Superconducting Dome in Ionic Liquid Gated Homoepitaxial Strontium Titanate Thin Films

TL;DR

This work demonstrates ionic-liquid gating of a surface 2DEG formed on a homoepitaxial SrTiO$_3$ thin film, achieving a superconducting transition up to $T_c\approx$503 mK at $N_H\approx3\times10^{13} {\rm cm}^{-2}$ and revealing conventional BCS scaling across the superconducting dome. By combining high-quality hMBE growth with precise electrostatic tuning, the authors map coherence length $\xi$, thickness $d$, and mean free path $L_{MFP}$, confirming 2D superconductivity with $d\ll\xi$ and a single heavy band occupancy. The superconducting fluctuations above $T_c$ collapse onto a universal AL-MT paraconductivity curve with a total-energy cutoff and a fitted MT parameter, indicating conventional fluctuation physics governs the transition broadening. Collectively, the results highlight the viability of STO thin films as tunable, BCS-like superconductors and point to avenues for further Tc enhancement via strain engineering and confinement design, with implications for oxide-based devices and potential Majorana platforms.

Abstract

In this work, we patterned a two-dimensional electron gas (2DEG) on the surface of a SrTiO$_3$ thin film grown homoepitaxially on SrTiO$_3$ by hybrid molecular beam epitaxy (hMBE). We explored the superconducting dome in this material system by tuning electron density with ionic liquid gating. We found superconducting transitions up to 503 mK near an optimal electron density of approximately 3 $\times$ 10$^{13}$ cm$^{-2}$. This is a meaningful increase from the typical optimal transition near 350 mK in similar 2DEGs on SrTiO$_3$ single crystal substrate surfaces. Systematic tuning of 2DEG electron density revealed a consistent BCS scaling between superconducting critical temperature, coherence length, and electron mean free path. Substantial variation of transition width across the dome was described by a paraconductivity model combining Aslamazov-Larkin and Maki-Thompson contributions.

Figures (14)

• Figure 1: Film growth and ionic liquid gated device. (A) High-resolution x-ray diffraction (XRD) 2$\theta$-$\omega$ coupled scan, overlapped SrTiO$_3$ substrate and film peaks. (B) Atomic Force Microscopy (AFM) image of the sample after growth. (C) Picture of the device with ionic liquid deposited on the device. (D) Picture of the fabricated Hall Bar device. The dark, light and golden ($\textcolor{#31332E}{\blacksquare}$, $\textcolor{#67644B}{\blacksquare}$, $\textcolor{#E5CA72}{\blacksquare}$) areas correspond to the insulating SiO$_2$, exposed STO, and Au/Ti contacts, respectively. Scale bars in (B), (C), (D) are 1 $\mu$m, 1 mm and 50 $\mu$m, respectively.
• Figure 2: Superconducting dome in ionic liquid gated homoepitaxial SrTiO$_3$ films. (A) Superconducting transitions with temperature. Solid and dot-dashed lines are for the 10 and 40 $\mathrm{\mu}$m channels respectively. For the lowest carrier density, measurements were across the entire hall bar with the 10, 20, and 40 $\mu$m channel width segments in series. (B), (C) Critical temperature as a function of magnetic field shown along with dashed line fits to G-L theory using Equations \ref{['eqbperp']} and \ref{['eqbpar']}. $\xi$ and $d$ fit values are also indicated for each fit. (D) Critical temperatures extracted as the point at which $R_{\mathrm{s}} = 0.5\times R_{\mathrm{n}}$ ($R_{\mathrm{n}}$ is the normal-state resistance). Shaded region shows the width of the transitions as $T(R_{\mathrm{s}} = 0.9\times R_{\mathrm{n}})-T(R_{\mathrm{s}} = 0.1\times R_{\mathrm{n}})$. $\textcolor{#8B0000}{\bullet}$ and $\textcolor{#FF8D02}{\blacktriangle}$ show the critical temperature for the 10 and 40 $\mathrm{\mu}$m channels respectively. The blue line is a guide to the eye. LAO/STO data are from 10_shalom_physrevlet. The LAO/STO dome interpolation follows 22_bergeal_advmatint23_mikheev_natelec. Data for ionic liquid gated devices on STO single crystal surfaces are from 21_mikheev_sciadv. (E) Critical magnetic field ($B_{\mathrm{c}}$) extrapolated from the fits shown as a function of $N_{\mathrm{H}}$. (F) $\xi$, $L_{\mathrm{MFP}}$ and $d$ dependence with $N_{\mathrm{H}}$. $\xi$ was obtained from G-L framework and through independent measurement and calculations based on BCS theory (shown with the dashed line using interpolated points).
• Figure 3: Paraconductivity above the superconducting transition. (A) Normalized transitions are shown for the 10 $\mu$m channel. The arrow signifies that the broader transitions have a higher $T_{\mathrm{c}}$. In subsequent plots, the $\bullet$ and $\blacktriangle$ markers show the 10 and 40 $\mu$m channels respectively. The inset shows the width of the transition taken as $\Delta T = T(R = 0.9*R_{\mathrm{n}}) - T_{\mathrm{c}}$. The dashed line is the expected width from AL-MT fluctuations ($\delta=0.22$ and $c=1$) while varying only $R_{\mathrm{n}}$. (B) Superconducting transitions shown as change in conductivity with $\varepsilon=ln(T/T_{\mathrm{c}})$. Dashed lines show the AL contribution only (red), AL and MT contributions combined using $\delta=0.22$ (grey), same AL and MT contributions with a total-energy cutoff parametrized by $c=1$ (black). (C) The dependence of $\delta$ with $R_{\mathrm{n}}$ is shown. The $\textcolor{#696969}{\bullet}, \textcolor{#696969}{\blacksquare}, \textcolor{#696969}{\blacktriangle}$ markers show the measured data along with dashed lines as linear fits for other superconductors 72_crow_physrevlett.
• Figure S1: Device fabrication steps. Optical images of the device taken after lift-off procedure for the (A) ohmic contacts, (B) mesa insulation, (C) gate contact. All scale bars are 50 microns.
• Figure S2: Tuning carrier density and Hall Effect measurements. (A) Carrier density as a function of $V_{\mathrm{GIL}}$. Hall Effect measurements are shown at 165 K and at a base temperature of 1.6 K. (B) Sheet resistance during the cooldown with $N_{\mathrm{H}} = 3.8\times10^{13} \mathrm{cm}^{-2}$. The different traces correspond to four terminal measurement with adjacent voltage probes along the Hall bar channel in Fig. \ref{['SM_figure_device']}C, with the local channel width indicated in the legend.