Gap suppression at a Lifshitz transition in a multi-condensate superconductor

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Superconductivity in multi-orbital materials involves coupled condensates.
• Quantum confinement in 2D oxide interfaces allows band structure engineering.
• Electrostatic doping can control 3d orbital occupancy.

Purpose of the Study:

• To investigate the superfluid stiffness of the LaAlO3/SrTiO3 interface across its phase diagram.
• To explore the transition between single and two-condensate superconductivity.
• To understand the role of multi-orbital physics and electrostatic doping.

Main Methods:

• Resonant microwave transport measurements to determine superfluid stiffness.
• Continuous and reversible electrostatic doping.
• Numerical simulations of quantum wells to analyze band structure and Lifshitz transitions.

Main Results:

• Evidence of a transition from single-condensate to two-condensate superconductivity.
• This transition is driven by electrostatic doping and linked to a Lifshitz transition between 3d bands.
• Superconducting gap suppression occurs upon population of the second band, challenging BCS theory.

Conclusions:

• Electrostatic doping enables control over multiple condensates in superconducting interfaces.
• Oppositely signed superconducting order parameters in two condensates, due to repulsive coupling, explain the observed phenomena.
• Findings provide a new approach to tune multi-orbital physics in superconducting systems.