To explain the so-called fractional quantum Hall effect for electrons moving in a strong magnetic field, R.B. Laughlin suggested in 1983 a many-body wave function that bears his name and earned him a Nobel prize. A crucial property of this function, supported by numerical and experimental evidence, is its extreme rigidity with respect to perturbations by impurities or external fields. In the talk a recent proof of one aspect of this rigidity will be discussed: reasonable modifications of the wave function cannot compress the one-particle density beyond a fixed value. The proof is based on two-dimensional potential theory. This is joint work with Elliott Lieb and Nicolas Rougerie.
Strongly correlated quantum particles with half-integer spin are of growing interest in many fields, including condensed matter, dense plasmas and ultracold atoms. From a theory point of view these systems are very challenging. Also, ab initio quantum simulations such as DMRG are essentially limited to the 1D case. Here, I will present an example of recent breakthroughs we could achieve using
a Nonequilibrium Green functions approach [1] that has recently allowed us to simulate the nonequilibrium transport in 2D and 3D fully including strong correlation effects. We achieve, for the first time, excellent agreement with ultracold atom experiments [2]. I will close by discussing prospects of using NEGF for transport and optics of solids as well as solids in contact with a plasma [3].
[1] Michael Bonitz, "Quantum Kinetic Theory", 2nd edition, Springer 2016.
[2] Niclas Schlünzen, Sebastian Hermanns, Michael Bonitz, and Claudio Verdozzi, Phys. Rev. B 93, 035107 (2016).
[3] Karsten Balzer, Niclas Schlünzen, Jan-Philip Joost, and Michael Bonitz, Phys. Rev. B (2016).
Mi
14.12.2016
Seminar room, F.04.122
Theoretische Physik
13:00
Cosmology Seminar Goettingen
Giorgio Arcadi
MPI für Kernphysik, Heidelberg
Evading Direct Detection constraints in theoretically motivated WIMP models
Weakly Interacting Massive Particles (WIMPs) are among the most popular Dark Matter (DM) candidates. Customarily they are assumed to be the only species of a new "dark" particle sector and to feature pair interactions, induced by suitable mediator fields, with the Standard Models states. These kinds of constructions are dubbed "simplified models" or "dark portals". This class of models allows to maximally profit of the complementarity between different DM search strategies. On the other hand, most realizations suffer increasing tensions with limits from DM Direct Detection experiments. Although conclusive statements are premature, a possible reason of this outcome relies on the oversimplified structure of dark portals. Indeed, more theoretically motivated frameworks typically predict the existence of a richer particle spectrum in the dark sector. In this case, it is possible to weaken the tight correlation between the Dark Matter relic density and direct Detection, for example through the introduction of new "dark" annihilation channels. I will discuss some concrete examples in which it is possible to preserve the validity of the WIMP paradigm compatibly with current direct detection limits and even in the case of absence of signals at next generation Direct Detection experiments.
Topological states of quantum matter such as topological insulators and superconductors have been an active field of research in physics for many years. The recent experimental progress on their realization with ultracold atomic gases raises natural questions about the notion of topological quantum matter far from thermal equilibrium. In this talk, we present our theoretical findings in this context.
We first discuss the non-equilibrium Hall response of a system
initialized in a topologically trivial state before its Hamiltonian is
ramped into a Chern insulator phase, comparing the coherent dynamics
with effects of dephasing. In the second part of the talk, we discuss
non-equilibrium properties of driven systems. In particular, we show how a perfect spin momentum locking, so far only known from edge states of two-dimensional topological insulators can emerge in the stroboscopic dynamics of one-dimensional Floquet lattice systems. Finally, we discuss dynamically defined topological quantum numbers that have no non-equilibrium counterpart, and report on their very recent experimental observation.
Januar2017
Di
10.01.2017
Seminarraum A3.101
Theoretische Physik
14:15
Theoretisch-physikalisches Seminar
Lisa Markhof
RWTH Aachen
Tomonaga-Luttinger model: spectral function and going beyond linear
dispersion
In the first part of my talk, I will focus on the momentum- and
energy-resolved single-particle spectral function of the Tomonaga-Luttinger model. Previous results mostly rely on an approximation where the two-particle interaction V(q) is taken as a constant. But if the momentum is fixed away from the Fermi points, the details of V(q) start to matter [1]. Using a method to calculate the momentum-resolved spectral function numerically exact [2], we could provide strong evidence that any curvature of the two-particle interaction at small transferred momentum q destroys power-law scaling of the momentum-resolved spectral function as a function of energy [3].
In the second part, I will extend the model Hamiltonian by allowing
non-linear terms in the fermionic dispersion. I will systematically
describe different approaches to calculate the dynamic structure factor of such a system. Finally, I will discuss the implications of our findings on the non-linear Luttinger liquid phenomenology [4].
[1] V. Meden, Phys. Rev. B 60, 4571 (1999)
[2] K. Schönhammer and V. Meden, Phys. Rev. B 47, 16205 (1993)
[3] L. Markhof and V. Meden, Phys. Rev. B 93, 085108 (2016)
[4] A. Imambekov, T. L. Schmidt, and L.I. Glazman, Rev. Mod. Phys.,
84(3):1253 (2012)