Andreev mapping studies of NbSe2 using Scanning Tunneling Microscopy
POSTER
Abstract
We report spatially resolved measurements of non-contact Andreev reflection (NCAR) in the layered superconductor NbSe₂ using scanning tunneling microscopy (STM). Unlike point-contact Andreev reflection (PCAR), which lacks spatial resolution and disturbs the surface, NCAR operates fully in the tunneling regime and enables site-by-site mapping of Andreev processes at the atomic scale.
By analyzing the tip-sample distance dependence of the tunneling conductance, we extract changes in the normalized decay rate (κ/κ₀) as a function of energy. Enhanced κ/κ₀ within the superconducting gap indicates Andreev reflection. We find that this signal varies significantly across the surface: it is stronger on Nb atoms than on Se, revealing the role of orbital-selective tunneling in coupling the STM tip to the superconducting condensate.
Beyond atomic contrast, we observe spatial modulations of the Andreev signal near crystallographic defects and within charge density wave (CDW) domains. These features are not accompanied by changes in the gap size, indicating that NCAR is sensitive to local variations in the electron-hole conversion probability, rather than simply density of states. This provides additional information inaccessible to conventional tunneling spectroscopy.
NbSe₂ is a known s-wave superconductor, but whether the gap is isotropic or anisotropic remains debated. The NCAR signal contains contributions from multiple Andreev processes and is sensitive to pairing symmetry. With improved energy resolution at lower temperatures, this technique may resolve subtle differences between isotropic and nodal s-wave states.
Our results demonstrate that NCAR enables direct, non-invasive mapping of superconducting correlations with atomic precision. It complements conventional STM spectroscopy and opens new possibilities for probing gap structure, orbital effects, and the influence of disorder and competing electronic orders in quantum materials.
By analyzing the tip-sample distance dependence of the tunneling conductance, we extract changes in the normalized decay rate (κ/κ₀) as a function of energy. Enhanced κ/κ₀ within the superconducting gap indicates Andreev reflection. We find that this signal varies significantly across the surface: it is stronger on Nb atoms than on Se, revealing the role of orbital-selective tunneling in coupling the STM tip to the superconducting condensate.
Beyond atomic contrast, we observe spatial modulations of the Andreev signal near crystallographic defects and within charge density wave (CDW) domains. These features are not accompanied by changes in the gap size, indicating that NCAR is sensitive to local variations in the electron-hole conversion probability, rather than simply density of states. This provides additional information inaccessible to conventional tunneling spectroscopy.
NbSe₂ is a known s-wave superconductor, but whether the gap is isotropic or anisotropic remains debated. The NCAR signal contains contributions from multiple Andreev processes and is sensitive to pairing symmetry. With improved energy resolution at lower temperatures, this technique may resolve subtle differences between isotropic and nodal s-wave states.
Our results demonstrate that NCAR enables direct, non-invasive mapping of superconducting correlations with atomic precision. It complements conventional STM spectroscopy and opens new possibilities for probing gap structure, orbital effects, and the influence of disorder and competing electronic orders in quantum materials.
*U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering DivisioN
Presenters
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Nirjhar Sarkar
- Oak Ridge National Laboratory