Condensed Matter Physics Seminar
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2022-11-18 (Friday)
room 1.02, Pasteura 5 at 12:15 
Maciej Fidrysiak (UJ, Cracow)Robust quantum collective excitations in high-temperature copper-oxide superconductors
Layered copper oxides are prototypical strongly correlated electron systems, evolving with chemical doping from the antiferromagnetic Mott insulating state, through the high-temperature superconducting phase, to the correlated metallic regime. Thanks to developments in spectroscopic techniques, a fairly complete mapping of quantum spin and charge fluctuations across the phase diagram has now become available for several cuprate families, which provides a testing ground for present theoretical models of those materials. I will summarize our recent theoretical study of collective spin (paramagnon) and charge (plasmon) excitations in the correlated metallic state of copper oxides.We find that paramagnon excitations persist down to the hole-overdoped regime and are less coherent on the electron-doped side of the phase diagram, in agreement with experiment. The role of strong electronic correlations and the underlying fermiology in stabilizing the magnetic excitations will be discussed. Finally, I will mention possible extensions to other classes of strongly correlated condensed-matter systems.
2022-11-04 (Friday)
room 1.02, Pasteura 5 at 12:15
Maciej Łebek (IFT UW)Prethermalization in one-dimensional Bose gases
When driven out of equilibrium, isolated many-body quantum systems eventually thermalize, reaching state that can be predicted using statistical ensemble theory. This relaxation towards thermal state is with few exceptions universal. Sometimes however, the final thermalization is preceded by appearance of long-lived, metastable pre-thermal state that may be very far from typical states corresponding to thermal equilibrium.In my talk, I will analyse non-equilibrium dynamics of the system involving many quasi one-dimensional Bose gases that are coupled together with long-range interactions. Such arrays of one-dimensional tubes are realized in experimental setups. Thanks to the underlying integrability of a single tube, a Boltzmann-equation formalism used to describe the dynamics was recently developed. By studying the properties of collision integral, I will identify the emergent integrals of motion and show that the system exhibits pre-thermal behavior.
2022-10-28 (Friday)
room 1.02, Pasteura 5 at 12:15
Marcin Wysokiński (MagTop Warsaw)Non-Abelian Berry phase induced entanglement between qubits in QED cavity
Geometric phases, generated by cyclic evolutions of quantum systems with degenerate groundstate providing qubit basis, offer realization of the non-Abelian statistics. As compared to dynamic gates, non-Abelian geometric control over such a qubit should benefit from tolerance to fluctuations. On the example of dynamically driven hole-spin 3/2 immersed in QED cavity we have shown that the interplay between the non-Abelian Berry phases generated by local time-dependent electrical fields and the shared photons allows for fast manipulation, detection, and long-range entanglement of the qubits. Moreover, we have demonstrated that owing to its geometrical structure, such a scheme is indeed robust against external noises. Additionally, we have confirmed that the found mechanism also applies to quantum dot qubits where single-gate holonomic operations are executed by (i) cyclic control of the position of confining potential on the two dimensional plane as well as by (ii) cyclic control of position along one dimension together with periodic change of spin-orbit interaction.
2022-10-21 (Friday)
room 1.02, Pasteura 5 at 12:15
Ivan Balog (Institute of Physics, Zagreb)Two applications of the nonperturbative functional renormalization group
After briefly introducing the nonperturbative functional renormalization group method, I explain two of its applications, for which the functional nature of the method is crucial. First of all I address the the question of how to calculate the probability distribution of the order parameter in critical systems. Since the first works of Wilson on the topic of the renormalization group it was understood that the quantities calculated within the RG should represent probabilities of observables. However no practical implementation for the calculation of probabilities was developed before our work. I will show how to calculate the distribution of critical order parameter and compare the results to the Monte Carlo calculations. Second application I will talk about is the analytical description of the approach to the lower critical dimension. This is the dimensionality where the transition at a finite temperature disappears due to entropy effects. These entropy effects are statistically rare and localized excitations such as kinks or instantons in systems with discrete symmetry or vertices and spin waves in the systems with continuous symmetry. The fundamental interest of this line of research is to understand how the localized excitations translate into long wavelength behavior within the functional RG approach.
2022-10-14 (Friday)
room 1.02, Pasteura 5 at 12:15
Aleksandra Ziółkowska (University of Oxford, UK)Hydrodynamic Approach to Integrable Quantum Field Theory
Hydrodynamics allow for efficient computation of many-body dynamics and have been successfully used in the study of black hole horizons, collective behaviour of QCD matter in heavy ion collisions, and non-equilibrium behaviour in strongly-interacting condensed matter systems.I will present the application of hydrodynamics to quantum field theory with an infinite number of local conservation laws. Such an integrable system can be described within a recently developed framework of the generalised hydrodynamics. I will present the key assumptions of generalised hydrodynamics as well as summarise some recent developments in this field. In particular, I will concentrate on the study of the $SU(3)_2$-Homogeneous sine-Gordon model. Thanks to the hydrodynamic approach, we were able to identify the key dynamical signatures of unstable excitations in this integrable quantum field theory and simulate the real time RG-flow of the theory between interacting and free conformal regimes.