Strongly Correlated Quantum Materials and
High-Temperature Superconductors Series

In the 2020-2021 academic year, the CMSA will be hosting a lecture series on Strongly Correlated Materials and High Tc Superconductor. All talks will take place from 10:30-12:00pm ET virtually on Zoom.

Cuprate high-temperature superconductors are a classic quantum material system to demonstrate the beauty of “Emergence and Entanglement” in the quantum phases of matter. Merely by adding more holes into an antiferromagnetic insulator, several fascinating phases emerge, including a d-wave superconductor, a pseudo-gap metal, and strange metal. After intensive studies from experimental, theoretical, and numerical communities for more than three decades, remarkable progress has been made, but basic questions remain:

  1. What is the origin of the superconductivity? What are the relative contributions of electron-phonon coupling, spin fluctuations, or resonating-valence-bonds? 
  2. How do we explain the pseudo-gap and the Fermi arc in the underdoped region above the critical temperature? Are they from some symmetry breaking order parameters, or do we need an unconventional picture involving fractionalization?
  3. Is the strange metal at optimal doping associated with a quantum critical point? And if so, what is the driving force of this phase transition?

The cuprate quantum materials have been a major source for many new concepts in modern condensed matter physics, such as quantum spin liquids, topological order, and non-Fermi liquids. In the coming years, it is clear that the study of the cuprates will continually motivate new concepts and development of new techniques. In this seminar series, we hope to accelerate this process by bringing together deeper conversations between experimental, theoretical, and numerical experts with different backgrounds and perspectives.

The Strongly Correlated Quantum Materials and High-Temperature Superconductors series is a part of the Quantum Matter in Mathematics and Physics seminar.

Seminar organizers: Juven Wang (Harvard CMSA) and Yahui Zhang (Harvard). 

Scientific program advisors: Professor Subir Sachdev (Harvard), Professor Patrick Lee (MIT).

In order to learn how to attend this series, please fill out this form.

For more information, please contact Juven Wang (jw@cmsa.fas.harvard.edu) and Yahui Zhang (yahui_zhang@g.harvard.edu)

Upcoming Talks

September 2, 2020 | 10:30am ET

Subir Sachdev (Harvard)

TitleMetal-to-metal quantum phase transitions not described by symmetry-breaking orders

Abstract: Numerous experiments have explored the phases of the cuprates with increasing doping density p from the antiferromagnetic insulator. There is now strong evidence that the small p region is a novel phase of matter, often called the pseudogap metal, separated from conventional Fermi liquid at larger p by a quantum phase transition. Symmetry-breaking orders play a spectator role, at best, at this quantum phase transition. I will describe trial wavefunctions across this metal-metal transition employing hidden layers of ancilla qubits (proposed by Ya-Hui Zhang). Quantum fluctuations are described by a gauge theory  of ghost fermions that carry neither spin nor charge. I will also
describe a separate approach to this transition in a t-J model with random exchange interactions in the limit of large dimensions. This approach leads to a partly solvable SYK-like critical theory of holons and spinons, and a linear in temperature resistivity from time reparameterization fluctuations. Near criticality, both approaches have in common emergent fractionalized excitations, and a significantly larger entropy than naively expected.

Video

September 23, 2020 | 10:30am ET

Subir Sachdev (Harvard)

Title: Metal-to-metal quantum phase transitions not described by symmetry-breaking orders II

Abstract: In this second talk, I will focus on (nearly) solvable models of metal-metal transition in random systems. The t-J model with random and all-to-all hopping and exchange can be mapped onto a quantum impurity model coupled self-consistently to an environment (the mapping also applies to a t-J model in a large dimension lattice,  with random nearest-neighbor exchange). Such models will be argued to exhibit metal-metal quantum phase transitions in the universality class of the SYK model, accompanied by a linear-in-T resistivity from time reparameterization  fluctuations. I will also present the results of exact diagonalization of random t-J clusters, obtained recently with Henry Shackleton, Alexander Wietek, and Antoine Georges.

September 24, 2020 | 12:00pm ET

Inna Vishik (University of California, Davis)

Title: Universality vs materials-dependence in cuprates: ARPES studies of the model cuprate Hg1201

Abstract: The cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures (T c ), and after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa 2 CuO 4+d (Hg1201) is a single-layer cuprate with a particularly high optimal T c and a simple crystal structure; yet there exists little information from ARPES about the electronic properties of this model system. I will present recent ARPES studies of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales which establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material, and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

October 15, 2020 | 10:30am ET

Louis Taillefer (Université de Sherbrooke)

Title: TBA




October 28, 2020 | 10:30am ET

Patrick Lee (MIT)

Title: TBA




November 4, 2020 |10:30am ET

Zhi-Xun Shen (Stanford University)

Title: TBA




November 12, 2020 |10:30am ET

Chandra Varma (Visting Professor, University of California, Berkeley.
Emeritus Distinguished Professor, University of California, Riverside.)

Title: TBA




November 18, 2020 |10:30am ET

Antoine Georges (Collège de France)

Title: TBA




November 19, 2020 |10:30am ET

Eduardo Fradkin (University of Illinois at Urbana-Champaign)

Title: TBA




November 25, 2020 |10:30am ET

Qimiao Si (Rice University)

Title: TBA




December 2, 2020 |10:30am ET

Andrey Chubukov (University of Minnesota)

Title: TBA




December 9, 2020 |10:30am ET

David Hsieh (Caltech)

Title: TBA




December 16, 2020 |10:30am ET

Zheng-Yu Weng (Tsinghua University)

Title: TBA




December 17, 2020 |10:30am ET

Steven Kivelson (Stanford University)

Title: TBA






January 20, 2021 |10:30am ET

Thomas Peter Devereaux (Stanford University)

Title: TBA






February 4, 2021 |10:30am ET

Senthil Todadri (MIT)

Title: TBA




April 1, 2021 |10:30am ET

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Naoto Nagaosa (University of Tokyo)

Title: TBA




May 12, 2021 |10:30am ET

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André-Marie Tremblay (Université de Sherbrooke)

Title: TBA




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