Ultrasound evidence for multicomponent superconducting order parameter in Ba1−xKxFe2As2 with electron quadrupling phase

Abstract Experiments have pointed to the formation of the electron quadrupling condensate in Ba 1− x K x Fe 2 As 2 at x ~ 0.8. The state spontaneously breaks time-reversal symmetry and is sandwiched between two critical points, separating it from the broken time-reversal symmetry (BTRS) superconducting state at $${T}_{{\rm{c}}}^{U(1)}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>U</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:math> and a normal-metal state at $${T}_{{\rm{c}}}^{{\rm{Z2}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Z2</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> . We report a theory of the acoustic effects spectroscopy of systems with an electron quadrupling phase based on ultrasound-velocity measurements. We show that the experimental results are consistent with BTRS superconductivity at x ~ 0.8, fulfilling the necessary condition for the formation of electron quadrupling in Ba 1− x K x Fe 2 As 2 . We provide the theoretical basis and the experimental strategy to study the order parameter symmetry of emerging quadrupling condensates in superconductors.