Tracking a spin-polarized superconducting bound state across a quantum phase transition

Area of Science:

• Condensed Matter Physics
• Quantum Materials
• Superconductivity

Background:

• Magnetic impurities in superconductors form Yu-Shiba-Rusinov (YSR) states.
• These states exhibit a quantum phase transition (QPT) with increasing exchange interaction.
• Observing the ground state change across the QPT is experimentally challenging.

Purpose of the Study:

• To develop a direct method for identifying the ground state of YSR states across a QPT.
• To investigate the spectral changes associated with the transition from a free spin to a screened spin ground state.
• To characterize the transition regime where YSR excitation energy interacts with Zeeman energy.

Main Methods:

• Utilizing ultralow temperature scanning tunneling microscopy (UT-STM).
• Probing the excitation spectrum of YSR states around a spin-1/2 impurity in a magnetic field.
• Analyzing the number and evolution of spectral peaks.

Main Results:

• The excitation spectrum transitions from two peaks (doublet, free spin) to four peaks (singlet, screened spin) across the QPT.
• A distinct transition regime was identified where YSR excitation energy is less than Zeeman energy.
• Direct observation of ground state changes across the QPT was achieved.

Conclusions:

• UT-STM provides a straightforward method for unambiguously identifying the ground state of spin-1/2 YSR states.
• Spectral peak evolution serves as a clear indicator of the YSR ground state across the QPT.
• The findings offer new insights into impurity physics in superconductors and quantum phase transitions.