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Condensed Matter Physics Seminar

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room 1.02, Pasteura 5 at 12:15

(IFT UW)

Students' talks

Dawid Ciszewski
"Superconductivity in a chemical capacitor setup"

Eryk Imos
"Teaching a polariton embedded neural network"

room 1.02, Pasteura 5 at 12:15

Inti Sodemann (University of Leipzig)

Chasing Kitaev’s Quantum Spin Liquids

Quantum spin liquids are states of matter with non-trivial patterns of quantum entanglement which feature remarkable properties, such as the emergence of particles with anyonic statistics. While the existence of these states has been theoretically anticipated for many decades since the visionary ideas of Phillip Anderson, a new wave of interest has been triggered by the exactly solvable honeycomb model of Alexei Kitaev which has a quantum spin liquid state with two kinds of emergent particles: an itinerant fermion (known as the spinon) and a vortex-like particle (known as the vison) which has non-abelian anyon exchange statistics. This model is approximately realized in alpha-RuCl_3, but understanding this material and its connection to the ideal Kitaev model has remained elusive partly because of our poor understanding of the impact of perturbations that destroy its exact solvability. In this talk, I will present our understanding of the impact of a Zeeman field on the Kitaev model. By analyzing in detail the behavior of its emergent anyon particles, we will see that there are very different fates for the ferromagnetic and anti-ferromagnetic Kitaev models. The ferromagnetic Kitaev spin liquid likely undergoes a single continuous phase transition directly into the ordinary spin-polarized state as the Zeeman field increases. We will see, however, that the anti-ferromagnetic Kitaev spin liquid likely goes through a series of phase transitions through other non-trivial intermediate quantum spin liquids before transitioning into the trivial spin-polarized state as the Zeeman field increases. We will critically discuss the possible connections of these results to experiments in alpha-RuCl_3.

room 1.02, Pasteura 5 at 12:15

Piotr Chankowski (IFT FUW)

Towards the thermal profile of the Stoner phase transition

I will report on our computations using the effective field theory of the free energy of the diluted gas of spin s=1/2 fermions interacting through a short range spin independent repulsive potential.

room 1.02, Pasteura 5 at 12:15

Emanuel Gull (University of Michigan)

Let's Get Real - Adapting the Toolkit of Many-Body Theory to Realistic Material Simulation

Quantum many-body theories are used to describe the physics of quantum systems with many strongly interacting particles. In condensed matter physics, these theories are typically applied to effective low-energy lattice models, which are designed to capture only the essential degrees of freedom of a solid. Such models contain phenomenological parameters and are often not predictive.This talk will summarize recent progress on solving the many-body problem without adjustable parameters and without the construction of effective low-energy models. We will showcase algorithmic and computational advances that have enabled high-precision calculations of solids with strong quantum effects. A path towards controlled and adaptive many-body simulations is outlined.

room 1.02, Pasteura 5 at 12:15

Jan Skolimowski (MagTop IF PAN Warsaw)

Real-space analysis of Hatsugai-Kohmoto interaction

Exactly solvable models play an important role in the understanding of physical phenomena, as they often provide reference points and intuitions for more accurate, but usually not solvable in general, models. One such model is the Hasugai-Kohmoto model. It circumvents the biggest obstacle of the Hubbard model, the non-commutative nature of the interaction and kinetic terms, by postulating Hubbard-like interaction in the momentum space. Despite this simplification, it is capable of describing a Mott gap opening and the model belongs to the same high-temperature universality class as the Hubbard model. Yet, the exact solvability of this model relies on a silent assumption of the periodicity of the system under consideration. In my talk, I will address this interaction from the real-space perspective and discuss its peculiarities. One of which is that it is highly sensitive to the boundary conditions and their influence is felt even in the thermodynamic limit. For example, the presence of the hard edges leads to the formation of a ferromagnetic ground state of the gapped phase, which is not the case for the periodic system. I will also present other properties of this model and put them in the context of the analogous results for the Hubbard model.

room 1.02, Pasteura 5 at 12:15

Carmine Autieri (MagTop PAS, Warsaw)

Altermagnetism: from the non-relativistic spin-splitting to the staggered Dzyaloshinskii-Moriya interaction

The Kramers’ degeneracy was born in the field of spectroscopy for systems with time-reversal symmetry. Under the additional condition of the inversion symmetry was applied also to the field of the solid-state physics for non-magnetic systems. Recently, it was shown that the extension of the Kramers’degeneracy to the antiferromagnetic systems has some limitations. Without spin-orbit coupling, some antiferromagnets does not present Kramers’degeneracy but a large non-relativistic spin-splitting due to the breaking of time-reversal symmetry. This antiferromagnetism without Kramers degeneracy was named altermagnetism. Altermagnetic compounds behave as conventional antiferromagnets in the real space and as ferromagnets in the k-space paving the way for new technological applications.The presence of the altermagnetic phase strongly depends on the magnetic space group. We investigate the altermagnetic properties of strongly-correlated transition metal oxides analyzing the Mott insulators Ca2RuO4 and YVO3. In both cases, the orbital physics is extremely relevant in the t2g subsector with the presence of an orbital-selective Mott physics in the first case and of a robust orbital-order in the second case. I will briefly mention how the nonsymmorphic symmetries and the dimensionality affect the properties of the altermagnetic phase.Including the spin-orbit coupling, we study the effect of Dzyaloshinskii–Moriya interaction (DMI) in centrosymmetric and noncentrosymmetric altermagnets. Once time-reversal symmetry is broken in altermagnets, the DMI can produce weak ferromagnetism or weak ferrimagnetism from a purely relativistic effect. The DMI that generated weak ferromagnetism in altermagnets has a staggered structure and the DMI can be enhanced by adapting to the staggered geometry the same strategies used to increase DMI in ferromagnetic multilayers. The weak ferromagnetism from a purely relativistic effect is a property exclusively of the altermagnets that is not found in either ferromagnets or conventional antiferromagnets.

room 1.02, Pasteura 5 at 12:15

Carmine Autieri (MagTop PAS, Warsaw)

Altermagnetism: from the non-relativistic spin-splitting to the staggered Dzyaloshinskii-Moriya interaction

room 1.02, Pasteura 5 at 12:15

Alvise Bastianello (TU Munich)

Confinement meets nonequilibrium in quantum spin chains

In recent years, the promise of a large-scale programmable quantum computer has become more and more concrete at an impressive pace, but the final goal is still beyond the horizon. In the meanwhile, analogic quantum simulators provide highly tunable tabletop realizations of exciting phenomena traditionally belonging to high-energy physics, or admittedly simplified versions of them. In this talk, I will discuss how confinement of excitations can be realized in spin chains and analyze some facets of their rich nonequilibrium dynamics, ranging from short-time features to atypically slow late-time thermalization.

room 1.02, Pasteura 5 at 12:15

Maciej Łebek (IFT UW)

Navier-Stokes equations for weakly perturbed integrable models

Hydrodynamics is an effective theory which describes non-equilibrium dynamics at large scales of space and time. The famous Navier-Stokes equations have a universal form and the microscopic details of the model enter only through transport coefficients, which are non-trivial to calculate. In my talk, I will recall classic results from kinetic theory regarding derivation of Navier-Stokes equations and calculation of transport coefficients for weakly interacting classical gas. Then, I will show that these methods can be generalized to the case of weakly perturbed integrable models, which are strongly correlated. Our solution builds on the exactly known thermodynamics and hydrodynamics of integrable systems and can be understood as a perturbation theory around interacting, integrable theories.

room 1.02, Pasteura 5 at 12:15

Carlos Mejuto Zaera (SISSA, Triste)

Local and Non-Local Interactions in Strongly Correlated Materials: Embedding Through Auxiliary Particles

Strong electronic correlation underlies the behaviour of solids and molecules of technological interest, such as multiferroics, magnetic materials or enzymatic centres. These are characterized by a huge tuneability: they can change between markedly different electronic states through slight variations in external parameters like temperature and pressure, enhancing their potential for the design of more efficient and sustainable devices. Computational models should play a central role in this design, alleviating the challenge of searching for and trying out different compounds in the lab. However, theoretically describing strongly correlated electrons is an extremely complex endeavour, requiringspecialized methods. While various techniques for accurately capturing strong correlation exist, they are typically associated with a steep computational cost, limiting the possible exploration of material families. For this reason, the development of more approximate formulations, reducing the computational overhead while remaining qualitatively accurate, is an important complementary strategy towards realizing the technological promise of correlated materials. In this talk,I will discuss the basis and application of one such model: the ghost Gutzwiller (gGut) framework. This can be seen as an embedding approach deriving from the traditional Gutzwiller variational Ansatz. gGut captures strong electronic correlation in terms of an effective, non-interacting quasi-particle Hamiltonian. The key here is the introduction of auxiliary states, the eponymous ghosts, which describe correlation in terms of one-body fluctuations, and which are ultimately projected-out when computing observables. Crucially, despite this comparatively simple structure, gGut can still provide qualitatively accurate spectra of correlated models in both low and high energy regimes. I will show how it reproduces some of the key phenomenological ingredients in multi-orbital models relevant for describing iron pnictides or perovskites. Furthermore, I will introduce a new approximation to recover non-local correlation effects, which are crucial to understanding the properties of enzymatic centres and multi-layered materials. I will discuss the scope and limitations of this approximation, and exemplify its quality on examples of bond dissociation in small molecules. The qualitative reliability of the gGut framework and its comparatively modest computational cost make it a promising addition to the theoretical tool-set for material exploration.

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