Calendar of physics seminars and colloquia
Physics colloqium page
Seminar: Fri 2013-12-06 10:00
S3020 (Soliden, floor 3)
Dr. Jan Budich
Fluctuation-driven topological Hund insulators
We discuss the role of electron-electron interaction in a two-band Hubbard model based on the Bernevig-Hughes-Zhang Hamiltonian exhibiting the quantum spin Hall (QSH) effect. By means of dynamical mean-field theory, a system with topologically trivial noninteracting parameters can be driven into a QSH phase at finite interaction strength by virtue of local dynamical fluctuations. For very strong interaction, the system reenters a trivial insulating phase by going through a Mott transition. The topological phase diagram is obtained by direct calculation of the bulk topological invariant of the interacting system in terms of its single-particle Green's function. In the interest of self-consistency, the talk will also provide a general introduction to both band structure topology and its reformulation in terms of single particle Green's functions for interacting systems.
Host: Henrik Johannesson
Seminar: Tue 2013-12-03 11:00
Anomalous thermodynamics at the micro-scale
Particle motion at the micro-scale is driven by thermal fluctuations and applied forces on one side, and damped by the strong resistance due to the fluid viscosity on the other. Friction is so strong that completely neglecting inertia - the overdamped approximation - gives an excellent effective description of the actual particle motion. In contrast, we here show that the overdamped approximation fails when thermodynamic quantities such as the entropy production in the environment is considered, in presence of temperature gradients. In the limit of vanishingly small, yet finite inertia, we find that the entropy production is dominated by an anomalous contribution that has no counterpart in the overdamped approximation. This phenomenon, that we call entropic anomaly, is due to a symmetry-breaking that occurs when moving to the small, finite inertia limit. Production of ‘anomalous entropy’ is traced back to intense sweeps down the temperature gradient.
Host: Bernhard Mehlig
Licenciate: Tue 2013-10-15 13:15
The one-dimensional Kondo Lattice in the Majorana Fermion Representation
This thesis analyses the possibilities offered by the representation of quantum mechanical operators in terms of Majorana fermions. These objects can be imagined as algebraic constituents of the fermionic degrees of freedom, allowing for a different representation of many Hamiltonians. Given a strongly correlated electron system, the Majorana fermion representation can be used to define new fundamental modes that grant a more convenient perspective on the problem and that can be used in the framework of standard analytical and numerical techniques. The main reason behind the usefulness of this approach is the advantageous form taken by the group of the unitary transformations of the Hilbert space, when represented in terms of Majorana fermions. To test such a new approach to strongly correlated quantum systems, the one dimensional Kondo lattice at zero temperature has been studied. Using the Majorana fermion representation a good description of the ferromagnetic phases of the model is obtained, already at mean-field level. This is possible because even very involved many-body processes, such as the emergence of the spin-selective Kondo insulator, or the deconfinement process of a fermion-spinon bound state, are described in a simple way in terms of Majorana fermions. These results prove that thanks to the redefinition of the degrees of freedom used in the analysis of the system, it becomes possible to obtain quite non-trivial results already at mean field level, or to consider very involved (but meaningful) correlated quantum states via simple variational trial states. This will potentially permit a more judicious and profitable choice of the fundamental degrees of freedom, allowing for an improvement of the efficiency of the analytical and numerical techniques used in the analysis of many strongly correlated quantum systems.
Lecture for PhD students: Fri 2013-10-11 13:15
MV:L15 (Matematiska vetenskaper)
Dr. Hans-Peter Eckle
Two Lectures on Simple Quantum Integrable Models from the Yang-Baxter Relations (II)
The quantum Rabi model, describing the interaction between light and matter in the simplest possible setting, has been applied to physical systems ranging from quantum optics to condensed matter, and quantum information theory. In the language of quantum information theory the Rabi model describes a qubit interacting with a single bosonic degree of freedom. In these lectures, we are concerned with the exact solution of the Rabi model. We shall show how the Rabi model with rotating wave approximation, the Jaynes-Cummings model and, especially, its N qubit generalization, the Tavis-Cummings model, fit into the framework of the Yang-Baxter relations and the algebraic Bethe ansatz. Furthermore, we shall demonstrate that also the full Rabi model, i.e. without the rotating wave approximation, is amenable to an exact solution, albeit by methods different from the algebraic Bethe ansatz.
Host: Henrik Johannesson
PhD minicourse: Thu 2013-10-10 13:15
Dr. Andrew Mitchell
University of Oxford
Lecture 2 on Numerical Renormalization Group
'Quantum Impurity Problems' are classic paradigms for strong electron correlations in condensed matter physics. They underpin the theoretical description of magnetic impurities in metals, nanodevices such as quantum dots, and appear as effective models within the dynamical mean field theory of correlated materials. Non-perturbative quantum many-body methods must be employed to solve such problems. In this course, we provide the conceptual framework of the Numerical Renormalization Group, discuss technical/practical details of the calculation, and present relevant applications.
Host: Mats Granath