Thermodynamic Evidence for Density Wave Order in a Two Dimensional ^{4}He Supersolid.

Area of Science:

• Condensed Matter Physics
• Quantum Fluids

Background:

• Two-dimensional (2D) helium-4 (⁴He) films on graphite exhibit exotic supersolid behavior.
• A supersolid displays intertwined density wave and superfluid order.

Purpose of the Study:

• To provide direct experimental evidence for the density wave order in a 2D ⁴He supersolid.
• To investigate the ground state properties of the second layer of ⁴He adsorbed on graphite.

Main Methods:

• Doping the 2D ⁴He film with a small concentration of helium-3 (³He) atoms (impuritons).
• Measuring heat capacity, magnetization, and NMR relaxation times of ³He over a wide temperature range (200 μK to 500 mK).

Main Results:

• Observed changes in the ground state of the second layer film with varying ⁴He amounts and fixed ³He doses.
• Obtained clear evidence for a solid second layer film.

Conclusions:

• The solid second layer film is consistent with the regime of superfluid response previously observed in pure ⁴He films.
• The study confirms the existence of density wave order in 2D ⁴He supersolids.