Sept. Seminar: Cascade of new even-denominator fractional quantum Hall states — Sept. 29

Professor of Electrical and Computer Engineering
Princeton University

Date: September 29, 2025
Time: 3:00 - 4:00 PM ET
Location: Grier 34-401A
Reception to follow

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One of the most remarkable phenomena in physics arises when two-dimensional (2D) electrons are subjected to a strong magnetic field: they exhibit a myriad of fractional quantum Hall states (FQHSs), each marked by a quantized Hall resistance given by (h/e²)/f, where f is a fraction. Arising from an interplay between topology and interaction, these have given rise to an amazing body of new concepts.

A large majority of FQHSs have odd denominators and are understood in terms of integer quantum Hall effect of particles called composite fermions. The excitations of these states are neither fermions nor bosons but are “anyons.” The even-denominator FQHSs are predicted to produce even more complex excitations, anyons that obey “non-Abelian” statistics. These fascinating excitations possess non-trivial braiding properties and have been advanced as a potential platform for fault-tolerant topological quantum computing. Until recently, even-denominator FQHSs had been rare, mostly observed in a half-filled excited (N = 1) Landau level for f = 5/2 in GaAs 2D electrons and its analogs in other materials including bilayer graphene. The exception had been a FQHS in the lowest (N = 0) Landau level with f = ½ in wide GaAs quantum wells.

Shayegan will show that his group's new ultrahigh-quality GaAs/AlAs samples, which are some of the purest solid-state materials (residual impurity concentration well below 1 per 10 billion atoms), reveal new even-denominator FQHSs at ¾, 3/8, 3/10,1/4, 1/6 and 1/8, thereby tremendously enriching the FQHS physics and expanding the reach of non-Abelian statistics. They also observe an f = ½ FQHS in their wide GaAs wells with a transport energy gap of about 6 K, the highest gap reported for any even-denominator FQHS. These developments also highlight how progress in physics is intimately connected to their ability to grow high quality materials.

Based on work done in collaboration with Chengyu Wang, Adbhut Gupta, Pranav Thekke Madathil, Siddharth K. Singh, Chia-Tse Tai, Yoon Jang (Edwin) Chung, Roland Winkler, Kirk Baldwin, and Loren Pfeiffer.

Mansour Shayegan received his B.S., M.S., and Ph.D. degrees in Electrical Engineering and Computer Science from the Massachusetts Institute of Technology. Since 1985 he has been a faculty member in the Department of Electrical Engineering at Princeton University where he teaches physics and electrical engineering courses.

Shayegan's research is in solid state physics with an emphasis on the fabrication of low-dimensional semiconductor structures and measurements of their electronic properties and collective phenomena. Shayegan has won numerous awards, including an Alfred P. Sloan Fellowship, and NSP Presidential Young Investigator Award, and IBM Faculty Development Award, and Alexander von Humboldt Prize, a Fullbright Fellowship, and a Princeton University Graduate Student Mentoring Award. He is an elected Fellow of the American Physical Society.