Sommerfeld Theory Colloquium (ASC)

Sommerfeld Theory Colloquium, Absolute temperature, that is the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special con- ditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. In this talk, I will present a negative temperature state for motional degrees of freedom: By tailoring the Bose-Hubbard Hamiltonian we exper- imentally created an attractively interacting ensemble of ultracold bosons, which is stable against collapse for arbitrary atom numbers. In this negative temperature state, the quasi-momentum distribu- tion develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Nega- tive temperatures imply negative pressures and open up new param- eter regimes for cold atoms, enabling fundamentally new many-body states and counterintuitive effects such as Carnot engines above unity efficiency. In addition, this system enabled us to study the dynam- ics of the phase transition from Mott insulator to superfluid and to experimentally investigate how fast phase coherence can spread.

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium, A single potential impurity can drastically change a low-temperature transport of strongly interacting particles in one dimension - a sys- tem which is known as the Luttinger liquid. An arbitrary weak backscattering of fermions from the impurity totally destroys their zero-temperature current while even a very strong backscattering of bosons makes no impact on their flow. On the other hand, a more complicated impurity (like a quantum dot or a double-barrier struc- ture with a resonant level) can preserve an ideal resonant conductance of fermions, or conversely lead (in a different geometry) to an ideal (i.e. infinite) resonant resistance. I emphasize the role of a so-called duality in these transport effects. The duality was related to the integrability of the Luttinger liquid with an impurity. I will show that - surprisingly - duality survives the addition of a non-local and retarded interaction (like electron-phonon) which almost certainly destroys the integrability.

Sommerfeld Theory Colloquium, The restricted three body problem has an intriguing dynamics. Glo- bal surfaces of section are a tool to reduce the study of the dynamics on a three dimensional energy hypersurface to the study of an area preserving map of a two dimensional surface. The existence of such a global surface of section is far from obvious. So far mostly per- turbative methods have been used to prove its existence. However, recently developed global tools originating from Gromov-Witten the- ory combined with the fact that energy hypersurfaces are of contact type can be applied to produce new global surfaces of section. In this talk I explain the restricted three body problem, the notion of a global surface of section and the new results we can obtain by bringing holomorphic curves and the contact form into play.

Sommerfeld Theory Colloquium, The physics of elementary particles is governed by symmetries. A particular symmetry stands out: the one between left and right, called parity. Its breaking in beta decay created a bombshell more than fifty years ago, and ultimately led to the creation of the Standard Model of particle interactions, whose final crowning confirmation is to be provided by the Large Hadron Collider (LHC) at CERN. The Standard Model is based on the premise of parity being broken al- ways, at all energies. I argue, on the contrary, that in nature left-right symmetry is fundamental, and that at high energies of the LHC one could actually see its restoration in full glory. I show how this would probe the nature of neutrino, through the spectacular signatures of lepton number violation.

Sommerfeld Theory Colloquium, Ultra-fast optical spectroscopy is a powerful tool for the observation of dynamical processes in several kind of materials. The basic time- resolved optical experiment is the so-called “pump-probe”: a first light pulse, the “pump”, resonantly triggers a photo-induced process. The subsequent system evolution can be monitored, for example, by the time–dependent transmission changes of a delayed “probe” pulse. The pump pulse photon energy, spectral width and peak intensity cre- ates a certain density of electron-hole pairs in a more or less localized region of space. After the creation of the initial carrier density the time evolution of the single-particle and many-particle excitations is now governed by a non-trivial interplay between electron-electron and electron-phonon scatterings. In this talk I will present a novel approach based on the merging of Non-Equilibrium Green’s func- tion theory and Density Functional Theory to investigate the carrier dynamics following a pump excitation. The case of bulk Silicon, a paradigmatic indirect gap semiconductor, is studied by using the Baym-Kadanoff equations. Both the electron-electron (e-e) and elec- tronphonon (e-p) self-energies are calculated fully Ab-Initio by using a semi-static GW approximation in the e-e case and a Fan self-energy in the e-p case. By using the generalized Baym-Kadanoff ansatz the two-time evolution is replaced by the only dynamics on the macro- scopic time axis. The enormous numerical difficulties connected with a real-time simulation of realistic systems is overcomed by using a completed collision approximation that further simplifies the mem- ory effects connected to the time evolution. The carrier dynamics is shown to reduce in such a way to have stringent connections to the well-known equilibrium electron-electron and electron-phonon self- energies. This link allows to use general arguments to motivate the relative balance between the e-e and e-p scattering channels on the basis of the carrier energies.

Sommerfeld Theory Colloquium, In Regensburg there exists a large Lattice-QCD group (SFB/TR-55) working in many fields, ranging from the development of energy efficient super-computers to specialized Lattice studies of SU(N) gauge theories with N>3 for matching to AdS/CFT predictions. The bulk of the work is focused on hadron phenomenology [generalized parton distributions (GPDs), distribution amplitudes (DAs), transverse momentum dependent parton distributions (TMDs)] and QCD thermodynamics, especially in a magnetic background field. (Like a chemical potential a B-field affects quark and gluon degrees of freedom differently thus allowing to investigate their connection. In contrast to the chemical potential, however, it is easy to implement on the lattice. The aim of the talk is to give an overview and to possibly identify topics of mutual interest.

Sommerfeld Theory Colloquium, In the next few years new data from ground and space will push the limit of cosmological observations to new frontiers. Combining CMB, weak lensing, redshift clustering and supernovae data, we will be able to constrain the properties of dark energy and its interactions from the background to the non-linear level. In this talk I discuss these methods and the future expected constraints on dark energy and modified gravity.

Sommerfeld Theory Colloquium, If systems characterized by slow (algebraic) relaxation are prepared in an out-of-equilibrium

initial state, one can observe a "physical aging regime" in the ensuing approach to equilibrium

that is governed by broken time translation invariance and non-trivial, often universal scaling laws. Dynamical systems near a critical point constitute prototypical and now well-understood

examples. Indeed, measuring critical exponents in the intermediate aging rather than the asymptotic stationary temporal regime is now a standard numerical tool. In this talk, I will first apply these concepts to simple driven lattice gases that relax towards non-equilibrium stationary systems displaying generic scale invariance. The expected simple aging behavior in the two-time density auto-correlation function is verified through Monte Carlo simulations in one, two, and three dimensions. Next I shall address the continuous non-equilibrium phase transition in driven Ising lattice gases in two dimensions. Whereas the temporal scaling of the density auto-correlation function in the non-equilibrium steady state does not allow a precise measurement of the associated critical exponents, these can be accurately determined from the aging scaling of the two-time auto-correlations and the order parameter evolution following a quench to the critical point. In the second part of the talk, I will present numerical results for the non-equilibrium relaxation kinetics of interacting magnetic flux lines in disordered type-II superconductors at low temperatures and low magnetic fields, represented by means of a three-dimensional elastic line model. Investigating the vortex density and height auto-correlations as well as the flux line mean-square displacement allows us to carefully disentangle different relaxation mechanisms (e.g., vortex line fluctuations and positional relaxation), and to assess their relative impact on the kinetics of dilute vortex matter at low temperatures. We observe the emergence of genuine glassy dynamics, caused by the competing effects of vortex pinning and long-range repulsive interactions between the flux lines. We contrast the effects of random point-like pinning centers and correlated columnar defects. We also compare data from Monte Carlo simulations with results from Langevin molecular dynamics.