Microsystems for time-resolved light and electron cryomicroscopy

Dr. Thomas Burg from the Max Planck Institute for Biophysical Chemistry, Goettingen

In this talk, I will present recent advances of our group towards connecting live-cell imaging and electron microscopy (EM) with millisecond time resolution. Before entering the high vacuum of an electron microscope, biological samples generally need to be fixed chemically or frozen. Due to the time required for this preparation, it is currently not possible to precisely correlate high-resolution EM images of cell structure with dynamics previously observed in the light microscope. We have been able to overcome this limitation by cryofixing cells and small model organisms with millisecond time-control directly in the light microscope through ultra-rapid cooling. Ice crystallization is strongly suppressed by freezing at more than ~10^4 °C/s. Next, in order to image rapidly frozen samples by high numerical aperture cryo-fluorescence microscopy, we designed a new type of light microscope that allows immersion imaging below the glass transition of water (-135 °C). We show that our technology can be integrated with conventional correlative light and electron microscopy workflows to open the path for understanding temporal relationships between cell activation and response in fields from neuroscience and immunology to pharmaceutical sciences.

Observation of the Higgs mode in a strongly interacting fermionic superfluid

Dr. Kiuji Gao (Physikalisches Institut, University of Bonn, Germany)

Higgs and Goldstone modes are possible collective modes of an order parameter upon spontaneously breaking a continuous symmetry. Whereas the low-energy Goldstone (phase) mode is always stable, additional symmetries are required to prevent the Higgs (amplitude) mode from rapidly decaying into low-energy excitations. In the realm of condensed-matter physics, particle-hole symmetry can play the role of Lorentz invariance in high energy physics and a Higgs mode has been observed in weaklyinteracting superconductors. However, whether the Higgs mode is also stable for strongly-correlated superconductors in which particle-hole symmetry is not precisely fulfilled or whether this mode becomes overdamped has been subject of numerous discussions. Experimental evidence is still lacking, in particular owing to the difficulty to excite the Higgs mode directly. Here, we observe the Higgs mode in a strongly interacting superfluid Fermi gas. By inducing a periodic modulation of the amplitude of the superconducting order parameter, we observe an excitation resonance at frequency 2Δ/h. For strong coupling, the peak width broadens and eventually the mode disappears when the Cooper pairs turn into tightly bound dimers signalling the eventual instability of the Higgs mode.

Imaging and tuning Schrödinger and Dirac charge carriers dynamics inside nanodevices

Prof.dr. Benoit Hackens (Institut de la Matière Condensée et des Nanosciences (IMCN), Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgium)

Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe local-scale electron behaviour inside the devices. Here, we will discuss how to get real-space information on quantum transport phenomena inside various mesoscopic devices. The results were obtained using low temperature scanning gate microscopy (SGM), which consists in mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity [1]. If the tip-induced perturbation is relatively small, SGM is an imaging technique giving access to the local density of states inside mesoscopic devices [2]. When the tip-induced perturbation is larger, it can be viewed as a moving scatterer whose shape can be tuned by varying the tip voltage and tip-sample distance. We will illustrate how both SGM modes helped to unveil peculiar aspects of relativistic charge carrier dynamics within graphene devices [3], and within devices carved out from semiconductor heterostructure hosting "conventional" two-dimensional electron systems [4]. Depending on the type of charge carriers, different fundamental scattering phenomena determine their interaction with the scattering potential, and this can also be evidenced and studied in detail thanks to SGM experiments.

[1] M. Topinka et al., Science 289, 671 (2000).
[2] F. Martins et al., Phys. Rev. Lett. 99, 136807 (2007).
[3] D. Cabosart et al., Nano Lett. 17, 1344 (2017).
[4] S. Toussaint et al., Sci. Rep. 8, 3017 (2018).

Sound propagation in nerves

Prof. Thomas Heimburg (Niels Bohr Institute, University of Copenhagen, Denmark)

It is a central paradigm in biology that excitatory events in nerve cells are of purely electrical nature. The nervous impulse is attributed to the electrical activity of a class of macromolecules called voltage-gated ion channels, implying that one searches for the origin of biological function at a molecular scale.  

However, it is widely unknown that during the nerve pulse also the temperature, the thickness and the length of nerves change, i.e., properties that do not manifest themselves on the molecular scale and are of general thermodynamic nature. Furthermore, in contrast to expectations from an electrical theory one finds no dissipation of energy in experiments on nerves. Many properties of nerve pulses rather resemble those of adiabatic processes such as sound or solitons, respectively. Solitons are localized sound-pulses that travel without changes in shape and without dissipation of energy. They are a consequence of the nonlinear elastic properties of biomembranes close to melting transitions. The electrical pulses in classical electrophysiology and electromechanical solitons differ largely in their physical implications. The description of solitary pulses requires the language of thermodynamics and hydrodynamics rather than that of electrical circuits. The framework presented here contains a mechanism for the occurrence of fluctuating currents across membranes, which resemble ion channels but are rather a consequence of thermal fluctuations in membrane area. It also contains a straight-forward explanation for the phenomenon of anesthesia based on a generic physical mechanism.

Dissipationless electron transport, multicomponent superfluidity, and the superfluid BCS-BEC crossover in electron-hole double bilayer graphene

Prof.dr. David Neilson (University of Camerino, Italy, and University of New South Wales, Sidney, Australia)

The recent observation in zero magnetic field of electron-hole superfluidity at temperatures of a few Kelvin in a pair of atomically close (1-2 nanometre separation) yet electrically isolated, n- and p-doped bilayer graphene sheets[1], as had been proposed in Ref. [2], raises exciting prospects in the quest for heat-free electron transport in nano devices[3] and for the investigation of the superfluid BCS-BEC crossover in a solid state device.[2] By sweeping the carrier densities using metallic gates, the long-range Coulomb pairing interaction strength can be continuously tuned through the crossover regime. The perpendicular electric field from the gates opens up small and continuously tuneable band gaps between the conduction and valence bands of each sheet. Interesting physics effects arise from the long-range nature of the superfluid pairing interaction, from the associated competition between screening and superfluidity (the condensate pairs are neutral)[4, 5], from multicondensate effects arising from the small band gaps[6], and from the potential for high transition temperatures, exploiting the extremely strong pairing interactions. I will attempt to provide an overview of each of these issues.

[1] Burg, G. W. et al., arXiv (2018). 1802.07331.
[2] Perali, A., Neilson, D. & Hamilton, A. R. , Phys. Rev. Lett. 110, 146803 (2013).
[3] https://www.fleet.org.au/innovate/excitonic-dissipationless/.
[4] Neilson, D., Perali, A. & Hamilton, A. R. , Phys. Rev. B 89, 060502 (2014).
[5] L´opez R´ıos, P., Andrea, P., Needs, R. J. & Neilson, D. ,Phys. Rev. Lett. (2018). (to appear).
[6] Conti, S., Perali, A., Peeters, F. M. & Neilson, D. , Phys. Rev. Lett. 119, 257002 (2017).

Uniturbulence: Turbulent behavior in astrophysical plasmas from unidirectionally propagating MHD waves

Prof.dr. Tom Van Doorsselaere (Centre for mathematical Plasma Astrophysics, KULeuven)

We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations. We simulate the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfvenic waves. Simpler versions of this scenario have been extensively studied as the phenomenon of phase mixing. We show that, by generalizing the textbook version of phase mixing, interesting phenomena are obtained, such as turbulence-like behavior and complex current-sheet structure. This is a novelty in longitudinally homogeneous plasma excited by unidirectionally propagating waves and we call this new phenomenon uniturbulence. This simulation is particularly relevant for the study of transverse waves in a solar coronal hole, which is the source region of the fast solar wind. However, the uniturubulence constitutes an important finding for turbulence-related phenomena in astrophysics in  general, relaxing the conditions that have to be fulfilled in order to generate turbulent behavior.

Search for a new kind of matter: The SoLid experiment

Prof. Antonin Vacheret - the SoLid Collaboration & Imperial college London

The aim of the SoLid experiment is to address persistent anomalies in oscillation data that could be explained by the existence of a new type of neutral particle called a sterile neutrino. The experiment uses a new detector technology deployed at very short distance from the SCK•CEN BR2 reactor to achieve high sensitivity in the search for oscillation in this new state. SoLid will test the reactor anomaly by measuring the antineutrino spectrum as a function of distance and energy to confirm or reject the hypothesis of a new neutrino. As BR2 uses High Enriched Uranium fuel, SoLid will provide, at the same time, a well measured Uranium-235 antineutrino spectrum, shedding some light on the current flux shape uncertainties.

The detector technology proposed by SoLid utilises ten of thousands of small size PVT cubes covered with LiF:ZnS(Ag) sheets to construct a highly segmented antineutrino detector. The PVT acts as a target for antineutrinos and is used to detect the positron with good energy resolution. The LiF:ZnS(Ag) layers provide both a neutron signal insensitive to gamma-rays and a precise localisation of the neutron capture. The result is an antineutrino detector with high detection efficiency and containment of energy designed to measure precisely meter scale antineutrino oscillation. In addition to high spatial resolution in measuring the signal, the segmentation also provide high discrimination of external backgrounds that resemble antineutrino interactions. The first phase of the experiment has started in January 2018 with the deployment of 5 modules corresponding to around 1.6 tonnes of fiducial mass at 6.2m from the BR2 reactor core.

In this presentation I will review the development of the SoLid detector technology construction phase and commissioning of the experiment. I will then present the current status of SoLid and the physics reach of the first phase.