Micrometer-Scale Ballistic Transport of Electron Pairs in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ Nanowires

High-mobility complex-oxide heterostructures and nanostructures offer new opportunities for extending the paradigm of quantum transport beyond the realm of traditional III-V or carbon-based materials. Recent quantum transport investigations with ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$-based quantum dots reveal the existence of a strongly correlated phase in which electrons form spin-singlet pairs without becoming superconducting. Here, we report evidence for the micrometer-scale ballistic transport of electron pairs in quasi-1D ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ nanowire cavities. In the paired phase, Fabry-Perot-like quantum interference is observed, in sync with conductance oscillations observed in the superconducting regime (at a zero magnetic field). Above a critical magnetic field $B_p$, the electron pairs unbind and the conductance oscillations shift with the magnetic field. These experimental observations extend the regime of ballistic electronic transport to strongly correlated phases.

Figure 1

Device schematic and Fabry-Perot oscillations. (a) Schematic of the cavity device defined by two barriers separated by a length $L$. Interference due to coherent scattering in the cavity results in conductance oscillations periodic in the Fermi momentum. (b) The background-subtracted zero-bias differential conductance ($dI/dV$) of the cavity [between voltage leads 3 and 4 in (a)] and the open wire (between leads 2 and 3) in the superconducting (red), paired (green), and normal (blue) phases of device $A$ clearly reveals that large oscillations are only present in the cavity.

Figure 2

Magnetic field dependence of the conductance oscillations for three devices. Left panels: transconductance $dG/d{V}_{\mathrm{SG}}$ from a lock-in amplifier measurement of $G$ at small ($100\mu \mathrm{V}$) bias versus $B$ and $V_{\mathrm{SG}}$. The alternating red and blue regions correspond to conductance oscillations. Right panels: the linecuts of $G$ versus $B$, at $V_{\mathrm{SG}}=0$, $-2$, and 0 mV for (a), (b), and (c), respectively, show a sharp peak attributed to superconductivity at $|B|<B_c\approx 0.2\mathrm{T}$ (shaded red), while the conductance in the paired (shaded green) and normal (shaded blue) phases is reduced.

Figure 3

Fabry-Perot interference signatures at finite bias for an $L=4\mu \mathrm{m}$ cavity (device $E$). (a),(c),(e), Zero-bias and finite-bias $dG/d{V}_{\mathrm{SG}}$ linecuts as a function of $V_{\mathrm{SG}}$ at $B=0\mathrm{T}$ [(a) SC], $B=1\mathrm{T}$ [(c) $P$], and $B=7\mathrm{T}$ [(e) $N$]. (b),(d),(f) $dG/d{V}_{\mathrm{SG}}$ vs $V_{4\mathrm{T}}$ and $V_{\mathrm{SG}}$ in the superconducting phase [(b) $B=0\mathrm{T}$], paired phase [(d) $B=1\mathrm{T}$], and normal, unpaired electron phase [(f) $B=7\mathrm{T}$], showing checkerboard features in each phase.