Exciton condensate at a total filling factor of one in Corbino two-dimensional electron bilayers

Magnetotransport and drag measurements on a quasi-Corbino two-dimensional electron bilayer at a total filling factor of 1 $({\nu }_T=1)$ reveal a drag voltage that is equal in magnitude to the drive voltage as soon as the two layers begin to form the expected ${\nu }_T=1$ exciton condensate. The identity of both voltages remains present even at elevated temperatures of $0.25\mathrm{K}$. The conductance of the drive layer vanishes only in the limit of strong coupling between the two layers and at $T\to 0\mathrm{K}$ which suggests the presence of an excitonic circular current.

Figure 1

Schematic view of the Corbino geometry used in this experiment. Application of appropriate voltages to the back gates (marked as “BG”) and front gates (“FG”) will lead to contact separation; i.e., contacts 1–5 will connect to the upper quantum well and $1^{*}–5^{*}$ to the lower one.

Figure 2

(Color online) Top panel: measured drive (solid line) and drag (dash-dotted line) voltages at $T_{\mathit{\text{bath}}}=15\mathrm{mK}$ on sample B. The (integer) filling factors $\nu ⩽2$ and ${\nu }_T=1$ $(d∕l_B=1.62)$ are labeled. Bottom panel: measured current in the drive layer. The inset plots the temperature dependence of the radial conductance $G$; the line is a fit using $G\propto \exp (-E_{\mathit{\text{gap}}}∕T)$.

Figure 3

(Color online) Top panel: drive (solid line) and drag (dash-dotted line) voltages versus the magnetic field at $T=0.25\mathrm{K}$ measured on sample A. Bottom panel: the current in the drive layer measured simultaneously. The inset illustrates the conductance $G$ of the drive layer as a function of the magnetic field and the total density. Clearly visible is how the conductance at ${\nu }_T=1$ decreases as the total density $n_T$ is reduced.

Figure 4

(Color online) Drive (solid dots) and drag (open dots) voltages at ${\nu }_T=1$ versus the parameter $d∕l_B$ at $T_{\mathit{\text{bath}}}=0.25\mathrm{K}$. Above $d∕l_B=1.65$, the amplitudes of drive and drag voltage at total filling factor 1 diverge. The inset shows the corresponding field sweep for the last pair of points at $d∕l_B=1.73$.