A topological excitation of quantum Hall ferromagnets
Illustrated by Cory R. Dean
A miniature graphene device mounted on fridge probe.
I am an Assistant Professor in the Physics Department at Cornell University, and I am building a lab studying quantum materials. I specialize in 2D materials, vdW heterostructures, quantum transport, and scanning tunneling microscopy. Before joining Cornell, I was a PCCM postdoc fellow at Princeton and I obtained my PhD from Harvard in 2019. You can read more about my research direction on my faculty profile. My publications can be found on google scholar.
I am also an amateur photographer and maker. Some hobby works are displayed on this site.
In 2D systems under strong magnetic fields, cyclotron motion of electrons create highly degenerate Landau levels. In these flat Landau levels, the effect of Coulomb interaction is greatly enhanced, which can lead to exotic quantum phenomena such as the famous fractional quantum Hall effect.
More intriguing effects can be realized by placing two 2D layers closeby (2-4nm) so they can interact through Coulomb interaction while direct tunneling is prohibited. In my PhD thesis, I pioneered many discoveries in the graphene double-layer system, which will be introduced below.
An exciton is a pair of an electron and a hole bound by the Coulomb interaction. Because they are bosons, many excitons can reside on the same quantum state to form a macroscopic quantum object. This so called exciton condensate state has many exotic properties such as superfluidity.
In this work, we establish exciton condensation in graphene double-layers by creating pairing between electrons in one layer and holes in a closeby layer. When the condensation happens, interlayer coherence is detected by a large quantized Coulomb drag signal.
Nature Physics 13, 746–750 (2017)
Superfluidity and superconductivity are among the most facinating phenomena, where fluid can flow without resistance. Superfluidity in Helium is caused by the Bose Einstein condensation (BEC) of He4 atoms, while conventional superconductors are described by the BCS theory, where weak attractions between electrons enable them to form loose pairs, and avoid impurities through a harmonic waltz.
In this work, we continued our investigation into exciton condensate. By tuning the attraction between electrons and holes, we could access both the BEC and BCS regime. When the attraction is strong, electrons and holes form tightly bound molecules resembling He4 atoms. When the attraction is weak, loose pairing occurs on the Fermi surfaces of the two Fermi liquids, similar to a BCS superconductor. Superfluid states are identified in both regimes and the highest transition temperature is found in the crossover region. This work paved ways to further understand exotic fermion condensate in a unified framework.
Science 375, 205-209 (2022)
In a partially filled Landau level, electrons arrange in intricate ways to reduce the Coulomb energy, resulting in the Nobel prize winning fractional quantum Hall effect (FQHE). More intriguingly, it hosts a new type of quasiparticles -- anyons, unique to two-dimensions.
Here, we discovered two component FQHE in graphene double-layer, conspired by intra-layer and inter-layer Coulomb interactions. These interactions can be neutralized by the formation of composite fermions, where an electron bind with two intralayer flux and one interlayer flux. We also discovered a semi-quantized state unique to the two component system, explained by anyon pairing between the two layers.
Nature Physics 15, 893–897 (2019)
When two 2D materials are directly stacked onto each other, interlayer tunneling significantly modifies the bandstructure through the formation of moire superlattices. Applying moire band engineering, people have created flat bands and discovered many exotic new phases by twisting two monolayer graphene at the magic angle.
Here, we twist and stack two Bernal bilayer graphene. Thanks to the tunable bandstructure of bilayer graphene with electric field, we could continuously change the bandstructure across the flat band condition in a single device, which allows us to closely inspect how correlated electrons states emerges from the flatband.
Nature 583, 221–225 (2020)
In this following study of twisted double bilayer graphene (TDBG), scanning tunneling microscope is used to visualize the moire superlattice structure of TDBG. Through scanning tunneling spectroscopy, we directly demonstrate the tunability of the band structure of TDBG with an electric field. We also show spectroscopic signatures of electronic correlations and topology for its flat band.
Nature Communications 12, 2732 (2021)
When 2D electrons are subjected to strong magnetic fields, enhanced Coulomb interactions not only cause fractional quantum Hall effect, but can also induce quantum Hall ferromagnetism, where degeneracy over internal degrees of freedom such as spin is lifted by exchange interactions.
In our most recent work, our spectroscopy measurement and spectroscopic imaging reveal the nature of spin-valley SU(4) quantum Hall ferromagnetism in graphene and a topological excitation called Skyrmion.
Science 375, 321-326 (2022)
I really enjoy hand on building. I had the privilege of learning many different building techniques, including CAD, CNC, 3D printing, composite, custom made circuit boards from the famous MIT course: How to Make (Almost) Anything.
For the final project, I designed and built a iPhone gimbal (stablizer) from accelerometers, brushless motors and a microcontroller. I wrote the control algorithem, which direct the motors to compensate for roll/tilt/yaw motion detected by the sensors. A demo can be seen here and the detailed design and making process can be found here.
Engines are some of the most fascinating machines to me. This model steam engine runs on compressed air. When compressed air feed into the inlet, it pushes a piston to drive the aluminum fly wheel. This was the project for Harvard Machine shop certification.
Here is a video of it in live action.
