Side Gate Tunable Josephson Junctions at the LaAlO3/SrTiO3 Interface

Abstract

Novel physical phenomena arising at the interface of complex oxide heterostructures offer exciting opportunities for the development of future electronic devices. Using the prototypical LaAlO₃/SrTiO₃ interface as a model system, we employ a single-step lithographic process to realize gate-tunable Josephson junctions through a combination of lateral confinement and local side gating. The action of the side gates is found to be comparable to that of a local back gate, constituting a robust and efficient way to control the properties of the interface at the nanoscale. We demonstrate that the side gates enable reliable tuning of both the normal-state resistance and the critical (Josephson) current of the constrictions. The conductance and Josephson current show mesoscopic fluctuations as a function of the applied side gate voltage, and the analysis of their amplitude enables the extraction of the phase coherence and thermal lengths. Finally, we realize a superconducting quantum interference device in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently via the side gates.

Keywords: Josephson junction; Oxide heterostructures; SQUID; field-effect; side gates.

Figure 1

(a) 3D schematic of a side gated constriction. α-LAO: amorphous LAO; c-LAO: crystalline LAO. (b) AFM image of a typical device showing the constriction and two side gates (SG₁ and SG₂). The 2DES is formed only at the interface between c-LAO and STO. (c) Height profile along the black line in panel b, showing a constriction width of approximately 50 nm.

Figure 2

(a) Sketch of the device geometry showing the electrical connections for transport measurements. (b) Spatial map of the out-of-plane electric polarization (P) for V_SG^1,2 = −50 mV, obtained by finite-element simulations. (c and d) Evolution of P and ε_STO, respectively, across the constriction (along the white line in panel b) for different values of V_SG^1,2. (e) Value of P at the center of the constriction (x = 0 nm) as a function of V_SG^1,2. (f) ΔP as a function of V_SG^1,2. Color code as in panel e. Inset: electric polarization profiles across the constriction.

Figure 3

(a) Four-probe resistance (R) of as a function of side gate voltage V_SG^1,2 measured for different V_BG. (b) Map of R as a function of V_SG¹ and V_SG². The voltage step is 0.2 mV. (c) R as a function of side gate voltage. Dev1: V_SG¹ = V_SG² (green). Dev2: V_SG¹ = V_SG² (blue), V_SG² = 0 mV (red), and V_SG¹ = 0 mV (black).

Figure 4

(a) Differential resistance (dV/dI) plotted as a function of bias current I_bias and side gate voltage V_SG^1,2, measured at V_BG = −11 V and T = 50 mK. (b) Fluctuations of the conductance G and the critical current I_c as a function of applied side gate voltage V_SG^1,2.

Figure 5

(a) AFM image of the SQUID device which comprises a left (SG_L) and right (SG_R) side gate electrodes. Inset: c-JJ of the left arm and the respective side gate electrode. (b) Tunability of the SQUID oscillations. Left column: V_SG^R = 0 mV and different values of V_SG^L. Right column: V_SG^L = 0 mV and different values of V_SG^R. B₀ is an experimentally determined offset in the magnetic field and has an uncertainty greater than one oscillation period.