November 27, 2017
Freezing electrons makes them get in line
WEST LAFAYETTE, Ind. — New research published in Nature Communications suggests that electrons in a two-dimensional gas can undergo a semi-ordered (nematic) to mostly-ordered (smectic) phase transition, which has been discussed in physics theory but never seen in practice before.
“Imagine that we could build an ice skating rink for electrons, and the electrons had to move along the surface; they couldn’t move up and down, so they just had to skate around each other,” said Michael Manfra, the Bill and Dee O’Brien Chair Professor of physics and astronomy at Purdue. “When you cool them down to very low temperatures and place them in a magnetic field, they sort of lock in step with each other; they line up like soldiers on a battlefield.”
The Purdue team used a high-purity growth technique, called molecular beam epitaxy, to grow a semiconductor material (gallium arsenide) that confines electrons to a two-dimensional layer. Then they cooled the sample down to near absolute zero in a dilution refrigerator. Voltage probes along the length of the conductor measured resistance, which is a function of temperature and magnetic field.
The researchers believe they induced a transition from a nematic to a smectic state. To make this happen, conditions had to be perfect – extremely cold, on a very clean surface, in a perpendicular magnetic field.
“At high temperatures, it looks isotropic, like electrons randomly dancing around one another,” Manfra said. “As it starts to cool down, the electrons lock into columns, and we see the resistance take off and peak. Then as we go to very low temperatures, it turns over again. This indicates a transition from a nematic to smectic phase.”
These results aren’t what the team originally went looking for – it was an accidental discovery that helps them better understand how electrons interact with one another in controlled environments. Discoveries in physics often happen this way; in the right conditions, serendipitous events can occur.
Geoff Gardner and graduate student Saeed Fallahi grew the material that allowed the electrons to behave as if they were on a two-dimensional plane. Graduate students Qi Qian and James Nakamura performed the experiment. They are all members of the Quantum Semiconductor Systems Group at Birck Nanotechnology Center in Purdue’s Discovery Park.
Writer: Kayla Zacharias, 765-494-9318, firstname.lastname@example.org
Source: Michael Manfra, 765-494-3016, email@example.com
Note to Journalists: The journal article is available here.
Possible nematic to smectic phase transition in a two-dimensional electron gas at half-filling
Q. Qian, J. Nakamura, S. Fallahi, G.C. Gardner, M.J.Manfra
Liquid crystalline phases of matter permeate nature and technology, with examples ranging from cell membranes to liquid-crystal displays. Remarkably, electronic liquid-crystal phases can exist in two-dimensional electron systems (2DES) at half Landau-level filling in the quantum Hall regime. Theory has predicted the existence of a liquid-crystal smectic phase that breaks both rotational and translational symmetries. However, previous experiments in 2DES are most consistent with an anisotropic nematic phase breaking only rotational symmetry. Here we report three transport phenomena at half-filling in ultra-low disorder 2DES: a non-monotonic temperature dependence of the sample resistance, dramatic onset of large time-dependent resistance fluctuations, and a sharp feature in the differential resistance suggestive of depinning. These data suggest that a sequence of symmetry-breaking phase transitions occurs as temperature is lowered: first a transition from an isotropic liquid to a nematic phase and finally to a liquid-crystal smectic phase.