Kolloquium der Theoretischen Physik
Wintersemester 2011
jeden zweiten Dienstag jeweils von 16-18h im Hörsaal F
um zahlreiches Erscheinen wird gebeten
25.10.2011
Walter Schirmacher (Universität Mainz)
The Blue From The Sky: Rayleigh scattering, longtime tails and the spectral properties of disordered solids
[show abstract]
Rayleigh scattering, which is the origin of the blue color of the sky, does not only arise due
to the presence of “scattering particles” but also due to any inhomogeneity in the wavecarrying
medium, as pointed out already by Rayleigh himself. So Rayleigh scattering is the
dominant long-wavelength scattering mechanism for waves in a statically ("quenched")
disordered system. Exploiting the mathematical analogy between single-particle diffusion
and waves (both obey a Helmholtz equation in frequency space) it will be shown that the
appearance of a "long-time tail" in the velocity autocorrelation function of single-particle
motion in a glass (e.g. in a lithium battery) has the same origin as Rayleigh scattering.
Another anomaly in wave motion in disordered systems, the ”boson peak” will be shown
to have its diffusive analogy in a transition from normal diffusion to anomalous diffusion.
The boson-peak anomaly arises from "level repulsion" caused by the disorder-induced local
violation the translational and rotational symmetry.
A field-theoretical treatment of these anomalies, which is in good agreement with measured
spectra will be given.
08.11.2011
Frederic Schuller (MPI für Gravitationsphysik Potsdam)
Matter, Spacetime, and all that
[show abstract]
All we know about spacetime, we infer from the properties of matter. For two hundred years, spacetime had been custom-tailored to the motion paths of apples. Then for one hundred years to light. Now the OPERA neutrino experiment may well have shown that the spacetime geometry must be re-tailored again.
In this talk, we systematically develop the kinematics and dynamics of any spacetime geometry that can carry predictive matter field equations and admits observers. Thus we are freed from the assumption that spacetime be a Lorentzian manifold solving the Einstein equations, but maintain the same degree of physical soundness and technical control.
A discussion of pertinent implications of this rather sweeping result -- for the interpretation of the OPERA neutrino experiment, the evaluation of astrophysical observations, and the search for classical signatures of quantum gravity -- will conclude this talk aimed at the non-specialist.
29.11.2011
Winston Fairbairn (FAU Erlangen, Mathematik)
On algebraic string actions
[show abstract]
I will introduce and discuss an algebraic first order formulation of the
bosonic string first derived two decades ago by Balachandran and
collaborators. Focusing on the case of a flat four-dimensional target
space-time, I will show how the framework allows for a description in
terms of self-dual variables reminiscent of Ashtekar's self-dual
formulation of four-dimensional gravity. I will then discuss the
relation to the Nambu-Goto string and show that the two theories are
classically equivalent if the algebraic string is restricted to its
self-dual or anti-self-dual sector.
13.12.2011
Christoph Bruder (Universität Basel)
Quantum control
[show abstract]
Quantum mechanics courses discuss how the initial state of a
system evolves if the Hamiltonian is time-dependent. In quantum
control, the question is reversed: how should the time-dependent
Hamiltonian be chosen to generate a prescribed unitary transformation
at some final time? After a general introduction, this talk will in
particular address the problem of how many quantum bits of a quantum
computer one needs to control to run an arbitrary quantum algorithm.
10.01.2012
no talk
24.01.2012
Klaus Richter (Universität Regensburg)
Mesoscopic Spintronics
[show abstract]
A major issue in semiconductor-based spintronics
is the creation and control of spin-polarized currents.
Coherence in mesoscopic conductors of reduced dimensionality
gives rise to quantum interference and thereby opens up novel
means to manipulate the electron spin degree of freedom.
I will discuss alternative concepts to achieve spin-polarized
currents in non-magnetic mesoscopic systems, ranging from
nanostructures with spin orbit interaction to spin-polarized
edge currents in graphene and topological insulators.
31.01.2012
Ignacio Wilson-Rae (TU München)
Phonon-tunneling dissipation of mechanical resonators
[show abstract]
State of the art optomechanical and nanomechanical setups
are close to allowing for the observation of quantum effects in a
"macroscopic" mechanical system. A major challenge in this context,
is understanding and controlling mechanical dissipation at low
temperatures. Following a broad introduction, I will present an
analysis of the dissipation induced by the unavoidable coupling of a
mechanical resonator to the vibrations of its substrate (known as
clamping losses). Our "phonon tunneling" approach leads to a
"master formula" for the design-limited dissipation 1/Q that is
applicable to a very wide range of high-Q resonators ranging from
microscale MEMS devices to carbon-nanotube nanoresonators. Based on
this master formula, we have developed an efficient FEM-enabled solver
that can be used as an aid to the design of complex resonators. We
have applied this concept to "free-free" micromirror structures
relevant for Fabry-Perot based optomechanics. A design which has
allowed us to rigorously test the theory. Finally, I will discuss the
case of high-stress membrane nanoresonators for which the Q-values of
different harmonics present a striking non-monotonic behavior due to
interference between the radiated elastic waves.
07.02.2012
Klaus Kroy (Universität Leipzig)
Hot Brownian Motion
[show abstract]
I introduce a Markovian description of the nonequilibrium Brownian
motion of a heated nanoparticle in a simple solvent with a
temperature-dependent viscosity. Analytical results for the generalized
fluctuation-dissipation and Stokes-Einstein relations compare favorably
with molecular dynamics simulations and measurements of laser-heated
gold nanoparticles. They provide a practical rational basis for emerging
photothermal tracer and nanoparticle trapping and tracking techniques.
