Electrical Modulation of Superconducting Critical Temperature in Liquid-Gated Thin Niobium Films

Abstract: We demonstrate that the superconducting critical temperature (Tc) of thin niobium films can be electrically modulated in a liquid-gated geometry device. Tc can be suppressed and enhanced by applying positive and negative gate voltage, respectively, in a reversible manner within a range of about 0.1 K. At a fixed temperature below Tc, we observed that the superconducting critical current can be modulated by gate voltage. This result suggests a possibility of an electrically-controlled switching device operating at or above liquid helium temperature, where superconductivity can be turned on or off solely by the applied gate voltage.

• The paper reports that reversible, polarity-dependent voltage gating modulates the superconducting critical temperature by approximately 0.1 K in 8 nm niobium films.
• The methodology employs ionic liquid gating with a Hall bar geometry and four-probe measurements in a high-vacuum cryostat to ensure precise control at cryogenic temperatures.
• The results suggest the potential for non-volatile, voltage-controlled superconducting switches, paving the way for advanced applications in cryogenic computing and quantum electronics.

Electrical Modulation of Superconducting Critical Temperature in Liquid-Gated Thin Niobium Films

Introduction

This study reports the direct modulation of the superconducting critical temperature ($T_c$) and critical current in niobium (Nb) thin films via an ionic liquid-gated field effect device. Employing a Hall bar geometry with an 8 nm sputtered Nb film on a sapphire substrate, the results demonstrate reversible, polarity-dependent shifts in $T_c$ and supercurrent near liquid helium temperatures. The findings detail a substantial, reversible modulation of $T_c$ on the order of 0.1 K—nearly three orders of magnitude larger than prior reports for metallic films—thereby establishing a pathway for electrically switchable superconducting devices viable for operation at technologically relevant temperatures.

Experimental Methodology

The Nb films were deposited with controlled stress below 200 MPa to preserve film integrity and coherence, with thickness (8 nm) tuned such that $T_c$ was near 4.2 K. Standard optical lithography and Ar:SF$_6$ RIE defined the Hall bar geometry, followed by careful removal of residual resist. The devices were covered with the ionic liquid DEME-TFSI in a glovebox and measured in a high-vacuum cryostat using four-probe techniques. Gate voltages were applied at 230 K, above the glass transition of the ionic liquid, to ensure mobile ions and formation of the electrical double layer (EDL) at the Nb surface.

Main Results

Gate-Modulated Superconducting Transition

Application of positive (negative) gate voltages resulted in reproducible suppression (enhancement) of $T_c$, with the modulation reversible and consistent over repeated voltage cycles except for minor irreversible resistance changes due to anodization at large negative biases. The maximum observed tunability ($\sim$ 0.1 K) is orders of magnitude greater than that achieved in classical dielectric gate structures or previous metallic channel experiments.

Supercurrent Modulation

At fixed sub-critical temperatures (e.g., 1.9 K), gate voltage also modulates the critical current ($I_c$), with changes of $\sim$ 20 $\mu$A out of a maximum $I_c$ of $\sim$ 208 $\mu$A in a 10 $\mu$m wide strip. Thus, switching between superconducting and resistive states can be achieved solely by voltage control, with measurable changes in transport characteristics.

Behavior in Thick Films

Experiments on 120 nm thick Nb films yielded similar $T_c$ modulation, despite the expectation that EDL gating would only affect a surface layer in conventional electrostatic mechanisms. This observation implies that field-induced effects cannot be ascribed purely to straightforward carrier density modulation and points toward more complex interfacial phenomena, potentially involving electro-mechanical coupling via electrostriction of the frozen ionic liquid.

Implications and Theoretical Perspectives

The results position ionic liquid gating as an effective tool for tuning superconductivity in metallic systems, circumventing classical screening limitations by leveraging large electric fields at the EDL. The magnitude and polarity dependence of the $T_c$ shift, coupled with the ability to modulate $I_c$, establish a foundation for non-volatile, voltage-controlled superconducting switches operational at 4.2 K. The demonstration with Nb films— the industrial standard for superconducting electronics— enhances viability for integration into logic, memory, and reconfigurable quantum-classical interface architectures.

However, the mechanism underlying the field effect modulation is not conclusively resolved. The comparable response in both thin and thick Nb films indicates non-electrostatic contributions, suggesting extrinsic or interface-specific mechanisms, such as ion-induced interfacial stresses or subtle changes in local chemical environment. This ambiguity highlights a need for further studies, potentially employing surface-sensitive probes and advanced modeling, to clarify the fundamental origins of the observed effects.

Future Directions

Several research avenues emerge from these results:

• Mechanistic Elucidation: Investigations employing synchrotron XPS, STM, or in-situ TEM could resolve field-induced interfacial modifications and disentangle electrostatic versus electro-mechanical or electrochemical modes.
• Material System Extension: Applying analogous liquid gating protocols to other conventional and unconventional superconductors may reveal universal or material-specific responses, expanding the applicability of the approach.
• Device Engineering: The demonstration of switchable superconductivity invites development of integrated, fast, and low-energy digital elements for cryogenic computing and quantum technologies.
• Scaling and Stability: Studies on long-term device endurance, noise characteristics, miniaturization, and device integration will be critical for technological translation.

Conclusion

This work evidences efficient and reversible modulation of both $T_c$ and $I_c$ in Nb thin films via ionic liquid gating, with a substantial amplitude of effect, clear bias polarity dependence, and practical operational temperature range. While the physical mechanism demands further clarification, the findings unlock new device paradigms based on voltage-gated superconductivity, with far-reaching implications for superconducting electronics and field-effect-based device control.