Electric Field Control of the LaAlO$_{3}$/SrTiO$_{3}$ Interface Ground State (0807.0585v1)

Abstract: Interfaces between complex oxides are emerging as one of the most interesting playgrounds in condensed matter physics. In this special setting, in which translational symmetry is artificially broken, a variety of novel electronic phases can be promoted. Theoretical studies predict complex phase diagrams and suggest the key role of the carrier density in determining the systems ground states. A particularly fascinating system is the interface between the insulators LaAlO${3}$ and SrTiO${3}$, which displays conductivity with high mobility. Recently two possible ground states have been experimentally identified: a magnetic state and a two dimensional (2D) superconducting condensate. In this Letter we use the electric field effect to explore the phase diagram of the system. The electrostatic tuning of the carrier density allows an on/off switching of superconductivity and drives a quantum phase transition (QPT) between a 2D superconducting state and an insulating state (2D-QSI). Analyses of the magnetotransport properties in the insulating state are consistent with weak localisation and do not provide evidence for magnetism. The electric field control of superconductivity demonstrated here opens the way to the development of novel mesoscopic superconducting circuits

• The paper demonstrates electrostatic tuning of the LAO/STO interface, controlling superconducting and insulating ground states.
• It employs gate voltages from -300 V to 320 V, reaching a max superconducting critical temperature of approximately 310 mK.
• The study reveals a critical resistance near 4.5 kΩ/□ and notable magnetotransport behavior, key to quantum phase transitions.

Electric Field Control of the LaAlO₃/SrTiO₃ Interface Ground State

The interaction between LaAlO₃ (LAO) and SrTiO₃ (STO) at their interface is pivotal in understanding novel electronic phases within condensed matter physics. The current paper examines the tunability of electronic states at the LAO/STO interface via electric field manipulation, demonstrating a quantum phase transition (QPT) between a superconducting state and an insulating state.

Main Findings

The LAO/STO interface exhibits high mobility conductivity, despite the individual insulating properties of LAO and STO. Recent experimental evidence identifies two primary ground states: a magnetic state and a two-dimensional (2D) superconducting state. One key finding of the research is the electrostatic tuning of the carrier density through applying an electric field, which allows for the modulation and on/off switching of superconductivity, leading to a QPT between a 2D superconducting state and a 2D insulating state.

Numerical Results

1. Superconductivity Tuning: Through applied gate voltages ranging from -300 V to 320 V, the superconducting critical temperature reaches a maximum of approximately 310 mK.
2. Critical Resistance: At the transition between superconducting and insulating states, the critical sheet resistance is identified as $R_c \approx 4.5 \, \text{k}\Omega/\square$, close to the quantum of resistance for charge $2e$ bosons ($R_Q = h/4e^2 \approx 6.45 \, \text{k}\Omega$).
3. Magnetotransport Properties: Convincing evidence of weak localization is observed, with a large negative magnetoresistance increasing in the insulating phase, reaching over -40% at 8 T for lower carrier concentrations.

Implications

The ability to control the superconducting phase via electric fields is significant for the development of mesoscopic circuits and could pave the way for advanced quantum technologies that exploit this tunable conductance. Furthermore, understanding the dichotomy between the superconducting and insulating phases deepens comprehension of complex oxides and their potential for strategic innovations in electronics.

Theoretical and Practical Implications

Electrostatic doping, as showcased in this paper, emerges as an ideal tool for probing the phase diagram of the LAO/STO system, circumventing issues with intrinsic and extrinsic doping methods. This work may inspire future theoretical frameworks addressing the quantum characteristics of oxide interfaces and stimulate technological advancements in electric field-tuned superconducting devices.

The strong correlation of $z\bar{\nu} = 2/3$ with prior studies on similar transitions suggests alignment with the 3D-XY model indicative of a relatively clean electronic system. The findings establish a solid foundation for understanding quantum critical points in 2D systems and suggest further explorations into interface engineering for electronic tailoring.

Future Developments

Further investigations might explore the comprehensive characterization of the quantum critical region, using more advanced methods to discern the precise nature of the insulating phase and its relationship with localized electron pairs. Additionally, ongoing studies could explore the broader applicability of electric field modulation in other oxide systems, which could lead to a broader class of functional electronic materials with controllable phases.

The discerned transition at the LAO/STO interface exemplifies the complex behavior and potential utility of oxide heterostructures as a versatile platform for future research and application in the fields of condensed matter physics and electronic engineering.