Topological ultranodal pair states in iron-based superconductors

Setty, Chandan; Bhattacharyya, Shinibali; Cao, Yifu; Kreisel, Andreas; Hirschfeld, P. J.

doi:10.1038/s41467-020-14357-2

Cited by 58 publications

(41 citation statements)

References 23 publications

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“…The pairing of electrons with additional internal degrees of freedom resulting, e.g., from different orbitals or basis sites, can lead to qualitatively new pairing states. Such pairing states have for example been proposed for iron-based superconductors [2][3][4][5][6][7][8][9], Cu x Bi 2 Se 3 [10,11], cubic systems such as half-Heusler compounds [12][13][14][15][16][17][18][19][20][21], UPt 3 [22,23], transition-metal dichalcogenides [24,25], and twisted bilayer graphene [26][27][28]. It has been shown that in centrosymmetric multiband superconductors that break TRS, point and line nodes are generically "inflated," by interband pairing, into Fermi surfaces of Bogoliubov quasiparticles [29,30].…”

Section: Introductionmentioning

confidence: 99%

Experimental consequences of Bogoliubov Fermi surfaces

2020

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Superconductors involving electrons with internal degrees of freedom beyond spin can have internally anisotropic pairing states that are impossible in single-band superconductors. As a case in point, in even-parity multiband superconductors that break time-reversal symmetry, nodes of the superconducting gap are generically inflated into two-dimensional Bogoliubov Fermi surfaces. The detection and characterization of these quasiparticle Fermi surfaces requires the understanding of their experimental consequences. In this paper, we derive the low-energy density of states for a broad range of possible nodal structures. Based on this, we calculate the low-temperature form of observables that are commonly employed for the characterization of nodal superconductors, i.e., the single-particle tunneling rate, the electronic specific heat and Sommerfeld coefficient, the thermal conductivity, the magnetic penetration depth, and the NMR spin-lattice relaxation rate, in the clean limit. We also address the question whether the topological invariant of the Bogoliubov Fermi surfaces is associated with topologically protected surface states, with negative results. This work is meant to serve as a guide for experimental searches for Bogoliubov Fermi surfaces in time-reversal-symmetry-breaking superconductors. arXiv:1909.10370v1 [cond-mat.supr-con]

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Section: Introductionmentioning

confidence: 99%

Experimental consequences of Bogoliubov Fermi surfaces

2020

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“…The existence of a Bogoliubov Fermi surface in this system would naturally explain why a relatively clean superconductor can support a finite density of quasiparticles, as reflected in the residual Sommerfeld coefficient and the differential conductance seen in STM [246,250]. Clearly, independent verification of the assumed time reversal symmetry breaking is needed before such an explanation can be accepted above more conventional ones, but the idea is intriguing.…”

Section: Bogoliubov Fermi Surface Scenariomentioning

confidence: 92%

“…One intriguing solution to this puzzle was put forward in reference [246], where it was suggested that the system might naturally make a transition into a topological state that manifested a so-called Bogoliubov Fermi surface, a locus of points in k-space that supported zero energy excitations at low temperature in the superconducting state. Note this manifold has the same dimension as that of the underlying normal state Fermi surface, i.e., a 2D patch in a system of three spatial dimensions, etc., as distinct from an unconventional superconductor with line or point nodes.…”

Section: Bogoliubov Fermi Surface Scenariomentioning

confidence: 99%

On the Remarkable Superconductivity of FeSe and Its Close Cousins

2020

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Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

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“…Such a Fermi surface is called the Bogoliubov Fermi surface (BFS). In superfluid 3 He and in cuprate superconductors [7][8][9][10], the BFS appears in the presence of superflow, while in systems with multiband energy spectrum or with broken time-reversal symmetry the BFS may exist even in the absence of superflow [11][12][13][14][15][16][17][18]. Note that in a nodal topological superfluid or in a cuprate superconductor the superflow explicitly breaks time-reversal and inversion symmetries, and thus origin of the BFS can be considered on a common ground in different systems.…”

Section: Introductionmentioning

confidence: 99%

Exceeding the Landau speed limit with topological Bogoliubov Fermi surfaces

Autti

Mäkinen

Rysti

et al. 2020

Phys. Rev. Research

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A common property of topological systems is the appearance of topologically protected zero-energy excitations. In a superconductor or superfluid, such states set the critical velocity of dissipationless flow v cL , proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of p-wave superfluid 3 He, which features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. The fluid is driven by rotation of the whole cryostat, and superflow breakdown is seen as the appearance of single-or half-quantum vortices. Vortices are detected using the relaxation rate of a Bose-Einstein condensate of magnons, created within the fluid. The superflow in the polar phase is found to be stable up to a finite critical velocity v c ≈ 0.2 cm/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above v cL but below v c is provided by the accumulation of the flow-induced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase, this surface has nontrivial topology which includes two pseudo-Weyl points. Vortices forming above the critical velocity are strongly pinned in the confining matrix, used to stabilize the polar phase, and hence stable macroscopic superflow can be maintained even when the external drive is brought to zero.

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Topological ultranodal pair states in iron-based superconductors

Cited by 58 publications

References 23 publications

Experimental consequences of Bogoliubov Fermi surfaces

Experimental consequences of Bogoliubov Fermi surfaces

On the Remarkable Superconductivity of FeSe and Its Close Cousins

Exceeding the Landau speed limit with topological Bogoliubov Fermi surfaces

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