MatSE Welcomes

2:00 - 3:00 p.m. on Thursday, May 28 in 402 Steidle Building

"Integrating Statistical Physics and Data-Driven Methods for Modeling Inelastic Material Behavior"

Weilun Qiu, Ph.D. Candidate, Mechanical Engineering and Applied Mechanics, University of Pennsylvania

Abstract

This talk will focus on Weilun Qiu's doctoral research and consists of two parts.

In the first part of the seminar, Qiu will introduce his work on data-driven discovery of internal variables. Internal variable theory has been highly successful in continuum mechanics, but its reliance on phenomenological intuition—often without a direct connection to lower-scale structure—can lead to black-box models with uncertain applicability and limited generalizability. Inspired by recent developments in Stochastic Thermodynamics with Internal Variables (STIV), which provides a first-principles-based definition of internal variables, their dynamics, and thermodynamic quantities such as entropy and non-equilibrium free energy, proposing IB-VONNs: a data-driven framework for identifying internal variables as functions of microscopic degrees of freedom, along with the associated constitutive relations. Physical constraints and thermodynamic consistency are incorporated into the framework. Demonstrating the approach using a one-dimensional phase-transforming system governed by Langevin dynamics, Qui will also discuss ongoing applications to the rheology of two-dimensional colloidal systems.

In the second part of the seminar, Qui will discuss his ongoing work on non-equilibrium entropy. A broad class of dynamical systems can be formulated within the framework of the General Equation for Non-Equilibrium Reversible–Irreversible Coupling (GENERIC), which is grounded in statistical mechanics. Building on this structure, he establishes a general identity and an associated numerical method for computing non-equilibrium entropy differences from mesoscopic fluctuation data. This differs from well-established fluctuation theorems, such as the Jarzynski equality, which are primarily used to extract equilibrium free energy differences.

About the Speaker

Weilun Qiu is a fifth-year Ph.D. candidate in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania (Penn), working with Professor Celia Reina. His research interests include scientific machine learning and statistical mechanics for scale-bridging modeling of mechanics of inelastic materials. Qui received his bacholor of science degree from Peking University.

3:00 - 5:30 p.m. on Thursday, May 28 in 402 Steidle Building

"Accurate Electron–Phonon Interactions from Advanced Density Functional Theory in Correlated
and Energy Materials"

Ruiqi Zhang, Research Assistant Professor, Tulane University

Abstract

Electron–phonon coupling plays a central role in charge transport, band renormalization, and superconductivity, yet its first-principles description remains challenging in complex quantum materials with d-electrons. In this talk, Zhang will present recent development of advanced SCAN/r2SCAN-based density-functional approaches for predictive, parameter-free calculations of electron–phonon interactions. Showing that r2SCAN accurately captures electronic structures, phonons, and strong electron–phonon coupling across a broad range of materials, including correlated oxides, ferroelectric perovskites, semiconductors, and phonon-mediated superconductors, without invoking empirical Hubbard parameters.

Zhang will then focus on high-temperature superconducting materials, where magnetism qualitatively reshapes electron–phonon coupling. In infinite-layer nickelates, finding that local magnetic moments enhance the coupling strength by nearly a factor of four, driven not only by oxygen modes but also by low-frequency La/Ni phonons coupled to flat Ni-derived bands. In cuprates, calculations reveal an even more striking result: magnetism activates strong coupling to oxygen rotational, circularly polarized phonons, which dominate the electron–phonon interaction and generate quasiparticle kinks and peak–dip–hump features consistent with ARPES experiments. These findings suggest that magnetism-enhanced electron–phonon coupling is a key missing ingredient in understanding spectral anomalies, pseudogap physics, and superconductivity in correlated materials, and opening a practical route toward high-throughput discovery of quantum materials with strong lattice-electronic interactions.

About the Speaker
Ruiqi Zhang completed his Ph.D. in 2018 at the University of Science and Technology of China. He started his postdoc appointment in 2018 at Tulane University and was promoted to be a research assistant professor in 2023. His recent focus involves investigating the electronic structures of strongly correlated materials like cuprates and nickelates, utilizing advanced density functional theory (DFT) with symmetry-breaking and Hamiltonian model calculations. His significant contributions in this field have led to an invitation to deliver an invited talk at the 2024 APS March Meeting. In very recent, Zhang has shown remarkable transferability of r2SCAN functional in providing accurate and efficient predictions of electron–phonon coupling for a wide array of materials.

Department of Materials Science and Engineering

2:00 - 3:00 p.m. on Thursday, May 28 in 402 Steidle Building

"Integrating Statistical Physics and Data-Driven Methods for Modeling Inelastic Material Behavior"

Weilun Qiu, Ph.D. Candidate, Mechanical Engineering and Applied Mechanics, University of Pennsylvania

3:00 - 5:30 p.m. on Thursday, May 28 in 402 Steidle Building

"Accurate Electron–Phonon Interactions from Advanced Density Functional Theory in Correlatedand Energy Materials"

Ruiqi Zhang, Research Assistant Professor, Tulane University

"Accurate Electron–Phonon Interactions from Advanced Density Functional Theory in Correlated
and Energy Materials"