Role of Fermi surface anisotropy in the study of gap anisotropy using magnetic-field-angle dependence of thermal oscillations in AyFe2-xSe2 superconductors

TL;DR

This study numerically investigates how Fermi surface anisotropy influences the angular dependence of thermal oscillations in AyFe2-xSe2 superconductors, revealing complex behaviors that challenge simple gap structure interpretations.

Contribution

It demonstrates that Fermi surface anisotropy can cause significant oscillations and sign reversals in thermodynamic measurements, even with isotropic pairing, highlighting the complexity in analyzing gap anisotropy.

Findings

Oscillations occur even with isotropic pairing on anisotropic Fermi surfaces.

Double sign reversal of oscillations can happen due to competing anisotropies in multiband systems.

Field-angle measurements alone may not reliably determine gap anisotropy.

Abstract

We present a numerical study of the field-angle resolved oscillations of the thermal conductivity and specific heat under rotated magnetic field in the $A_y$Fe$_{2-x}$Se$_2$ [$A$=K,Rb,Cs,(Tl,K)] superconductors, using realistic two-band Fermi surface parameterization. Our key finding is that even for isotropic pairing on an anisotropic Fermi surface, the thermodynamic quantities exhibit substantial oscillatory behavior in the superconducting state, even much below the upper critical field. Furthermore, in multiband systems the competition of anisotropies between two Fermi surfaces can cause a double sign reversal of oscillations as a function of temperature, irrespective of gap anisotropy. Our findings put severe constraints on simple interpretations of field-angle resolved measurements widely used to identify the angular structure of the superconducting gap.