New frontiers in topological materials

Phase transitions in solids are often accompanied by structural and/or spin order changes. Subtle lattice distortions, for example, can remain hidden from conventional crystallographic probes, hindering the identification of the correct order parameter. I will demonstrate an extremely sensitive metrology for identifying hidden symmetry breaking in complex quantum materials through the interplay of topology, quantum geometry, and nonlinear quantum transport. I will introduce our theoretical and experimental results on a correlated polar metal Ca3Ru2O7 (hidden structural transition) and an antiferromagnetic semiconductor CrSBr (hidden spin transition).

Professor Binghai Yan

Pennsylvania State University, USA

Professor Binghai Yan

Pennsylvania State University, USA

Binghai Yan is a theoretical physicist studying quantum materials, including topological materials and chiral materials, at the Pennsylvania State University. After completing his PhD at Tsinghua University in 2008, he worked as a Humboldt postdoc at Bremen University and later at Stanford University. He was a group leader in the Max Planck Institute in Dresden during 2012 – 2016, assistant and associate professor at Weizmann Institute during 2017 – 2024. He joined the Penn State University as a professor of physics in 2025. He was awarded the ARCHES Prize in Germany in 2013, the Israel Physical Society Prize for Young Scientist in 2017 and recognised as a Highly Cited Researcher every year since 2019.

I will present three experiments in the making where we hope to employ ultracold atoms to explore interesting phenomena in condensed-matter and mesoscopic physics. First, I will summarise recent work on transport properties of ultracold atoms within triangular two-dimensional lattices, highlighting effects of geometric singularities at band-touching points and of geometric frustration of atomic motion in flat bands. I will update you on recent efforts to place ultracold Fermi gases in such lattices. Second, I will describe how we hope to use ultracold transition-metal atoms to realise novel states of matter such as topological superfluids, and will present experimental progress in cooling titanium atoms for this purpose. Last, I will describe experiments using atoms trapped in optical tweezer arrays where we return to the study of symmetry breaking transitions related to the Dicke model, but, now, with digital control over the atom number in a distinctly mesoscopic regime.

Professor Dan Stamper-Kurn

University of California, Berkeley, USA

Professor Dan Stamper-Kurn

University of California, Berkeley, USA

Dan Stamper-Kurn is a Professor of Physics at the University of California, Berkeley, and also a Faculty Scientist in the Materials Sciences Division of Lawrence Berkeley National Laboratory. Together with his research team at Berkeley, Professor Stamper-Kurn is an experimentalist who studies quantum mechanical phenomena in systems composed of ultracold atoms and quantum states of light. Current areas of interest include quantum simulation of geometrically frustrated materials and other condensed-matter systems, symmetry breaking dynamics in mesoscopic systems composed of a small number of atoms, quantum information processing in the neutral-atom platform, and bringing new atomic elements to the ultracold temperature regime. Professor Stamper-Kurn initiated and now directs the Challenge Institute for Quantum Computation, a collaboration centred at several universities across California, which focuses on fundamental scientific and engineering challenges in quantum information science.

Ultracold atoms in optical lattices are one of the major platforms for experimental quantum simulations and many-body physics, including topology. One particularly useful feature of ultracold atoms is the possibility to employ Bose-Einstein condensates as very localised probes in momentum space.

I will start by reviewing our previous work on using atom interferometry to directly measure Berry flux in a graphene-type hexagonal optical lattice, where we could directly detect the topologically protected Berry flux associated with a Dirac point. Beyond single bands, the resulting Berry phases generalise to matrix-valued Wilson loops, which give rise to even richer physics. I will present out current status towards applying these methods to study the multiband topology and Euler class in the Kagome lattice.

In a second part, I will discuss how quasiperiodic potentials such as the Aubry-André-Harper chain can give rise to a fractal band structure where each effective band can support topologically protected quantised charge pumping. Surprisingly, the quantised currents are extremely robust and remain quantised even when additional disorder closes the relevant gaps.

Professor Ulrich Schneider

University of Cambridge, UK

Professor Ulrich Schneider

University of Cambridge, UK

Professor Ulrich Schneider is an experimental physicist studying quantum simulation and many-body physics using ultracold atoms in optical lattices. He is a reader in many-body physics at the Cavendish Laboratory of the University of Cambridge and a fellow of Jesus College Cambridge. Earlier positions include the Ludwig-Maximilians Universität in Munich and the Johannes-Gutenberg Universität in Mainz.

He has worked extensively on non-equilibrium dynamics in optical lattices and the realisation of many-body localisation. Other works include studies of quantum transport, the dynamics of a quantum phase transition, fermionic Mott insulators, and Negative Absolute Temperatures. He also developed interferometric probes for topology and realized optical quasicrystals. He is the winner of the 2015 Rudolf-Kaiser prize, holds an ERC starting grant, and is part of the AION consortium, the DesOEQ programme grant and the UK Quantum Technology Hub in Computing & Simulation.

Most superconductors are thermals insulators. A disordered chiral p-wave superconductor, however, can make a transition to a thermal metal phase. Because heat is then transported by Majorana fermions, this phase is referred to as a Majorana metal. We will discussion numerical evidence that the mechanism for the phase transition with increasing electrostatic disorder is the percolation of boundaries separating domains of different Chern number. We construct the network of domain walls using the spectral localiser as a "topological landscape function", and obtain the thermal metal insulator phase diagram from the percolation transition.

Professor Carlo Beenakker

Leiden University, The Netherlands

Professor Carlo Beenakker

Leiden University, The Netherlands

Carlo Beenakker is a theoretical physicist at the Instituut-Lorentz of Leiden Univrsity, where he studies topological states of matter, in particular in view of applications to quantum computing. He is a member of the Royal Netherlands Academy of Arts and Sciences, and a Knight in the order of the Dutch Lion.

Cold atomic gases have the potential to explore bosonic version of fractional quantum Hall states. To achieve such phases, it is of interest to generate optical lattices that host Chern bands with narrow energy dispersion and certain geometric properties which promote the realisation of strongly correlated phases. One approach is to use Raman coupling of internal (spin) states to form optical flux lattices that are analogous to Landau levels. I shall present recent results which show how one can design optical flux lattices to make them both very narrow in energy and of "ideal" geometry.

Professor Nigel Cooper

University of Cambridge, UK

Professor Nigel Cooper

University of Cambridge, UK

Nigel Cooper received a DPhil from the University of Oxford in 1994. He held research positions at Harvard University and the Institut Laue Langevin, and was a Royal Society University Research Fellow and Lecturer at the University of Birmingham before moving to Cambridge in 2000. He was awarded the 2007 Maxwell Medal and Prize by the Institute of Physics, a Humboldt Research Award (2013), an EPSRC Established Career Fellowship (2013), a Simons Investigator Award (2017) and the 2019 Lord Rayleigh Prize of the IOP.

Recent experiments of transition-metal dichalcogenide moiré heterostructures have highlighted the intricate interplay between strong electronic correlations, topology and Bloch band geometry in highly tunable systems. Thus far, its understanding rests on mappings to Landau levels and the fractional quantum Hall effect, while competing and time-reversal-symmetric phases due to nontrivial quantum geometry remain largely unexplored. In this talk, I will first describe how strong interactions in fractionally filled time-reversal-symmetric bands can realise a new class of topological Mott insulators on emergent Kondo lattices, with possible realisations in twisted MoTe₂ and WSe². These phases feature an interaction-driven segregation of the active topological bands into a partial Wannier basis of localised magnetic orbitals as well as itinerant topological states, closely resembling Kondo systems, albeit with a topological twist. I will then argue that the interplay of quantum geometry and electronic interactions can be diagnosed via low-frequency optical conductivity measurements of correlated metals. At dilute fillings near a topological band inversion, a quantum-geometric Fermi surface contribution can lead to drastically enhanced terahertz optical absorption and enrich the physics of Fermi liquids.

Professor Martin Claassen

University of Pennsylvania, USA

Professor Martin Claassen

University of Pennsylvania, USA

Martin Claassen is a condensed matter theorist at the University of Pennsylvania. His research focuses on the emergent collective phenomena of quantum materials in and far from thermal equilibrium, and the interactions of light and matter. After obtaining a PhD at Stanford University in 2017, he worked as a postdoctoral research fellow at the Center for Computational Quantum Physics of the Simons Foundation Flatiron Institute in New York. Since 2020, he is an assistant professor in the Department of Physics and Astronomy at the University of Pennsylvania.

Localisation, or its absence, is a fundamental distinguishing feature of any many-body system, affecting the ways that mass, charge, energy, and momentum move and respond to inhomogeneity. A current frontier in this venerable field is the study of transport in driven matter: subjecting a quantum system to a time-dependent Hamiltonian can generated a rich array of localising and delocalising dynamics. I will discuss results from a sequence of recent cold-atom experiments on kicked and driven quantum matter, highlighting data on anomalous transport, the interplay between dynamical and disorder-induced localisation, and quantum simulation of integer quantum Hall matter illuminated by light of arbitrary polarisation. The chiral nature of the latter experiment enables drive-synthesised topological phenomena and opens pathways to spectroscopic probes of the quantum geometry of higher-dimensional systems with lower-dimensional ones. In a surprise pivot to practical applications, I will close with a brief description of how Floquet band engineering, a celebrated tool for synthesising topological quantum matter, can also be used to build compact precise force sensors based on matter-wave interferometry.

Professor David Weld

University of California, Santa Barbara, USA

Professor David Weld

University of California, Santa Barbara, USA

David received a BA in physics from Harvard University and a PhD in physics from Stanford University; after working as a postdoctoral researcher and research scientist at MIT he joined the faculty at UC Santa Barbara. Current research interests of the Weld group include quantum and classical transport, Floquet phases of matter, quasicrystals, and quantum interactive dynamics. The Weld group's research in experimental ultracold atomic physics has been recognised with an NSF CAREER award, a Presidential Early Career Award for Scientists & Engineers (PECASE), a Sloan Fellowship, a Young Investigator Prize form the Air Force Office of Scientific Research, and an Experimental Physics Investigator award from the Moore Foundation. David is currently a visiting scholar in Jesus College at Cambridge University.