Inverse electronic compressibility of magic-angle twisted bilayer graphene as a function of moiré filling factor and perpendicular magnetic field. The data show multiple interpenetrating sequences of Chern insulators (bright lines), as well as regions of negative compressibility (dark blue features), which indicate phase transitions between them and correspond to changes in occupation of the spin/valley flavors.
Inverse electronic compressibility of magic-angle twisted bilayer graphene as a function of moiré filling factor and perpendicular magnetic field. The data show multiple interpenetrating sequences of Chern insulators (bright lines), as well as regions of negative compressibility (dark blue features), which indicate phase transitions between them and correspond to changes in occupation of the spin/valley flavors.

Novel Chern insulators in magic-angle bilayer graphene

August 2021

Dr. Ben Feldman, Assistant Professor of Physics at Stanford University, has measured the local electronic compressibility of magic-angle twisted bilayer graphene (MATBG) using a scanning single-electron transistor microscope. 

The high-resolution data reveal several previously unobserved correlated Chern insulating states as well as sharp phase transitions between them. These behaviors arise due to the interplay between the crystalline moiré superlattice and the applied magnetic field, together with various symmetry-breaking terms, and the team reconstructs the many-body spectrum and phase diagram of MATBG as a function of carrier density and magnetic field. 

The work was completed in collaboration with the group of Dr. Zhi-Xun Shen, Professor of Physics at Stanford University who corroborated the appearance of the Chern insulators with microwave impedance microscopy, and with theoretical insight provided in part by Dr. Philip Phillips, Professor of Physics at the University of Illinois, Urbana-Champaign, highlighting the multi-institution collaborative nature of the research.

 

QSQM 

QSQM Principal Investigator Mason Elected to National Academy of Sciences

April 2021

Physics professor Nadya Mason is among 120 newly elected U.S. members – 59 of whom are women, the most elected in a single year – and 30 international members in recognition of their distinguished and continuing achievements in original research.

“The historic number of women elected this year reflects the critical contributions that they are making in many fields of science, as well as a concerted effort by our academy to recognize those contributions and the essential value of increasing diversity in our ranks,” said National Academy of Sciences President Marcia McNutt. “I am pleased to welcome all of our new members, and I look forward to engaging with them in the work of the National Academies.”

At the QSQM Center, Nadya Mason is fabricating TCI-superconductor interface materials as well as arrays for benchmarking the DR scanner instrument, and is also performing transport measurements of candidate topological superconductors as well as strange metal-normal metal interfaces.

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Figure A: Schematic picture of coincidence scattering process; B 2e ARPES photoelectron counting rate in the normal and superconducting phases for two photoelectrons with momenta k and -k. The encircled peak at zero energy directly reflects the magnitude of the superconducting condensate.
Figure: A Schematic picture of coincidence scattering process; B 2e ARPES photoelectron counting rate in the normal and superconducting phases for two photoelectrons with momenta k and -k. The encircled peak at zero energy directly reflects the magnitude of the superconducting condensate.

A Theoretical Model for 2e ARPES

April 2021

Dr. Dirk Morr, Professor of Physics at the University of Illinois, Chicago, has developed a theoretical model for 2-electron coincidence spectroscopy (2e ARPES) that can directly measure the center of mass momentum, spin pairing state and energy gap of Cooper pairs in a superconductor. 

This ability to distinguish between different superconducting pairing states provides an unprecedented insight into superconducting pairing mechanism.

Dr. Morr is working in a team funded by the Department of Energy (DoE) as an Energy Frontier Research Center (EFRC) for Quantum Sensing and Quantum Materials (QSQM). Other team members include Dr. Peter Abbamonte, Director of the Center and Dr. Fahad Mahmood both at the University of Illinois, and Dr. Tom Devereaux at the Stanford University and the SLAC National Accelerator Laboratory.