equilibrium. The characteristic feature of these models is the presence of a fluctuationless configuration which the dynamics cannot leave, which has proved a rather stringent requirement in experiments. Recently, we proposed to seek such transitions in highly tuneable systems of cold atomic gases, which offers to probe this physics and, at the same time, to investigate the robustness of these transitions to quantum coherent effects. Here we specifically focus on the interplay between classical and quantum fluctuations in a simple driven open quantum model which, in the classical limit, reproduces a contact process, which is known to undergo a continuous transition in the "directed percolation" universality class. For strong coherent drive, this system features a line of non-equilibrium first order phase transitions, which terminate at a bicritical point. In this talk, I will present our recent results on the non-equilibrium phase transitions in this model with a special emphasis on the bicritical scaling regime and the dynamics close to the non-equilibrium first order transition.

Room 1D1 iCal Event
19 Apr 2017
04:30 PM

### IMPRS Seminar

Richard Hartmann (TUD)

Seminarroom 4 iCal Event