Researchers at the University of Cincinnati have created a strontium titanate (SrTiO₃) platform achieving electron mobility up to 7400 in two-dimensional electron gases. Despite systematically exploring electron densities expected to induce superconductivity, the team observed no superconducting transition even when cooling the material to approximately 10 mK, a temperature incredibly close to absolute zero. This suggests that vertical confinement and electronic orbital rearrangement may suppress superconductivity in high mobility SrTiO₃ systems, while the fabrication technique offers a promising path toward new quantum devices.

High Mobility SrTiO₃ 2DEG Fabrication via Hydrogen Plasma Exposure

Commercial strontium titanate (SrTiO₃) single crystals served as the foundation for fabricating two-dimensional electron gases (2DEGs) using a cost-effective hydrogen plasma exposure technique, eliminating the need for complex thin film growth processes. This fabrication method proved compatible with electron-beam lithography patterning at the few-micron scale, suggesting potential for further down-scaling towards mesoscopic and quantum device sizes. Device channel widths varied from 5 to 40 micrometers, alongside channel-gate gaps of 5, 20, and 80 micrometers, allowing for systematic exploration of device geometry effects.

Researchers fabricated and tested devices labeled A through H, documenting the measured Hall density ranges achieved across different cooldown cycles. The team systematically aged the 2DEGs by mildly heating the samples between cooldowns, observing changes in electron density and exploring a range of values, including those expected to induce superconductivity in SrTiO₃.

This absence of superconductivity, despite achieving high electron mobilities up to 7400, presents a surprising result given the established expectation of superconductivity in similar SrTiO₃ 2DEG systems. The observed lack of a transition prompts questions about the interplay between superconductivity and clean transport within the material. Further investigation focused on electrostatic gate modulation within the 2DEG system, with researchers systematically exploring its scaling with electron density and side gate geometry.

Contrary to initial expectations, the team found that larger separations between the side gate and the channel actually improved the total achievable modulation. This counterintuitive finding suggests that conventional design principles regarding gate control may not fully apply in this high-mobility 2DEG system. At low electron densities, stochastic channel pinch-off events were observed, creating quasi-ballistic constrictions with irregular conductance quantization.

Data, python notebooks used during original data collection, and python notebooks for generation of manuscript figures are available as Supplementary material. The observed differences in aging timescales between devices appeared linked to the 2DEG area, leading to speculation that larger lateral dimensions facilitate gradual oxygen diffusion towards oxygen vacancies.

Preliminary tests involving room temperature sputtering of a 30 nm silicon oxide capping layer onto SrTiO₃, followed by hydrogen plasma exposure, yielded no measurable conductivity. This work details these findings and highlights the open questions of why STO superconductivity and clean transport appear to be in competition, and how to reach the desirable regime of their coexistence.

Suppression of Superconductivity in Quasi-Two-Dimensional SrTiO₃ Systems

The fabrication of high-mobility two-dimensional electron gases (2DEG) on strontium titanate (SrTiO₃) surfaces consistently yielded electron mobilities reaching 7400, a performance level achieved through hydrogen plasma exposure. Despite this success in creating highly conductive channels, systematic exploration of electron densities within the expected range for superconductivity revealed no transition to the superconducting state, even at temperatures as low as 10 mK. This absence of superconductivity suggests a suppression mechanism at play within the high-mobility, quasi-two-dimensional SrTiO₃ system, potentially linked to the vertical confinement of electrons and a rearrangement of their orbital configurations.

At low electron density, stochastic channel pinch-off events were observed, creating quasi-ballistic constrictions with irregular conductance quantization. The investigation into the suppression of superconductivity considered the possibility that the electron density was simply too low to support the phenomenon. However, a conservative lower bound for volume electron density in the devices, 5× 10¹⁷ cm^-3, falls within the range where superconductivity is expected in uniformly-doped bulk SrTiO₃, with a critical temperature (Tc) around 100 mK.

This eliminates a simple density-based explanation, prompting consideration of more subtle factors. The team points to band order inversion towards single doubly-degenerate d_xz/yz band filling as a potential cause, a phenomenon observed in other high-mobility 2DEG systems where superconductivity is suppressed. The SrTiO₃ 2DEGs in this work appear to also belong to the d_xz/yz filling archetype, as suggested by the relatively high electron mass from quantum oscillations.

This epitaxy-free and high mobility oxide material platform offers a promising new route towards patterning quantum devices. The team’s findings suggest that achieving superconductivity in this material system requires careful consideration of both electron density and the electronic structure of the 2DEG. Further research is needed to fully understand the interplay between these factors and to develop strategies for overcoming the observed suppression mechanisms, ultimately clearing the way for realizing the full potential of quantum devices.

Electron Density Control Through Sample Aging and HPE Technique

This unexpectedly high mobility, a key characteristic of the fabricated two-dimensional electron gases (2DEGs), contrasted sharply with the absence of superconductivity within the expected density range for the material. Researchers systematically varied electron density through controlled aging of the samples, transitioning from ambient storage lasting months to accelerated aging via mild heating on a hot plate, to map the potential superconducting dome. The ability to tune the 2DEG’s carrier concentration was important in this search, allowing exploration across a broad density range.

The team’s approach to density control involved monitoring sheet resistance at room temperature, enabling estimation of electron density under the assumption of a constant electron mobility of 5.5. This method allowed for a reliable proxy for the Hall electron density measured at milliKelvin temperatures, confirming the validity of the aging process as a means of systematic exploration.

Gradual 2DEG aging timescales were readily adjustable, providing a versatile platform for investigating the relationship between electron density and superconductivity. The evolution of Hall density matched the electron density estimated from room temperature measurements, further validating the technique. Alongside the search for superconductivity, the researchers also characterized electrostatic side gating, assessing its potential for developing mesoscopic oxide devices such as quantum constrictions, junctions, wires and dots.

Monitoring gate leakage current during initial sweeps defined the upper bound of the accessible gate voltage range, with the threshold for leakage onset consistently increasing as electron density decreased with device aging. “Fig. S6 (b) shows electron density evolution of the side gate leakage onset threshold (set at 20 pA),” according to supplemental materials detailing the process. This observation suggests a robust control mechanism over the 2DEG’s behavior, even at low carrier densities.

The data revealed a substantial improvement in gate leakage at the lowest carrier densities, indicating the potential for precise electrostatic manipulation of the 2DEG. The ability to accelerate the aging process by mildly heating the sample to 70-90 °C on a hot plate in air allowed for efficient exploration of electron densities between 2-3× 10¹⁴ cm^-2 and approximately 5× 10¹³ cm^-2, the point at which non-linearity in the metal-2DEG contact resistance became significant. The systematic exploration of both sample aging and electrostatic side gating provides a comprehensive understanding of the factors influencing the behavior of high-mobility SrTiO₃ 2DEGs, despite the unexpected absence of superconductivity.

Electrostatic Side Gate Tuning and Channel Geometry Scaling

High electron mobility, reaching 7400 in the fabricated strontium titanate (SrTiO₃) devices, initially suggested promising conditions for superconductivity, but subsequent investigation revealed an unexpected outcome. This counterintuitive finding challenges assumptions about electrostatic control in two-dimensional electron gas (2DEG) systems and opens new avenues for device design. The team’s work focused on 2DEG formed by hydrogen plasma exposure on SrTiO₃ substrates, allowing for adjustable aging timescales ranging from months to hours.

This control over aging facilitated a broad exploration of electron density, specifically targeting the range expected to induce superconductivity within the material. The absence of superconductivity, despite the high mobility and extensive density range tested, points to suppression mechanisms likely linked to the unique characteristics of this high-mobility 2DEG, including vertical confinement and electronic orbital rearrangement. Further characterization revealed unusual behavior at low electron densities within the narrowest 2DEG channels, measuring 5 μm in lithographic width.

Stochastic channel pinch-off events created quasi-ballistic constrictions exhibiting irregular conductance quantization, a phenomenon that could be harnessed for novel mesoscopic device applications. Side-gate modulation appeared to be dominated by tuning mobility and confinement, rather than through simple capacitive charge modulation, a distinction important for understanding the device’s operational principles. The observed improvement in electrostatic modulation with increased side gate separation is particularly noteworthy. According to the research, this suggests a viable pathway for controlling nanoscale channels with long gate-channel gaps, mitigating leakage and enhancing device performance.

The team reported, detailing the subtle relationship between Hall density and gate voltage. This finding is significant because it demonstrates that careful control of device geometry can overcome limitations typically associated with electrostatic gating in 2DEG systems.

The adjustable aging timescale, coupled with the observed improvements in electrostatic modulation through optimized gate geometry, provides a versatile platform for investigating and controlling nanoscale channels. This platform’s potential extends beyond fundamental research, offering a pathway to create functional quantum devices with tailored properties.

Observation of Quasi-Ballistic Constrictions at Low Electron Density

At electron densities below 5 x 10¹³ cm^-2, the strontium titanate (SrTiO₃) two-dimensional electron gas (2DEG) exhibited stochastic channel pinch-off, resulting in quasi-ballistic constrictions and irregular conductance quantization. These constrictions, characterized by conductance fluctuations, bear resemblance to those intentionally created in other semiconductors, though precise assignment of step features to multiples of the conductance quantum proved challenging due to uncertainties in series resistance. Subtracting a constant 4 kΩ series resistance revealed faint step features clustering near 1, 2, 3, and 5 e²/h*, suggesting complex electron behavior within these constricted channels.

The creation of these quasi-ballistic constrictions occurred alongside systematic exploration of electrostatic side gating and its scaling with channel geometry. This control allowed researchers to investigate the material’s behavior in the density regime expected to exhibit superconductivity, despite ultimately observing no superconducting transition down to 10 mK.

The absence of superconductivity, even at temperatures incredibly close to absolute zero, is unexpected given the high electron mobilities achieved, up to 7400, in these devices. The researchers note, highlighting the deviation from established behavior in similar materials. Analysis of the data revealed an effective electron mass of 3.1 m_e, suggesting occupation of the heavy-mass d_xz/yz states, consistent with previous observations.

The team also observed a discrepancy between the electron density determined through Shubnikov-de Haas oscillations and Hall measurements, a common feature in SrTiO₃ 2DEGs. The epitaxy-free fabrication process and the resulting high-mobility oxide material platform offer a promising new route toward patterning quantum devices.

👉 More information
🗞 Suppression of superconductivity and electrostatic side gate tuning in high mobility SrTiO₃ surface electron gas
✍️ Dickson Boahen, Sushant Padhye, Gayan De Silva, Eshanvi Rao and Evgeny Mikheev
🧠 DOI: http://link.aps.org/doi/10.1103/z6lp-8pqw

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