Tip Pressure Induced Incoherent Energy Gap in CaFe2As2

TL;DR

This study demonstrates that applying local tip pressure via STM can induce an incoherent energy gap and local Cooper pairing in CaFe2As2, revealing pressure-sensitive electronic and structural phase changes.

Contribution

It shows that STM tip pressure can locally induce an incoherent energy gap and Cooper pairing in CaFe2As2, providing insights into pressure effects on its electronic phases.

Findings

Incoherent energy gap appears with increased tip pressure.

Tip pressure induces rhombic distortion of the As lattice.

The induced gap disappears below the bulk magnetic transition temperature.

Abstract

In CaFe2As2, superconductivity can be achieved by applying a modest c-axis pressure of several kbar. Here we use scanning tunneling microscopy/spectroscopy (STM/S) to explore the STM tip pressure effect on single crystals of CaFe2As2. When performing STM/S measurements, the tip-sample interaction can be controlled to act repulsive with reduction of the junction resistance, thus to apply a tip pressure on the sample. We find that an incoherent energy gap emerges at the Fermi level in the differential conductance spectrum when the tip pressure is increased. This energy gap is of the similar order of magnitude as the superconducting gap in the chemical doped compound Ca0.4Na0.6Fe2As2 and disappears at the temperature well below that of the bulk magnetic ordering. Moreover, we also observe the rhombic distortion of the As lattice, which agrees with the orthorhombic distortion of the underlying Fe lattice. These findings suggest that the STM tip pressure can induce the local Cooper pairing in the orthorhombic phase of CaFe2As2.