Institut für Theoretische Physik
Start / Aktuell
November  2014
Mo
03.11.2014
Sitzungszimmer Mathematik
Theoretische Physik
14:15

We show that Liouville integrable Hamiltonian systems with one-and-a-half degree of freedom can be described by the method of hydrodynamic reductions. The key object is the Vlasov kinetic equation.

Kontakt: K.-H. Rehren
Mo
03.11.2014
Max-Born-Hörsaal (HS2), Friedrich-Hund-Platz 1
Fakultät für Physik
16:30
Göttinger Physikalisches Kolloquium

Prof. Dr. Angela Rizzi
IV. Physikalisches Institut Göttingen

Physics Nobel Price 2014
EFFICIENT BLUE LIGHT-EMITTING DIODES LEADING
TO BRIGHT AND ENERGY-SAVING WHITE LIGHT SOURCES

The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”

Light-emitting diodes (LEDs) are narrow-band light sources based on semiconductor components, with wavelengths ranging from the infrared to the ultraviolet. The first LEDs were studied and constructed during the 1950s and 1960s in several laboratories. They emitted light at different wavelengths, from the infrared to the green. However, emitting blue light proved to be a difficult task, which took three more decades to achieve. It required the development of techniques for the growth of high-quality crystals as well as the ability to control p-doping of semiconductors with high bandgap, which was achieved with gallium-nitride (GaN) only at the end of the 1980s. The development of efficient blue LEDs also required the production of GaN-based alloys with different compositions and their integration into multilayer structures such as heterojunctions and quantum wells. The invention of efficient blue LEDs has led to white light sources for illumination. When exciting a phosphor material with a blue LED, light is emitted in the green and red spectral ranges, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of complementary colours (red, green and blue) can be used together. Both of these technologies are used in today's high-efficiency white electroluminescent light sources. These light sources, with very long lifetimes, have begun to replace incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20-30% of our electrical energy consumption, and since these new white light sources require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings, of great benefit to mankind.

Di
04.11.2014
Auditorium MPS
MPI für Sonnensystemforschung
11:00
Solar-Seminar

Benjamin Beeck
MPS

3D MHD simulations of stellar magnetoconvection



Mi
05.11.2014
Seminarraum A 03.101
Theoretische Physik
12:00
Statistische Mechanik komplexer Systeme (Forschungsseminar M.Phy.410)

Abhinav Sharma
3rd Institute of Physics - Biophysics, Göttingen

Elastic response of Filamentous networks and their remodeling in presence of active forces

My talk will consist of two parts. First I focus on composite networks. Experiments have shown that elasticity of disordered filamentous networks with compliant crosslinks is very different from networks with rigid crosslinks. Filamentous networks are modeled as a collection of randomly oriented rigid filaments connected to each other by flexible crosslinks that are modeled as wormlike chains. We show that for sufficiently high crosslink density, the network linear elastic response is affine on the scale of the filaments’ length. The nonlinear regime can become highly nonaffine and is characterized by a divergence of the elastic modulus at finite strain. In contrast to the prior predictions, we do not find an asymptotic regime in which the differential elastic modulus scales linearly with the stress, although an approximate linear dependence can be seen in a transition from entropic to enthalpic regimes. In the second part of my talk I focus on remodeling of networks in presence of active forces. First we show experimentally that myosin motors contract crosslinked actin polymer networks to clusters with a scale-free size distribution. This critical behaviour occurs over an unexpectedly broad range of crosslink concentrations. To understand this robustness, we developed a quantitative model of contractile networks that takes into account network restructuring: motors reduce connectivity by forcing crosslinks to unbind. Paradoxically, to coordinate global contractions, motor activity should be low. Otherwise, motors drive initially well-connected networks to a critical state where ruptures form across the entire network.

Kontakt: Marcus Müller
Mi
05.11.2014
Seminarraum A 03.101
Theoretische Physik
12:00



Kontakt: Marcus Müller
Mi
05.11.2014
Auditorium MPS
MPI für Sonnensystemforschung
14:00
S3-Seminar

Martin Bo Nielsen
IMPRS

Asteroseismic determination of internal rotation in Sun-like stars



Kontakt: Sonja Schuh
Mi
05.11.2014
Auditorium MPS
MPI für Sonnensystemforschung
14:40
S3-Seminar

Malte Venzmer
IMPRS

Analysis of plasma and magnetic field solar wind data and extrapolation modeling for deriving near-sun estimates based on empirical data analysis



Kontakt: Sonja Schuh
Mi
05.11.2014
Auditorium MPS
MPI für Sonnensystemforschung
15:20
S3-Seminar

Sijie Yu
IMPRS

Nonlinear force-free modeling of coronal magnetic fields



Kontakt: Sonja Schuh
Do
06.11.2014
MPS - Equeluues + Fornax
MPI für Sonnensystemforschung
11:00
Planetary-Seminar

Andreas Pack
Uni Göttingen

Using oxygen isotopes in planetary sciences

Oxygen isotope ratios give unique insights into processes in the early solar system and the early Earth. As result of new analytical developments we are now able to identify very small variations in oxygen isotopes that allow to address important scientific questions. I will present our recent data on lunar rocks and on the oldest rocks from Earth.

Kontakt: Urs Mall
Do
06.11.2014
Hörsaal 3
Fakultät für Physik
16:00
Joint Physics Seminar

Prof. Michael Horn von Hoegen
University of Duisburg-Essen, Department of Physics & Center for Nanointegration CENIDE

Electron diffraction at surfaces: by now ultrafast!

Ultrafast structural phenomena at surfaces are studied by electron diffraction using a combination of a pulsed electron gun with a fs laser system in a pump probe scheme under ultra-high vacuum conditions. The grazing incidence of electrons of 7 – 30 keV in a reflection high energy electron diffraction (RHEED) geometry ensures high surface sensitivity. The sample is excited with 800 nm photons with pulses of 80 fs length, an energy of 1 mJ at 5 kHz repetition rate and a fluence of 1 – 8 mJ/cm2. Applying a tilted pulse front scheme for the laser pulses the temporal resolution of the entire setup has recently been improved to less than 400 femtoseconds! The huge potential of this technique is demonstrated with the non-equilibrium dynamics of the In induced (8x2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 130 K from a (8x2) ground state, which is accompanied by the formation of a charge density wave (CDW), to a (4x1) excited state. Upon excitation by the fs-laser pulse the (8x2) groundstate is driven into the excited (4x1) state at a sample temperature of 30 K. The surface is only excited electronically, the CDW is lifted by photo doping in less than 400 fs, but the surface remains for almost one nanosecond in a super-cooled excited (4x1) high temperature state. An activation barrier of ~40 meV for the collective motion of the In atoms hinders the immediate recovery of the (8x2) groundstate. This metastable situation – a hidden state of matter far away from equilibrium – is only accessible through the ultra-fast excitation by the fs-laser pulse. Relaxation to the (8x2) ground state is delayed on a timescale of  ~ 500 picoseconds and is triggered by remnant (8x2) areas pinned at adsorbates that act as nucleation seeds – the same way that super-cooled water in a bottle freezes upon the insertion of seeds. The surface unit cells fall back into their ground state, one at a time, like a row of falling dominoes. The phase front propagates at about 100 m/s.

Kontakt: Dr. Robert Maaß
Fr
07.11.2014
SR 4, Institut für Theoretische Physik, A03.101
SFB 937
14:00
Seminar

Roland Netz
FU Berlin

DNA dynamics, protein force spectroscopy and viscoelastic properties of polymeric networks



Kontakt: Glormann
Fr
07.11.2014
Seminarraum 11, C3.101
SFB 1073
14:15
SFB Seminar

Dr. Henning Ulrichs
Universität Göttingen, I. Physik

Spin-wave instabilities on the nanoscale

In this seminar I will discuss magnetization dynamics in strongly excited ferromagnetic systems. The research was motivated by application, specifically within the context of magnonic spintronics. In a magnonic device, one aims to utilize spin waves as information carriers, instead of charge currents. In particular in metallic magnetic systems compatible with standard CMOS technology, spin waves suffer from large damping. Thus compensating these losses is a practical necessity. I will discuss two strategies (inductive excitation and spin currents) which allow to reach this goal. When losses are compensated, the amplitudes of the spin waves increase exponentially. Understanding such instability processes is of fundamental importance for the realization of a useful magnonic device. An example for such a device will be presented in the end.


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