Topological superconductivity facilitated by exchange interaction on the surface of FeTe_0.5Se_0.5

Abstract

FeTe1−x Se x is a family of iron-based superconductors with a well-known critical temperature (T c) of 14.5 K for x = 0.45. Also well-established is the presence of topological surface states of FeTe0.55Se0.45 in which topological superconductivity sets in. By using density functional calculations and the Bardeen–Cooper–Schrieffer (BCS) theory for traditional superconductors we calculated the T c of the surface layers of FeTe0.5Se0.5 with the exchange interaction as the source of attractive force to form the Cooper pairs. The estimated T c is in reasonable agreement with the experimental value and suggests that the exchange interaction is behind the topological superconductivity on the surface of FeTe0.55Se0.45.

Pressure-dependent topological superconductivity on the surface of FeTe0.5Se0.5

“…Theoretical analysis [1][2][3] of FeTe 0.5 Se 0.5 , which is only slightly different from FeTe 0.55 Se 0.45 but much easier treated in computation, also confirms topological superconductivity. Dirac-cone type surface states within the gap of bulk states have been derived using density functional theory (DFT), and a Z 2 invariant equal to 1 is obtained by the parity of orbitals at time-reversal-invariant momenta in the first Brillouin zone.…”

“…where E s (E t ) is the total energy of the two electrons from intralayer Fe atoms at singlet (triplet) state and ψ k (r) is the Bloch wave function for an electron with crystal momentum k. The two-particle Coulomb interaction e 2 /ε |r − r |, with e the absolute value of the electron charge and ε the dielectric constant, indicates the origin of the exchange interaction. Calculated exchange energy using GGA functional at 1.08 GPa for nearest neighbors (J 1 ) of electrons is −15.4 meV, whose magnitude is considerably higher than the value (−11.8 meV) without the associated strain [3]. Exchange energy for second nearest neighbors (J 2 ) is −12.9 meV, which is also stronger in magnitude than J 2 at zero pressure.…”

“…As will be seen, pressure increases rapidly by compression along the z axis, with a and b kept constant. Throughout our investigation ab initio DFT calculations were executed using the VASP code [23,24] with general gradient approximation (GGA) [25] chosen for exchange-correlation functional, which helps straight comparison with references [2,3].…”

“…Research has shown that FeTe 1−x Se x [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] is a superconductor with its T c dependent on x, the proportional content of Se. An especially interesting example is FeTe 0.55 Se 0.45 , which has been confirmed by spin-polarized and angle-resolved photoelectron spectroscopy [1] to have a Dirac cone associated with its (001) surface states.…”

“…Antiferromagnetic (AFM) exchange interaction [4] or spin fluctuation [5] has been proposed for the mechanism of the unconventional superconductivity. Our previous calculation [3] based on DFT and maximally localized Wannier functions was able to extract the AFM exchange energy and used it as the attractive interaction between electrons of the Cooper pair in the BCS model. The derived T c and superconducting gap for FeTe 0.5 Se 0.5 are close to those of FeTe 0.55 Se 0.45 , proving theoretically that singlet exchange mediated by phonons is the source of topological superconductivity for the compound.…”