Electron-Hole Condensation

By Dennis Pleskot
(video included at bottom of page)

Electrons are crucial to the modern world.  They power our computers, cell phones, and almost everything else we encounter in our daily lives.  The physics of electrons have been extensively studied since their discovery, but never before has there been a better opportunity to observe the quantum properties of electrons than in atomic layer materials (ALMs).  ALMs are materials that are bound together by strong covalent bonds within a plane and weak van der Waals forces out of the plane, allowing them to be separated into sheets that are only a few atoms thick.  Due to their very nature, these materials confine the motion of electrons, allowing for the observation of novel electronic phenomena.  One such phenomenon is the condensation of electrons and holes from a non-interacting gas phase to a highly interacting liquid phase.  Our group has extensively studied the properties of this condensate in atomically thin MoTe2.

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Figure 1: Schematic of a MoTe2 device and experimental measurement.

As shown in Figure 1, we assemble a 3-layered structure of ALMs consisting of MoTe2 sandwiched between two layers of graphene.  Electrical contacts are attached to the top and bottom graphene sheets, and we measure the current across all three layers in response to excitation from a pulsed laser.  The laser, focused down to a microscopic area, scans across the surface of the device, while we measure the current. This allows us to map out the response of the sample to light. When a photon of light from the laser interacts with a semiconductor such as MoTe2, it gets absorbed by the material and generates an electron-hole pair.  The electron can then either recombine with the hole or be drawn out of the material as observable photocurrent through the contacts.  In some cases, however, the electrons and holes can exhibit new and interesting behavior before these processes occur.

As we increase the power of the laser and decrease the time between pulses hitting the device, more electron-hole pairs are created in a shorter amount of time.  Because of this, the density of electron-hole pairs increases until the distance between each pair becomes comparable to the size of a single pair.  Once this happens, the electron-hole pairs begin to interact with each other and form small droplets, as shown in Figure 2 (on the right), which then grow into larger droplets, similar to the condensation of water from air that can be observed on the morning dew of plants.  Eventually, as these droplets grow and merge together, a new electron-hole liquid phase emerges.

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Figure 2: Evolution of electron-hole interactions with increasing density.

By altering the conditions of the experiment, we can discover some basic properties of this condensed electronic phase.  For instance, applying a voltage across the device pulls apart the electron-hole pairs and effectively cause the liquid to evaporate back into an electron-hole gas.  By carefully controlling this voltage, we can determine the energy of the liquid state.  Currently, we are exploring the effects of other parameters on this electronic liquid phase, such as temperature and magnetic field, which could yield greater insight into the physics of electrons and holes in ALMs.

For more info, see our brief video summary: