Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

Fernandes, Rafael M.; Vafek, Oskar

doi:10.1103/physrevb.90.214514

Cited by 58 publications

(96 citation statements)

References 49 publications

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“…Substituting (25) into (24) we find that the renormalization brings in additional pairing terms to originally local Hubbard-Hund interaction, in the form…”

Section: Supplementary Materials a Pairing In The Orbital And Bandmentioning

confidence: 99%

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Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides

Vafek

Chubukov

2017

Phys. Rev. Lett.

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We present a novel mechanism of s−wave pairing in Fe-based superconductors. The mechanism involves holes near dxz/dyz pockets only and is applicable primarily to strongly hole doped materials. We argue that as long as the renormalized Hund's coupling J exceeds the renormalized inter-orbital Hubbard repulsion U ′ , any finite spin-orbit coupling gives rise to s-wave superconductivity. This holds even at weak coupling and regardless of the strength of the intra-orbital Hubbard repulsion U . The transition temperature grows as the hole density decreases. The pairing gaps are four-fold symmetric, but anisotropic, with the possibility of eight accidental nodes along the larger pocket. The resulting state is consistent with the experiments on KFe2As2.

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“…Substituting (25) into (24) we find that the renormalization brings in additional pairing terms to originally local Hubbard-Hund interaction, in the form…”

Section: Supplementary Materials a Pairing In The Orbital And Bandmentioning

confidence: 99%

“…1). The effective Hamiltonian H = H 0 + H int for the low-energy states near Γ can be obtained, quite generally, using the method of invariants [24,25], without the need to assume a particular microscopic model. The non-interacting part is…”

Section: Pacs Numbersmentioning

confidence: 99%

Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides

Vafek

Chubukov

2017

Phys. Rev. Lett.

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“…As discussed in Ref. [46], there are two possible nematic couplings: λ 1 , which couples φ q to the onsite energy difference between the d xz and d yz orbitals, and λ 2 , which couples φ q to the hopping anisotropy between nearest-neighbor d xy orbitals (see Fig. 1b):…”

Section: Microscopic Modelmentioning

confidence: 99%

“…At the non-interacting level, this is accomplished by the atomic spin orbit coupling λ SOC S · L, which couples the d xz (d yz ) orbital associated with the Y (X) pocket to the d xy orbital associated with the X (Y ) pocket [46]:…”

Section: Microscopic Modelmentioning

confidence: 99%

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Kang

Fernandes

2016

Phys. Rev. Lett.

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The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both s-wave and d-wave symmetries are equally favored, and Tc is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the s-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li1−x(OH)xFeSe.In most iron-based superconductors (FeSC), superconductivity is found in close proximity to a magnetically ordered state, suggesting that magnetic fluctuations play an important role in binding the Cooper pairs [1][2][3][4]. Indeed, the fact that the Fermi surface of these materials is composed of small hole pockets and electron pockets separated by the magnetic ordering vector led to the proposal of a sign-changing s +− wave state, in which the gap function has different signs in the hole and in the electron pockets. However, the recent observation of superconductivity over 70 K in monolayer FeSe brought new challenges to the field [5][6][7][8][9][10][11][12][13][14]. In contrast to the standard FeSC, no long-range magnetic order is observed in thin films or even bulk FeSe [15], and the Fermi surface of monolayer FeSe consists of electron pockets only [6,7,11,16]. Since T c in monolayer FeSe is the highest among all FeSC, the elucidation of its origin is a fundamental step in the search for higher T c in these systems.One of the proposed scenarios to explain the dramatic ten-fold increase of T c in monolayer FeSe with respect to the 8 K value in bulk FeSe [17] was the strong coupling to an optical phonon mode of the SrTiO 3 (STO) substrate [16,18,19], which is manifested by replica bands observed in ARPES [16]. Although such a coupling can certainly enhance T c [20][21][22][23][24], recent experiments indicate that the STO substrate may not be essential to achieve the high-T c state. In particular, T c up to 40 K was observed in electrostatically-gated films of FeSe with different thickness grown both on STO and MgO substrates [25]. Similar values of T c were reported in FeSe coated with potassium [26,27] and in the bulk sample Li 1−x (OH) x FeSe [28,29], which consists of intercalated FeSe layers. In common to all these systems is the fact that their Fermi surface consists of electron pockets only, suggesting that doping by negative charge carriers plays a fundamental role in stabilizing the high-T c state.Importantly, recent experiments in K-coated bulk FeSe [26] revealed that, besides shifting the chemical potential, electron-doping also su...

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“…1(a)] [3][4][5][6][7], most microscopic theories for iron-based superconductors are focused on the role of spin [8][9][10], orbital [11], or nematic [7,12] fluctuations to the electron pairing and superconductivity. Although angleresolved photoemission spectroscopy (ARPES) experiments on different families of iron-based superconductors have identified the presence of SOC through observation of electronic band splitting at the Brillouin zone center (ZC) below T s [13][14][15], much is unknown concerning the role of SOC to the AF order, nematic phase, electron pairing mechanism, and superconductivity [16][17][18][19].In addition to its impact on the Fermi surface and electronic band dispersions, SOC also brings lattice anisotropies into anisotropies of magnetic fluctuations [20,21] [46][47][48][49][50][51]. Although polarized INS experiments have conclusively established the presence of SOC induced lowenergy spin excitation anisotropy near the resonance mode in different families of iron-based superconductors [23][24][25][26][27][28][29][30][31][32][33][34], the spin excitation anisotropy persists in the paramagnetic tetragonal state, and becomes isotropic at temperatures well above T N and T s [25,…”

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confidence: 99%

Spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe2(As0.7P0.3)2

Zhang

Yuan

et al. 2017

Phys. Rev. B

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We use neutron polarization analysis to study spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe 2 (As 0.7 P 0.3 ) 2 (T c = 30 K). Different from optimally hole-and electron-doped BaFe 2 As 2 , where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above T c , we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe 2 (As 0.7 P 0.3 ) 2 . Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center Q AF = (1,0,L = odd), while the magnetic spectrum is isotropic at the zone boundary Q = (1,0,L = even). By comparing the temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe 2 (As 0.7 P 0.3 ) 2 and hole-doped Ba 0.67 K 0.33 Fe 2 As 2 (T c = 38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity. DOI: 10.1103/PhysRevB.96.180503 Spin-orbit coupling (SOC) is an interaction of an electron's spin with its motion. While the importance of SOC to the electronic properties of the 4d and 5d correlated electron materials such as Sr 2 RuO 4 and Sr 2 IrO 4 is long recognized [1,2], its relevance to the physics of the 3d correlated electron materials such as iron pnictide superconductors is much less clear. Since iron pnictide superconductors are derived from metallic parent compounds exhibiting antiferromagnetic (AF) order at T N below a tetragonal-to-orthorhombic structural transition temperature T s associated with orbital ordering and nematic phase [ Fig. 1(a)] [3][4][5][6][7], most microscopic theories for iron-based superconductors are focused on the role of spin [8][9][10], orbital [11], or nematic [7,12] fluctuations to the electron pairing and superconductivity. Although angleresolved photoemission spectroscopy (ARPES) experiments on different families of iron-based superconductors have identified the presence of SOC through observation of electronic band splitting at the Brillouin zone center (ZC) below T s [13][14][15], much is unknown concerning the role of SOC to the AF order, nematic phase, electron pairing mechanism, and superconductivity [16][17][18][19].In addition to its impact on the Fermi surface and electronic band dispersions, SOC also brings lattice anisotropies into anisotropies of magnetic fluctuations [20,21] [46][47][48][49][50][51]. Although polarized INS experiments have conclusively established the presence of SOC induced lowenergy spin excitation anisotropy near the resonance mode in different families of iron-based superconductors [23][24][25][26][27][28][29][30][31][32][33][34], the spin excitation anisotropy persists in the paramagnetic tetragonal state, and becomes isotropic at temperatures well above T N and T s [25,26,[30][31][32]. Therefore, it is still unclear how SOC is coupled to spin fluctuation anisotropy and superconductivity.To resolve this problem, we used polarized INS to study low-energy spin ...

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Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

Cited by 58 publications

References 49 publications

Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides

Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe2(As0.7P0.3)2

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