TY - JOUR AB - Motivated by properties-controlling potential of the strain, we investigate strain dependence of structure, electronic, and magnetic properties of Sr2IrO4 using complementary theoretical tools: ab-initio calculations, analytical approaches (rigid octahedra picture, Slater-Koster integrals), and extended t−J model. We find that strain affects both Ir-Ir distance and Ir-O-Ir angle, and the rigid octahedra picture is not relevant. Second, we find fundamentally different behavior for compressive and tensile strain. One remarkable feature is the formation of two subsets of bond- and orbital-dependent carriers, a compass-like model, under compression. This originates from the strain-induced renormalization of the Ir-O-Ir superexchange and O on-site energy. We also show that under compressive (tensile) strain, Fermi surface becomes highly dispersive (relatively flat). Already at a tensile strain of 1.5%, we observe spectral weight redistribution, with the low-energy band acquiring almost purely singlet character. These results can be directly compared with future experiments. AU - Paerschke, Ekaterina AU - Chen, Wei-Chih AU - Ray, Rajyavardhan AU - Chen, Cheng-Chien ID - 12213 JF - npj Quantum Materials KW - Condensed Matter Physics KW - Electronic KW - Optical and Magnetic Materials TI - Evolution of electronic and magnetic properties of Sr₂IrO₄ under strain VL - 7 ER - TY - JOUR AB - We review our theoretical results of the sound propagation in two-dimensional (2D) systems of ultracold fermionic and bosonic atoms. In the superfluid phase, characterized by the spontaneous symmetry breaking of the U(1) symmetry, there is the coexistence of first and second sound. In the case of weakly-interacting repulsive bosons, we model the recent measurements of the sound velocities of 39K atoms in 2D obtained in the weakly-interacting regime and around the Berezinskii–Kosterlitz–Thouless (BKT) superfluid-to-normal transition temperature. In particular, we perform a quite accurate computation of the superfluid density and show that it is reasonably consistent with the experimental results. For superfluid attractive fermions, we calculate the first and second sound velocities across the whole BCS-BEC crossover. In the low-temperature regime, we reproduce the recent measurements of first-sound speed with 6Li atoms. We also predict that there is mixing between sound modes only in the finite-temperature BEC regime. AU - Salasnich, Luca AU - Cappellaro, Alberto AU - Furutani, Koichiro AU - Tononi, Andrea AU - Bighin, Giacomo ID - 12154 IS - 10 JF - Symmetry KW - Physics and Astronomy (miscellaneous) KW - General Mathematics KW - Chemistry (miscellaneous) KW - Computer Science (miscellaneous) SN - 2073-8994 TI - First and second sound in two-dimensional bosonic and fermionic superfluids VL - 14 ER - TY - THES AB - In this Thesis, I study composite quantum impurities with variational techniques, both inspired by machine learning as well as fully analytic. I supplement this with exploration of other applications of machine learning, in particular artificial neural networks, in many-body physics. In Chapters 3 and 4, I study quasiparticle systems with variational approach. I derive a Hamiltonian describing the angulon quasiparticle in the presence of a magnetic field. I apply analytic variational treatment to this Hamiltonian. Then, I introduce a variational approach for non-additive systems, based on artificial neural networks. I exemplify this approach on the example of the polaron quasiparticle (Fröhlich Hamiltonian). In Chapter 5, I continue using artificial neural networks, albeit in a different setting. I apply artificial neural networks to detect phases from snapshots of two types physical systems. Namely, I study Monte Carlo snapshots of multilayer classical spin models as well as molecular dynamics maps of colloidal systems. The main type of networks that I use here are convolutional neural networks, known for their applicability to image data. AU - Rzadkowski, Wojciech ID - 10759 SN - 2663-337X TI - Analytic and machine learning approaches to composite quantum impurities ER - TY - JOUR AB - Recently it was shown that anyons on the two-sphere naturally arise from a system of molecular impurities exchanging angular momentum with a many-particle bath (Phys. Rev. Lett. 126, 015301 (2021)). Here we further advance this approach and rigorously demonstrate that in the experimentally realized regime the lowest spectrum of two linear molecules immersed in superfluid helium corresponds to the spectrum of two anyons on the sphere. We develop the formalism within the framework of the recently experimentally observed angulon quasiparticle AU - Brooks, Morris AU - Lemeshko, Mikhail AU - Lundholm, Douglas AU - Yakaboylu, Enderalp ID - 10585 IS - 4 JF - Atoms KW - anyons KW - quasiparticles KW - Quantum Hall Effect KW - topological states of matter TI - Emergence of anyons on the two-sphere in molecular impurities VL - 9 ER - TY - JOUR AB - Area-dependent quantum field theory is a modification of two-dimensional topological quantum field theory, where one equips each connected component of a bordism with a positive real number—interpreted as area—which behaves additively under glueing. As opposed to topological theories, in area-dependent theories the state spaces can be infinite-dimensional. We introduce the notion of regularised Frobenius algebras in Hilbert spaces and show that area-dependent theories are in one-to-one correspondence to commutative regularised Frobenius algebras. We also provide a state sum construction for area-dependent theories. Our main example is two-dimensional Yang–Mills theory with compact gauge group, which we treat in detail. AU - Runkel, Ingo AU - Szegedy, Lorant ID - 8816 IS - 1 JF - Communications in Mathematical Physics SN - 00103616 TI - Area-dependent quantum field theory VL - 381 ER -