Physics · the experiment
Read the vortex lattice
Two sheets of graphene, a superfluid of neutral pairs, a few extra electrons. Four questions, from beginner to expert, on what the spacing of a lattice can reveal.
1 · Say what the whirlpools carry
Beginner
In this exciton superfluid, what do the vortices carry, unlike those of a superconductor?
2 · Predict the spacing
Predicted spacing of the lattice: a = √(4π l_B² / (√3 ṽ)), where l_B depends only on the magnetic field and ṽ is the extra charge, counted as a fraction of a Landau level. At 9 tesla and ṽ = 0.14, the measured spacing is about 60 nm.
Intermediate
In the same field, what spacing does the formula give for ṽ = 0.035?
3 · Move charge between the sheets
Total extra charge held fixed, at ṽ = 0.14. The team moves charge from one sheet to the other: each sheet's filling changes by up to 30%. A Wigner crystal in the top sheet would shift its spacing by about 20 nm.
Expert
What does the observed lattice do?
4 · Explain the triangles
Theory predicts a honeycomb: vortices with their charge in the top sheet, antivortices with theirs in the bottom sheet. The microscope shows a triangular pattern.
Expert
Why does the microscope show triangles rather than a honeycomb?
Lattice decoded
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