Interatomic Coulomb interaction and electron nematic bond order in FeSe

Abstract

Despite having the simplest atomic structure, bulk FeSe has an observed electronic structure with the largest deviation from the band theory predictions among all Fe-based superconductors and exhibits a low temperature nematic electronic state without intervening magnetic order. We show that the Fe-Fe interatomic Coulomb repulsion V offers a natural explanation for the puzzling electron correlation effects in FeSe superconductors. It produces a strongly renormalized low-energy band structure where the van Hove singularity sits remarkably close to Fermi level in the hightemperature electron liquid phase as observed experimentally. This proximity enables the quantum fluctuations in V to induce a rotational symmetry breaking electronic bond order in the d-wave channel. We argue that this emergent low-temperature d-wave bond nematic state, different from the commonly discussed ferro-orbital order and spin-nematicity, has been observed recently by several angle resolved photoemission experiments detecting the lifting of the band degeneracies at high symmetry points in the Brillouin zone. We present a symmetry analysis of the space group and identify the hidden antiunitary T -symmetry that protects the band degeneracy and the electronic order/interaction that can break the symmetry and lift the degeneracy. We show that the d-wave nematic bond order, together with the spin-orbit coupling, provide a unique explanation of the temperature dependence, momentum space anisotropy, and domain effects observed experimentally. We discuss the implications of our findings on the structural transition, the absence of magnetic order, and the intricate competition between nematicity and superconductivity in FeSe superconductors.

Effects of strain on the electronic structure, superconductivity, and nematicity in FeSe studied by angle-resolved photoemission spectroscopy

“…This is inferred from the negative correlation of ∆E nem to ∆E h−e [ Fig. 3(i)], which is also consistent with a recent theoretical study reporting the Pomeranchuk-type nematic instability at ∆E h−e ∼ 0 [27].…”

Robust short-range-ordered nematicity in FeSe evidenced by high-pressure NMR

“…These findings support the orbitalorder driven electronic nematicity in bulk FeSe, with possible momentum space anisotropy 22,25 . However, there are also controversies on the origin of nematicity in FeSe.…”

“…Recently, it was proposed that the nematicity in FeSe may be caused by local Hund's rule couplings 50 , or by interatomic Coulomb repulsion 25 . It remains to be seen how such microscopic interaction parameters, as well as the wavefunction overlap that governs the electronic structure, are affected by hydrostatic pressure.…”

Sign-Reversing Orbital Polarization in the Nematic Phase of FeSe due to theC2Symmetry Breaking in the Self-Energy

“…Microscopically, the orbital order is expressed by the symmetry breaking in the self-energy. In the mean-field level approximations, however, the self-energy is constant in k-space unless large inter-site Coulomb interactions are introduced [13]. For this reason, we have to study the non-local correlation effect beyond the mean-field theory, based on the realistic Hubbard model with onsite Coulomb interaction.…”

“…The spontaneous symmetry breaking from C 4 -to C 2 -symmetry, so called the electronic nematic transition, is one of the fundamental unsolved electronic properties in Fe-based superconductors. To explain this nematicity, both the spin-nematic scenario [1][2][3][4][5][6] and the orbital order scenario [7][8][9][10][11][12][13] have been studied intensively. Above the structural transition temperatures T str , large enhancement of the electronic nematic susceptibility predicted by both scenarios [1,10] is actually observed by the measurements of the softening of the shear modulus C 66 [1,10,14,15], Raman spectroscopy, [16][17][18], and in-plane resistivity anisotropy ∆ρ [19].…”