Title: Superconductivity in Overdoped Cuprates Can be Understood from a BCS Perspective!

Abstract:
I will summarize key experimental studies of the low energy properties of overdoped cuprate high temperature superconductors and conclude that a theoretical understanding of the “essential physics” is achievable in terms of a conventional Fermi-liquid treatment of the normal state, and a BCS mean-field treatment of the (d-wave) superconducting state. For this perspective to be consistent, it is necessary to posit that there is a crossover from a strongly correlated underdoped regime (where a different theoretical perspective is necessary) to the more weakly correlated over-doped regime. It is also necessary to argue that the various observed features of the overdoped materials that are inconsistent with this perspective can be attributed to the expected effects of the intrinsic disorder associated with most of the materials being solid state solutions (alloys). As a test of this idea, we make a series of falsifiable predictions concerning the expected behavior of “ideal” (disorder free) overdoped cuprates.

This work is based on a paper with the same title - which will probably have appeared on the arXiv by the time of this conference - written in collaboration with Brad Ramshaw.

Title: Recent Progress on the Theory of the 2DES at larger r_s

Abstract:
Theoretical and experimental studies of two-dimensional electron systems (2DES) have played a pivotal role in the history of quantum condensed matter physics. It is also an old, and long-studied problem, about which much is known. In this talk, I will describe some new results obtained by some brilliant collaborators of mine concerning the quantum phases of the 2DES at large and intermediate values of r_s, where r_s is the Wigner-Seitz radius of the 2DES in units of the effective Bohr radius. These include two new results for the simple 2DES: 1) A more complete analysis of tunneling processes deep in the Wigner crystal (WC) phase (large r_s), which in particular provides suggestive evidence of a self-doping instability. 2) A variational Monte Carlo (VMC) study of the phase diagram in the presence of screening gates, yielding a quantitative estimate of the critical gate-distance necessary for the existence of a WC. We also explore several new aspects of multi-component 2DES (i.e. that arises in a multi-valley semiconductor): 1) An asymptotic large r_s examination of valley ordering in the WC phase. 2) Combined VMC and approximate analytic studies of generalized ferromagnetism in the fluid phases at intermediate r_s, including the observation of a spin unpolarized electron-nematic phase in a two-valley 2DES of the sort that occurs in AlAs heterostructures.

Ref: Phys. Rev. B 109, 235130 (2024); Fiz. Nyzk. Temp. 49, 747-769 (2023); Phys. Rev. Lett. 129, 227202 (2022); Phys. Rev. Lett. 132, 266501 (2024); arXiv:2507.05344 (2025); arXiv:2501.16430 (2025); PRB 111, 155135 (2025).

Bio:
Steven Allan Kivelson is an American theoretical physicist known for contributions to condensed matter theory - mostly done in collaboration with a string of brilliant students, post docs and more established collaborators. He is the son of Margaret Kivelson, a distinguished Space Physicists at UCLA, and Daniel Kivelson, a prominent Physical Chemist, also at UCLA. He received his Ph.D. from Harvard University in 1979, working with C. Daniel Gelatt, Jr., and then spent three years as a postdoctoral researcher working with J. Robert Schrieffer, first at the University of Pennsylvania and then at the Institute for Theoretical Physics at the University of California, Santa Barbara. He joined the faculty of the State University of New York at Stony Brook in 1982 and then moved to the University of California, Los Angeles in 1989. In 2004, he joined the faculty at Stanford University, where he was appointed Prabhu Goel Family Professor of Physics in 2012.

Kivelson was awarded the Guggenheim fellowship in 1995, was elected to the American Academy of Arts and Sciences in 2001, and to the National Academy of Sciences in 2010. He won the APS Buckley Prize of 2025 in Condensed Matter Physics. He is the co-editor in Chief of Nature Partner Journal Quantum Materials, published in partnership with Nanjing University. He is particularly proud of having published scholarly papers in collaboration with his father on the glass transition, and his daughter, Sophia Kivelson, on emergence and complexity. His son, Paul Kivelson, is the author of the science fiction masterpiece, “Weapons of the Mind,” and his wife, Pamela Davis, is an artist whose vibrant portrait of the late Shoucheng Zhang hangs in the Geballe Laboratory for Advanced Materials at Stanford University.