In van der Waals heterostructures layer distance between the constituent layers is an important parameter. It determines coupling between the two layers of graphene in bilayer graphene, the strength of moiré patterns or the size of proximity effects (e.g. induced exchange interaction). Since hopping parameters heavily depend on the layer distance controlling the layer distance is in important knob in the band structure engineering of van der Waals heterostructures. We have developed a unique pressure cell setup, where the layer distance in van der Waals heterostructures can be tuned using hydrostatic pressure. Our setup allows nanodevices up to 12 contacts, that can be bonded on a circuit board. We have found that hBN protects the heterostructures from degradation in the pressure mediating liquid, kerosene. The setup allows hydrostatic pressures up to 2 GPa, in the temperature range of 50mK-300K, vector magnetic field 9/3T or up to 17T. We are open for collaborations on studying van der Waals structures under hydrostatic pressure [1].

The hydrostatic pressure changes preferentially the interlayer distance and hence enhances interactions between the layers.

In our earlier studies we found that if graphene placed on WSe2, a spin-orbit coupling can be induced in graphene. By applying hydrostatic pressure on WSe2/Gr heterostructure, we have found that the spin-orbit coupling can be increased.  By looking magneto conductance measurements at low temperature a clear transition from weak localization to weak anti-localization have been found clearly demonstrating the increase of spin-orbit coupling [2].

Measurements on a WSe2/Gr heterostructure. Whereas the conductance curves do not change much with pressure (left), the quantum correction undergoes a large change (right). The transitions from WL to WAL signal signifies increased spin-orbit interaction.

Finally, hydrostatic pressures in the range of 2GPa can substantially change the band structure of twisted graphene structures. We demonstrate this on twisted doubly bilayer graphene, two bilayer graphene layers place stacked together with a rotation angle of 1.07, close to the magic angle. At this small rotation angles, the two layers strongly hybridized and flat bands, that are electric field tunable, arise at low energies. These narrow bands are separated from higher lying bands by moiré gaps. Our calculations shown that a pressure of 2GPa is enough to close these bandgaps in the spectrum, and substantially altering the spectrum of the system.

Band structure of twisted double bilayer graphene (with twist angle of 1.067) at ambient and large pressures. A strong modification of the bandstructure is observed. Right: artistic view of a device, with the green arrow showing the hydrostatic pressure.

References

[1]  Bálint Fülöp, Albin Márffy, Simon Zihlmann, Martin Gmitra, Endre Tóvári, Bálint Szentpéteri, Máté Kedves, Kenji Watanabe, Takashi Taniguchi, Jaroslav Fabian, Christian Schönenberger, Péter Makk, Szabolcs Csonka, Boosting proximity spin orbit coupling in graphene/WSe2 heterostructures via hydrostatic pressure, arXiv:2103.13325

[2] Bálint Fülöp, Albin Márffy, Endre Tóvári, Máté Kedves, Simon Zihlmann, David Indolese, Zoltán Kovács-Krausz, Kenji Watanabe, Takashi Taniguchi, Christian Schönenberger, István Kézsmárki, Péter Makk, Szabolcs Csonka, Transport measurements on van der Waals heterostructures under pressure, arXiv:2103.14617