Many-body enhanced nonlinear conductance resonance in quantum channels

We measure a strong enhancement of the nonlinear differential conductance ($g=dI/dV$), the amplitude of which exceeds the universal quantum conductance ($2e^2/h$), under finite bias voltage in quantum point contacts (QPCs). By developing a spin-based model in the low-electron-density limit, we demonstrate that this resonance is an intrinsic nonequilibrium phenomenon that arises from many-body induced modifications to the QPC potential. A comparison with the linear conductance ($G=I/V$) shows that this phenomenon is driven by many-body dynamics within a single one-dimensional sub-band.

Figure 1

(a) $g=dI/d{V}_{\mathrm{sd}}$ measured vs. $V_g$ (up and down sweeps) for $V_{\mathrm{sd}}=0$ and $V_{\mathrm{sd}}=+4$ mV at 22 mK, QPC A. (Inset) Typical QPC before deposition of the top gate. (b) $g\left(V_{\mathrm{sd}}\right)$ measured at different $V_g$, showing $g>G_0$, QPC A. (c) Color plot of $g(V_{\mathrm{sd}},V_g)$. $g>G_0$ values in red. (Red line) $g(V_{\mathrm{sd}}=+4$ mV) of (a), (blue line) $g(V_g=0.38V)$ of (b), QPC A. (d) Temperature dependence of the conductance peak, QPC B. (Inset) Linear conductance showing the 0.7 anomaly. (e) Nonlinear conductance peak for $V_{\mathrm{sd}}=+4.5$ mV at $B=0$ T and 8 T, QPC B.

Figure 2

(a) Schematic $I$-$V_{\mathrm{sd}}$ relation. As a pinched-off QPC [in (d)] becomes conducting [in (e)] the potential barrier shifts downward (dashed to solid curve) due to the energy gain via coupling to spin fluctuations and the current becomes enhanced. The current should be bound by the maximum current, $I_{\max }=G_0{V}_{\mathrm{sd}}$, if only one sub-band is responsible. Once the conductance saturates [in (f)], the potential shift does not affect the plateau. (b) Corresponding differential conductance. (c) Measured $I$-$V_{\mathrm{sd}}$ curves. [(d)–(f)] Energy configurations at different gate voltages. $E_b$ is the potential barrier height. (g) Linear-conductance regime.

Figure 3

(a) Calculated $g$ at finite bias $V_{\mathrm{sd}}=1$ mV with several spin-coupling constants $J_0$. $J_0=10,11,12,12.5,13$ meV from bottom to top curves. (b) $g$ at several $V_{\mathrm{sd}}$ ($0,0.25,0.5,1$ meV from bottom to top) for $J_0=12.5$ meV. Absence of conductance peak in the mean-field approximation (dash-dotted red curve) demonstrates the importance of spin fluctuations. (c) The conductance peak with strong temperature and magnetic field (inset) dependence within the experimental range. $J_0=12.5$ meV; $V_{\mathrm{sd}}=1$ meV. (d) Strong enhancement of conductance peak for flatter potential barrier (inset). Increment of $\alpha$ is 0.2 [defined below Eq. (2)].