Gapless superconductivity in Nb thin films probed by terahertz spectroscopy

Area of Science:

• Condensed matter physics
• Superconductivity
• Quantum phenomena

Background:

• Time reversal symmetry (TRS) breaking is a known mechanism for generating exotic quantum phases.
• In superconductors, magnetic field-induced TRS breaking can lead to a novel gapless superconducting state.
• Understanding this state is crucial for advancing condensed matter physics.

Purpose of the Study:

• To investigate the gapless superconducting state in niobium (Nb) thin films.
• To present a complete functional form of the superconducting order parameter under an arbitrary magnetic field.
• To explore the Lifshitz topological phase transition in this system.

Main Methods:

• Magneto-terahertz spectroscopy was employed to probe the superconducting state.
• Experimental data was analyzed to understand the behavior of the superconducting order parameter.
• Theoretical frameworks were challenged by the observed phenomena.

Main Results:

• A Lifshitz topological phase transition was observed, characterized by a vanishing quasiparticle gap across the Fermi surface.
• The superconducting order parameter was shown to transition smoothly from a gapped to a gapless regime.
• Magnetic pair-breaking effects in Nb were observed, contradicting traditional perturbative theories.

Conclusions:

• Magneto-terahertz spectroscopy offers a unique method to study the gapless superconducting state.
• The findings challenge existing theoretical models for magnetic pair-breaking effects.
• This work opens new avenues for exploring and manipulating gapless superconductivity.