We are hosting a series of seminars presented by both visitors to the department and department members (graduate, postdoctoral, and faculty).  These seminars are meant to be informal, with many opportunities for interruptions and for asking plenty of questions.  If you are interested in presenting, or have someone in mind you would like to invite, let us know!

Upcoming seminars

18 October 2018

Location: V-102 (V-wing)
Time: 12:30 pm (pizza will be served)

Title: Vortices and their dynamics in superfluid helium-3

Jere Mäkinen (Aalto University)

Helium-3 undergoes phase transition to superfluid phase at temperatures of the order of 1 mK via spin-triplet pairing. The triplet pairing allows for multiple superfluid phases, three of which can be realized in bulk geometry. The first part of the talk concentrates on dynamics of quantized vortices in the fully-gapped topological phase, known as the B phase. In particular, I will cover experiments on the response of an equilibrium vortex lattice to perturbations such as a rapid stopping of the rotational drive (spin-down) or application of an AC component on top of the DC drive. Remarkably, the spin-down measurements show finite dissipation in the zero-temperature limit, interpreted as overheating of the vortex-core-bound fermion states by helical excitations of the vortex core. Moreover, the AC measurements reveal an energy-cascade originating from wave turbulence of quantized vortices.

The second part concentrates on vortices seen in novel superfluid phases encountered in restricted geometry. In the experiments the sample volume contains a nanostructured material consisting on nearly parallel strands smaller in diameter than the coherence length. Under these conditions, the highest-temperature superfluid phase is the polar-phase containing a Dirac nodal line in the quasiparticle energy spectrum. The polar phase supports a different variety of vortices than the bulk phases. Particular attention is paid to half-quantum vortices (HQVs) - vortices containing half a quantum of circulation plus winding of the spin space. Strong pinning of HQV cores to the confining strands allows taking the HQVs through phase transitions to other superfluid phases encountered only within nanoconfined samples - where the existence of HQVs has connections to other fields of physics such as topological quantum computing and cosmology.

20 September 2018

Location: CCIS L1-029
Time: 2:00 pm

Title: Ultrafast videography of individual molecules

Prof. Rupert Huber (University of Regensburg)

Using the carrier wave of terahertz light pulses to transiently bias the junction of a scanning tunnelling microscope has enabled ultrafast atom-scale videography [1-3]. I will review how the combination of lightwave electronics with low-temperature single-molecule scanning tunnelling microscopy has allowed us to take the first femtosecond snapshot images of an individual molecular orbital and to resolve ultrafast vibrations of a single molecule with sub-angstrom precision [2]. Furthermore, I will present latest results demonstrating how single quantum events can be traced one by one in ultrafast action spectroscopy and the actual transient terahertz voltage can be determined without fitting parameters. We anticipate that these results pave an unprecedentedly direct way to the nanocosm, directly resolving the essential elementary dynamics of key processes in molecular and solid-state physics, as well as in materials sciences, chemistry and biology.

[1]   T.L. Cocker, V. Jelic, M. Gupta, S.J. Molesky, J.A.J. Burgess, G. De Los Reyes, L.V. Titova, Y.Y. Tsui, M.R. Freeman, F.A. Hegmann, Nature Photonics 7, 620 (2013)
[2]    T.L. Cocker, D. Peller, P. Yu, J. Repp, and R. Huber, Nature 539, 263 (2016)
[3]    V. Jelic, K. Iwaszczuk, P.H. Nguyen, C. Rathje, G.J. Hornig, H.M. Sharum, J.R. Hoffman, M.R. Freeman, and F.A. Hegmann, Nature Physics 13, 591 (2017).