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How to Whip Up New Physics

Sometimes, science can be a bit like making a good sandwich — but one a little more complex than your average PB&J.

Like good chefs, physicists creatively experiment with their ingredients. Chefs attempting a fresh take on the humble sandwich, for example, will test different layers — meats, cheeses, condiments — pressed between slices of wheat or rye. Physicists exploring two-dimensional materials create their own quantum concoctions called van der Waals heterostructures, layers of 1-atom-thin materials, weakly bonded, that are arranged in a very particular way. They put these sandwiches inside a high magnetic field, where the electrons start doing the darndest things.

Below are some of the "recipes" scientists have used to make exciting physics discoveries with heterostructures. Whether you don a lab coat or a white toque, we hope these recipes whet your appetite for science.

Hofstadter Hoagie

Recipe Note: Three groups of creative chefs simultaneously developed this zesty recipe, each adding its own special techniques and flavoring. Here’s the basic recipe:

Preparation:

  • Precool experimental chamber to -273 °C.
  • Ramp up magnetic field to 35 teslas.

Ingredients:

  • 1 graphene flake
  • 1 layer boron nitride

Yield:

A beautiful fractal energy pattern and validation of a decades-old physics prediction.

Directions:

  1. Gently place the graphene flake atop the boron nitride to create a lovely moiré pattern.
  2. Carefully place this combination inside the precooled, high-field experimental space.
  3. When your data reveals a fractional quantum Hall state, your sandwich is done and your data is ready to share with Science and Nature!
  4. Turn off the magnet to find that the graphene has become a semiconductor with a band gap — potentially useful as a transistor.

Detailed instructions:

The Columbia University variation:

Hofstadter's butterfly and the fractal quantum Hall effect in moiré superlattices,, Nature, Vol. 497 (30 May 2013).

The Chefs: C.R. Dean, L. Wang, P. Maher, C. Forsythe, F. Ghahari, Y. Gao, J. Katoch, M. Ishigami, P. Moon, M. Koshino, T. Taniguchi, K. Watanabe, K. L. Shepard, J. Hone & P. Kim.

The University of Manchester variation:

Cloning of Dirac fermions in graphene superlattices,, Nature Vol. 497 (30 May 2013).

The Chefs: L.A. Ponomarenko, R.V. Gorbachev, G.L. Yu, D.C. Elias, R. Jalil, A.A. Patel, A. Mishchenko, A.S. Mayorov, C.R. Woods, J.R. Wallbank, M. Mucha-Kruczynski, B.A. Piot, M. Potemski, I.V. Grigorieva, K.S. Novoselov, F. Guinea, V. I. Fal'ko & A.K. Geim.

The MIT variation:

Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure,, Science, Vol. 340, Issue 6139 (21 Jun 2013).

The Chefs: B. Hunt, J.D. Sanchez- Yamagishi, A.F. Young, M. Yankowitz, B.J. LeRoy, L. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo- Herrero, R.C. Ashoori.

graphic divider

Tungsten Diselenide Double-Decker

Recipe Note: The tungsten diselenide (WSe2) layer is a sandwich within a sandwich, a tungsten layer between selenium ions. It rejects most electrodes, turning the interface with them into an insulator. So rather than run electric currents through it to measure the material’s properties, chefs use strong electric fields.

Preparation:

  • Pre-cool experimental chamber to -273 °C.
  • Ramp up magnetic field to 35 teslas.

Ingredients:

  • Tungsten (W) and selenium (Se) pellets, in 1:2 molar ratio
  • Crystals of graphite and hexagonal boron nitride (BN)

Yield:

A comprehensive picture of the electronic structure of a new 2D material: a transition metal dicalcogenide.

Chefs:

M.V. Gustafsson, M. Yankowitz, C. Forsythe, D. Rhodes, K. Watanabe, T. Taniguchi, J. Hone, X. Zhu & C.R. Dean

Directions:

  1. Heat W and Se together at 1000 °C for two days.
  2. Place resulting WSe2 crystals in a vacuum and anneal at
    450 °C to melt off excess Se.
  3. Use Scotch tape to remove single layers from WSe2, BN and graphite.
  4. Stack in the following order: graphite, BN, WSe2, graphite, BN.
  5. Place the sandwich inside the pre-cooled magnet.
  6. Use a single-electron transistor to control the electric field across the WSe2.
  7. The way electric fields pass through the sample should give a stunning picture of energy levels — enough to publish in Nature Materials.

Detailed Instructions:

Ambipolar Landau levels and strong bandselective carrier interactions in monolayer WSe2, Nature Materials, Vol. 17 (26 March 2018).

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Black Phosphorus Panini

Preparation:

  • Precool experimental chamber to -267 °C.
  • Ramp up magnetic field to 35 teslas.

Ingredients:

  • 1 fresh black phosphorus crystal
  • 1 teaspoon acrylic

Yield:

A beautiful demonstration of 2D behavior in the elemental semiconductor known as black phosphorus.

Chefs:

V. Tayari, N. Hemsworth, I. Fakih, A. Favron, E. Gaufrès, G.Gervais, R. Martel & T. Szkopek

Directions:

  1. Gently peel black phosphorus in a nitrogen atmosphere.
  2. Sparingly apply a thin acrylic coating. Be sure to cover the entire sample.
  3. Carefully place your lightly glazed sample inside the pre-cooled, high-field experimental space.
  4. As soon as your phosphorus shows Shubnikov-de Haas oscillations, you are ready to share your data with Nature Communications.

Detailed instructions:

Two-dimensional magnetotransport in a black phosphorus naked quantum well, Nature Communications, Vol. 6, Article number 7702 (7 July 2015).

graphic divider

Ion-spread Sub

Preparation:

  • Place the Si/SiO2 wafer on top of the WO3 powder in a furnace.
  • Heat furnace to 150 °C and expose wafer to a sulfur gas.
  • Wait for a single layer of tungsten disulfide (WS2) to grow, then slowly cool down the furnace.
  • Structure the layer into a device and put electric contacts on it.
  • Precool the cryostat to -50 °C.

Ingredients:

  • 1 silicon-silicon oxide (Si/SiO2) wafer
  • 1 crucible of hot sulfur
  • 1 pinch of tungsten trioxide (WO3) powder
  • 1 pot of ion-spread*

Yield:

A plethora of electronic phases in a beautiful color map.

Chefs:

J.M. Lu, O. Zheliuk, Q. Chen, I. Leermakers, N.F.Q. Yuan, N.E. Hussey, U. Zeitler, K. T. Law, J.T. Ye

Directions:

  1. Be brave enough to immerse your beautiful micro device into the ion-spread.
  2. Place it inside the precooled cryostat.
  3. Apply a voltage between the spread (still damp) and the sample.
  4. Shock-cool to -269 °C within a couple of minutes.
  5. Observe superconductivity in the device.
  6. Try (and fail) to kill the superconductivity with magnetic fields as high as 37 teslas.
  7. Share these amazing results via Science and PNAS with the rest of the world.

Detailed instructions:

Full superconducting dome of strong Ising protection in gated monolayer WS2, PNAS, Vol. 115 (14), (April 3, 2018).

Evidence for two-dimensional Ising superconductivity in gated MoS2, Science, Vol. 350, Issue 6266 (11 Dec 2015).

*The precise makeup of the ion spread should be: N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethyl sulphonyl)-imide