Intrinsic Negative-U Centers in Freestanding LaAlO3/SrTiO3 Micro-membranes

Abstract

The LaAlO3/SrTiO3 (LAO/STO) interface hosts a rich range of electronic phenomena, including unconventional electron pairing that in quantum dots gives rise to a negative effective charging energy U. Here, we show freestanding LAO/STO micro-membranes naturally hosting negative-U centers, where lateral confinement arises intrinsically, rather than from engineered nanostructures. These centers coexist with gate-tunable superconductivity and can remain stable upon thermal cycling from millikelvin temperatures to room temperature. Transport is in excellent agreement with calculations based on a negative-U Anderson model, and electrostatic simulations indicate characteristic center sizes of 20-80 nm. Our findings suggest that negative-U centers may arise from the intrinsic interfacial inhomogeneity typical of LAO/STO, and should therefore be considered a general feature of the LAO/STO interface. This could have important consequences for the microwave response of interfacial superconducting devices.

Figures (5)

• Figure 1: a False color scanning electron micrograph of a micro-membrane on SiO$_2$. b Device schematic together with a representation of the array of conductive puddles at the LAO/STO interface (not to scale). A nanometer-sized puddle (red) may represent the intrinsic origin of the observed negative-$U$ center. Inset: electrochemical potentials of the leads and transitions in a negative-$U$ center (black) and their evolution with magnetic field (red). c Differential resistance of Dev#1 as a function of $V_{\text{G}}$ and $I_{\text{SD}}$, with $B_{\parallel} = 25\ \mathrm{mT}$, to suppress Al-contact superconductivity ($T_{\text{c}}^{\text{Al}}\approx$1.2 K, $B_{\text{c}}^{\text{Al}}\approx$10 mT). d Gate dependence of switching current $I_{\text{s}}$, normal-state resistance $R_{\text{N}}$ and their product $I_{\text{s}} \cdot R_\text{N}$. e Normal-state conductance at $B_{\perp} = 0.5\ \mathrm{T}$ vs $V_\text{G}$ (black trace). The red trace shows the residual $\Delta G$ after background subtraction.
• Figure 2: a Measured residual conductance, $\Delta G$, of Dev#1 as a function of $V_\text{G}$ and $V_\text{SD}$, at $B_{\perp} = 0.5\ \mathrm{T}$ (white dashed line in panel c). b Measured zero-bias $\Delta G$ versus $V_\text{G}$ and $B_{\parallel}$. The white curves show Lorentzian fits of a selected peak/peak pair at $B_{\parallel}=1.5,\ 2.5,\ 3.5,\ 4.5,\ 5.5\ \mathrm{T}$. The symbols correspond to those in Fig. 3. c Measured zero-bias $\Delta G$ versus $V_\text{G}$ and $B_{\perp}$. Panels b and c are shifted along $V_\text{G}$ by $\approx1.5\ \mathrm{V}$ and $\approx1.3\ \mathrm{V}$ respectively, to align the low-field peak center with panel a; the shift arises from device drift during measurements. d Simulated conductance versus $V_\text{G}$ and $V_\text{SD}$. e Simulated zero-bias conductance versus $V_\text{G}$ and $B_{\parallel}$. f Simulated zero-bias conductance versus $V_\text{G}$ and $B_{\perp}$. The numbers on panel e and f indicate the ground-state charge occupancy $\mathcal{N}$.
• Figure 3: $B_{\parallel}$ dependence of the gate voltage separation between zero-bias conductance peaks in Dev#1 and Dev#2. The symbols correspond to those in Fig. 2b and S3. The pairing field, $B_{\text{P}}$, and the Landé $g$-factor, $g$, are extracted from linear fits (solid lines). For two peak pairs of Dev#2, data points at high fields (filled grey markers) are excluded from the fit. Note that in some cases the very low slope of $\Delta V_\text{G}(B)$ renders $B_\text{P}$ ill-defined.
• Figure 4: a Estimated center diameter as a function of gate capacitance $C_\text{G}$ for a contact aspect ratio $w/w_\text{t} = 0.2$ and various vacuum thicknesses $t$. The inset shows a schematic of the simulated geometry (top and side views). b Estimated center diameter as a function of $C_\text{G}$ for a fixed vacuum thickness $t=50\ \mathrm{nm}$ and varying contact aspect ratios $w/w_\text{t}$.
• Figure 5: a Thermal cycling sequence applied to Dev#1. b Residual conductance of Dev#1 after the first cool-down, after thermal cycling to 125 K, and after thermal cycling to room temperature with exposure to air, at $B_{\perp} = 0.5\ \mathrm{T}$. White dashed lines are guides to the eye.