Condensed Matter Physics Seminar
2006/2007 | 2007/2008 | 2008/2009 | 2009/2010 | 2010/2011 | 2011/2012 | 2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025
2023-06-16 (Friday)
room 0.03a, Pasteura 5 at 12:15 
Eugene Demler (ETH Zuerich)Quantum simulations: from the Fermi Hubbard model to quantum assisted NMR inference
I will discuss recent progress of optical lattice emulators of the Fermi Hubbard model, specifically the new availability of snapshots of many-body states with single particle resolution. I will review new insights from these experiments on the properties of doped Mott insulators, including the demonstration of magnetically mediated pairing. I will also present the idea of using quantum simulators to perform inference of NMR spectra for biological molecules.The seminar will be held as a part of the "Distinguished Lectures on Complex Systems and Quantum Physics" series ( https://www.fuw.edu.pl/news/news8237.html ).
2023-06-02 (Friday)
room 1.02, Pasteura 5 at 12:15
Jasper van Wezel (Universiteit van Amsterdam)Thermalization by a synthetic black hole horizon
Open problems in general relativity have motivated the search for analogue gravitational systems in condensed matter implementations. In one such system, a 1D tight-binding model with position-dependent hopping, the possibility of realizing an analogue gravitational horizon has recently been demonstrated. Here, we introduce the concept of emergent temperature, inspired by Unruh radiation, and show that it arises naturally in tight-binding lattice models featuring a horizon. Despite finding many similarities between the emergent lattice temperature and the gravitational Unruh temperature, we show that the nature of the thermal spectrum in the lattice theory is radically different from that in the continuum limit. Additionally, we provide a detailed analysis of outgoing radiation delineating the conditions under which the horizon behaves purely as a thermal source.
2023-05-26 (Friday)
room 1.02, Pasteura 5 at 12:15
Federico Becca (Università degli Studi di Trieste)Variational wave functions for spin models with anisotropic-exchange couplings or spin-phonon coupling
The definition of variational wave functions represents an invaluable tool to describe strongly-correlated systems. A promising playground where this kind of approach has been fruitfully applied lies in highly-frustrated spin models, where spin-liquid phases may emerge from the competition of various super-exchange interactions. Gutzwiller-projected wave functions (constructed from fermionic and bosonic constituents) have been employed since the pioneering suggestion by Anderson of the resonating-valence-bond theory. In the recent years, we demonstrated that the fermionic approach gives very accurate results for simple Heisenberg models with SU(2) spin symmetry. The next challenge is now to incorporate further ingredients in the microscopic Hamiltonians. Examples are given by the inclusion of the Dzyaloshinskii-Moriya interaction (or, more generally, anisotropic-exchange couplings, as present in Kitaev materials) or quantum phonons. Besides being relevant for an accurate description of real materials, these additional perturbations are extremely useful to assess the stability of spin-liquid phases, especially the gapless ones. In this talk, we discuss the accuracy of Gutzwiller-projected wave functions to determine the correct results of these extended models.
2023-05-19 (Friday)
room 1.02, Pasteura 5 at 12:15
Piotr Chankowski (IFT UW)Stoner phase transition at T=0 - the effective field theory approach
I will report on our recent computations of the ground state energy of the diluted gas of spin s fermions interacting through a short range repulsive potential. The result is used to study the impact the higher order corrections can have on the characteristics of the phase transition (at zero temperature) to the ordered phase (on the emergence of the itinerant ferromagnetism).
2023-05-05 (Friday)
room 1.02, Pasteura 5 at 12:15
Louk Rademaker (Universite de Geneve)Forget graphene: More exotic moiré physics with TMDs
The recent revolution in moiré materials started with the discovery of correlated insulators and superconductivity in twisted bilayer graphene. I will show that much stronger electron correlations appear in moiré bilayers of transition-metal dichalcogenides (TMDs). After introducing the origin of flat bands in TMD moirés (including ARPES results), I will discuss theoretical predictions for a range of exotic phenomena: the amorphous Wigner-Mott electron slush; the 3/4 chiral spin liquid; and metal-insulator criticality.
2023-04-28 (Friday)
room 1.02, Pasteura 5 at 12:15
Jan Skolimowski (MagTop IF PAN)Topological properties of QSH system in presence of strongly correlated flat bands
The magnetoresistive random-access memory (MRAM) is one of the promising candidates for faster and more efficient non-volatile computer memory. Among various realizations of MRAM technology some are based on the spin-orbit torque (SOT) mechanism. The family of ferromagnetic 2D systems with typical 2D crystal structures presents in-equivalent properties at K and K' offering the chance to manipulate the topology and show properties tailor-made for the development of this technology. In my talk, I will introduce an effective model of ferromagnetic 2D systems based on the Kane-Mele model, in which non-trivial topology and electron correlations are intertwined to obtain among other phases the ferrovalley insulator and quantum anomalous Hall insulators. The results for this model obtained using the cluster perturbation theory will be discussed with emphasis on the allowed topological phase transitions driven by the change of the model parameters. Among the materials to which we can apply this system, we have several proposed 2D ferromagnetic materials RuCl2, and a recently synthesized family of materials MA_2Z_4 (M,A,Z represent the Vand Cr magnetic transition metal atom, IV-element and V elements respectively) and their Janus phases.
2023-04-21 (Friday)
room 1.02, Pasteura 5 at 12:15
Wojciech Buraś, Mateusz Homenda, Antoni Lis (FUW)Students' Talks
1. Imperfect Bose gas in d dimensions,
2. In search of theory of nematic quantum criticality,
3. Aspherical electron density models in crystallography.
2. In search of theory of nematic quantum criticality,
3. Aspherical electron density models in crystallography.
2023-04-14 (Friday)
room 1.02, Pasteura 5 at 12:15
Ubaldo Cavazos-Olivas (IFT FUW)Polaron and bipolaron formation in a Bose gas
Ultracold quantum many-body systems constitute an interesting research playground due to their wide range of applications, from precision measurements to transport phenomena in the context of condensed matter. One particular example are hybrid systems of atoms and ions, which are rapidly developing. A distinctive property of these kind of systems at ultralow temperature is the emergence of the so-called polaron. A quantum bath composed of bosonic atoms weakly coupled to an ion can be properly described by means of Bogoliubov theory. Nevertheless, this approach is no longer valid as soon as the strong coupling regime is taken into account, leading to an instability with an infinite number of bosons collapsing into the ion. Ion-atom systems feature long-range interactions which drive the system to form a many-body bound state with high density and large atom number. In order to explore this physics and circumvent the bosons unstable behavior, based on, a variational approach is adopted. Employing a regularized potential that retains the correct long-distance behavior, we study the properties of interest in the formation of ionic Bose polaron and bipolaron, such as their energy, the number of bosons that takes part in the cloud formation, and the induced interactions which are tunable by the potential parameters.
2023-04-05 (Wednesday)
room 1.02, Pasteura 5 at 12:15
Teresa Kulka (IFT FUW)Nature of spinons in the 1D Heisenberg antiferromagnet
In this study, we provide an intuitive understanding of the collective low-energy spin excitation of the one-dimensional spin-1/2 antiferromagnetic Heisenberg chain, known as the spinon. We demonstrate that a single spinon can be excited by adding one extra spin to the ground state wave function of the Heisenberg chain with an even number of sites. This procedure accurately reproduces all key features of the spinon's dispersion, and provides a natural explanation for its spin-1/2. We also show that the spinon dispersion can be approximately reproduced if we replace the true ground state with the simplest valence-bond solid. This proves that the spinon of the one-dimensional Heisenberg model can be understood as a single spin flowing through a valence-bond solid.
2023-03-31 (Friday)
room 1.02, Pasteura 5 at 12:15
Michał Suchorowski (IFT UW)Rotation of a molecule in two-dimensional condensate: angulon properties and spontaneous vortex formation
In this study, we explore the behavior of a rotating molecular impurity in a two-dimensional Bose-Einstein condensate. Using a Gross-Pitaevskii-type equation, we investigate how the impurity-bath interaction and size of the condensate affect the angular momentum distribution, density deformation, and density currents in the system. In line with experimental results and angulon theory, we show that the impurity's effective moment of inertia is modified by its interaction with the quantum solvent, leading to a slowing down of its rotation for low angular momentum states. Additionally, we observe the emergence of collective excitations such as solitons and vortices in the system after rapid rotation of the impurity. Our model provides insight into the behavior of impurities in two-dimensional quantum gases and can be seen as an effective description of a rotating molecule on the surface of a superfluid helium droplet.