Add to Calendar 2025-11-20T10:00:00 2025-11-20T11:00:00 Open Quantum System Theory Event Information: Non-unitary operations have tremendous potential for use in quantum technology. On the one hand, they can be used to describe the imperfections of experiments which are not fully isolated from the environment. They also describe operations which are necessary for quantum information technology such as qubit-reset, mixed-state preparation and autonomous quantum error correction. In this talk, I will discuss emerging ideas in open system theory. Firstly, I will introduce the Lindblad master equation and non-Hermitian Hamiltonians as effective descriptions of the environmental coupling. This non-Hermitian description naturally gives rise to an open-system topological invariant, the subject of recent theoretical debate. Secondly, I will introduce a bath model that goes beyond the simplified Lindblad and non-Hermitian descriptions. This may have applications as a spectrally dependent switch, which implements decay processes for systems with energies in a fixed range. Lastly, I will discuss my future plans to determine general bounds on autonomous quantum error correction. Event Location: BRIM 311

Add to Calendar 2025-11-27T10:00:00 2025-11-27T11:00:00 Towards One Dimensional Heavy Fermions Event Information: Strong correlations between conduction electrons and magnetic moments reflect the competition between moment instabilities and magnetic order. Investigations of the resulting collective phases and complex phase diagrams in 3D compounds continue to reveal new behaviors, although similar efforts on 1D systems remain very limited due to a lack of suitable materials. We present here experimental evidence for 1D excitations in Ti4MnBi2, a moderately correlated metal consisting of well-separated chains of spin S = 1/2 moments. Inelastic neutron scattering (INS) measurements complemented by DMRG calculations show that Ti4MnBi2 is well described by the frustrated J1- J2 XXZ Hamiltonian, displaying a continuum of spinon excitations based on an underlying AF ↑↑↓↓ modulation of the Ising-like Mn moments along the chain. This is evidence for the one- dimensionality of the Mn subsystem. Fermi liquid behavior is observed in both the specific heat and electrical resistivity at low temperatures, indicating that the conduction electrons are three-dimensional, with magnetic correlations similar in strength to those found in metallic cuprates and ruthenates. Using a combination of inelastic neutron scattering measurements and DMRG calculations, we explore the phase diagram of Ti4MnBi2 as a function of magnetic fields transverse to the chain direction. Highlights include the collapse of the underlying antiferromagnetic phase at a quantum critical point (QCP), and the emergence of magnon pair excitations at the highest fields. We discuss the possible role of Kondo coupling of the Mn moments to the conduction electrons in determining the lifetimes of the spinons and magnons, and in restructuring their excitations near the AF QCP. This work is a collaboration with Xiyang Li, Igor Zaliznyak, Alberto Nocera, Kateryna Foyetsova, and George Sawatzky. This research was supported by the US National Science Foundation through grant NSF-DMR-1807451, by the Natural Sciences and Engineering Research Council of Canada (NSERC), and by the Stewart Blusson Quantum Matter Institute by the Canada First Research Excellence Fund (CFREF). Event Location: BRIM 311

Add to Calendar 2025-12-04T10:00:00 2025-12-04T11:00:00 The revival of Fe-based superconductors: cascades of screening processes and their implications Event Information: Understanding how renormalized quasiparticles emerge in strongly correlated electron materials provides a challenge for both experiment and theory. It has been predicted that distinctive spin and orbital screening mechanisms drive this process in multiorbital materials with strong Coulomb and Hund’s interactions. In this talk I will focus on hole-doped iron-based superconductors AFe2As2 (A=Rb,Cs), which are the most correlated members of this material class with large effective masses and strongly renormalized band dispersions. It is currently debated which role electronic correlations play in the complex phase diagrams of iron-based materials, which contain unconventional superconducting, nematic, and magnetic phases as well as strange metal behavior. In our angle-resolved photoemission spectroscopy (ARPES) study, we observe the existence of two screening processes. The emergence of low-energy Fe 3dxy quasiparticles at low temperatures is tied to spin screening. A second process changes the spectral weight at high energies up to room temperature. Supported by material-specific and model calculations, we propose that orbital screening of Fe 3d atomic excitations is responsible for it. These two cascading screening processes drive the temperature evolution from a bad metal to a correlated Fermi liquid [1,2].   [1] M-H. Chang et al., Nature Comm. 15, 9958 (2024). [2] M-H. Chang et al., Commun. Mater 6, 157 (2025). Event Location: BRIM 311