Harvard University Department of Physics

Congratulations to the 2016-17 Physics Graduates!

group photo of graduating physics PhD students

This year's graduating seniors and PhDs are...

Toward Mass-Producible Quantum Computers

artist's representation of a diamond-based quantum computer

Quantum computers are experimental devices that offer large speedups on some computational problems. One promising approach to building them involves harnessing nanometer-scale atomic defects in diamond materials...

Vafa Receives 2017 Ellis Island Medal of Honor

The Ellis Island Medals of Honor embody the spirit of America in their celebration of patriotism, tolerance, brotherhood and diversity. They recognize individuals who have made it their mission to share with those less fortunate their wealth of knowledge...

UPCOMING EVENTS

Tuesday, June 27th

CMP Special Seminar: Andrea Young (UCSB), hosted by Philip Kim
Start: 02:00pm - End: 03:00pm
- 02:00pm
- Title: New Fractional quantum Hall states in graphene heterostructures
  
  Abstract: I will discuss magnetocapacitance experiments from a new generation of ultra-clean graphene heterostructures, realized by replacing conventional gate metallization with single crystal graphite flakes. In the first part of the talk, I will discuss bilayer graphene as a platform for exploring the half-filled Landau level (LL), focusing on the observation of robust, single component even-denominator fractional quantum Hall states at filling ν=-5/2, -1/2, 3/2, and 7/2. Numerical simulations of the blayer graphene ZLL suggest this state is in the Pfaffian phase, consistent with the experimentally observation of a 7/13 and weak 8/17 incompressible state—predicted Pfaffian ‘daughter’ states. Unlike in conventional systems, magnetic field directly changes the single particle wavefunction, tuning the mixture of lowest and first LL amplitudes within a single component level. We observe this pseudopotential tuning as rise and subsequent near-collapse of the ½ gap at high field, indicating the presence of a nearby Pfaffian-composite fermi liquid transition. A similar transition can be tuned by engineering a level crossing between N=0 and N=1 LLs in different valleys. Here, we find that the Pfaffian-CFL transition is mediated by an unexpected gapped phase, characterized by large valley imbalance but a persistent charge gap.
  
  In the second part of the talk, I will describe the effects of a superlattice on the nature of the fractional quantum Hall effect. At high magnetic field in devices where the graphene is aligned with one or both adjacent hBN crystals, a substrate-induced moiré superlattice gives rise to a variety of Hofstadter bands with Chern numbers . Trajectories of energy gaps in the density () magnetic flux () plane follow linear trajectories parameterized by integers and , Contrary to single particle theory—which predicts a fractal band structure, with each band subdivided into a recursive hierarchy of minibands—we find robust incompressible states at fractional filling of these Chern bands, characterized by fractional and/or fractional . Basic arguments, supplemented by numerical DMRG calculations, support two kinds of correlated states: states with integer and fractional break lattice symmetry, while states with both fractional and fractional do not break lattice symmetry. These latter, known as fractional Chern insulators, constitute a fractional quantum Hall effect ‘without Landau levels.’
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Wednesday, June 28th

Prof. Mathias Kolle - MIT
Start: 05:30pm - End: 07:00pm
- 05:30pm
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