The LCN2 community is proud to present the Leiden Networks Day: a one-day symposium open to all researchers interested in networks, from Leiden, the Netherlands and beyond. The event will feature a number of excellent international speakers and is free More info
Friday September 23, 2016, plenary conference room, Poortgebouw
9:30 - 10:00 Arrival
10:00 - 10:05 Opening by Han de Winde, vice-dean of the Faculty of Science
10:05 - 10:15 Introduction by Frank den Hollander
10:15 - 11:00
Pierluigi Crescenzi (University of Florence)
A large graph mining round trip: From theory to practice and back
11:00 - 11:30 Break
11:30 - 12:15
Janos Kertesz (Central European University)
Social contagion: A truly complex phenomenon
12:15 - 13:00
Kees Stam (Free University Amsterdam)
Complex brain networks: from observations to models
13:00 - 14:15
(free if you are registered)
14:15 - 15:00
Mariangeles Serrano (Universitat de Barcelona)
Network geometry and gravity models in complex networks
15:00 - 15:45
Stefan Thurner (Medical University of Vienna)
How financial multilayer networks create systemic risk - and how to manage it
15:45 - 16:15 Break
16:15 - 17:00
Alessandro Vespignani (Northeastern University)
From spreading processes in networks to infectious disease forecast
There will be plenty of opportunity to meet with researchers interested in network science throughout the day. A lunch, refreshments and afternoon drinks and snacks will be provided.
Be sure to note September 23 in your calendar and register by clicking this link.
Nature publishes an article on a paradoxical discovery in superconductivity. Leiden physicist Jan Zaanen writes a News & Views article about this in the same issue of August 19th.
Superconductivity is a bizarre but useful physical phenomenon. By More info
cooling a material down to below a critical temperature, its electrical resistance suddenly disappears completely. That way, you can easily send electricity through a wire without any loss of energy. This comes in very handy for example in windmills or MRI scanners.
News & Views
The cooling however poses a problem, which finally does cost energy. That is why physicists are on the hunt for a material with superconductive behavior at not-too-low temperatures. In a News & Views article in Nature, Leiden physicist Jan Zaanen describes how a new discovery leads to an interesting paradox.
The discovery, made by Ivan Božović from Yale University, has to do with copper oxides. In principle these don’t conduct any current. Their electrons have too strong of an interaction and retain each other, like cars in a traffic jam. But taking away some electrons gives the rest some room to manoeuver. They do this in pairs. These electron pairs can actually move so well that superconductivity occurs, even though the temperature is relatively high.
By removing too many electrons, a surplus of empty spots arises. Electrons now have difficulty finding each other to form pairs, and the value for the critical temperature drops. Everything seems to indicate that the famous Bardeen-Cooper-Schrieffer (BCS) theory from 1957 is applicable in this case. This theory describes quantum physics of conventional superconductors very precisely. BCS counter-intuitively predicts that all electrons take part in superconductivity, even if the critical temperature value is extremely low. However, Božović sees the number of participating electrons diminish proportionally to the critical temperature value. Like Zaanen describes, this as an apparent paradox which cannot be explained with our current understanding of quantum physics.
On an apparently normal cube a pattern of hollows and bulges appears when the cube is compressed. Physicists from Leiden University and FOM Institute AMOLF together with colleagues from Tel Aviv University have developed a method to design such three-dimensional More info
structures and to construct these using simple building blocks. This paves the way for the use of 'machine materials' in, for example, prostheses and wearable technology. The researchers will publish their findings on 28 July in Nature.
Normally, atoms and molecules determine the properties of the materials they form. However that is different for 'metamaterials' designed by humans. "In the case of metamaterials, the spatial structure determines the material's behavior," explains group leader Martin van Hecke. "For example, a pattern of holes in a sheet of material gives rise to a mechanical response that is completely different than in the same material without holes. We also wanted to investigate this phenomenon for a three-dimensional pattern of holes."
Van Hecke and his colleagues designed a cube-shaped, flexible building block with a hole in it. If pressure is applied to such a block then some of the sides cave in, whereas others bulge out. By stacking several of these building blocks researchers could make three-dimensional structures. Van Hecke: "The orientation of the blocks in the metamaterial is important. Under pressure, all of the hollow and bulging sides must fit exactly together. Most of the stacks are 'frustrated': somewhere within two hollows or bulges meet. However a large number of fitting solutions for this three-dimensional puzzle were found."
Van Hecke's colleagues at Tel Aviv University calculated the number of possible, non-frustrated stacks for different cubes of building blocks. "For one cube of 14x14x14 building blocks that is a number with no less than 65 figures," says Van Hecke. "For each possible stack the deformation within the cube results in a specific pattern on the sides of the cube. By smartly combining the building blocks we can program the material such that every desired pattern appears on the sides of a compressed cube. Surprisingly such a cube can also be used to analyse patterns. If we press it against a pattern of hollows and bulges then we measure a force that is dependent on the pattern."
Although Van Hecke's research is fundamental in nature there are applications on the horizon. "This type of programmable 'machine materials' could be ideal for prostheses or wearable technology in which a close fit with the body is important," says Van Hecke. "If we can make the building blocks more complex or produce these from other materials then the possibilities really are endless."
To demonstrate that any pattern can be produced on a cube's surface, the researchers developed a cube of 10x10x10 blocks on which a smiley appears when the cube is compressed.
Credits: Corentin Coulais
Physicists have studied the astrophysical neutrino signal as reported by the IceCube collaboration from a different angle with their ANTARES detector. The Milky Way centre was an obvious prime suspect to be a source, but this hypothesis is now only More info
Gotta catch ‘em all! Physicists are always in the hunt for any kind of particle raining down from the sky. Amongst them are neutrinos—one of the hardest to catch. These ultralight particles are so difficult to detect because they penetrate through anything, including detectors. This also means that they are extremely interesting for scientists, because they travel from the inside of space objects directly to Earth, without getting deflected along the way. And with that, they keep a bunch of information safely stored inside them.
To catch them, scientists need massive detectors made of several cubic kilometers of ice or water, like IceCube on Antarctica or ANTARES in the Mediterranean Sea. IceCube has recently reported many detected neutrinos, with a higher number coming from the Southern sky. The centre of the Mily Way is located there, so our Galaxy’s core was an obvious prime suspect to be responsible for a good part of this neutrino influx. However, the signal events in IceCube have a limited resolution, so it remained unclear where the mysterious signal comes from.
Now an international team of physicists, including Leiden University’s Dorothea Samtleben, have used the ANTARES detector to look at the signal at high resolution from a better angle. They show that under certain plausible assumptions on the neutrino flux properties only two of the events detected by IceCube could originate from the Milky Way. ANTARES will now continue with a newly developed reconstruction method to also probe even higher energetic neutrino fluxes from the Milky Way as cause.
‘We are assuming that the so far detected astrophysical neutrinos come from sources with violent “explosions”,’ says Samtleben. ‘We don't know whether the detected neutrinos come from our own galaxy or from outside, and so far also no significant correlation of the neutrino directions could be found with any other known astrophysical source’.
The ANTARES team publishes their results in Physics Letters B on 10 September, but the article is already accessible online.
Melle T.J.J.M. Punter, Armando Hernandez-Garcia, Daniela J. Kraft, Renko de Vries,
and Paul van der Schoot (2016) Self-Assembly Dynamics of Linear Virus-Like Particles: Theory and
Experiment, J. Phys. Chem. B, 120, 6286-6297. [DOI]
Scott Waitukaitis and Martin van Hecke (2016) Origami building blocks: Generic and special four-vertices, Physical Review E, 93, 023003. [DOI][pdf]
25 Aug, 11:30, room 276 Instituut Lorentz
Theory seminar Petr Jizba, Czech Technical University: A new class of entropy-power-based uncertainty relations
6 Sept, 13:45, Academy building, Rapenburg 73
Thesis Defense Ke Liu - IL: Gauge Theory and Nematic Order. The Rich Landscape of Orientational Phase Transition. Promotor: Prof.dr. J. Zaanen
14 Sept, 18:00, Sitterzaal 032
Colloquium Ehrenfestii William Irvine (Chicago): TBA