Impossible devices with correlated electrons

Congratulations to Liam and Noah, whose paper is out in Science.

They report two fantastic advances studying a quantum point contact in graphene at high magnetic fields. First, they observe the universal T^2, V^2 scaling of the conductance when tunneling an electron into the fractionalized edge at filling factor 1/3—verifying a seminal theoretical prediction 33 years after the fact. Second, they use the counterintuitive properties of the strong tunneling regime between electrons and fractionally charge quasiparticles to realize an ‘impossible’ electronic device: a nearly dissipationless DC voltage step up transformer.

Yu and Fangyuan's paper published in Nature

Slightly belated congratulations to Yu, Fangyuan, and collaborators for their lovely work published recently in Nature (some additional coverage here and here). This paper sorted out a puzzle: why, in twisted bilayer graphene does the system seem to “know” about spin, valley, and sublattice symmetry breaking at high temperature, but does not always break all the symmetries at low temperatures? It turns out that the entropy of those degrees of freedom is higher than expected, favoring a disordered ferromagnet at high temperatures even as the system is paramagnetic. The physics is similar to that of the Pomeranchuk effect in 3He, in which the liquid freezes as it is heated due to the increased entropy of its nuclear spins.

Gregory's paper published in Nature!

Congratulations to Gregory and coauthors on their recent publication in Nature. They describe an effect in which a magnetic state can be reversed simply by application of a gate voltage, which arises from the extremely unusual properties of orbital magnets realized, in this case, in a moire graphene heterostructure. Unlike every magnet known since Thales of Miletus first described lodestones 2600 years ago, orbital magnets do not rely on aligning the electron spins, but rather the two dimensional orbits of the electrons themselves. That means they can be controlled in completely different ways, and this paper demonstrates a beautiful and clean way to do that.

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Yu's paper published in Nature Physics!

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Superconductivity in tBLG

…without a correlated insulator

Congratulations to Yu and William on the publication of their paper in Nature Physics,, which describes the observation of superconducting states in twisted bilayer graphene even when no correlated insulators are observed. This result points to disparate mechanisms for the correlated insulators and the superconductors—this contrasts with proposals that liken tBLG to the cuprates.

Quantum anomalous Hall paper published in Science

Marec, Charles, and Gregory’s paper was published in Science today. Congratulations to all coauthors—and a great way to end the year!

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An orbital quantum anomalous Hall effect

Quantized Hall effects arise when time reversal symmetry is broken., usually by an externally applied magnetic field. They can also arise at zero magnetic field in a magnetic system with strong spin orbit coupling. In our paper, we showed that this effect can arise from a purely orbital effect, in which electrons spontaneously polarized into valleys in a twisted bilayer graphene sample. The observed quantization persists to record high temperatures to boot!

Haoxin's paper published in Nature Physics

Haoxin’s paper was published in Nature Physics today! Congratulations to all coauthors!

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Skyrmion solids

When an electron gas is completely spin polarized, adding charge to the system necessarily involves changing the total spin—in a strongly interacting system, the new charges can have intricate spin textures known as skyrmions, in which the spin winds itself in a patter resembling a vortex. In this paper, Haoxin and collaborators used decay of charge neutral spin waves to find that these skyrmions can freeze into a solid, forming an absorbtive medium for the spin waves.