SEMINAR: Ashkin-Teller Physics: Multicritical Phase Diagrams, Algebraic Order, Chaos, Tripling, and Spinodals, d=1 Finite-Temperature Ordering from Renormalization-Group Theory

Guest: Nihat Berker, Kadir Has University

Title: Ashkin-Teller Physics: Multicritical Phase Diagrams, Algebraic Order, Chaos, Tripling, and Spinodals, d=1 Finite-Temperature Ordering from Renormalization-Group Theory (PHYS, MAT, MATH)

Date/Time: December 17, 2025, 13:40

Location: FENS G032

Abstract: Recent renormalization-group calculations have hyperfolded interesting Ashkin-Teller model physics. Global phase diagrams with bicritical, tricritical, critical-end points, bounding first- and second-order phase transitions bounding different types of conventional and algebraic ordering have been calculated, including continuously variable XY degrees of freedom and 6-point interactions in triple-deck models.[1-3] With quenched-random ferro and antiferromagnetic couplings, two distinct types of spin-glass chaos have been found.[4] Spinodal and equilibrium phase diagrams have been calculated.[5] With d=1 with long-range interactions, the hybrid Thouless point for the onset of finite-temperature phase transitions and an equivalent-neighbor regime before reaching equivalence has been found. [6]

[1] XY-Ashkin-Teller Phase Diagram in d=3, A. Türkoğlu and A.N. Berker, Physica A 674, 130715 (2025).

[2] Multiplicity of Algebraic Order from Fixed Lines of Potential Surfaces: XY-Ashkin-Teller in Spatial Dimension d = 2, E.C. Artun and A.N. Berker, Phys. Rev. E 112, 024126 (2025).

[3] 6-Point Tripled Ashkin-Teller Global Phase Diagrams in Two and Three Dimensions, D.I. Zeynioğlu and A.N. Berker, Physica A 680, 131049 (2025).

[4] Spin-Glass Phases and Multichaos in the Ashkin-Teller Model, A. Saray and A. N. Berker, Chaos, Solitons and Fractals 202, 117458 (2025).

[5] Spinodal and Equilibrium Global Phase Diagram of the d=3 Merged Potts-Cubic-Clock Model: First-Order Equilibrium and Second-Order Spinodal Boundaries with Hidden Topologies from Renormalization-Group Theory, U. Açıkel and A.N. Berker, arXiv:2505.18785 [cond-mat.stat-mech] (2025).

[6] Ferromagnetic and Spin-Glass Finite-Tempeature Order but no Antiferromagetic Order in the d=1 Ising Model with Long-Range Power-Law Interactions, E.C. Artun and A.N. Berker, arXiv:2508.11168 [cond-mat.stat-mech] (2025).

Bio: Prof. Nihat Berker is a distinguished theoretical physicist whose research spans statistical physics, phase transitions and critical phenomena, spin glasses and complex systems, and renormalization group methods. His work has also explored fractal structures in ethnomusicology, brain signals, and spin-glass chaos, reflecting a uniquely interdisciplinary scientific perspective. Prof. Berker has held senior academic leadership and faculty positions at leading institutions worldwide. He is currently Dean of the Faculty of Engineering and Natural Sciences and Vice Rector at Kadir Has University, and Emeritus Professor of Physics at MIT, where he served as a faculty member for over two decades. He previously served as Rector of Sabancı University (2009–2016), was a Professor of Physics at Koç University, and served as Dean of the Faculty of Science and Letters and Chair of the Physics Department at Istanbul Technical University. He is the recipient of numerous prestigious awards, including the Humboldt Research Award, is a member of Academia Europaea, and is widely recognized for his lasting contributions to theoretical physics and academic leadership.